@peteriman you marked #1005 as fixed by this, but are you sure that is the case? I saw your comment in the thread, but have not had time to verify it.

@thernstig I'm quite sure there were multiple concurrent log4js.shutdown() calls based on your logs in #1005.

Nevertheless, please take some time to verify my comments in #1005.
If possible, I will be grateful if you could apply the changes in this PR to be an extra pair of eyes to verify the fix.

Read onwards for the rationale of this fix:

The issue is due to log4js.shutdown() doesn't reset the current appenders.
It does a clone to appendersToCheck for appenders traversal to close the resources but does not clear appenders.

log4js.shutdown() will loop the appendersToCheck and call app.shutdown() which writes '' before closing the file.

On the second log4js.shutdown(), as traverses the same appendersToCheck, the error will occur as it attempts to write to an already closed file, resulting in Error [ERR_STREAM_WRITE_AFTER_END]: write after end.

The fix resets the appenders, releasing the resources.
That prevents subsequent log4js.shutdown() calls from re-visiting them again, after all they have already been closed.

   // Clone out to maintain a reference
   const appendersToCheck = Array.from(appenders.values());

+  // Reset immediately to prevent leaks
+  appenders.init();
+  categories.init();

   // Call each of the shutdown functions in parallel
   const shutdownFunctions = appendersToCheck.reduceRight(
     (accum, next) => (next.shutdown ? accum + 1 : accum),
     0
   );

I think it looks good, not sure if @nomiddlename wants to LGTM this or not.

@thernstig Your questions were of great help. I should add a few more automated test cases to cover those.
After that, I can merge it in. 😁

Released in log4js@6.4.0.