[PATCH v2 1/4] rebase: start implementing it as a builtin

[Pratik Karki <predatoramigo@gmail.com>]

This commit imitates the strategy that was used to convert the
difftool to a builtin, see be8a90e (difftool: add a skeleton for the
upcoming builtin, 2017-01-17) for details: This commit renames the
shell script `git-rebase.sh` to `git-legacy-rebase.sh` and hands off to
it by default.

The current version of the builtin rebase does not, however, make full
use of the internals but instead chooses to spawn a couple of Git
processes to find out if we run the builtin or legacy rebase as that
keeps the directory that we are in correct. There remains a lot
of room for improvement, left for a later date. The following commits
will recreate the functionality of the shell script, in pure C.

We intentionally avoid reading the config directly to avoid
messing up the GIT_* environment variables when we need to fall back to
exec()ing the shell script. The test of builtin rebase can be done by
`git -c rebase.useBuiltin=true rebase ...`

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
---
 .gitignore                            |  1 +
 Makefile                              |  3 +-
 builtin.h                             |  1 +
 builtin/rebase.c                      | 55 +++++++++++++++++++++++++++
 git-rebase.sh => git-legacy-rebase.sh |  0
 git.c                                 |  6 +++
 6 files changed, 65 insertions(+), 1 deletion(-)
 create mode 100644 builtin/rebase.c
 rename git-rebase.sh => git-legacy-rebase.sh (100%)

diff --git a/.gitignore b/.gitignore
index 3284a1e9b..ec2395901 100644
--- a/.gitignore
+++ b/.gitignore
@@ -78,6 +78,7 @@
 /git-init-db
 /git-interpret-trailers
 /git-instaweb
+/git-legacy-rebase
 /git-log
 /git-ls-files
 /git-ls-remote
diff --git a/Makefile b/Makefile
index 0cb6590f2..e88fe2e5f 100644
--- a/Makefile
+++ b/Makefile
@@ -609,7 +609,7 @@ SCRIPT_SH += git-merge-one-file.sh
 SCRIPT_SH += git-merge-resolve.sh
 SCRIPT_SH += git-mergetool.sh
 SCRIPT_SH += git-quiltimport.sh
-SCRIPT_SH += git-rebase.sh
+SCRIPT_SH += git-legacy-rebase.sh
 SCRIPT_SH += git-remote-testgit.sh
 SCRIPT_SH += git-request-pull.sh
 SCRIPT_SH += git-stash.sh
@@ -1059,6 +1059,7 @@ BUILTIN_OBJS += builtin/prune.o
 BUILTIN_OBJS += builtin/pull.o
 BUILTIN_OBJS += builtin/push.o
 BUILTIN_OBJS += builtin/read-tree.o
+BUILTIN_OBJS += builtin/rebase.o
 BUILTIN_OBJS += builtin/rebase--helper.o
 BUILTIN_OBJS += builtin/receive-pack.o
 BUILTIN_OBJS += builtin/reflog.o
diff --git a/builtin.h b/builtin.h
index 0362f1ce2..44651a447 100644
--- a/builtin.h
+++ b/builtin.h
@@ -202,6 +202,7 @@ extern int cmd_prune_packed(int argc, const char **argv, const char *prefix);
 extern int cmd_pull(int argc, const char **argv, const char *prefix);
 extern int cmd_push(int argc, const char **argv, const char *prefix);
 extern int cmd_read_tree(int argc, const char **argv, const char *prefix);
+extern int cmd_rebase(int argc, const char **argv, const char *prefix);
 extern int cmd_rebase__helper(int argc, const char **argv, const char *prefix);
 extern int cmd_receive_pack(int argc, const char **argv, const char *prefix);
 extern int cmd_reflog(int argc, const char **argv, const char *prefix);
diff --git a/builtin/rebase.c b/builtin/rebase.c
new file mode 100644
index 000000000..c0c9d6cd2
--- /dev/null
+++ b/builtin/rebase.c
@@ -0,0 +1,55 @@
+/*
+ * "git rebase" builtin command
+ *
+ * Copyright (c) 2018 Pratik Karki
+ */
+
+#include "builtin.h"
+#include "run-command.h"
+#include "exec-cmd.h"
+#include "argv-array.h"
+#include "dir.h"
+
+static int use_builtin_rebase(void)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+	struct strbuf out = STRBUF_INIT;
+	int ret;
+
+	argv_array_pushl(&cp.args,
+			 "config", "--bool", "rebase.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_rebase(int argc, const char **argv, const char *prefix)
+{
+	/*
+	* NEEDSWORK: Once the builtin rebase has been tested enough
+	* and git-legacy-rebase.sh is retired to contrib/, this preamble
+	* can be removed.
+	*/
+	if (!use_builtin_rebase()) {
+		const char *path = mkpath("%s/git-legacy-rebase",
+					  git_exec_path());
+
+		if (sane_execvp(path, (char **)argv) < 0)
+			die_errno("could not exec %s", path);
+
+		return 0;
+	}
+
+	if (argc != 2)
+		die("Usage: %s <base>", argv[0]);
+	prefix = setup_git_directory();
+	trace_repo_setup(prefix);
+	setup_work_tree();
+
+	die("TODO");
+}
diff --git a/git-rebase.sh b/git-legacy-rebase.sh
similarity index 100%
rename from git-rebase.sh
rename to git-legacy-rebase.sh
diff --git a/git.c b/git.c
index 9dbe6ffaa..bacfefb2d 100644
--- a/git.c
+++ b/git.c
@@ -518,6 +518,12 @@ static struct cmd_struct commands[] = {
 	{ "pull", cmd_pull, RUN_SETUP | NEED_WORK_TREE },
 	{ "push", cmd_push, RUN_SETUP },
 	{ "read-tree", cmd_read_tree, RUN_SETUP | SUPPORT_SUPER_PREFIX},
+	/*
+	 *NEEDSWORK: Until the rebase is independent and needs no redirection
+	 to rebase shell script this is kept as is, then should be changed to
+	 RUN_SETUP | NEED_WORK_TREE
+	*/
+	{ "rebase", cmd_rebase },
 	{ "rebase--helper", cmd_rebase__helper, RUN_SETUP | NEED_WORK_TREE },
 	{ "receive-pack", cmd_receive_pack },
 	{ "reflog", cmd_reflog, RUN_SETUP },
-- 
2.18.0