[PATCH v4 3/4] sequencer: refactor the code to detach HEAD to checkout.c

[Pratik Karki <predatoramigo@gmail.com>]

In the upcoming builtin rebase, we will have to start by detaching
the HEAD, just like shell script version does. Essentially, we have
to do the same thing as `git checkout -q <revision>^0 --`, in pure C.

The aforementioned functionality was already present in `sequencer.c`
in `do_reset()` function. But `do_reset()` performs more than detaching
the HEAD, and performs action specific to `sequencer.c`.

So this commit refactors out that part from `do_reset()`, and moves it
to a new function called `detach_head_to()`. As this function has
nothing to do with the sequencer, and everything to do with what `git
checkout -q <revision>^0 --` does, we move that function to checkout.c.

This refactoring actually introduces a slight change in behavior:
previously, the index was locked before parsing the argument to the
todo command `reset`, while it now gets locked *after* that, in the
`detach_head_to()` function.

It does not make a huge difference, and the upside is that this closes
a few (unlikely) code paths where the index would not be unlocked upon
error.

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
---
 checkout.c  | 64 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 checkout.h  |  3 +++
 sequencer.c | 58 +++++-------------------------------------------
 3 files changed, 72 insertions(+), 53 deletions(-)

diff --git a/checkout.c b/checkout.c
index bdefc888ba..da68915fd7 100644
--- a/checkout.c
+++ b/checkout.c
@@ -2,6 +2,11 @@
 #include "remote.h"
 #include "refspec.h"
 #include "checkout.h"
+#include "unpack-trees.h"
+#include "lockfile.h"
+#include "refs.h"
+#include "tree.h"
+#include "cache-tree.h"
 
 struct tracking_name_data {
 	/* const */ char *src_ref;
@@ -42,3 +47,62 @@ const char *unique_tracking_name(const char *name, struct object_id *oid)
 	free(cb_data.dst_ref);
 	return NULL;
 }
+
+int detach_head_to(struct object_id *oid, const char *action,
+		   const char *reflog_message)
+{
+	struct strbuf ref_name = STRBUF_INIT;
+	struct tree_desc desc;
+	struct lock_file lock = LOCK_INIT;
+	struct unpack_trees_options unpack_tree_opts;
+	struct tree *tree;
+	int ret = 0;
+
+	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
+		return -1;
+
+	memset(&unpack_tree_opts, 0, sizeof(unpack_tree_opts));
+	setup_unpack_trees_porcelain(&unpack_tree_opts, action);
+	unpack_tree_opts.head_idx = 1;
+	unpack_tree_opts.src_index = &the_index;
+	unpack_tree_opts.dst_index = &the_index;
+	unpack_tree_opts.fn = oneway_merge;
+	unpack_tree_opts.merge = 1;
+	unpack_tree_opts.update = 1;
+
+	if (read_cache_unmerged()) {
+		rollback_lock_file(&lock);
+		strbuf_release(&ref_name);
+		return error_resolve_conflict(_(action));
+	}
+
+	if (!fill_tree_descriptor(&desc, oid)) {
+		error(_("failed to find tree of %s"), oid_to_hex(oid));
+		rollback_lock_file(&lock);
+		free((void *)desc.buffer);
+		strbuf_release(&ref_name);
+		return -1;
+	}
+
+	if (unpack_trees(1, &desc, &unpack_tree_opts)) {
+		rollback_lock_file(&lock);
+		free((void *)desc.buffer);
+		strbuf_release(&ref_name);
+		return -1;
+	}
+
+	tree = parse_tree_indirect(oid);
+	prime_cache_tree(&the_index, tree);
+
+	if (write_locked_index(&the_index, &lock, COMMIT_LOCK) < 0)
+		ret = error(_("could not write index"));
+	free((void *)desc.buffer);
+
+	if (!ret)
+		ret = update_ref(reflog_message, "HEAD", oid,
+				 NULL, 0, UPDATE_REFS_MSG_ON_ERR);
+
+	strbuf_release(&ref_name);
+	return ret;
+
+}
diff --git a/checkout.h b/checkout.h
index 9980711179..6ce46cf4e8 100644
--- a/checkout.h
+++ b/checkout.h
@@ -10,4 +10,7 @@
  */
 extern const char *unique_tracking_name(const char *name, struct object_id *oid);
 
