SL-18330: Merge commit '6b53036' into DRTVWR-587-maint-V

Bring over part of the LLEventDispatcher work inspired by DRTVWR-558.

12 files changed, 828 insertions, 177 deletions

diff --git a/indra/llcommon/CMakeLists.txt b/indra/llcommon/CMakeLists.txt
index 54020a4231..91ae2440fa 100644
--- a/indra/llcommon/CMakeLists.txt
+++ b/indra/llcommon/CMakeLists.txt
@@ -17,6 +17,7 @@ include(Tracy)
 
 set(llcommon_SOURCE_FILES
     indra_constants.cpp
+    lazyeventapi.cpp
     llallocator.cpp
     llallocator_heap_profile.cpp
     llapp.cpp
@@ -115,11 +116,13 @@ set(llcommon_SOURCE_FILES
 set(llcommon_HEADER_FILES
     CMakeLists.txt
 
+    apply.h
     chrono.h
     classic_callback.h
     ctype_workaround.h
     fix_macros.h
     indra_constants.h
+    lazyeventapi.h
     linden_common.h
     llalignedarray.h
     llallocator.h
@@ -290,9 +293,9 @@ if (LL_TESTS)
 
   #set(TEST_DEBUG on)
   set(test_libs llcommon)
-  LL_ADD_INTEGRATION_TEST(bitpack "" "${test_libs}")
   LL_ADD_INTEGRATION_TEST(classic_callback "" "${test_libs}")
   LL_ADD_INTEGRATION_TEST(commonmisc "" "${test_libs}")
+  LL_ADD_INTEGRATION_TEST(lazyeventapi "" "${test_libs}")
   LL_ADD_INTEGRATION_TEST(llbase64 "" "${test_libs}")
   LL_ADD_INTEGRATION_TEST(llcond "" "${test_libs}")
   LL_ADD_INTEGRATION_TEST(lldate "" "${test_libs}")
diff --git a/indra/llcommon/lazyeventapi.cpp b/indra/llcommon/lazyeventapi.cpp
new file mode 100644
index 0000000000..028af9f33f
--- /dev/null
+++ b/indra/llcommon/lazyeventapi.cpp
@@ -0,0 +1,72 @@
+/**
+ * @file   lazyeventapi.cpp
+ * @author Nat Goodspeed
+ * @date   2022-06-17
+ * @brief  Implementation for lazyeventapi.
+ * 
+ * $LicenseInfo:firstyear=2022&license=viewerlgpl$
+ * Copyright (c) 2022, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+// Precompiled header
+#include "linden_common.h"
+// associated header
+#include "lazyeventapi.h"
+// STL headers
+// std headers
+#include <algorithm>                // std::find_if
+// external library headers
+// other Linden headers
+#include "llevents.h"
+#include "llsdutil.h"
+
+LL::LazyEventAPIBase::LazyEventAPIBase(
+    const std::string& name, const std::string& desc, const std::string& field)
+{
+    // populate embedded LazyEventAPIParams instance
+    mParams.name = name;
+    mParams.desc = desc;
+    mParams.field = field;
+    // mParams.init and mOperations are populated by subsequent add() calls.
+
+    // Our raison d'etre: register as an LLEventPumps::PumpFactory
+    // so obtain() will notice any request for this name and call us.
+    // Of course, our subclass constructor must finish running (making add()
+    // calls) before mParams will be fully populated, but we expect that to
+    // happen well before the first LLEventPumps::obtain(name) call.
+    mRegistered = LLEventPumps::instance().registerPumpFactory(
+        name,
+        [this](const std::string& name){ return construct(name); });
+}
+
+LL::LazyEventAPIBase::~LazyEventAPIBase()
+{
+    // If our constructor's registerPumpFactory() call was unsuccessful, that
+    // probably means somebody else claimed the name first. If that's the
+    // case, do NOT unregister their name out from under them!
+    // If this is a static instance being destroyed at process shutdown,
+    // LLEventPumps will probably have been cleaned up already.
+    if (mRegistered && ! LLEventPumps::wasDeleted())
+    {
+        // unregister the callback to this doomed instance
+        LLEventPumps::instance().unregisterPumpFactory(mParams.name);
+    }
+}
+
+LLSD LL::LazyEventAPIBase::getMetadata(const std::string& name) const
+{
+    // Since mOperations is a vector rather than a map, just search.
+    auto found = std::find_if(mOperations.begin(), mOperations.end(),
+                              [&name](const auto& namedesc)
+                              { return (namedesc.first == name); });
+    if (found == mOperations.end())
+        return {};
+
+    // LLEventDispatcher() supplements the returned metadata in different
+    // ways, depending on metadata provided to the specific add() method.
+    // Don't try to emulate all that. At some point we might consider more
+    // closely unifying LLEventDispatcher machinery with LazyEventAPI, but for
+    // now this will have to do.
+    return llsd::map("name", found->first, "desc", found->second);
+}
diff --git a/indra/llcommon/lazyeventapi.h b/indra/llcommon/lazyeventapi.h
new file mode 100644
index 0000000000..a815b119f0
--- /dev/null
+++ b/indra/llcommon/lazyeventapi.h
@@ -0,0 +1,204 @@
+/**
+ * @file   lazyeventapi.h
+ * @author Nat Goodspeed
+ * @date   2022-06-16
+ * @brief  Declaring a static module-scope LazyEventAPI registers a specific
+ *         LLEventAPI for future on-demand instantiation.
+ * 
+ * $LicenseInfo:firstyear=2022&license=viewerlgpl$
+ * Copyright (c) 2022, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+#if ! defined(LL_LAZYEVENTAPI_H)
+#define LL_LAZYEVENTAPI_H
+
+#include "apply.h"
+#include "lleventapi.h"
+#include "llinstancetracker.h"
+#include <boost/signals2/signal.hpp>
+#include <string>
+#include <tuple>
+#include <utility>                  // std::pair
+#include <vector>
+
+namespace LL
+{
+    /**
+     * Bundle params we want to pass to LLEventAPI's protected constructor. We
+     * package them this way so a subclass constructor can simply forward an
+     * opaque reference to the LLEventAPI constructor.
+     */
+    // This is a class instead of a plain struct mostly so when we forward-
+    // declare it we don't have to remember the distinction.
+    class LazyEventAPIParams
+    {
+    public:
+        // package the parameters used by the normal LLEventAPI constructor
+        std::string name, desc, field;
+        // bundle LLEventAPI::add() calls collected by LazyEventAPI::add(), so
+        // the special LLEventAPI constructor we engage can "play back" those
+        // add() calls
+        boost::signals2::signal<void(LLEventAPI*)> init;
+    };
+
+    /**
+     * LazyEventAPIBase implements most of the functionality of LazyEventAPI
+     * (q.v.), but we need the LazyEventAPI template subclass so we can accept
+     * the specific LLEventAPI subclass type.
+     */
+    // No LLInstanceTracker key: we don't need to find a specific instance,
+    // LLLeapListener just needs to be able to enumerate all instances.
+    class LazyEventAPIBase: public LLInstanceTracker<LazyEventAPIBase>
+    {
+    public:
+        LazyEventAPIBase(const std::string& name, const std::string& desc,
+                         const std::string& field);
+        virtual ~LazyEventAPIBase();
+
+        // Do not copy or move: once constructed, LazyEventAPIBase must stay
+        // put: we bind its instance pointer into a callback.
+        LazyEventAPIBase(const LazyEventAPIBase&) = delete;
+        LazyEventAPIBase(LazyEventAPIBase&&) = delete;
