Merge branch '59-git-tracing' into 'master' v3.6.2

Enable GIT_TRACE_PERFORMANCE through a config variable.

The value of the variable must an absolute path needs to exist so we’re able to check if we can write in that file. Because in the 
case we cannot write we’ll throw a warning to the output of the users.

```sh
~/dev/gitlab/local/pacoguzman/gitlab-ce (master=)$ git push origin master
warning: could not open '/wadus' for tracing: Permission denied
Everything up-to-date
```

Closes #59

See merge request !91

2 files changed, 30 insertions, 0 deletions

diff --git a/lib/gitlab_config.rb b/lib/gitlab_config.rb
index beaf173..781c706 100644
--- a/lib/gitlab_config.rb
+++ b/lib/gitlab_config.rb
@@ -50,4 +50,8 @@ class GitlabConfig
   def git_annex_enabled?
     @config['git_annex_enabled'] ||= false
   end
+
+  def git_trace_log_file
+    @config['git_trace_log_file']
+  end
 end
diff --git a/lib/gitlab_shell.rb b/lib/gitlab_shell.rb
index 971b22f..f72ce74 100644
--- a/lib/gitlab_shell.rb
+++ b/lib/gitlab_shell.rb
@@ -1,4 +1,5 @@
 require 'shellwords'
+require 'pathname'
 
 require_relative 'gitlab_net'
 
@@ -150,6 +151,14 @@ class GitlabShell
       env.merge!({ 'GIT_ANNEX_SHELL_LIMITED' => '1' })
     end
 
+    if git_trace_available?
+      env.merge!({
+        'GIT_TRACE' => @config.git_trace_log_file,
+        'GIT_TRACE_PACKET' => @config.git_trace_log_file,
+        'GIT_TRACE_PERFORMANCE' => @config.git_trace_log_file,
+      })
+    end
+
     Kernel::exec(env, *args, unsetenv_others: true)
   end
 
@@ -232,6 +241,23 @@ class GitlabShell
     end
   end
 
+  def git_trace_available?
+    return false unless @config.git_trace_log_file
+
+    if Pathname(@config.git_trace_log_file).relative?
+      $logger.warn "gitlab-shell: is configured to trace git commands with #{@config.git_trace_log_file.inspect} but an absolute path needs to be provided"
+      return false
+    end
+
+    begin
+      File.open(@config.git_trace_log_file, 'a') { nil }
+      return true
+    rescue => ex
+      $logger.warn "gitlab-shell: is configured to trace git commands with #{@config.git_trace_log_file.inspect} but it's not possible to write in that path #{ex.message}"
+      return false
+    end
+  end
+
   def repo_path=(repo_path)
     raise ArgumentError, "Repository path not provided. Please make sure you're using GitLab v8.10 or later." unless repo_path
     raise InvalidRepositoryPathError if File.absolute_path(repo_path) != repo_path