eventlog: Fix racy flushing

Previously no attempt was made to avoid multiple threads writing their
capability-local eventlog buffers to the eventlog writer simultaneously.
This could result in multiple eventlog streams being interleaved. Fix
this by documenting that the EventLogWriter's write() and flush()
functions may be called reentrantly and fix the default writer to
protect its FILE* by a mutex.

Fixes #18210.

1 files changed, 20 insertions, 1 deletions

diff --git a/rts/eventlog/EventLogWriter.c b/rts/eventlog/EventLogWriter.c
index b19e617a4c..5387f932eb 100644
--- a/rts/eventlog/EventLogWriter.c
+++ b/rts/eventlog/EventLogWriter.c
@@ -28,6 +28,17 @@ static pid_t event_log_pid = -1;
 // File for logging events
 static FILE *event_log_file = NULL;
 
+#if defined(THREADED_RTS)
+// Protects event_log_file
+static Mutex event_log_mutex;
+
+static void acquire_event_log_lock(void) { ACQUIRE_LOCK(&event_log_mutex); }
+static void release_event_log_lock(void) { RELEASE_LOCK(&event_log_mutex); }
+#else
+static void acquire_event_log_lock(void) {}
+static void release_event_log_lock(void) {}
+#endif
+
 static void initEventLogFileWriter(void);
 static bool writeEventLogFile(void *eventlog, size_t eventlog_size);
 static void flushEventLogFile(void);
@@ -89,6 +100,9 @@ initEventLogFileWriter(void)
     }
 
     stgFree(event_log_filename);
+#if defined(THREADED_RTS)
+    initMutex(&event_log_mutex);
+#endif
 }
 
 static bool
@@ -97,15 +111,17 @@ writeEventLogFile(void *eventlog, size_t eventlog_size)
     unsigned char *begin = eventlog;
     size_t remain = eventlog_size;
 
+    acquire_event_log_lock();
     while (remain > 0) {
         size_t written = fwrite(begin, 1, remain, event_log_file);
         if (written == 0) {
+            release_event_log_lock();
             return false;
         }
         remain -= written;
         begin += written;
     }
-
+    release_event_log_lock();
     return true;
 }
 
@@ -124,6 +140,9 @@ stopEventLogFileWriter(void)
         fclose(event_log_file);
         event_log_file = NULL;
     }
+#if defined(THREADED_RTS)
+    closeMutex(&event_log_mutex);
+#endif
 }
 
 const EventLogWriter FileEventLogWriter = {