+        LazyEventAPIBase& operator=(const LazyEventAPIBase&) = delete;
+        LazyEventAPIBase& operator=(LazyEventAPIBase&&) = delete;
+
+        // capture add() calls we want to play back on LLEventAPI construction
+        template <typename... ARGS>
+        void add(const std::string& name, const std::string& desc, ARGS&&... rest)
+        {
+            // capture the metadata separately
+            mOperations.push_back(std::make_pair(name, desc));
+            // Use connect_extended() so the lambda is passed its own
+            // connection.
+            // We can't bind an unexpanded parameter pack into a lambda --
+            // shame really. Instead, capture it as a std::tuple and then, in
+            // the lambda, use apply() to convert back to function args.
+            mParams.init.connect_extended(
+                [name, desc, rest = std::make_tuple(std::forward<ARGS>(rest)...)]
+                (const boost::signals2::connection& conn, LLEventAPI* instance)
+                {
+                    // we only need this connection once
+                    conn.disconnect();
+                    // Our add() method distinguishes name and desc because we
+                    // capture them separately. But now, because apply()
+                    // expects a tuple specifying ALL the arguments, expand to
+                    // a tuple including add_trampoline() arguments: instance,
+                    // name, desc, rest.
+                    // apply() can't accept a template per se; it needs a
+                    // particular specialization.
+                    apply(&LazyEventAPIBase::add_trampoline<const std::string&, const std::string&,  ARGS...>,
+                          std::tuple_cat(std::make_tuple(instance, name, desc),
+                                         rest));
+                });
+        }
+
+        // The following queries mimic the LLEventAPI / LLEventDispatcher
+        // query API.
+
+        // Get the string name of the subject LLEventAPI
+        std::string getName() const { return mParams.name; }
+        // Get the documentation string
+        std::string getDesc() const { return mParams.desc; }
+        // Retrieve the LLSD key we use for dispatching
+        std::string getDispatchKey() const { return mParams.field; }
+
+        // operations
+        using NameDesc = std::pair<std::string, std::string>;
+
+    private:
+        // metadata that might be queried by LLLeapListener
+        std::vector<NameDesc> mOperations;
+
+    public:
+        using const_iterator = decltype(mOperations)::const_iterator;
+        const_iterator begin() const { return mOperations.begin(); }
+        const_iterator end()   const { return mOperations.end(); }
+        LLSD getMetadata(const std::string& name) const;
+
+    protected:
+        // Params with which to instantiate the companion LLEventAPI subclass
+        LazyEventAPIParams mParams;
+
+    private:
+        // true if we successfully registered our LLEventAPI on construction
+        bool mRegistered;
+
+        // actually instantiate the companion LLEventAPI subclass
+        virtual LLEventPump* construct(const std::string& name) = 0;
+
+        // Passing an overloaded function to any function that accepts an
+        // arbitrary callable is a PITB because you have to specify the
+        // correct overload. What we want is for the compiler to select the
+        // correct overload, based on the carefully-wrought enable_ifs in
+        // LLEventDispatcher. This (one and only) add_trampoline() method
+        // exists solely to pass to LL::apply(). Once add_trampoline() is
+        // called with the expanded arguments, we hope the compiler will Do
+        // The Right Thing in selecting the correct LLEventAPI::add()
+        // overload.
+        template <typename... ARGS>
+        static
+        void add_trampoline(LLEventAPI* instance, ARGS&&... args)
+        {
+            instance->add(std::forward<ARGS>(args)...);
+        }
+    };
+
+    /**
+     * LazyEventAPI provides a way to register a particular LLEventAPI to be
+     * instantiated on demand, that is, when its name is passed to
+     * LLEventPumps::obtain().
+     *
+     * Derive your listener from LLEventAPI as usual, with its various
+     * operation methods, but code your constructor to accept
+     * <tt>(const LL::LazyEventAPIParams& params)</tt>
+     * and forward that reference to (the protected)
+     * <tt>LLEventAPI(const LL::LazyEventAPIParams&)</tt> constructor.
+     *
+     * Then derive your listener registrar from
+     * <tt>LazyEventAPI<your LLEventAPI subclass></tt>. The constructor should
+     * look very like a traditional LLEventAPI constructor:
+     *
+     * * pass (name, desc [, field]) to LazyEventAPI's constructor
+     * * in the body, make a series of add() calls referencing your LLEventAPI
+     *   subclass methods.
+     *
+     * You may use any LLEventAPI::add() methods, that is, any
+     * LLEventDispatcher::add() methods. But the target methods you pass to
+     * add() must belong to your LLEventAPI subclass, not the LazyEventAPI
+     * subclass.
+     *
+     * Declare a static instance of your LazyEventAPI listener registrar
+     * class. When it's constructed at static initialization time, it will
+     * register your LLEventAPI subclass with LLEventPumps. It will also
+     * collect metadata for the LLEventAPI and its operations to provide to
+     * LLLeapListener's introspection queries.
+     *
+     * When someone later calls LLEventPumps::obtain() to post an event to
+     * your LLEventAPI subclass, obtain() will instantiate it using
+     * LazyEventAPI's name, desc, field and add() calls.
+     */
+    template <class EVENTAPI>
+    class LazyEventAPI: public LazyEventAPIBase
+    {
+    public:
+        // for subclass constructor to reference handler methods
+        using listener = EVENTAPI;
+
+        LazyEventAPI(const std::string& name, const std::string& desc,
+                     const std::string& field="op"):
+            // Forward ctor params to LazyEventAPIBase
+            LazyEventAPIBase(name, desc, field)
+        {}
+
+    private:
+        LLEventPump* construct(const std::string& /*name*/) override
+        {
+            // base class has carefully assembled LazyEventAPIParams embedded
+            // in this instance, just pass to LLEventAPI subclass constructor
+            return new EVENTAPI(mParams);
+        }
+    };
+} // namespace LL
+
+#endif /* ! defined(LL_LAZYEVENTAPI_H) */
diff --git a/indra/llcommon/lleventapi.cpp b/indra/llcommon/lleventapi.cpp
index ff5459c1eb..3d46ef1034 100644
--- a/indra/llcommon/lleventapi.cpp
+++ b/indra/llcommon/lleventapi.cpp
@@ -35,6 +35,7 @@
 // external library headers
 // other Linden headers
 #include "llerror.h"
+#include "lazyeventapi.h"
 
