SL-18330: Make LLSDSerialize::deserialize() default to notation.

LLSDSerialize::serialize() emits a header string, e.g. "<? llsd/notation ?>"
for notation format. Until now, LLSDSerialize::deserialize() has required that
header to properly decode the input stream.

But none of LLSDBinaryFormatter, LLSDXMLFormatter or LLSDNotationFormatter
emit that header themselves. Nor do any of the Python llsd.format_binary(),
format_xml() or format_notation() functions. Until now, you could not use
LLSD::deserialize() to parse an arbitrary-format LLSD stream serialized by
anything but LLSDSerialize::serialize().

Change LLSDSerialize::deserialize() so that if no header is recognized,
instead of failing, it attempts to parse as notation. Add tests to exercise
this case.

The tricky part about this processing is that deserialize() necessarily reads
some number of bytes from the input stream first, to try to recognize the
header. If it fails to do so, it must prepend the bytes it has already read to
the rest of the input stream since they're probably the beginning of the
serialized data.

To support this use case, introduce cat_streambuf, a std::streambuf subclass
that (virtually) concatenates other std::streambuf instances. When read by a
std::istream, the sequence of underlying std::streambufs appears to the
consumer as a single continuous stream.

1 files changed, 25 insertions, 2 deletions

diff --git a/indra/llcommon/llstreamtools.h b/indra/llcommon/llstreamtools.h
index 1b04bf91d7..997f738840 100644
--- a/indra/llcommon/llstreamtools.h
+++ b/indra/llcommon/llstreamtools.h
@@ -27,8 +27,10 @@
 #ifndef LL_STREAM_TOOLS_H
 #define LL_STREAM_TOOLS_H
 
+#include <deque>
 #include <iostream>
 #include <string>
+#include <vector>
 
 // unless specifed otherwise these all return input_stream.good()
 
@@ -113,6 +115,27 @@ LL_COMMON_API std::streamsize fullread(
 
 LL_COMMON_API std::istream& operator>>(std::istream& str, const char *tocheck);
 
-#endif
-
+/**
+ * cat_streambuf is a std::streambuf subclass that accepts a variadic number
+ * of std::streambuf* (e.g. some_istream.rdbuf()) and virtually concatenates
+ * their contents.
+ */
+// derived from https://stackoverflow.com/a/49441066/5533635
+class cat_streambuf: public std::streambuf
+{
+private:
+    std::deque<std::streambuf*> mInputs;
+    std::vector<char> mBuffer;
+
+public:
+    // only valid for std::streambuf* arguments
+    template <typename... Inputs>
+    cat_streambuf(Inputs... inputs):
+        mInputs{inputs...},
+        mBuffer(1024)
+    {}
+
+    int underflow();
+};
 
+#endif