improve preserving message from failed pull request

 - If text editor aborts, message is discarded.
 - If the API request to create the PR fails, message is kept.
 - If the API request succeeds, message is discarded.
 - When kept message is reused for subsequent PR, only its title and
   body get inserted. The PR instructions and changelog still get
   regenerated.

Fixes #178

features/pull_request.feature

@@ -80,3 +80,70 @@ Feature: hub pull-request
       """
     When I successfully run `hub pull-request useragent`
     Then the output should contain exactly "the://url\n"
+
+  Scenario: Text editor adds title and body
+    Given the "origin" remote has url "git://github.com/mislav/coral.git"
+    And the text editor adds:
+      """
+      This title comes from vim!
+
+      This body as well.
+      """
+    Given the GitHub API server:
+      """
+      post('/repos/mislav/coral/pulls') {
+        { :title => 'This title comes from vim!',
+          :body  => 'This body as well.'
+        }.each do |param, value|
+          if params[param] != value
+            halt 422, json(
+              :message => "expected %s to be %s; got %s" % [
+                param.inspect,
+                value.inspect,
+                params[param].inspect
+              ]
+            )
+          end
+        end
+        json :html_url => "https://github.com/mislav/coral/pull/12"
+      }
+      """
+    When I successfully run `hub pull-request`
+    Then the output should contain exactly "https://github.com/mislav/coral/pull/12\n"
+    And the file ".git/PULLREQ_EDITMSG" should not exist
+
+  Scenario: Failed pull request preserves previous message
+    Given the "origin" remote has url "git://github.com/mislav/coral.git"
+    And the text editor adds:
+      """
+      This title will fail
+      """
+    Given the GitHub API server:
+      """
+      post('/repos/mislav/coral/pulls') {
+        halt 422 if params[:title].include?("fail")
+        halt 422 unless params[:body] == "This title will fail"
+        json :html_url => "https://github.com/mislav/coral/pull/12"
+      }
+      """
+    When I run `hub pull-request`
+    Then the exit status should be 1
+    And the stderr should contain exactly:
+      """
+      Error creating pull request:  (HTTP 422)\n
+      """
+    Given the text editor adds:
+      """
+      But this title will prevail
+      """
+    When I successfully run `hub pull-request`
+    Then the file ".git/PULLREQ_EDITMSG" should not exist
+
+  Scenario: Text editor fails
+    Given the "origin" remote has url "git://github.com/mislav/coral.git"
+    And the text editor exits with error status
+    And an empty file named ".git/PULLREQ_EDITMSG"
+    When I run `hub pull-request`
+    Then the stderr should contain "error using text editor for pull request message"
+    And the exit status should be 1
+    And the file ".git/PULLREQ_EDITMSG" should not exist

features/steps.rb

@@ -159,3 +159,18 @@
 Given /^the remote commit state of "(.*?)" "(.*?)" is nil$/ do |proj, ref|
   step %{the remote commit states of "#{proj}" "#{ref}" are:}, "[ ]"
 end
+
+Given /^the text editor exits with error status$/ do
+  text_editor_script "exit 1"
+end
+
+Given /^the text editor adds:$/ do |text|
+  text_editor_script <<-BASH
+    file="$3"
+    contents="$(cat "$file" 2>/dev/null || true)"
+    { echo "#{text}"
+      echo
+      echo "$contents"
+    } > "$file"
+  BASH
+end

features/support/env.rb

@@ -35,6 +35,8 @@ def close_streams
   set_env 'HUB_TEST_HOST', '127.0.0.1:0'
   # ensure we use fakebin `open` to test browsing
   set_env 'BROWSER', 'open'
+  # ensure we use fakebin `vim` as text editor
+  set_env 'GIT_EDITOR', 'vim'
 
   author_name  = "Hub"
   author_email = "hub@test.local"
@@ -56,6 +58,7 @@ def close_streams
 After do
   @server.stop if defined? @server and @server
   processes.each {|_, p| p.close_streams }
+  FileUtils.rm_f("#{bin_dir}/vim")
 end
 
