sequencer: refactor the code to detach HEAD to checkout.c

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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: prertik

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;
+
+}

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 */

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,