[PATCH 2/2] difftool: add a feature flag for the builtin vs scripted version

[Johannes Schindelin <johannes.schindelin@gmx.de>]

The popular difftool command was just converted into a builtin, for
better performance on Windows as well as to reduce the number of Perl
scripts (so that we may, in the very long run, be able to ship Git for
Windows without any Perl interpreter at all).

However, it would be sloppy practice to simply switch over from the
tried-and-tested (albeit slow) scripted version to the spanking new
builtin version that has not seen a whole lot of real-world testing.

So let's add a feature flag.

If the file `use-builtin-difftool` exists in Git's exec path, Git will
now automagically use the builtin version of the difftool, without
requiring the user to call `git builtin-difftool <args>`. This comes in
particularly handy when the difftool command is used from within
scripts.

If the file `use-builtin-difftool` is absent from Git's exec path, which
is the default, Git will use the scripted version as before.

The original idea was to use an environment variable
GIT_USE_BUILTIN_DIFFTOOL, but the test suite resets those variables, and
we do want to use that feature flag to run the tests with, and without,
the feature flag.

Besides, the plan is to add an opt-in flag in Git for Windows'
installer. If we implemented the feature flag as an environment
variable, we would have to modify the user's environment, in order to
make the builtin difftool the default when called from Git Bash, Git CMD
or third-party tools.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 .gitignore        |  1 +
 git-difftool.perl |  7 +++++++
 git.c             | 20 ++++++++++++++++++++
 3 files changed, 28 insertions(+)

diff --git a/.gitignore b/.gitignore
index 4f54531..91bfd09 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
+/use-builtin-difftool
 /GIT-BUILD-OPTIONS
 /GIT-CFLAGS
 /GIT-LDFLAGS
diff --git a/git-difftool.perl b/git-difftool.perl
index a5790d0..28e47d8 100755
--- a/git-difftool.perl
+++ b/git-difftool.perl
@@ -23,6 +23,13 @@ use File::Temp qw(tempdir);
 use Getopt::Long qw(:config pass_through);
 use Git;
 
+if (-e Git::exec_path() . '/use-builtin-difftool') {
+	unshift(@ARGV, "builtin-difftool");
+	unshift(@ARGV, "git");
+	exec(@ARGV);
+	die("Could not execute builtin difftool");
+}
+
 sub usage
 {
 	my $exitcode = shift;
diff --git a/git.c b/git.c
index eaa0f67..7a0df7a 100644
--- a/git.c
+++ b/git.c
@@ -2,6 +2,7 @@
 #include "exec_cmd.h"
 #include "help.h"
 #include "run-command.h"
+#include "dir.h"
 
 const char git_usage_string[] =
 	"git [--version] [--help] [-C <path>] [-c name=value]\n"
@@ -542,6 +543,22 @@ static void strip_extension(const char **argv)
 #define strip_extension(cmd)
 #endif
 
+static int use_builtin_difftool(void)
+{
+	static int initialized, use;
+
+	if (!initialized) {
+		struct strbuf buf = STRBUF_INIT;
+		strbuf_addf(&buf, "%s/%s", git_exec_path(),
+			    "use-builtin-difftool");
+		use = file_exists(buf.buf);
+		strbuf_release(&buf);
+		initialized = 1;
+	}
+
+	return use;
+}
+
 static void handle_builtin(int argc, const char **argv)
 {
 	struct argv_array args = ARGV_ARRAY_INIT;
@@ -551,6 +568,9 @@ static void handle_builtin(int argc, const char **argv)
 	strip_extension(argv);
 	cmd = argv[0];
 
+	if (!strcmp("difftool", cmd) && use_builtin_difftool())
+		cmd = "builtin-difftool";
+
 	/* Turn "git cmd --help" into "git help --exclude-guides cmd" */
 	if (argc > 1 && !strcmp(argv[1], "--help")) {
 		int i;
-- 
2.10.1.583.g721a9e0