Merge #260: event loop: tolerate unexpected exceptions in post() callbacks

b8a48c6 event loop: tolerate unexpected exceptions in `post()` callbacks (hulxv)

Pull request description:

  If a function posted via `EventLoop::post()` threw an exception, the event loop would exit without resetting `m_post_fn` or notifying the condition variable, permanently deadlocking the calling thread in `post()`.

  This change catches the exception instead, logs it, and keeps the event loop running so the caller is unblocked and other I/O events continue to be processed.

  Fix #259

ACKs for top commit:
  ryanofsky:
    Code review ACK b8a48c6. Since last review just squashed commits and added missing thread safety annotation. This change is a probably an improvement from a debugging perspective since it will now always log an error when an unexpected exception happens. It is not a clear improvement in behavior since continuing after an unexpected failure could potentially be worse than hanging, but the difference is unlikely to be significant. Probably the best thing would be to [catch and rethrow](#260 (comment)) the exception.

Tree-SHA512: 5cf5481227b810ce09801eebf6bcde44ee3152545738cbd8068365a2d988ddd78486cc08906e0df3125be83f06acb0ae40e718c3b1d28c602e821ae1989c317e

Author: ryanofsky

src/mp/proxy.cpp

@@ -22,6 +22,7 @@
 #include <kj/async-prelude.h>
 #include <kj/common.h>
 #include <kj/debug.h>
+#include <kj/exception.h>
 #include <kj/function.h>
 #include <kj/memory.h>
 #include <kj/string.h>
@@ -245,7 +246,12 @@ void EventLoop::loop()
         if (read_bytes != 1) throw std::logic_error("EventLoop wait_stream closed unexpectedly");
         Lock lock(m_mutex);
         if (m_post_fn) {
-            Unlock(lock, *m_post_fn);
+            // m_post_fn throwing is never expected. If it does happen, the caller
+            // of EventLoop::post() will return without any indication of failure,
+            // which will likely cause other bugs. Log the error and continue.
+            KJ_IF_MAYBE(exception, kj::runCatchingExceptions([&]() MP_REQUIRES(m_mutex) { Unlock(lock, *m_post_fn); })) {
+                MP_LOG(*this, Log::Error) << "EventLoop: m_post_fn threw: " << kj::str(*exception).cStr();
+            }
             m_post_fn = nullptr;
             m_cv.notify_all();
         } else if (done()) {