 LLEventAPI::LLEventAPI(const std::string& name, const std::string& desc, const std::string& field):
     lbase(name, field),
@@ -43,6 +44,13 @@ LLEventAPI::LLEventAPI(const std::string& name, const std::string& desc, const s
 {
 }
 
+LLEventAPI::LLEventAPI(const LL::LazyEventAPIParams& params):
+    LLEventAPI(params.name, params.desc, params.field)
+{
+    // call initialization functions with our brand-new instance pointer
+    params.init(this);
+}
+
 LLEventAPI::~LLEventAPI()
 {
 }
diff --git a/indra/llcommon/lleventapi.h b/indra/llcommon/lleventapi.h
index 5991fe8fd5..25f6becd8b 100644
--- a/indra/llcommon/lleventapi.h
+++ b/indra/llcommon/lleventapi.h
@@ -35,6 +35,11 @@
 #include "llinstancetracker.h"
 #include <string>
 
+namespace LL
+{
+    class LazyEventAPIParams;
+}
+
 /**
  * LLEventAPI not only provides operation dispatch functionality, inherited
  * from LLDispatchListener -- it also gives us event API introspection.
@@ -65,19 +70,6 @@ public:
     std::string getDesc() const { return mDesc; }
 
     /**
-     * Publish only selected add() methods from LLEventDispatcher.
-     * Every LLEventAPI add() @em must have a description string.
-     */
-    template <typename CALLABLE>
-    void add(const std::string& name,
-             const std::string& desc,
-             CALLABLE callable,
-             const LLSD& required=LLSD())
-    {
-        LLEventDispatcher::add(name, desc, callable, required);
-    }
-
-    /**
      * Instantiate a Response object in any LLEventAPI subclass method that
      * wants to guarantee a reply (if requested) will be sent on exit from the
      * method. The reply will be sent if request.has(@a replyKey), default
@@ -150,16 +142,20 @@ public:
          * @endcode
          */
         LLSD& operator[](const LLSD::String& key) { return mResp[key]; }
-		
-		 /**
-		 * set the response to the given data
-		 */
-		void setResponse(LLSD const & response){ mResp = response; }
+
+         /**
+         * set the response to the given data
+         */
+        void setResponse(LLSD const & response){ mResp = response; }
 
         LLSD mResp, mReq;
         LLSD::String mKey;
     };
 
+protected:
+    // constructor used only by subclasses registered by LazyEventAPI
+    LLEventAPI(const LL::LazyEventAPIParams&);
+
 private:
     std::string mDesc;
 };
diff --git a/indra/llcommon/lleventdispatcher.cpp b/indra/llcommon/lleventdispatcher.cpp
index cd0ab6bc29..0c3bb35cfe 100644
--- a/indra/llcommon/lleventdispatcher.cpp
+++ b/indra/llcommon/lleventdispatcher.cpp
@@ -40,15 +40,24 @@
 // other Linden headers
 #include "llevents.h"
 #include "llerror.h"
+#include "llexception.h"
 #include "llsdutil.h"
 #include "stringize.h"
 #include <memory>                   // std::auto_ptr
 
 /*****************************************************************************
+*   DispatchError
+*****************************************************************************/
+struct DispatchError: public LLException
+{
+    DispatchError(const std::string& what): LLException(what) {}
+};
+
+/*****************************************************************************
 *   LLSDArgsSource
 *****************************************************************************/
 /**
- * Store an LLSD array, producing its elements one at a time. Die with LL_ERRS
+ * Store an LLSD array, producing its elements one at a time. It is an error
  * if the consumer requests more elements than the array contains.
  */
 class LL_COMMON_API LLSDArgsSource
@@ -74,8 +83,7 @@ LLSDArgsSource::LLSDArgsSource(const std::string function, const LLSD& args):
 {
     if (! (_args.isUndefined() || _args.isArray()))
     {
-        LL_ERRS("LLSDArgsSource") << _function << " needs an args array instead of "
-                                  << _args << LL_ENDL;
+        LLTHROW(DispatchError(stringize(_function, " needs an args array instead of ", _args)));
     }
 }
 
@@ -88,8 +96,8 @@ LLSD LLSDArgsSource::next()
 {
     if (_index >= _args.size())
     {
-        LL_ERRS("LLSDArgsSource") << _function << " requires more arguments than the "
-                                  << _args.size() << " provided: " << _args << LL_ENDL;
+        LLTHROW(DispatchError(stringize(_function, " requires more arguments than the ",
+                                        _args.size(), " provided: ", _args)));
     }
     return _args[_index++];
 }
@@ -163,7 +171,8 @@ public:
     /// default values
     LLSDArgsMapper(const std::string& function, const LLSD& names, const LLSD& defaults);
 
-    /// Given arguments map, return LLSD::Array of parameter values, or LL_ERRS.
+    /// Given arguments map, return LLSD::Array of parameter values, or
+    /// trigger error.
     LLSD map(const LLSD& argsmap) const;
 
 private:
@@ -195,7 +204,7 @@ LLSDArgsMapper::LLSDArgsMapper(const std::string& function,
 {
     if (! (_names.isUndefined() || _names.isArray()))
     {
-        LL_ERRS("LLSDArgsMapper") << function << " names must be an array, not " << names << LL_ENDL;
+        LLTHROW(DispatchError(stringize(function, " names must be an array, not ", names)));
     }
     auto nparams(_names.size());
     // From _names generate _indexes.
@@ -218,8 +227,8 @@ LLSDArgsMapper::LLSDArgsMapper(const std::string& function,
         // defaults is a (possibly empty) array. Right-align it with names.
         if (ndefaults > nparams)
         {
-            LL_ERRS("LLSDArgsMapper") << function << " names array " << names
-                                      << " shorter than defaults array " << defaults << LL_ENDL;
+            LLTHROW(DispatchError(stringize(function, " names array ", names,
+                                            " shorter than defaults array ", defaults)));
         }
 
         // Offset by which we slide defaults array right to right-align with
@@ -256,14 +265,14 @@ LLSDArgsMapper::LLSDArgsMapper(const std::string& function,
         }
         if (bogus.size())
         {
-            LL_ERRS("LLSDArgsMapper") << function << " defaults specified for nonexistent params "
-                                      << formatlist(bogus) << LL_ENDL;
+            LLTHROW(DispatchError(stringize(function, " defaults specified for nonexistent params ",
+                                            formatlist(bogus))));
         }
     }
     else
     {
-        LL_ERRS("LLSDArgsMapper") << function << " defaults must be a map or an array, not "
-                                  << defaults << LL_ENDL;
+        LLTHROW(DispatchError(stringize(function, " defaults must be a map or an array, not ",
+                                        defaults)));
     }
 }
 
@@ -271,8 +280,8 @@ LLSD LLSDArgsMapper::map(const LLSD& argsmap) const
 {
     if (! (argsmap.isUndefined() || argsmap.isMap() || argsmap.isArray()))
     {
-        LL_ERRS("LLSDArgsMapper") << _function << " map() needs a map or array, not "
-                                  << argsmap << LL_ENDL;
+        LLTHROW(DispatchError(stringize(_function, " map() needs a map or array, not ",
+                                        argsmap)));
     }
     // Initialize the args array. Indexing a non-const LLSD array grows it
     // to appropriate size, but we don't want to resize this one on each
@@ -369,8 +378,8 @@ LLSD LLSDArgsMapper::map(const LLSD& argsmap) const
     // by argsmap, that's a problem.
     if (unfilled.size())
     {
-        LL_ERRS("LLSDArgsMapper") << _function << " missing required arguments "
-                                  << formatlist(unfilled) << " from " << argsmap << LL_ENDL;
+        LLTHROW(DispatchError(stringize(_function, " missing required arguments ",
+                                        formatlist(unfilled), " from ", argsmap)));
     }
 
     // done
@@ -420,7 +429,7 @@ struct LLEventDispatcher::LLSDDispatchEntry: public LLEventDispatcher::DispatchE
         std::string mismatch(llsd_matches(mRequired, event));
         if (! mismatch.empty())
         {
-            LL_ERRS("LLEventDispatcher") << desc << ": bad request: " << mismatch << LL_ENDL;
+            LLTHROW(DispatchError(stringize(desc, ": bad request: ", mismatch)));
         }
         // Event syntax looks good, go for it!
         mFunc(event);
@@ -577,13 +586,6 @@ void LLEventDispatcher::add(const std::string& name, const std::string& desc,
                                     new LLSDDispatchEntry(desc, callable, required))));
 }
 
-void LLEventDispatcher::addFail(const std::string& name, const std::string& classname) const
-{
-    LL_ERRS("LLEventDispatcher") << "LLEventDispatcher(" << mDesc << ")::add(" << name
-                                 << "): " << classname << " is not a subclass "
-                                 << "of LLEventDispatcher" << LL_ENDL;
-}
-
 /// Unregister a callable
 bool LLEventDispatcher::remove(const std::string& name)
 {
@@ -596,48 +598,90 @@ bool LLEventDispatcher::remove(const std::string& name)
     return true;
 }
 