 RSpec::Matchers.define :be_successful_command do
@@ -124,6 +127,14 @@ def edit_hub_config
     File.open(config, 'w') { |cfg| cfg << YAML.dump(data) }
   end
 
+  define_method(:text_editor_script) do |bash_code|
+    File.open("#{bin_dir}/vim", 'w', 0755) do |exe|
+      exe.puts "#!/bin/bash"
+      exe.puts "set -e"
+      exe.puts bash_code
+    end
+  end
+
   def run_silent cmd
     in_current_dir do
       command = SimpleCommand.run(cmd)

lib/hub/commands.rb

@@ -206,8 +206,9 @@ def pull_request(args)
             [format, base_branch, remote_branch]
         end
 
-        options[:title], options[:body] = pullrequest_editmsg(commit_summary) { |msg|
-          msg.puts default_message if default_message
+        options[:title], options[:body] = pullrequest_editmsg(commit_summary) { |msg, initial_message|
+          initial_message ||= default_message
+          msg.puts initial_message if initial_message
           msg.puts ""
           msg.puts "# Requesting a pull to #{base_project.owner}:#{options[:base]} from #{options[:head]}"
           msg.puts "#"
@@ -218,8 +219,6 @@ def pull_request(args)
 
       pull = api_client.create_pullrequest(options)
 
-      delete_pullreq_editmsg_file
-
       args.executable = 'echo'
       args.replace [pull['html_url']]
     rescue GitHubAPI::Exceptions
@@ -230,6 +229,8 @@ def pull_request(args)
         warn "Are you sure that #{base_url} exists?"
       end
       exit 1
+    else
+      delete_editmsg
     end
 
     # $ hub clone rtomayko/tilt
@@ -997,29 +998,41 @@ def page_stdout
     end
 
     def pullrequest_editmsg(changes)
-      message_file = pullreq_editmsg_file
-
-      unless File.exists?(message_file)
-        File.open(message_file, 'w') { |msg|
-          yield msg
-          if changes
-            msg.puts "#\n# Changes:\n#"
-            msg.puts changes.gsub(/^/, '# ').gsub(/ +$/, '')
-          end
-        }
+      message_file = pullrequest_editmsg_file
+
+      if File.exists?(message_file)
+        title, body = read_editmsg(message_file)
+        previous_message = [title, body].compact.join("\n\n") if title
       end
 
+      File.open(message_file, 'w') { |msg|
+        yield msg, previous_message
+        if changes
+          msg.puts "#\n# Changes:\n#"
+          msg.puts changes.gsub(/^/, '# ').gsub(/ +$/, '')
+        end
+      }
+
       edit_cmd = Array(git_editor).dup
       edit_cmd << '-c' << 'set ft=gitcommit' if edit_cmd[0] =~ /^[mg]?vim$/
       edit_cmd << message_file
       system(*edit_cmd)
-      abort "can't open text editor for pull request message" unless $?.success?
+
+      unless $?.success?
+        # writing was cancelled, or the editor never opened in the first place
+        delete_editmsg(message_file)
+        abort "error using text editor for pull request message"
+      end
 
       title, body = read_editmsg(message_file)
       abort "Aborting due to empty pull request title" unless title
       [title, body]
     end
 
+    def pullrequest_editmsg_file
+      File.join(git_dir, 'PULLREQ_EDITMSG')
+    end
+
     def read_editmsg(file)
       title, body = '', ''
       File.open(file, 'r') { |msg|
@@ -1035,12 +1048,8 @@ def read_editmsg(file)
       [title =~ /\S/ ? title : nil, body =~ /\S/ ? body : nil]
     end
 
-    def delete_pullreq_editmsg_file
-      File.delete(pullreq_editmsg_file) if File.exists?(pullreq_editmsg_file)
-    end
-
-    def pullreq_editmsg_file
-      File.join(git_dir, 'PULLREQ_EDITMSG')
+    def delete_editmsg(file = pullrequest_editmsg_file)
+      File.delete(file) if File.exist?(file)
     end
 
     def expand_alias(cmd)