Fix potential hang with parallelize: false

We were missing a case in `HookRunner#wait_for_slot` where if no slots
were available we would infinite loop on the thread that "lost" if
another thread took all the slots. This would result in the losing
thread never releasing the mutex, preventing the winning thread from
release the slots! Thus the hook run would never terminate.

Fix by `wait`ing in the case where there are no slots available so we
release the lock awaiting a signal.

Fixes sds#476

Author: sds

CHANGELOG.md

@@ -5,6 +5,7 @@
 * Fix `forwarding to private method` warning on Ruby 2.4.x
 * Update `childprocess` dependency to 0.6.x series
 * Auto-sign configuration file when installing hooks for the first time
+* Fix potential hang when a hook's `parallelize` option was set to `false`
 
 ## 0.38.0
 

lib/overcommit/hook_runner.rb

@@ -107,6 +107,9 @@ def wait_for_slot(hook)
 
             # Wait for a signal from another thread to try again
             @resource.wait(@lock)
+          else
+            # Otherwise there are not slots left, so just wait for signal
+            @resource.wait(@lock)
           end
         end
       end

spec/integration/parallelize_spec.rb

@@ -0,0 +1,25 @@
+require 'spec_helper'
+require 'timeout'
+
+describe 'running a hook with parallelism disabled' do
+  subject { shell(%w[git commit --allow-empty -m Test]) }
+
+  let(:config) { <<-YML }
+    CommitMsg:
+      TrailingPeriod:
+        enabled: true
+        parallelize: false
+  YML
+
+  around do |example|
+    repo do
+      File.open('.overcommit.yml', 'w') { |f| f.write(config) }
+      `overcommit --install > #{File::NULL}`
+      example.run
+    end
+  end
+
+  it 'does not hang' do
+    Timeout.timeout(5) { subject }
+  end
+end