TEMPORARY: re-add opt-in for builtin difftool

Upstream Git already picked up the builtin difftool (it took only six
iterations this time to get our changes into upstream) and it will be
part of v2.12.0.

In the meantime, to make maintenance of Git for Windows easier, let's
just add back the opt-in, and then drop this patch as soon as it is
time to release Git for Windows v2.12.0.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.gitignore

@@ -76,6 +76,7 @@
 /git-init-db
 /git-interpret-trailers
 /git-instaweb
+/git-legacy-difftool
 /git-log
 /git-ls-files
 /git-ls-remote

Makefile

@@ -527,6 +527,7 @@ SCRIPT_LIB += git-sh-setup
 SCRIPT_LIB += git-sh-i18n
 
 SCRIPT_PERL += git-add--interactive.perl
+SCRIPT_PERL += git-legacy-difftool.perl
 SCRIPT_PERL += git-archimport.perl
 SCRIPT_PERL += git-cvsexportcommit.perl
 SCRIPT_PERL += git-cvsimport.perl

builtin/difftool.c

@@ -616,10 +616,34 @@ static int run_file_diff(int prompt, const char *prefix,
 	exit(ret);
 }
 
+/*
+ * NEEDSWORK: this function can go once the legacy-difftool Perl script is
+ * retired.
+ *
+ * We intentionally avoid reading the config directly here, to avoid messing up
+ * the GIT_* environment variables when we need to fall back to exec()ing the
+ * Perl script.
+ */
+static int use_builtin_difftool(void) {
+	struct child_process cp = CHILD_PROCESS_INIT;
+	struct strbuf out = STRBUF_INIT;
+	int ret;
+
+	argv_array_pushl(&cp.args,
+			 "config", "--bool", "difftool.usebuiltin", NULL);
+	cp.git_cmd = 1;
+	if (capture_command(&cp, &out, 6))
+		return 0;
+	strbuf_trim(&out);
+	ret = !strcmp("true", out.buf);
+	strbuf_release(&out);
+	return ret;
+}
+
 int cmd_difftool(int argc, const char **argv, const char *prefix)
 {
 	int use_gui_tool = 0, dir_diff = 0, prompt = -1, symlinks = 0,
-	    tool_help = 0;
+	    tool_help = 0, nongit;
 	static char *difftool_cmd = NULL, *extcmd = NULL;
 	struct option builtin_difftool_options[] = {
 		OPT_BOOL('g', "gui", &use_gui_tool,
@@ -647,6 +671,25 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	/*
+	 * NEEDSWORK: Once the builtin difftool has been tested enough
+	 * and git-legacy-difftool.perl is retired to contrib/, this preamble
+	 * can be removed.
+	 */
+	if (!use_builtin_difftool()) {
+		const char *path = mkpath("%s/git-legacy-difftool",
+					  git_exec_path());
+
+		if (sane_execvp(path, (char **)argv) < 0)
+			die_errno("could not exec %s", path);
+
+		return 0;
+	}
+	prefix = setup_git_directory_gently(&nongit);
+	trace_repo_setup(prefix);
+	if (!nongit)
+		setup_work_tree();
+
 	git_config(difftool_config, NULL);
 	symlinks = has_symlinks;
 
@@ -657,6 +700,8 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 	if (tool_help)
 		return print_tool_help();
 
+	if (nongit)
+		die("Not a git repository (or any of the parent directories): .git");
 	/* NEEDSWORK: once we no longer spawn anything, remove this */
 	setenv(GIT_DIR_ENVIRONMENT, absolute_path(get_git_dir()), 1);
 	setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);

git.c

@@ -424,7 +424,12 @@ static struct cmd_struct commands[] = {
 	{ "diff-files", cmd_diff_files, RUN_SETUP | NEED_WORK_TREE },
 	{ "diff-index", cmd_diff_index, RUN_SETUP },
 	{ "diff-tree", cmd_diff_tree, RUN_SETUP },
-	{ "difftool", cmd_difftool, RUN_SETUP | NEED_WORK_TREE },
+	/*
+	 * NEEDSWORK: Once the redirection to git-legacy-difftool.perl in
+	 * builtin/difftool.c has been removed, this entry should be changed to
+	 * RUN_SETUP | NEED_WORK_TREE
+	 */
+	{ "difftool", cmd_difftool },
 	{ "fast-export", cmd_fast_export, RUN_SETUP },
 	{ "fetch", cmd_fetch, RUN_SETUP },
 	{ "fetch-pack", cmd_fetch_pack, RUN_SETUP },

t/t7800-difftool.sh

@@ -23,13 +23,41 @@ prompt_given ()
 	test "$prompt" = "Launch 'test-tool' [Y/n]? branch"
 }
 
+for use_builtin_difftool in false true
+do
+
+case "$use_builtin_difftool" in
+true)
+	GIT_CONFIG_PARAMETERS="'difftool.usebuiltin=true'"
+	export GIT_CONFIG_PARAMETERS
+	;;
+false)
+	test_have_prereq PERL || {
+		say 'Skipping scripted difftool (missing PERL)'
+		continue
+	}
+	;;
+esac
+
+test_expect_success 'verify we are running the correct difftool' '
+	if test true = '$use_builtin_difftool'
+	then
+		test_must_fail ok=129 git difftool -h >help &&
+		grep "g, --gui" help
+	else
+		git difftool -h >help &&
+		grep "g|--gui" help
+	fi
+'
+
 test_expect_success 'basic usage requires no repo' '
-	test_expect_code 129 git difftool -h >output &&
+	code='$(test "$use_builtin_difftool" = true && echo 129 || echo 0)' &&
+	test_expect_code $code git difftool -h >output &&
 	grep ^usage: output &&
 	# create a ceiling directory to prevent Git from finding a repo
 	mkdir -p not/repo &&
 	test_when_finished rm -r not &&
-	test_expect_code 129 \
+	test_expect_code $code \
 	env GIT_CEILING_DIRECTORIES="$(pwd)/not" \
 	git -C not/repo difftool -h >output &&
 	grep ^usage: output
@@ -616,4 +644,13 @@ test_expect_success SYMLINKS 'difftool --dir-diff symlinked directories' '
 	)
 '
 
+test true != $use_builtin_difftool || break
+
+test_expect_success 'tear down for re-run' '
+	rm -rf * .[a-z]* &&
+	git init
+'
+
+done
+
 test_done