-/// Call a registered callable with an explicitly-specified name. If no
-/// such callable exists, die with LL_ERRS.
+/// Call a registered callable with an explicitly-specified name. It is an
+/// error if no such callable exists.
 void LLEventDispatcher::operator()(const std::string& name, const LLSD& event) const
 {
-    if (! try_call(name, event))
+    std::string error{ try_call_log(std::string(), name, event) };
+    if (! error.empty())
     {
-        LL_ERRS("LLEventDispatcher") << "LLEventDispatcher(" << mDesc << "): '" << name
-                                     << "' not found" << LL_ENDL;
+        callFail(event, error);
     }
 }
 
-/// Extract the @a key value from the incoming @a event, and call the
-/// callable whose name is specified by that map @a key. If no such
-/// callable exists, die with LL_ERRS.
+/// Extract the @a key value from the incoming @a event, and call the callable
+/// whose name is specified by that map @a key. It is an error if no such
+/// callable exists.
 void LLEventDispatcher::operator()(const LLSD& event) const
 {
-    // This could/should be implemented in terms of the two-arg overload.
-    // However -- we can produce a more informative error message.
-    std::string name(event[mKey]);
-    if (! try_call(name, event))
+    std::string error{ try_call_log(mKey, event[mKey], event) };
+    if (! error.empty())
     {
-        LL_ERRS("LLEventDispatcher") << "LLEventDispatcher(" << mDesc << "): bad " << mKey
-                                     << " value '" << name << "'" << LL_ENDL;
+        callFail(event, error);
+    }
+}
+
+void LLEventDispatcher::callFail(const LLSD& event, const std::string& msg) const
+{
+    static LLSD::String key{ "reply" };
+    if (event.has(key))
+    {
+        // Oh good, the incoming event specifies a reply pump -- pass back a
+        // response that includes an "error" key with the message.
+        sendReply(llsd::map("error", msg), event, key);
     }
 }
 
 bool LLEventDispatcher::try_call(const LLSD& event) const
 {
-    return try_call(event[mKey], event);
+    return try_call_log(mKey, event[mKey], event).empty();
 }
 
 bool LLEventDispatcher::try_call(const std::string& name, const LLSD& event) const
 {
+    return try_call_log(std::string(), name, event).empty();
+}
+
+std::string LLEventDispatcher::try_call_log(const std::string& key, const std::string& name,
+                                            const LLSD& event) const
+{
+    std::string error{ try_call(key, name, event) };
+    if (! error.empty())
+    {
+        LL_WARNS("LLEventDispatcher") << error << LL_ENDL;
+    }
+    return error;
+}
+
+// This internal method returns empty string if the call succeeded, else
+// non-empty error message.
+std::string LLEventDispatcher::try_call(const std::string& key, const std::string& name,
+                                        const LLSD& event) const
+{
     DispatchMap::const_iterator found = mDispatch.find(name);
     if (found == mDispatch.end())
     {
-        return false;
+        if (key.empty())
+        {
+            return stringize("LLEventDispatcher(", mDesc, "): '", name, "' not found");
+        }
+        else
+        {
+            return stringize("LLEventDispatcher(", mDesc, "): bad ", key, " value '", name, "'");
+        }
+    }
+
+    try
+    {
+        // Found the name, so it's plausible to even attempt the call.
+        found->second->call(stringize("LLEventDispatcher(", mDesc, ") calling '", name, "'"),
+                            event);
+    }
+    catch (const DispatchError& err)
+    {
+        return err.what();
     }
-    // Found the name, so it's plausible to even attempt the call.
-    found->second->call(STRINGIZE("LLEventDispatcher(" << mDesc << ") calling '" << name << "'"),
-                        event);
-    return true;                    // tell caller we were able to call
+    return {};                      // tell caller we were able to call
 }
 
 LLSD LLEventDispatcher::getMetadata(const std::string& name) const
@@ -655,8 +699,17 @@ LLSD LLEventDispatcher::getMetadata(const std::string& name) const
 
 LLDispatchListener::LLDispatchListener(const std::string& pumpname, const std::string& key):
     LLEventDispatcher(pumpname, key),
-    mPump(pumpname, true),          // allow tweaking for uniqueness
-    mBoundListener(mPump.listen("self", boost::bind(&LLDispatchListener::process, this, _1)))
+    // Do NOT tweak the passed pumpname. In practice, when someone
+    // instantiates a subclass of our LLEventAPI subclass, they intend to
+    // claim that LLEventPump name in the global LLEventPumps namespace. It
+    // would be mysterious and distressing if we allowed name tweaking, and
+    // someone else claimed pumpname first for a completely unrelated
+    // LLEventPump. Posted events would never reach our subclass listener
+    // because we would have silently changed its name; meanwhile listeners
+    // (if any) on that other LLEventPump would be confused by the events
+    // intended for our subclass.
+    LLEventStream(pumpname, false),
+    mBoundListener(listen("self", [this](const LLSD& event){ return process(event); }))
 {
 }
 
diff --git a/indra/llcommon/lleventdispatcher.h b/indra/llcommon/lleventdispatcher.h
index 9e1244ef5b..6d1df86fea 100644
--- a/indra/llcommon/lleventdispatcher.h
+++ b/indra/llcommon/lleventdispatcher.h
@@ -61,7 +61,6 @@ static const auto& nil(nil_);
 #include <boost/function.hpp>
 #include <boost/bind.hpp>
 #include <boost/iterator/transform_iterator.hpp>
-#include <boost/utility/enable_if.hpp>
 #include <boost/function_types/is_nonmember_callable_builtin.hpp>
 #include <boost/function_types/parameter_types.hpp>
 #include <boost/function_types/function_arity.hpp>
@@ -166,11 +165,12 @@ public:
      * When calling this name, pass an LLSD::Array. Each entry in turn will be
      * converted to the corresponding parameter type using LLSDParam.
      */
-    template<typename Function>
-    typename boost::enable_if< boost::function_types::is_nonmember_callable_builtin<Function>
-                               >::type add(const std::string& name,
-                                           const std::string& desc,
-                                           Function f);
+    // enable_if usage per https://stackoverflow.com/a/39913395/5533635
+    template<typename Function,
+             typename = typename std::enable_if<
+                 boost::function_types::is_nonmember_callable_builtin<Function>::value
+             >::type>
+    void add(const std::string& name, const std::string& desc, Function f);
 
     /**
      * Register a nonstatic class method with arbitrary parameters.
@@ -189,12 +189,13 @@ public:
      * When calling this name, pass an LLSD::Array. Each entry in turn will be
      * converted to the corresponding parameter type using LLSDParam.
      */
-    template<typename Method, typename InstanceGetter>
-    typename boost::enable_if< boost::function_types::is_member_function_pointer<Method>
-                               >::type add(const std::string& name,
-                                           const std::string& desc,
-                                           Method f,
-                                           const InstanceGetter& getter);
+    template<typename Method, typename InstanceGetter,
+             typename = typename std::enable_if<
+                 boost::function_types::is_member_function_pointer<Method>::value &&
+                 ! std::is_convertible<InstanceGetter, LLSD>::value
+             >::type>
+    void add(const std::string& name, const std::string& desc, Method f,
+             const InstanceGetter& getter);
 
