modified_files and modified_lines_in_file on CommitMsg (#727)

* Install debugger

* Extract stashing before running hooks to a module

This way, if another hook context needs this behavior it can be included
using Overcommit::HookContext::Helpers::StashUnstashedChanges.

* Extract getting file modifications into a module

Methods for determining which files were modified and on which lines can
be imported into hook context with
Overcommit::HookContext::Helpers::FileModifications.

* Refactor hook contexts to use new modules

* Apply Rubocop fixes

* Remove pry and pry-byebug from Gemfile

lib/overcommit/hook_context/commit_msg.rb

@@ -1,8 +1,15 @@
 # frozen_string_literal: true
 
+require_relative 'pre_commit'
+require_relative 'helpers/stash_unstaged_changes'
+require_relative 'helpers/file_modifications'
+
 module Overcommit::HookContext
   # Contains helpers related to contextual information used by commit-msg hooks.
   class CommitMsg < Base
+    include Overcommit::HookContext::Helpers::StashUnstagedChanges
+    include Overcommit::HookContext::Helpers::FileModifications
+
     def empty_message?
       commit_message.strip.empty?
     end

lib/overcommit/hook_context/helpers/file_modifications.rb

@@ -0,0 +1,78 @@
+# frozen_string_literal: true
+
+module Overcommit::HookContext
+  module Helpers
+    # This module contains methods for determining what files were changed and on what unique line
+    # numbers did the change occur.
+    module FileModifications
+      # Returns whether this hook run was triggered by `git commit --amend`
+      def amendment?
+        return @amendment unless @amendment.nil?
+
+        cmd = Overcommit::Utils.parent_command
+        return unless cmd
+        amend_pattern = 'commit(\s.*)?\s--amend(\s|$)'
+
+        # Since the ps command can return invalid byte sequences for commands
+        # containing unicode characters, we replace the offending characters,
+        # since the pattern we're looking for will consist of ASCII characters
+        unless cmd.valid_encoding?
+          cmd = Overcommit::Utils.
+            parent_command.
+            encode('UTF-16be', invalid: :replace, replace: '?').
+            encode('UTF-8')
+        end
+
+        return @amendment if
+          # True if the command is a commit with the --amend flag
+          @amendment = !(/\s#{amend_pattern}/ =~ cmd).nil?
+
+        # Check for git aliases that call `commit --amend`
+        `git config --get-regexp "^alias\\." "#{amend_pattern}"`.
+          scan(/alias\.([-\w]+)/). # Extract the alias
+          each do |match|
+            return @amendment if
+              # True if the command uses a git alias for `commit --amend`
+              @amendment = !(/git(\.exe)?\s+#{match[0]}/ =~ cmd).nil?
+          end
+
+        @amendment
+      end
+
+      # Get a list of added, copied, or modified files that have been staged.
+      # Renames and deletions are ignored, since there should be nothing to check.
+      def modified_files
+        unless @modified_files
+          currently_staged = Overcommit::GitRepo.modified_files(staged: true)
+          @modified_files = currently_staged
+
+          # Include files modified in last commit if amending
+          if amendment?
+            subcmd = 'show --format=%n'
+            previously_modified = Overcommit::GitRepo.modified_files(subcmd: subcmd)
+            @modified_files |= filter_modified_files(previously_modified)
+          end
+        end
+        @modified_files
+      end
+
+      # Returns the set of line numbers corresponding to the lines that were
+      # changed in a specified file.
+      def modified_lines_in_file(file)
+        @modified_lines ||= {}
+        unless @modified_lines[file]
+          @modified_lines[file] =
+            Overcommit::GitRepo.extract_modified_lines(file, staged: true)
+
+          # Include lines modified in last commit if amending
+          if amendment?
+            subcmd = 'show --format=%n'
+            @modified_lines[file] +=
+              Overcommit::GitRepo.extract_modified_lines(file, subcmd: subcmd)
+          end
+        end
+        @modified_lines[file]
+      end
+    end
+  end
+end

