Bug/650 traceback when setting parallelize to false (#656)

* Reproduce deadlock when parallize is false

* Fix potential deadlock when waiting for hooks

If a hook is running while another hook tries to start and there are
not enough slots left then it will wait for the first hook to stop and
release its slots. This first hook will not signal the second if there
are no more hooks left. This results in a deadlock.

This situation is far more likely to occur when parallelize is false
since the hook will consume all the available slots.

Author: steinybot

CHANGELOG.md

@@ -6,6 +6,7 @@
 * Add `RubySyntax` pre-commit hook
 * Relax `childprocess` dependency to allow version 1.x
 * Add `CodeSpellCheck` pre-commit hook
+* Fix deadlock which was more likely to occur when setting `parallelize` on a hook to `false`
 
 ## 0.48.1
 

lib/overcommit/hook_runner.rb

@@ -129,12 +129,9 @@ def release_slot(hook)
         slots_released = processors_for_hook(hook)
         @slots_available += slots_released
 
-        if @hooks_left.any?
-          # Signal once. `wait_for_slot` will perform additional signals if
-          # there are still slots available. This prevents us from sending out
-          # useless signals
-          @resource.signal
-        end
+        # Signal every time in case there are threads that are already waiting for
+        # these slots to be released
+        @resource.signal
       end
     end
 

spec/integration/parallelize_spec.rb

@@ -7,10 +7,16 @@
   subject { shell(%w[git commit --allow-empty -m Test]) }
 
   let(:config) { <<-YML }
+    concurrency: 20
     CommitMsg:
       TrailingPeriod:
         enabled: true
         parallelize: false
+        command: ['ruby', '-e', 'sleep 1']
+      TextWidth:
+        enabled: true
+        parallelize: true
+        processors: 1
   YML
 
   around do |example|
@@ -22,6 +28,7 @@
   end
 
   it 'does not hang' do
-    Timeout.timeout(5) { subject }
+    result = Timeout.timeout(5) { subject }
+    result.stderr.should_not include 'No live threads left. Deadlock?'
   end
 end