     /**
      * Register a free function with arbitrary parameters. (This also works
@@ -211,13 +212,12 @@ public:
      * an LLSD::Array using LLSDArgsMapper and then convert each entry in turn
      * to the corresponding parameter type using LLSDParam.
      */
-    template<typename Function>
-    typename boost::enable_if< boost::function_types::is_nonmember_callable_builtin<Function>
-                               >::type add(const std::string& name,
-                                           const std::string& desc,
-                                           Function f,
-                                           const LLSD& params,
-                                           const LLSD& defaults=LLSD());
+    template<typename Function,
+             typename = typename std::enable_if<
+                 boost::function_types::is_nonmember_callable_builtin<Function>::value
+             >::type>
+    void add(const std::string& name, const std::string& desc, Function f,
+             const LLSD& params, const LLSD& defaults=LLSD());
 
     /**
      * Register a nonstatic class method with arbitrary parameters.
@@ -240,42 +240,42 @@ public:
      * an LLSD::Array using LLSDArgsMapper and then convert each entry in turn
      * to the corresponding parameter type using LLSDParam.
      */
-    template<typename Method, typename InstanceGetter>
-    typename boost::enable_if< boost::function_types::is_member_function_pointer<Method>
-                               >::type add(const std::string& name,
-                                           const std::string& desc,
-                                           Method f,
-                                           const InstanceGetter& getter,
-                                           const LLSD& params,
-                                           const LLSD& defaults=LLSD());
+    template<typename Method, typename InstanceGetter,
+             typename = typename std::enable_if<
+                 boost::function_types::is_member_function_pointer<Method>::value &&
+                 ! std::is_convertible<InstanceGetter, LLSD>::value
+             >::type>
+    void add(const std::string& name, const std::string& desc, Method f,
+             const InstanceGetter& getter, const LLSD& params,
+             const LLSD& defaults=LLSD());
 
     //@}    
 
     /// Unregister a callable
     bool remove(const std::string& name);
 
-    /// Call a registered callable with an explicitly-specified name. If no
-    /// such callable exists, die with LL_ERRS. If the @a event fails to match
-    /// the @a required prototype specified at add() time, die with LL_ERRS.
+    /// Call a registered callable with an explicitly-specified name. It is an
+    /// error if no such callable exists. It is an error if the @a event fails
+    /// to match the @a required prototype specified at add() time.
     void operator()(const std::string& name, const LLSD& event) const;
 
     /// Call a registered callable with an explicitly-specified name and
     /// return <tt>true</tt>. If no such callable exists, return
-    /// <tt>false</tt>. If the @a event fails to match the @a required
-    /// prototype specified at add() time, die with LL_ERRS.
+    /// <tt>false</tt>. It is an error if the @a event fails to match the @a
+    /// required prototype specified at add() time.
     bool try_call(const std::string& name, const LLSD& event) const;
 
     /// Extract the @a key value from the incoming @a event, and call the
-    /// callable whose name is specified by that map @a key. If no such
-    /// callable exists, die with LL_ERRS. If the @a event fails to match the
-    /// @a required prototype specified at add() time, die with LL_ERRS.
+    /// callable whose name is specified by that map @a key. It is an error if
+    /// no such callable exists. It is an error if the @a event fails to match
+    /// the @a required prototype specified at add() time.
     void operator()(const LLSD& event) const;
 
     /// Extract the @a key value from the incoming @a event, call the callable
     /// whose name is specified by that map @a key and return <tt>true</tt>.
-    /// If no such callable exists, return <tt>false</tt>. If the @a event
-    /// fails to match the @a required prototype specified at add() time, die
-    /// with LL_ERRS.
+    /// If no such callable exists, return <tt>false</tt>. It is an error if
+    /// the @a event fails to match the @a required prototype specified at
+    /// add() time.
     bool try_call(const LLSD& event) const;
 
     /// @name Iterate over defined names
@@ -329,17 +329,16 @@ private:
     void addMethod(const std::string& name, const std::string& desc,
                    const METHOD& method, const LLSD& required)
     {
-        CLASS* downcast = dynamic_cast<CLASS*>(this);
-        if (! downcast)
-        {
-            addFail(name, typeid(CLASS).name());
-        }
-        else
-        {
-            add(name, desc, boost::bind(method, downcast, _1), required);
-        }
+        CLASS* downcast = static_cast<CLASS*>(this);
+        add(name, desc, boost::bind(method, downcast, _1), required);
     }
-    void addFail(const std::string& name, const std::string& classname) const;
+    std::string try_call_log(const std::string& key, const std::string& name,
+                             const LLSD& event) const;
+    std::string try_call(const std::string& key, const std::string& name,
+                         const LLSD& event) const;
+    // Implement "it is an error" semantics for attempted call operations: if
+    // the incoming event includes a "reply" key, log and send an error reply.
+    void callFail(const LLSD& event, const std::string& msg) const;
 
     std::string mDesc, mKey;
     DispatchMap mDispatch;
@@ -427,7 +426,25 @@ struct LLEventDispatcher::invoker
         // Instead of grabbing the first item from argsrc and making an
         // LLSDParam of it, call getter() and pass that as the instance param.
         invoker<Function, next_iter_type, To>::apply
-        ( func, argsrc, boost::fusion::push_back(boost::fusion::nil(), boost::ref(getter())));
+        ( func, argsrc, boost::fusion::push_back(boost::fusion::nil(), bindable(getter())));
+    }
+
+    template <typename T>
+    static inline
+    auto bindable(T&& value,
+                  typename std::enable_if<std::is_pointer<T>::value, bool>::type=true)
+    {
+        // if passed a pointer, just return that pointer
+        return std::forward<T>(value);
+    }
+
+    template <typename T>
+    static inline
+    auto bindable(T&& value,
+                  typename std::enable_if<! std::is_pointer<T>::value, bool>::type=true)
+    {
+        // if passed a reference, wrap it for binding
+        return std::ref(std::forward<T>(value));
     }
 };
 
@@ -446,9 +463,8 @@ struct LLEventDispatcher::invoker<Function,To,To>
     }
 };
 
-template<typename Function>
-typename boost::enable_if< boost::function_types::is_nonmember_callable_builtin<Function> >::type
-LLEventDispatcher::add(const std::string& name, const std::string& desc, Function f)
+template<typename Function, typename>
+void LLEventDispatcher::add(const std::string& name, const std::string& desc, Function f)
 {
     // Construct an invoker_function, a callable accepting const args_source&.
     // Add to DispatchMap an ArrayParamsDispatchEntry that will handle the
@@ -457,10 +473,9 @@ LLEventDispatcher::add(const std::string& name, const std::string& desc, Functio
                                 boost::function_types::function_arity<Function>::value);
 }
 
-template<typename Method, typename InstanceGetter>
-typename boost::enable_if< boost::function_types::is_member_function_pointer<Method> >::type
-LLEventDispatcher::add(const std::string& name, const std::string& desc, Method f,
-                       const InstanceGetter& getter)
+template<typename Method, typename InstanceGetter, typename>
+void LLEventDispatcher::add(const std::string& name, const std::string& desc, Method f,
+                            const InstanceGetter& getter)
 {
     // Subtract 1 from the compile-time arity because the getter takes care of
     // the first parameter. We only need (arity - 1) additional arguments.
@@ -468,20 +483,18 @@ LLEventDispatcher::add(const std::string& name, const std::string& desc, Method
                                 boost::function_types::function_arity<Method>::value - 1);
 }
 