lib/overcommit/hook_context/helpers/stash_unstaged_changes.rb

@@ -0,0 +1,141 @@
+# frozen_string_literal: true
+
+module Overcommit::HookContext
+  module Helpers
+    # This module contains behavior for stashing unstaged changes before hooks are ran and restoring
+    # them afterwards
+    module StashUnstagedChanges
+      # Stash unstaged contents of files so hooks don't see changes that aren't
+      # about to be committed.
+      def setup_environment
+        store_modified_times
+        Overcommit::GitRepo.store_merge_state
+        Overcommit::GitRepo.store_cherry_pick_state
+
+        # Don't attempt to stash changes if all changes are staged, as this
+        # prevents us from modifying files at all, which plays better with
+        # editors/tools which watch for file changes.
+        if !initial_commit? && unstaged_changes?
+          stash_changes
+
+          # While running hooks make it appear as if nothing changed
+          restore_modified_times
+        end
+      end
+
+      # Restore unstaged changes and reset file modification times so it appears
+      # as if nothing ever changed.
+      #
+      # We want to restore the modification times for each of the files after
+      # every step to ensure as little time as possible has passed while the
+      # modification time on the file was newer. This helps us play more nicely
+      # with file watchers.
+      def cleanup_environment
+        if @changes_stashed
+          clear_working_tree
+          restore_working_tree
+          restore_modified_times
+        end
+
+        Overcommit::GitRepo.restore_merge_state
+        Overcommit::GitRepo.restore_cherry_pick_state
+      end
+
+      private
+
+      # Stores the modification times for all modified files to make it appear like
+      # they never changed.
+      #
+      # This prevents (some) editors from complaining about files changing when we
+      # stash changes before running the hooks.
+      def store_modified_times
+        @modified_times = {}
+
+        staged_files = modified_files
+        unstaged_files = Overcommit::GitRepo.modified_files(staged: false)
+
+        (staged_files + unstaged_files).each do |file|
+          next if Overcommit::Utils.broken_symlink?(file)
+          next unless File.exist?(file) # Ignore renamed files (old file no longer exists)
+          @modified_times[file] = File.mtime(file)
+        end
+      end
+
+      # Returns whether the current git branch is empty (has no commits).
+      def initial_commit?
+        return @initial_commit unless @initial_commit.nil?
+        @initial_commit = Overcommit::GitRepo.initial_commit?
+      end
+
+      # Returns whether there are any changes to tracked files which have not yet
+      # been staged.
+      def unstaged_changes?
+        result = Overcommit::Utils.execute(%w[git --no-pager diff --quiet])
+        !result.success?
+      end
+
+      def stash_changes
+        @stash_attempted = true
+
+        stash_message = "Overcommit: Stash of repo state before hook run at #{Time.now}"
+        result = Overcommit::Utils.with_environment('GIT_LITERAL_PATHSPECS' => '0') do
+          Overcommit::Utils.execute(
+            %w[git -c commit.gpgsign=false stash save --keep-index --quiet] + [stash_message]
+          )
+        end
+
+        unless result.success?
+          # Failure to stash in this case is likely due to a configuration
+          # issue (e.g. author/email not set or GPG signing key incorrect)
+          raise Overcommit::Exceptions::HookSetupFailed,
+                "Unable to setup environment for #{hook_script_name} hook run:" \
+                "\nSTDOUT:#{result.stdout}\nSTDERR:#{result.stderr}"
+        end
+
+        @changes_stashed = `git stash list -1`.include?(stash_message)
+      end
+
+      # Restores the file modification times for all modified files to make it
+      # appear like they never changed.
+      def restore_modified_times
+        @modified_times.each do |file, time|
+          next if Overcommit::Utils.broken_symlink?(file)
+          next unless File.exist?(file)
+          File.utime(time, time, file)
+        end
+      end
+
+      # Clears the working tree so that the stash can be applied.
+      def clear_working_tree
+        removed_submodules = Overcommit::GitRepo.staged_submodule_removals
+
+        result = Overcommit::Utils.execute(%w[git reset --hard])
+        unless result.success?
+          raise Overcommit::Exceptions::HookCleanupFailed,
+                "Unable to cleanup working tree after #{hook_script_name} hooks run:" \
+                "\nSTDOUT:#{result.stdout}\nSTDERR:#{result.stderr}"
+        end
+
+        # Hard-resetting a staged submodule removal results in the index being
+        # reset but the submodule being restored as an empty directory. This empty
+        # directory prevents us from stashing on a subsequent run if a hook fails.
+        #
+        # Work around this by removing these empty submodule directories as there
+        # doesn't appear any reason to keep them around.
+        removed_submodules.each do |submodule|
+          FileUtils.rmdir(submodule.path)
+        end
+      end
+
+      # Applies the stash to the working tree to restore the user's state.
+      def restore_working_tree
+        result = Overcommit::Utils.execute(%w[git stash pop --index --quiet])
+        unless result.success?
+          raise Overcommit::Exceptions::HookCleanupFailed,
+                "Unable to restore working tree after #{hook_script_name} hooks run:" \
+                "\nSTDOUT:#{result.stdout}\nSTDERR:#{result.stderr}"
+        end
+      end
+    end
+  end
+end