Analysis server hangs with Linux 5.5 and IntelliJ based IDEs

3 thoughts on “Analysis server hangs with Linux 5.5 and IntelliJ based IDEs”

1. The following simple program would reproduce the hang on a compute instance with 5.5.x linux kernel.

   import 'dart:io' as io;
   import 'dart:convert';
   
   void main(List<String> args) async {
     if (args.length != 1) {
       print('Usage: run.dart child|parent');
       return;
     }
     if (args[0] == 'child') {
       for (int i = 0; i < 30000; i++) {
         print('line $i');
       }
       print('done');
       return;
     } else if (args[0] == 'parent') {
       final p = await io.Process.start(io.Platform.executable,[io.Platform.script.toFilePath(), 'child']);
       p.stdout.transform(utf8.decoder).listen((x) => print('stdout: $x'));
       p.stderr.transform(utf8.decoder).listen((x) => print('stderr: $x'));
       final exitCode = await p.exitCode;
       print('process exited with ${exitCode}');
     }
   }

   $ dart run.dart parent
   

   The parent process would then hang.

   Based on my cursory analysis (I have never looked at this part of the code before) I think this is a bug in our code – we don’t seem to be using epoll correctly. We use edge-triggered mode for file descriptors (except server sockets). This mode comes with some warnings in the man pages. Namely it warns that if you use EPOLLET you should only epoll_wait after you have received EAGAIN from read/write – I don’t see us following this rule. We drain some available amount of bytes from the file descriptor but we don’t really drain it until we hit EAGAIN (and we don’t even update our estimate of the available bytes as we are draining it – so we more bytes come while we are reading we are going to ignore that, which does not work with edge-triggered mode).

   I am not sure why this problem only surfaces now – but I see that there were some changes to the Kernel around epoll, so this might have caused it to surface.

   We should either stop using ET mode or we should fix our code to follow man page guidelines.

   Assigning to @zichangg for actual implementation work.

2. We should do a proper fix and backport it to stable.

3. A quick update:
   I have a fix ready(https://dart-review.googlesource.com/c/sdk/+/136322).
   But I still need to a review from @sortie before it lands.

Comments are closed.