+int detach_head_to(struct object_id *oid, const char *action,
+		   const char *reflog_message);
+
 #endif /* CHECKOUT_H */
diff --git a/sequencer.c b/sequencer.c
index 5354d4d51e..106d518f4d 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -29,6 +29,7 @@
 #include "oidset.h"
 #include "commit-slab.h"
 #include "alias.h"
+#include "checkout.h"
 
 #define GIT_REFLOG_ACTION "GIT_REFLOG_ACTION"
 
@@ -2756,14 +2757,7 @@ static int do_reset(const char *name, int len, struct replay_opts *opts)
 {
 	struct strbuf ref_name = STRBUF_INIT;
 	struct object_id oid;
-	struct lock_file lock = LOCK_INIT;
-	struct tree_desc desc;
-	struct tree *tree;
-	struct unpack_trees_options unpack_tree_opts;
-	int ret = 0, i;
-
-	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
-		return -1;
+	int i;
 
 	if (len == 10 && !strncmp("[new root]", name, len)) {
 		if (!opts->have_squash_onto) {
@@ -2789,56 +2783,14 @@ static int do_reset(const char *name, int len, struct replay_opts *opts)
 		if (get_oid(ref_name.buf, &oid) &&
 		    get_oid(ref_name.buf + strlen("refs/rewritten/"), &oid)) {
 			error(_("could not read '%s'"), ref_name.buf);
-			rollback_lock_file(&lock);
 			strbuf_release(&ref_name);
 			return -1;
 		}
 	}
 
-	memset(&unpack_tree_opts, 0, sizeof(unpack_tree_opts));
-	setup_unpack_trees_porcelain(&unpack_tree_opts, "reset");
-	unpack_tree_opts.head_idx = 1;
-	unpack_tree_opts.src_index = &the_index;
-	unpack_tree_opts.dst_index = &the_index;
-	unpack_tree_opts.fn = oneway_merge;
-	unpack_tree_opts.merge = 1;
-	unpack_tree_opts.update = 1;
-
-	if (read_cache_unmerged()) {
-		rollback_lock_file(&lock);
-		strbuf_release(&ref_name);
-		return error_resolve_conflict(_(action_name(opts)));
-	}
-
-	if (!fill_tree_descriptor(&desc, &oid)) {
-		error(_("failed to find tree of %s"), oid_to_hex(&oid));
-		rollback_lock_file(&lock);
-		free((void *)desc.buffer);
-		strbuf_release(&ref_name);
-		return -1;
-	}
-
-	if (unpack_trees(1, &desc, &unpack_tree_opts)) {
-		rollback_lock_file(&lock);
-		free((void *)desc.buffer);
-		strbuf_release(&ref_name);
-		return -1;
-	}
-
-	tree = parse_tree_indirect(&oid);
-	prime_cache_tree(&the_index, tree);
-
-	if (write_locked_index(&the_index, &lock, COMMIT_LOCK) < 0)
-		ret = error(_("could not write index"));
-	free((void *)desc.buffer);
-
-	if (!ret)
-		ret = update_ref(reflog_message(opts, "reset", "'%.*s'",
-						len, name), "HEAD", &oid,
-				 NULL, 0, UPDATE_REFS_MSG_ON_ERR);
-
-	strbuf_release(&ref_name);
-	return ret;
+	return detach_head_to(&oid, action_name(opts),
+			      reflog_message(opts, "reset", "'%.*s'",
+					     len, name));
 }
 
 static int do_merge(struct commit *commit, const char *arg, int arg_len,
-- 
2.18.0