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/**
* @file sync.h
* @author Nat Goodspeed
* @date 2019-03-13
* @brief Synchronize coroutines within a test program so we can observe side
* effects. Certain test programs test coroutine synchronization
* mechanisms. Such tests usually want to interleave coroutine
* executions in strictly stepwise fashion. This class supports that
* paradigm.
*
* $LicenseInfo:firstyear=2019&license=viewerlgpl$
* Copyright (c) 2019, Linden Research, Inc.
* $/LicenseInfo$
*/
#if ! defined(LL_SYNC_H)
#define LL_SYNC_H
#include "llcond.h"
#include "lltut.h"
#include "stringize.h"
#include "llerror.h"
#include "llcoros.h"
/**
* Instantiate Sync in any test in which we need to suspend one coroutine
* until we're sure that another has had a chance to run. Simply calling
* llcoro::suspend() isn't necessarily enough; that provides a chance for the
* other to run, but doesn't guarantee that it has. If each coroutine is
* consistent about calling Sync::bump() every time it wakes from any
* suspension, Sync::yield() and yield_until() should at least ensure that
* somebody else has had a chance to run.
*/
class Sync
{
LLScalarCond<int> mCond{0};
F32Milliseconds mTimeout;
public:
Sync(F32Milliseconds timeout=F32Milliseconds(10.0f)):
mTimeout(timeout)
{}
/**
* Bump mCond by n steps -- ideally, do this every time a participating
* coroutine wakes up from any suspension. The choice to bump() after
* resumption rather than just before suspending is worth calling out:
* this practice relies on the fact that condition_variable::notify_all()
* merely marks a suspended coroutine ready to run, rather than
* immediately resuming it. This way, though, even if a coroutine exits
* before reaching its next suspend point, the other coroutine isn't
* left waiting forever.
*/
void bump(int n=1)
{
// Calling mCond.set_all(mCond.get() + n) would be great for
// coroutines -- but not so good between kernel threads -- it would be
// racy. Make the increment atomic by calling update_all(), which runs
// the passed lambda within a mutex lock.
int updated;
mCond.update_all(
[&n, &updated](int& data)
{
data += n;
// Capture the new value for possible logging purposes.
updated = data;
});
// In the multi-threaded case, this log message could be a bit
// misleading, as it will be emitted after waiting threads have
// already awakened. But emitting the log message within the lock
// would seem to hold the lock longer than we really ought.
LL_DEBUGS() << llcoro::logname() << " bump(" << n << ") -> " << updated << LL_ENDL;
}
/**
* Set mCond to a specific n. Use of bump() and yield() is nicely
* maintainable, since you can insert or delete matching operations in a
* test function and have the rest of the Sync operations continue to
* line up as before. But sometimes you need to get very specific, which
* is where set() and yield_until() come in handy: less maintainable,
* more precise.
*/
void set(int n)
{
LL_DEBUGS() << llcoro::logname() << " set(" << n << ")" << LL_ENDL;
mCond.set_all(n);
}
/// suspend until "somebody else" has bumped mCond by n steps
void yield(int n=1)
{
return yield_until(STRINGIZE("Sync::yield_for(" << n << ") timed out after "
<< int(mTimeout.value()) << "ms"),
mCond.get() + n);
}
/// suspend until "somebody else" has bumped mCond to a specific value
void yield_until(int until)
{
return yield_until(STRINGIZE("Sync::yield_until(" << until << ") timed out after "
<< int(mTimeout.value()) << "ms"),
until);
}
private:
void yield_until(const std::string& desc, int until)
{
std::string name(llcoro::logname());
LL_DEBUGS() << name << " yield_until(" << until << ") suspending" << LL_ENDL;
tut::ensure(name + ' ' + desc, mCond.wait_for_equal(mTimeout, until));
// each time we wake up, bump mCond
bump();
}
};
#endif /* ! defined(LL_SYNC_H) */
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