difftool: print list of valid tools with '--tool-help'

Since bc7a96a (mergetool--lib: Refactor tools into separate files,
2011-08-18), it is possible to add a new diff tool by creating a simple
script in the '$(git --exec-path)/mergetools' directory.  Updating the
difftool help text is still a manual process, and the documentation can
easily go out of sync.

This commit teaches difftool the '--tool-help' option, which:
  - Reads the list of valid tools from 'mergetools/*'
  - Determines which of them are actually installed
  - Determines which are capable of diffing (i.e. not just a merge tool)
  - Prints the resulting list for the user

Signed-off-by: Tim Henigan <tim.henigan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-difftool.txt

@@ -36,11 +36,9 @@ OPTIONS
 
 -t <tool>::
 --tool=<tool>::
-	Use the diff tool specified by <tool>.
-	Valid diff tools are:
-	araxis, bc3, deltawalker, diffuse, emerge, ecmerge, gvimdiff,
-	kdiff3,	kompare, meld, opendiff, p4merge, tkdiff, vimdiff and
-	xxdiff.
+	Use the diff tool specified by <tool>.  Valid values include
+	emerge, kompare, meld, and vimdiff. Run `git difftool --tool-help`
+	for the list of valid <tool> settings.
 +
 If a diff tool is not specified, 'git difftool'
 will use the configuration variable `diff.tool`.  If the
@@ -68,6 +66,9 @@ of the diff post-image.  `$MERGED` is the name of the file which is
 being compared. `$BASE` is provided for compatibility
 with custom merge tool commands and has the same value as `$MERGED`.
 
+--tool-help::
+	Print a list of diff tools that may be used with `--tool`.
+
 -x <command>::
 --extcmd=<command>::
 	Specify a custom command for viewing diffs.

git-difftool.perl

@@ -15,12 +15,14 @@
 use warnings;
 use File::Basename qw(dirname);
 use File::Copy;
+use File::Find;
 use File::stat;
 use File::Path qw(mkpath);
 use File::Temp qw(tempdir);
 use Getopt::Long qw(:config pass_through);
 use Git;
 
+my @tools;
 my @working_tree;
 my $rc;
 my $repo = Git->repository();
@@ -30,7 +32,7 @@ sub usage
 {
 	my $exitcode = shift;
 	print << 'USAGE';
-usage: git difftool [-t|--tool=<tool>]
+usage: git difftool [-t|--tool=<tool>] [--tool-help]
                     [-x|--extcmd=<cmd>]
                     [-g|--gui] [--no-gui]
                     [--prompt] [-y|--no-prompt]
@@ -62,6 +64,51 @@ sub find_worktree
 
 my $workdir = find_worktree();
 
+sub filter_tool_scripts
+{
+	if (-d $_) {
+		if ($_ ne ".") {
+			# Ignore files in subdirectories
+			$File::Find::prune = 1;
+		}
+	} else {
+		if ((-f $_) && ($_ ne "defaults")) {
+			push(@tools, $_);
+		}
+	}
+}
+
+sub print_tool_help
+{
+	my ($cmd, @found, @notfound);
+	my $gitpath = Git::exec_path();
+
+	find(\&filter_tool_scripts, "$gitpath/mergetools");
+
+	foreach my $tool (@tools) {
+		$cmd  = "TOOL_MODE=diff";
+		$cmd .= ' && . "$(git --exec-path)/git-mergetool--lib"';
+		$cmd .= " && get_merge_tool_path $tool >/dev/null 2>&1";
+		$cmd .= " && can_diff >/dev/null 2>&1";
+		if (system('sh', '-c', $cmd) == 0) {
+			push(@found, $tool);
+		} else {
+			push(@notfound, $tool);
+		}
+	}
+
+	print "'git difftool --tool=<tool>' may be set to one of the following:\n";
+	print "\t$_\n" for (sort(@found));
+
+	print "\nThe following tools are valid, but not currently available:\n";
+	print "\t$_\n" for (sort(@notfound));
+
+	print "\nNOTE: Some of the tools listed above only work in a windowed\n";
+	print "environment. If run in a terminal-only session, they will fail.\n";
+
+	exit(0);
+}
+
 sub setup_dir_diff
 {
 	# Run the diff; exit immediately if no diff found
@@ -230,18 +277,22 @@ sub write_to_file
 
 # parse command-line options. all unrecognized options and arguments
 # are passed through to the 'git diff' command.
-my ($difftool_cmd, $dirdiff, $extcmd, $gui, $help, $prompt);
+my ($difftool_cmd, $dirdiff, $extcmd, $gui, $help, $prompt, $tool_help);
 GetOptions('g|gui!' => \$gui,
 	'd|dir-diff' => \$dirdiff,
 	'h' => \$help,
 	'prompt!' => \$prompt,
 	'y' => sub { $prompt = 0; },
 	't|tool:s' => \$difftool_cmd,
+	'tool-help' => \$tool_help,
 	'x|extcmd:s' => \$extcmd);
 
 if (defined($help)) {
 	usage(0);
 }
+if (defined($tool_help)) {
+	print_tool_help();
+}
 if (defined($difftool_cmd)) {
 	if (length($difftool_cmd) > 0) {
 		$ENV{GIT_DIFF_TOOL} = $difftool_cmd;

t/t7800-difftool.sh

@@ -319,6 +319,11 @@ test_expect_success PERL 'say no to the second file' '
 	echo "$diff" | stdin_doesnot_contain br2
 '
 
+test_expect_success PERL 'difftool --tool-help' '
+	tool_help=$(git difftool --tool-help) &&
+	echo "$tool_help" | stdin_contains tool
+'
+
 test_expect_success PERL 'setup change in subdirectory' '
 	git checkout master &&
 	mkdir sub &&