From: Patrick Steinhardt <ps@pks.im>
Date: Wed, 3 Jun 2026 16:14:03 +0000 (+0200)
Subject: reset: introduce dry-run mode
X-Git-Url: http://git.ipfire.org/gitweb/index.cgi?a=commitdiff_plain;h=c39ed0f69fba50a1604cb62b368ab9ba37b4c216;p=thirdparty%2Fgit.git

reset: introduce dry-run mode

In a subsequent commit we'll add add another caller to `reset_head()`
that wants to perform a dry-run check of whether it would be possible to
udpate the index and working tree when moving to a new commit. Introduce
a new flag that lets the caller perform this operation.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---

diff --git a/reset.c b/reset.c
index 9ff14f5ed1..a8d7eea4d6 100644
--- a/reset.c
+++ b/reset.c
@@ -92,11 +92,14 @@ int reset_head(struct repository *r, const struct reset_head_opts *opts)
 	unsigned reset_hard = opts->flags & RESET_HEAD_HARD;
 	unsigned refs_only = opts->flags & RESET_HEAD_REFS_ONLY;
 	unsigned update_orig_head = opts->flags & RESET_HEAD_ORIG_HEAD;
+	unsigned dry_run = opts->flags & RESET_HEAD_DRY_RUN;
 	struct object_id *head = NULL, head_oid;
 	struct tree_desc desc[2] = { { NULL }, { NULL } };
 	struct lock_file lock = LOCK_INIT;
 	struct unpack_trees_options unpack_tree_opts = { 0 };
 	struct tree *tree;
+	struct index_state scratch_index = INDEX_STATE_INIT(r);
+	struct index_state *istate;
 	const char *action;
 	int ret = 0, nr = 0;
 
@@ -109,7 +112,7 @@ int reset_head(struct repository *r, const struct reset_head_opts *opts)
 	if (opts->branch_msg && !opts->branch)
 		BUG("branch reflog message given without a branch");
 
-	if (!refs_only && repo_hold_locked_index(r, &lock, LOCK_REPORT_ON_ERROR) < 0) {
+	if (!refs_only && !dry_run && repo_hold_locked_index(r, &lock, LOCK_REPORT_ON_ERROR) < 0) {
 		ret = -1;
 		goto leave_reset_head;
 	}
@@ -124,16 +127,36 @@ int reset_head(struct repository *r, const struct reset_head_opts *opts)
 	if (!oid)
 		oid = &head_oid;
 
-	if (refs_only)
-		return update_refs(r, opts, oid, head);
+	if (refs_only) {
+		if (!dry_run)
+			return update_refs(r, opts, oid, head);
+		return 0;
+	}
+
+	if (dry_run) {
+		if (read_index_from(&scratch_index, r->index_file, r->gitdir) < 0 ||
+		    index_state_unmerged_to_stage0(&scratch_index) < 0) {
+			ret = error(_("could not read index"));
+			goto leave_reset_head;
+		}
+
+		istate = &scratch_index;
+	} else {
+		if (repo_read_index_unmerged(r) < 0) {
+			ret = error(_("could not read index"));
+			goto leave_reset_head;
+		}
+		istate = r->index;
+	}
 
 	action = reset_hard ? "reset" : "checkout";
 	setup_unpack_trees_porcelain(&unpack_tree_opts, action);
 	unpack_tree_opts.head_idx = 1;
-	unpack_tree_opts.src_index = r->index;
-	unpack_tree_opts.dst_index = r->index;
+	unpack_tree_opts.src_index = istate;
+	unpack_tree_opts.dst_index = istate;
 	unpack_tree_opts.fn = reset_hard ? oneway_merge : twoway_merge;
-	unpack_tree_opts.update = 1;
+	unpack_tree_opts.update = !dry_run;
+	unpack_tree_opts.dry_run = dry_run;
 	unpack_tree_opts.merge = 1;
 	unpack_tree_opts.preserve_ignored = 0; /* FIXME: !overwrite_ignore */
 	unpack_tree_opts.skip_cache_tree_update = 1;
@@ -141,11 +164,6 @@ int reset_head(struct repository *r, const struct reset_head_opts *opts)
 	if (reset_hard)
 		unpack_tree_opts.reset = UNPACK_RESET_PROTECT_UNTRACKED;
 
-	if (repo_read_index_unmerged(r) < 0) {
-		ret = error(_("could not read index"));
-		goto leave_reset_head;
-	}
-
 	if (!reset_hard && !fill_tree_descriptor(r, &desc[nr++], &head_oid)) {
 		ret = error(_("failed to find tree of %s"),
 			    oid_to_hex(&head_oid));
@@ -162,6 +180,9 @@ int reset_head(struct repository *r, const struct reset_head_opts *opts)
 		goto leave_reset_head;
 	}
 
+	if (dry_run)
+		goto leave_reset_head;
+
 	tree = repo_parse_tree_indirect(r, oid);
 	if (!tree) {
 		ret = error(_("unable to read tree (%s)"), oid_to_hex(oid));
@@ -181,6 +202,7 @@ int reset_head(struct repository *r, const struct reset_head_opts *opts)
 leave_reset_head:
 	rollback_lock_file(&lock);
 	clear_unpack_trees_porcelain(&unpack_tree_opts);
+	release_index(&scratch_index);
 	while (nr)
 		free((void *)desc[--nr].buffer);
 	return ret;
diff --git a/reset.h b/reset.h
index 97ced2601e..9f696382c1 100644
--- a/reset.h
+++ b/reset.h
@@ -21,6 +21,12 @@ enum reset_head_flags {
 
 	/* Update ORIG_HEAD as well as HEAD */
 	RESET_HEAD_ORIG_HEAD = (1 << 4),
+
+	/*
+	 * Perform a dry-run by performing the operation without updating
+	 * any user-visible state.
+	 */
+	RESET_HEAD_DRY_RUN = (1 << 5),
 };
 
 struct reset_head_opts {