Fail early on invalid input (raise ... unless ...)

This intention of this change is to make the normal flow of execution
easier to read, and to prevent mistakes in deeply nested if-else trees.

lib/gitlab_shell.rb

@@ -18,20 +18,18 @@ class GitlabShell
     if @origin_cmd
       parse_cmd
 
-      if git_cmds.include?(@git_cmd)
-        ENV['GL_ID'] = @key_id
+      raise DisallowedCommandError unless git_cmds.include?(@git_cmd)
 
-        access = api.check_access(@git_cmd, @repo_name, @key_id, '_any')
+      ENV['GL_ID'] = @key_id
 
-        if access.allowed?
-          process_cmd
-        else
-          message = "gitlab-shell: Access denied for git command <#{@origin_cmd}> by #{log_username}."
-          $logger.warn message
-          puts access.message
-        end
+      access = api.check_access(@git_cmd, @repo_name, @key_id, '_any')
+
+      if access.allowed?
+        process_cmd
       else
-        raise DisallowedCommandError
+        message = "gitlab-shell: Access denied for git command <#{@origin_cmd}> by #{log_username}."
+        $logger.warn message
+        puts access.message
       end
     else
       puts "Welcome to GitLab, #{username}!"
@@ -51,14 +49,12 @@ class GitlabShell
     @git_cmd = args.first
 
     if @git_cmd == 'git-annex-shell'
-      if @config.git_annex_enabled?
-        @repo_name = escape_path(args[2].sub(/\A\/~\//, ''))
+      raise DisallowedCommandError unless @config.git_annex_enabled?
 
-        # Make sure repository has git-annex enabled
-        init_git_annex(@repo_name)
-      else
-        raise DisallowedCommandError
-      end
+      @repo_name = escape_path(args[2].sub(/\A\/~\//, ''))
+
+      # Make sure repository has git-annex enabled
+      init_git_annex(@repo_name)
     else
       raise DisallowedCommandError unless args.count == 2
       @repo_name = escape_path(args.last)
@@ -73,24 +69,22 @@ class GitlabShell
     repo_full_path = File.join(repos_path, repo_name)
 
     if @git_cmd == 'git-annex-shell'
-      if @config.git_annex_enabled?
-        args = Shellwords.shellwords(@origin_cmd)
-        parsed_args =
-          args.map do |arg|
-            # Convert /~/group/project.git to group/project.git
-            # to make git annex path compatible with gitlab-shell
-            if arg =~ /\A\/~\/.*\.git\Z/
-              repo_full_path
-            else
-              arg
-            end
+      raise DisallowedCommandError unless @config.git_annex_enabled?
+
+      args = Shellwords.shellwords(@origin_cmd)
+      parsed_args =
+        args.map do |arg|
+          # Convert /~/group/project.git to group/project.git
+          # to make git annex path compatible with gitlab-shell
+          if arg =~ /\A\/~\/.*\.git\Z/
+            repo_full_path
+          else
+            arg
           end
+        end
 
-        $logger.info "gitlab-shell: executing git-annex command <#{parsed_args.join(' ')}> for #{log_username}."
-        exec_cmd(*parsed_args)
-      else
-        raise DisallowedCommandError
-      end
+      $logger.info "gitlab-shell: executing git-annex command <#{parsed_args.join(' ')}> for #{log_username}."
+      exec_cmd(*parsed_args)
     else
       $logger.info "gitlab-shell: executing git command <#{@git_cmd} #{repo_full_path}> for #{log_username}."
       exec_cmd(@git_cmd, repo_full_path)