-template<typename Function>
-typename boost::enable_if< boost::function_types::is_nonmember_callable_builtin<Function> >::type
-LLEventDispatcher::add(const std::string& name, const std::string& desc, Function f,
-                       const LLSD& params, const LLSD& defaults)
+template<typename Function, typename>
+void LLEventDispatcher::add(const std::string& name, const std::string& desc, Function f,
+                            const LLSD& params, const LLSD& defaults)
 {
     // See comments for previous is_nonmember_callable_builtin add().
     addMapParamsDispatchEntry(name, desc, make_invoker(f), params, defaults);
 }
 
-template<typename Method, typename InstanceGetter>
-typename boost::enable_if< boost::function_types::is_member_function_pointer<Method> >::type
-LLEventDispatcher::add(const std::string& name, const std::string& desc, Method f,
-                       const InstanceGetter& getter,
-                       const LLSD& params, const LLSD& defaults)
+template<typename Method, typename InstanceGetter, typename>
+void LLEventDispatcher::add(const std::string& name, const std::string& desc, Method f,
+                            const InstanceGetter& getter,
+                            const LLSD& params, const LLSD& defaults)
 {
     addMapParamsDispatchEntry(name, desc, make_invoker(f, getter), params, defaults);
 }
@@ -524,17 +537,21 @@ LLEventDispatcher::make_invoker(Method f, const InstanceGetter& getter)
  * LLEventPump name and dispatch key, and add() its methods. Incoming events
  * will automatically be dispatched.
  */
-class LL_COMMON_API LLDispatchListener: public LLEventDispatcher
+// Instead of containing an LLEventStream, LLDispatchListener derives from it.
+// This allows an LLEventPumps::PumpFactory to return a pointer to an
+// LLDispatchListener (subclass) instance, and still have ~LLEventPumps()
+// properly clean it up.
+class LL_COMMON_API LLDispatchListener:
+    public LLEventDispatcher,
+    public LLEventStream
 {
 public:
     LLDispatchListener(const std::string& pumpname, const std::string& key);
-
-    std::string getPumpName() const { return mPump.getName(); }
+    virtual ~LLDispatchListener() {}
 
 private:
     bool process(const LLSD& event);
 
-    LLEventStream mPump;
     LLTempBoundListener mBoundListener;
 };
 
diff --git a/indra/llcommon/llevents.cpp b/indra/llcommon/llevents.cpp
index 0a213bddef..1a305ec3dc 100644
--- a/indra/llcommon/llevents.cpp
+++ b/indra/llcommon/llevents.cpp
@@ -68,19 +68,78 @@
 LLEventPumps::LLEventPumps():
     mFactories
     {
-        { "LLEventStream",   [](const std::string& name, bool tweak)
+        { "LLEventStream",   [](const std::string& name, bool tweak, const std::string& /*type*/)
                              { return new LLEventStream(name, tweak); } },
-        { "LLEventMailDrop", [](const std::string& name, bool tweak)
+        { "LLEventMailDrop", [](const std::string& name, bool tweak, const std::string& /*type*/)
                              { return new LLEventMailDrop(name, tweak); } }
     },
     mTypes
     {
-        // LLEventStream is the default for obtain(), so even if somebody DOES
-        // call obtain("placeholder"), this sample entry won't break anything.
-        { "placeholder", "LLEventStream" }
+//      { "placeholder", "LLEventStream" }
     }
 {}
 
+bool LLEventPumps::registerTypeFactory(const std::string& type, const TypeFactory& factory)
+{
+    auto found = mFactories.find(type);
+    // can't re-register a TypeFactory for a type name that's already registered
+    if (found != mFactories.end())
+        return false;
+    // doesn't already exist, go ahead and register
+    mFactories[type] = factory;
+    return true;
+}
+
+void LLEventPumps::unregisterTypeFactory(const std::string& type)
+{
+    auto found = mFactories.find(type);
+    if (found != mFactories.end())
+        mFactories.erase(found);
+}
+
+bool LLEventPumps::registerPumpFactory(const std::string& name, const PumpFactory& factory)
+{
+    // Do we already have a pump by this name?
+    if (mPumpMap.find(name) != mPumpMap.end())
+        return false;
+    // Do we already have an override for this pump name?
+    if (mTypes.find(name) != mTypes.end())
+        return false;
+    // Leverage the two-level lookup implemented by mTypes (pump name -> type
+    // name) and mFactories (type name -> factory). We could instead create a
+    // whole separate (pump name -> factory) map, and look in both; or we
+    // could change mTypes to (pump name -> factory) and, for typical type-
+    // based lookups, use a "factory" that looks up the real factory in
+    // mFactories. But this works, and we don't expect many calls to make() -
+    // either explicit or implicit via obtain().
+    // Create a bogus type name extremely unlikely to collide with an actual type.
+    static std::string nul(1, '\0');
+    std::string type_name{ nul + name };
+    mTypes[name] = type_name;
+    // TypeFactory is called with (name, tweak, type), whereas PumpFactory
+    // accepts only name. We could adapt with std::bind(), but this lambda
+    // does the trick.
+    mFactories[type_name] =
+        [factory]
+        (const std::string& name, bool /*tweak*/, const std::string& /*type*/)
+        { return factory(name); };
+    return true;
+}
+
+void LLEventPumps::unregisterPumpFactory(const std::string& name)
+{
+    auto tfound = mTypes.find(name);
+    if (tfound != mTypes.end())
+    {
+        auto ffound = mFactories.find(tfound->second);
+        if (ffound != mFactories.end())
+        {
+            mFactories.erase(ffound);
+        }
+        mTypes.erase(tfound);
+    }
+}
+
 LLEventPump& LLEventPumps::obtain(const std::string& name)
 {
     PumpMap::iterator found = mPumpMap.find(name);
@@ -114,7 +173,7 @@ LLEventPump& LLEventPumps::make(const std::string& name, bool tweak,
         // Passing an unrecognized type name is a no-no
         LLTHROW(BadType(type));
     }
-    auto newInstance = (found->second)(name, tweak);
+    auto newInstance = (found->second)(name, tweak, type);
     // LLEventPump's constructor implicitly registers each new instance in
     // mPumpMap. But remember that we instantiated it (in mOurPumps) so we'll
     // delete it later.
diff --git a/indra/llcommon/llevents.h b/indra/llcommon/llevents.h
index ae6e5aabc9..c1dbf4392f 100644
--- a/indra/llcommon/llevents.h
+++ b/indra/llcommon/llevents.h
@@ -268,6 +268,45 @@ public:
     LLEventPump& make(const std::string& name, bool tweak=false,
                       const std::string& type=std::string());
 
