Avoid deadlocks in the Alarm class by releasing the lock before invoking
callbacks. This deadlock manifested itself in the ProgressManager:
1. On the main thread, the ProgressManager acquires its lock in
ProgressManager::Decrement and calls Alarm::Create.
2. On the main thread, the Alarm acquires its lock in Alarm::Create.
3. On the alarm thread, the Alarm acquires its lock after waiting on the
condition variable and calls ProgressManager::Expire.
4. On the alarm thread, the ProgressManager acquires its lock in
ProgressManager::Expire.
Note how the two threads are acquiring the locks in different orders.
Deadlocks can be avoided by always acquiring locks in the same order,
but since the two mutexes here are private implementation details,
belong to different classes, that's not straightforward. Luckily, we
don't need to have the Alarm mutex locked when invoking the callbacks.
That exactly how this patch solves the issue.
llvm-project/lldb/source/Host/common/Alarm.cpp:37:5:
error: 'lock_guard' may not intend to support class template argument deduction [-Werror,-Wctad-maybe-unsupported]
std::lock_guard alarm_guard(m_alarm_mutex);
^
The commit introduces a new, generic, Alarm class. The class lets you to
schedule functions (callbacks) that will execute after a predefined
timeout. Once scheduled, you can cancel and reset a callback, given the
timeout hasn't expired yet.
The alarm class worker thread that sleeps until the next timeout
expires. When the thread wakes up, it checks for all the callbacks that
have expired and calls them in order. Because the callback is called
from the worker thread, the only guarantee is that a callback is called
no sooner than the timeout. A long running callback could potentially
block the worker threads and delay other callbacks from getting called.
I intentionally kept the implementation as simple as possible while
addressing the needs for the use case of coalescing progress events as
discussed in [1]. If we want to rely on this somewhere else, we can
reassess whether we need to address this class' limitations.
[1] https://discourse.llvm.org/t/rfc-improve-lldb-progress-reporting/75717/
