difftool: add a skeleton for the upcoming builtin

This adds a builtin difftool that still falls back to the legacy Perl
version, which has been renamed to `legacy-difftool`.

The idea is that the new, experimental, builtin difftool immediately hands
off to the legacy difftool for now, unless the config variable
difftool.useBuiltin is set to true.

This feature flag will be used in the upcoming Git for Windows v2.11.0
release, to allow early testers to opt-in to use the builtin difftool and
flesh out any bugs.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 63 insertions, 0 deletions

diff --git a/builtin/difftool.c b/builtin/difftool.c
new file mode 100644
index 0000000..53870bb
--- /dev/null
+++ b/builtin/difftool.c
@@ -0,0 +1,63 @@
+/*
+ * "git difftool" builtin command
+ *
+ * This is a wrapper around the GIT_EXTERNAL_DIFF-compatible
+ * git-difftool--helper script.
+ *
+ * This script exports GIT_EXTERNAL_DIFF and GIT_PAGER for use by git.
+ * The GIT_DIFF* variables are exported for use by git-difftool--helper.
+ *
+ * Any arguments that are unknown to this script are forwarded to 'git diff'.
+ *
+ * Copyright (C) 2016 Johannes Schindelin
+ */
+#include "builtin.h"
+#include "run-command.h"
+#include "exec_cmd.h"
+
+/*
+ * 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)
+{
+	/*
+	 * 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();
+	trace_repo_setup(prefix);
+	setup_work_tree();
+
+	die("TODO");
+}