+    /// function passed to registerTypeFactory()
+    typedef std::function<LLEventPump*(const std::string& name, bool tweak, const std::string& type)> TypeFactory;
+
+    /**
+     * Register a TypeFactory for use with make(). When make() is called with
+     * the specified @a type string, call @a factory(name, tweak, type) to
+     * instantiate it.
+     *
+     * Returns true if successfully registered, false if there already exists
+     * a TypeFactory for the specified @a type name.
+     */
+    bool registerTypeFactory(const std::string& type, const TypeFactory& factory);
+    void unregisterTypeFactory(const std::string& type);
+
+    /// function passed to registerPumpFactory()
+    typedef std::function<LLEventPump*(const std::string&)> PumpFactory;
+
+    /**
+     * Register a PumpFactory for use with obtain(). When obtain() is called
+     * with the specified @a name string, if an LLEventPump with the specified
+     * @a name doesn't already exist, call @a factory(name) to instantiate it.
+     *
+     * Returns true if successfully registered, false if there already exists
+     * a factory override for the specified @a name.
+     *
+     * PumpFactory does not support @a tweak because it's only called when
+     * <i>that particular</i> @a name is passed to obtain(). Bear in mind that
+     * <tt>obtain(name)</tt> might still bypass the caller's PumpFactory for a
+     * couple different reasons:
+     *
+     * * registerPumpFactory() returns false because there's already a factory
+     *   override for the specified @name
+     * * between a successful <tt>registerPumpFactory(name)</tt> call (returns
+     *   true) and a call to <tt>obtain(name)</tt>, someone explicitly
+     *   instantiated an LLEventPump(name), so obtain(name) returned that.
+     */
+    bool registerPumpFactory(const std::string& name, const PumpFactory& factory);
+    void unregisterPumpFactory(const std::string& name);
+
     /**
      * Find the named LLEventPump instance. If it exists post the message to it.
      * If the pump does not exist, do nothing.
@@ -325,13 +364,13 @@ testable:
     typedef std::set<LLEventPump*> PumpSet;
     PumpSet mOurPumps;
     // for make(), map string type name to LLEventPump subclass factory function
-    typedef std::map<std::string, std::function<LLEventPump*(const std::string&, bool)>> PumpFactories;
+    typedef std::map<std::string, TypeFactory> TypeFactories;
     // Data used by make().
     // One might think mFactories and mTypes could reasonably be static. So
     // they could -- if not for the fact that make() or obtain() might be
     // called before this module's static variables have been initialized.
     // This is why we use singletons in the first place.
-    PumpFactories mFactories;
+    TypeFactories mFactories;
 
     // for obtain(), map desired string instance name to string type when
     // obtain() must create the instance
diff --git a/indra/llcommon/llleaplistener.cpp b/indra/llcommon/llleaplistener.cpp
index 11bfec1b31..471f52e91c 100644
--- a/indra/llcommon/llleaplistener.cpp
+++ b/indra/llcommon/llleaplistener.cpp
@@ -14,14 +14,16 @@
 // associated header
 #include "llleaplistener.h"
 // STL headers
-#include <map>
+#include <algorithm>                // std::find_if
 #include <functional>
+#include <map>
+#include <set>
 // std headers
 // external library headers
-#include <boost/foreach.hpp>
 // other Linden headers
-#include "lluuid.h"
+#include "lazyeventapi.h"
 #include "llsdutil.h"
+#include "lluuid.h"
 #include "stringize.h"
 
 /*****************************************************************************
@@ -110,7 +112,7 @@ LLLeapListener::~LLLeapListener()
     // value_type, and Bad Things would happen if you copied an
     // LLTempBoundListener. (Destruction of the original would disconnect the
     // listener, invalidating every stored connection.)
-    BOOST_FOREACH(ListenersMap::value_type& pair, mListeners)
+    for (ListenersMap::value_type& pair : mListeners)
     {
         pair.second.disconnect();
     }
@@ -208,31 +210,65 @@ void LLLeapListener::getAPIs(const LLSD& request) const
 {
     Response reply(LLSD(), request);
 
+    // first, traverse existing LLEventAPI instances
+    std::set<std::string> instances;
     for (auto& ea : LLEventAPI::instance_snapshot())
     {
-        LLSD info;
-        info["desc"] = ea.getDesc();
-        reply[ea.getName()] = info;
+        // remember which APIs are actually instantiated
+        instances.insert(ea.getName());
+        reply[ea.getName()] = llsd::map("desc", ea.getDesc());
+    }
+    // supplement that with *potential* instances: that is, instances of
+    // LazyEventAPI that can each instantiate an LLEventAPI on demand
+    for (const auto& lea : LL::LazyEventAPIBase::instance_snapshot())
+    {
+        // skip any LazyEventAPI that's already instantiated its LLEventAPI
+        if (instances.find(lea.getName()) == instances.end())
+        {
+            reply[lea.getName()] = llsd::map("desc", lea.getDesc());
+        }
     }
 }
 
+// Because LazyEventAPI deliberately mimics LLEventAPI's query API, this
+// function can be passed either -- even though they're unrelated types.
+template <typename API>
+void reportAPI(LLEventAPI::Response& reply, const API& api)
+{
+    reply["name"] = api.getName();
+    reply["desc"] = api.getDesc();
+    reply["key"]  = api.getDispatchKey();
+    LLSD ops;
+    for (const auto& namedesc : api)
+    {
+        ops.append(api.getMetadata(namedesc.first));
+    }
+    reply["ops"] = ops;
+}
+
 void LLLeapListener::getAPI(const LLSD& request) const
 {
     Response reply(LLSD(), request);
 
-    auto found = LLEventAPI::getInstance(request["api"]);
-    if (found)
+    // check first among existing LLEventAPI instances
+    auto foundea = LLEventAPI::getInstance(request["api"]);
+    if (foundea)
+    {
+        reportAPI(reply, *foundea);
+    }
+    else
     {
-        reply["name"] = found->getName();
-        reply["desc"] = found->getDesc();
-        reply["key"] = found->getDispatchKey();
-        LLSD ops;
-        for (LLEventAPI::const_iterator oi(found->begin()), oend(found->end());
-             oi != oend; ++oi)
+        // Here the requested LLEventAPI doesn't yet exist, but do we have a
+        // registered LazyEventAPI for it?
+        LL::LazyEventAPIBase::instance_snapshot snap;
+        auto foundlea = std::find_if(snap.begin(), snap.end(),
+                                     [api = request["api"].asString()]
+                                     (const auto& lea)
+                                     { return (lea.getName() == api); });
+        if (foundlea != snap.end())
         {
-            ops.append(found->getMetadata(oi->first));
+            reportAPI(reply, *foundlea);
         }
-        reply["ops"] = ops;
     }
 }
 
