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, 4 insertions, 0 deletions

diff --git a/includes/rts/EventLogWriter.h b/includes/rts/EventLogWriter.h
index 4975b72b07..38a0c06086 100644
--- a/includes/rts/EventLogWriter.h
+++ b/includes/rts/EventLogWriter.h
@@ -24,9 +24,13 @@ typedef struct {
     void (* initEventLogWriter) (void);
 
     // Write a series of events returning true on success.
+    // Note that this may be called by multiple threads simultaneously.
+    // The writer is responsible for concurrency control.
     bool (* writeEventLog) (void *eventlog, size_t eventlog_size);
 
     // Flush possibly existing buffers (may be NULL)
+    // Note that this may be called by multiple threads simultaneously.
+    // The writer is responsible for concurrency control.
     void (* flushEventLog) (void);
 
     // Close an initialized EventLogOutput (may be NULL)