Type: Bug

The race was reported in #159640 and partially fixed by #165560.

The fix doesn't fully cover the issue though. The problem is that the task identifier is added to _inProgressTasks just before terminalTaskSystem._executeTask() is called, and removed just after. But _executeTask will return (via "await") before updating _activeTasks. So _activeTasks won't be updated until the next tick (at the earliest, and possibly much much later).

This is a problem because although terminalTaskSystem updates _instances immediately (so it knows how many instances of a task are running), it ignores that unless there's an entry in _activeTasks.

As a result, a task can be re-submitted after its name has been removed from _inProgressTasks, but before the first task has been added to _activeTasks, bypass the instanceLimit check, and start a second, invalid instance. And this is the root cause of #159640.

Typically the race can be hard to hit - especially after #165560 - but if the task has a "dependsOn" that runs for more than a few milliseconds, it's easy to repro. Use these commands to create and open a new project with a suitable task.

mkdir -p test/.vscode
cd test
cat <<"EOF" > .vscode/tasks.json
{
  "version": "2.0.0",
  "tasks": [
    {
      "label": "sleep",
      "type": "shell",
      "command": "sleep 2"
    },
    {
      "label": "build",
      "type": "shell",
      "command": "echo 'Running build' && sleep 5 && echo 'Finished build'",
      "dependsOn": ["sleep"],
      "group": {
        "kind": "build",
        "isDefault": true
      }
    }
  ]
}
EOF
code .

Trust the workspace, then hit Cmd-Shift-B three times in less than 2 seconds (but not too fast, or _inProgressTasks will stop the second and third attempts). You will get three instances of the task running.

VS Code version: Code 1.77.3 (704ed70, 2023-04-12T09:19:37.325Z)
OS version: Darwin x64 22.4.0
Modes:
Sandboxed: No

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