Log error output when a git command fails

app/models/git_repository.rb

@@ -1,4 +1,7 @@
 # frozen_string_literal: true
+
+require 'tempfile'
+
 # Responsible for all git knowledge of a repo
 # Caches a local mirror (not a full checkout) and creates a workspace when deploying
 class GitRepository
@@ -184,13 +187,16 @@ def instance_cache(key)
   # success: stdout as string
   # error: nil
   def capture_stdout(*command, dir: repo_cache_dir)
-    success, output = Samson::CommandExecutor.execute(
-      *command,
-      whitelist_env: ['HOME', 'PATH'],
-      timeout: 30.minutes,
-      err: '/dev/null',
-      dir: dir
-    )
-    output.strip if success
+    Tempfile.create('git-stderr') do |error_file|
+      success, output = Samson::CommandExecutor.execute(
+        *command,
+        whitelist_env: ['HOME', 'PATH'],
+        timeout: 30.minutes,
+        err: error_file.path,
+        dir: dir
+      )
+      Rails.logger.error("Failed to run command #{command}: #{error_file.read}") unless success
+      output.strip if success
+    end
   end
 end

test/models/git_repository_test.rb

@@ -101,6 +101,12 @@ def call
       repository.commit_from_ref('NOT A VALID REF').must_be_nil
     end
 
+    it 'logs an error if ref does not exist' do
+      create_repo_with_tags
+      Rails.logger.expects(:error).with { |message| message.must_match(/^Failed to run command/) }
+      repository.commit_from_ref('NOT A VALID REF')
+    end
+
     it 'returns the commit of a branch' do
       create_repo_with_an_additional_branch('my_branch')
       repository.commit_from_ref('my_branch').must_match /^[0-9a-f]{40}$/