diff --git a/indra/llcommon/tests/lazyeventapi_test.cpp b/indra/llcommon/tests/lazyeventapi_test.cpp
new file mode 100644
index 0000000000..31b2d6d17f
--- /dev/null
+++ b/indra/llcommon/tests/lazyeventapi_test.cpp
@@ -0,0 +1,136 @@
+/**
+ * @file   lazyeventapi_test.cpp
+ * @author Nat Goodspeed
+ * @date   2022-06-18
+ * @brief  Test for lazyeventapi.
+ * 
+ * $LicenseInfo:firstyear=2022&license=viewerlgpl$
+ * Copyright (c) 2022, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+// Precompiled header
+#include "linden_common.h"
+// associated header
+#include "lazyeventapi.h"
+// STL headers
+// std headers
+// external library headers
+// other Linden headers
+#include "../test/lltut.h"
+#include "llevents.h"
+#include "llsdutil.h"
+
+// observable side effect, solely for testing
+static LLSD data;
+
+// LLEventAPI listener subclass
+class MyListener: public LLEventAPI
+{
+public:
+    // need this trivial forwarding constructor
+    // (of course do any other initialization your subclass requires)
+    MyListener(const LL::LazyEventAPIParams& params):
+        LLEventAPI(params)
+    {}
+
+    // example operation, registered by LazyEventAPI subclass below
+    void set_data(const LLSD& event)
+    {
+        data = event["data"];
+    }
+};
+
+// LazyEventAPI registrar subclass
+class MyRegistrar: public LL::LazyEventAPI<MyListener>
+{
+    using super = LL::LazyEventAPI<MyListener>;
+    using super::listener;
+public:
+    // LazyEventAPI subclass initializes like a classic LLEventAPI subclass
+    // constructor, with API name and desc plus add() calls for the defined
+    // operations
+    MyRegistrar():
+        super("Test", "This is a test LLEventAPI")
+    {
+        add("set", "This is a set operation", &listener::set_data);
+    }
+};
+// Normally we'd declare a static instance of MyRegistrar -- but because we
+// want to test both with and without, defer declaration to individual test
+// methods.
+
+/*****************************************************************************
+*   TUT
+*****************************************************************************/
+namespace tut
+{
+    struct lazyeventapi_data
+    {
+        lazyeventapi_data()
+        {
+            // before every test, reset 'data'
+            data.clear();
+        }
+        ~lazyeventapi_data()
+        {
+            // after every test, reset LLEventPumps
+            LLEventPumps::deleteSingleton();
+        }
+    };
+    typedef test_group<lazyeventapi_data> lazyeventapi_group;
+    typedef lazyeventapi_group::object object;
+    lazyeventapi_group lazyeventapigrp("lazyeventapi");
+
+    template<> template<>
+    void object::test<1>()
+    {
+        set_test_name("LazyEventAPI");
+        // this is where the magic (should) happen
+        // 'register' still a keyword until C++17
+        MyRegistrar regster;
+        LLEventPumps::instance().obtain("Test").post(llsd::map("op", "set", "data", "hey"));
+        ensure_equals("failed to set data", data.asString(), "hey");
+    }
+
+    template<> template<>
+    void object::test<2>()
+    {
+        set_test_name("No LazyEventAPI");
+        // Because the MyRegistrar declaration in test<1>() is local, because
+        // it has been destroyed, we fully expect NOT to reach a MyListener
+        // instance with this post.
+        LLEventPumps::instance().obtain("Test").post(llsd::map("op", "set", "data", "moot"));
+        ensure("accidentally set data", ! data.isDefined());
+    }
+
+    template<> template<>
+    void object::test<3>()
+    {
+        set_test_name("LazyEventAPI metadata");
+        MyRegistrar regster;
+        // Of course we have 'regster' in hand; we don't need to search for
+        // it. But this next test verifies that we can find (all) LazyEventAPI
+        // instances using LazyEventAPIBase::instance_snapshot. Normally we
+        // wouldn't search; normally we'd just look at each instance in the
+        // loop body.
+        const MyRegistrar* found = nullptr;
+        for (const auto& registrar : LL::LazyEventAPIBase::instance_snapshot())
+            if ((found = dynamic_cast<const MyRegistrar*>(&registrar)))
+                break;
+        ensure("Failed to find MyRegistrar via LLInstanceTracker", found);
+
+        ensure_equals("wrong API name", found->getName(), "Test");
+        ensure_contains("wrong API desc", found->getDesc(), "test LLEventAPI");
+        ensure_equals("wrong API field", found->getDispatchKey(), "op");
+        // Normally we'd just iterate over *found. But for test purposes,
+        // actually capture the range of NameDesc pairs in a vector.
+        std::vector<LL::LazyEventAPIBase::NameDesc> ops{ found->begin(), found->end() };
+        ensure_equals("failed to find operations", ops.size(), 1);
+        ensure_equals("wrong operation name", ops[0].first, "set");
+        ensure_contains("wrong operation desc", ops[0].second, "set operation");
+        LLSD metadata{ found->getMetadata(ops[0].first) };
+        ensure_equals("bad metadata name", metadata["name"].asString(), ops[0].first);
+        ensure_equals("bad metadata desc", metadata["desc"].asString(), ops[0].second);
+    }
+} // namespace tut
diff --git a/indra/llcommon/tests/lleventdispatcher_test.cpp b/indra/llcommon/tests/lleventdispatcher_test.cpp
index 466f11f52a..b38e47a773 100644
--- a/indra/llcommon/tests/lleventdispatcher_test.cpp
+++ b/indra/llcommon/tests/lleventdispatcher_test.cpp
@@ -20,6 +20,7 @@
 #include "../test/lltut.h"
 #include "llsd.h"
 #include "llsdutil.h"
+#include "llevents.h"
 #include "stringize.h"
 #include "tests/wrapllerrs.h"
 #include "../test/catch_and_store_what_in.h"
@@ -644,12 +645,45 @@ namespace tut
                    outer.find(inner) != std::string::npos);
         }
 
-        void call_exc(const std::string& func, const LLSD& args, const std::string& exc_frag)
+        std::string call_exc(const std::string& func, const LLSD& args, const std::string& exc_frag)
         {
-            std::string threw = catch_what<std::runtime_error>([this, &func, &args](){
-                    work(func, args);
-                });
-            ensure_has(threw, exc_frag);
+            // This method was written when LLEventDispatcher responded to
+            // name or argument errors with LL_ERRS, hence the name: we used
+            // to have to intercept LL_ERRS by making it throw. Now we set up
+            // to catch an error response instead. But -- for that we need to
+            // be able to sneak a "reply" key into args, which must be a Map.
+            if (! (args.isUndefined() or args.isMap()))
+                fail(stringize("can't test call_exc() with ", args));
+            LLEventStream replypump("reply");
+            LLSD reply;
+            LLTempBoundListener bound{
+                replypump.listen(
+                    "listener",
+                    [&reply](const LLSD& event)
+                    {
+                        reply = event;
+                        return false;
+                    }) };
+            LLSD modargs{ args };
+            modargs["reply"] = replypump.getName();
+            if (func.empty())
+            {
+                work(modargs);
+            }
+            else
+            {
+                work(func, modargs);
+            }
+            ensure("no error response", reply.has("error"));
+            ensure_has(reply["error"], exc_frag);
+            return reply["error"];
+        }
+
+        void call_logerr(const std::string& func, const LLSD& args, const std::string& frag)
+        {
+            CaptureLog capture;
+            work(func, args);
+            capture.messageWith(frag);
         }
 
         LLSD getMetadata(const std::string& name)
@@ -1031,13 +1065,7 @@ namespace tut
     {
         set_test_name("call with bad name");
         call_exc("freek", LLSD(), "not found");
-        // We don't have a comparable helper function for the one-arg
-        // operator() method, and it's not worth building one just for this
-        // case. Write it out.
-        std::string threw = catch_what<std::runtime_error>([this](){
-                work(LLSDMap("op", "freek"));
-            });
-        ensure_has(threw, "bad");
+        std::string threw = call_exc("", LLSDMap("op", "freek"), "bad");
         ensure_has(threw, "op");
         ensure_has(threw, "freek");
     }
@@ -1087,7 +1115,7 @@ namespace tut
             ensure_equals("answer mismatch", tr.llsd, answer);
             // Should NOT be able to pass 'answer' to Callables registered
             // with 'required'.
-            call_exc(tr.name_req, answer, "bad request");
+            call_logerr(tr.name_req, answer, "bad request");
             // But SHOULD be able to pass 'matching' to Callables registered
             // with 'required'.
             work(tr.name_req, matching);
@@ -1107,11 +1135,11 @@ namespace tut
         // args. We should only need to engage it for one map-style
         // registration and one array-style registration.
         std::string array_exc("needs an args array");
-        call_exc("free0_array", 17, array_exc);
-        call_exc("free0_array", LLSDMap("pi", 3.14), array_exc);
+        call_logerr("free0_array", 17, array_exc);
+        call_logerr("free0_array", LLSDMap("pi", 3.14), array_exc);
 
         std::string map_exc("needs a map");
-        call_exc("free0_map", 17, map_exc);
+        call_logerr("free0_map", 17, map_exc);
         // Passing an array to a map-style function works now! No longer an
         // error case!
 //      call_exc("free0_map", llsd::array("a", "b"), map_exc);
@@ -1158,7 +1186,7 @@ namespace tut
         {
             foreach(const llsd::MapEntry& e, inMap(funcsab))
             {
-                call_exc(e.second, tooshort, "requires more arguments");
+                call_logerr(e.second, tooshort, "requires more arguments");
             }
         }
     }