branch: add --dry-run for --delete-merged

"git branch --dry-run --delete-merged ..." prints one line per ref that
would be deleted without modifying refs or branch configuration.

--dry-run is only meaningful together with --delete-merged and is
rejected otherwise.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: HaraldNordgren

Documentation/git-branch.adoc

@@ -25,7 +25,7 @@ git branch (-m|-M) [<old-branch>] <new-branch>
 git branch (-c|-C) [<old-branch>] <new-branch>
 git branch (-d|-D) [-r] <branch-name>...
 git branch --edit-description [<branch-name>]
-git branch (--delete-merged <branch>)... [<pattern>...]
+git branch [--dry-run] (--delete-merged <branch>)... [<pattern>...]
 
 DESCRIPTION
 -----------
@@ -232,6 +232,12 @@ A branch that a surviving branch depends on through a chain of local
 upstreams is kept, so a branch is never deleted out from under stacked
 work.
 
+`--dry-run`::
+	With `--delete-merged`, print which branches would be
+	deleted and exit without touching any ref.  Useful for
+	sanity-checking a wide pattern like `'origin/*'` before
+	committing to the deletion.
+
 `-v`::
 `-vv`::
 `--verbose`::

builtin/branch.c

@@ -199,6 +199,7 @@ enum delete_branch_flags {
 	DELETE_BRANCH_QUIET = (1 << 1),
 	DELETE_BRANCH_SKIP_UNMERGED = (1 << 2),
 	DELETE_BRANCH_NO_HEAD_FALLBACK = (1 << 3),
+	DELETE_BRANCH_DRY_RUN = (1 << 4),
 };
 
 static int check_branch_commit(const char *branchname, const char *refname,
@@ -340,13 +341,20 @@ static int delete_branches(int argc, const char **argv, int kinds,
 		free(target);
 	}
 
-	if (refs_delete_refs(get_main_ref_store(the_repository), NULL, &refs_to_delete, REF_NO_DEREF))
+	if (!(flags & DELETE_BRANCH_DRY_RUN) &&
+	    refs_delete_refs(get_main_ref_store(the_repository), NULL, &refs_to_delete, REF_NO_DEREF))
 		ret = 1;
 
 	for_each_string_list_item(item, &refs_to_delete) {
 		char *describe_ref = item->util;
 		char *name = item->string;
-		if (!refs_ref_exists(get_main_ref_store(the_repository), name)) {
+		if (flags & DELETE_BRANCH_DRY_RUN) {
+			if (!(flags & DELETE_BRANCH_QUIET))
+				printf(remote_branch
+					? _("Would delete remote-tracking branch %s (was %s).\n")
+					: _("Would delete branch %s (was %s).\n"),
+					name + branch_name_pos, describe_ref);
+		} else if (!refs_ref_exists(get_main_ref_store(the_repository), name)) {
 			char *refname = name + branch_name_pos;
 			if (!(flags & DELETE_BRANCH_QUIET))
 				printf(remote_branch
@@ -899,6 +907,7 @@ int cmd_branch(int argc,
 	int delete = 0, rename = 0, copy = 0, list = 0,
 	    unset_upstream = 0, show_current = 0, edit_description = 0;
 	struct strvec delete_merged = STRVEC_INIT;
+	int dry_run = 0;
 	const char *new_upstream = NULL;
 	int noncreate_actions = 0;
 	/* possible options */
@@ -955,6 +964,8 @@ int cmd_branch(int argc,
 		OPT_CALLBACK_F(0, "delete-merged", &delete_merged, N_("branch"),
 			N_("delete merged branches whose upstream matches <branch> (repeatable)"),
 			PARSE_OPT_NONEG, parse_opt_strvec),
+		OPT_BOOL(0, "dry-run", &dry_run,
+			N_("with --delete-merged, only print which branches would be deleted")),
 		OPT__FORCE(&force, N_("force creation, move/rename, deletion"), PARSE_OPT_NOCOMPLETE),
 		OPT_MERGED(&filter, N_("print only branches that are merged")),
 		OPT_NO_MERGED(&filter, N_("print only branches that are not merged")),
@@ -1017,6 +1028,9 @@ int cmd_branch(int argc,
 	if (noncreate_actions > 1)
 		usage_with_options(builtin_branch_usage, options);
 
+	if (dry_run && !delete_merged.nr)
+		die(_("--dry-run requires --delete-merged"));
+
 	if (recurse_submodules_explicit) {
 		if (!submodule_propagate_branches)
 			die(_("branch with --recurse-submodules can only be used if submodule.propagateBranches is enabled"));
@@ -1057,7 +1071,8 @@ int cmd_branch(int argc,
 		goto out;
 	} else if (delete_merged.nr) {
 		ret = delete_merged_branches(&delete_merged, argv,
-					     quiet ? DELETE_BRANCH_QUIET : 0);
+					     (quiet ? DELETE_BRANCH_QUIET : 0) |
+					     (dry_run ? DELETE_BRANCH_DRY_RUN : 0));
 		goto out;
 	} else if (show_current) {
 		print_current_branch_name();

t/t3200-branch.sh

@@ -1900,6 +1900,19 @@ test_expect_success '--delete-merged deletes only selected merged branches' '
 		git checkout -b tracks-other other/main --track &&
 		sha=$(git rev-parse --short merged) &&
 
+		git branch --dry-run --delete-merged origin/next merged >actual 2>&1 &&
+		echo "Would delete branch merged (was $sha)." >expect &&
+		test_cmp expect actual &&
+		git rev-parse --verify refs/heads/merged &&
+
+		check_branches <<-\EOF &&
+		also-merged
+		main
+		merged
+		tracks-other
+		unmerged
+		EOF
+
 		git branch --delete-merged origin/next merged >actual 2>&1 &&
 		echo "Deleted branch merged (was $sha)." >expect &&
 		test_cmp expect actual &&
@@ -1948,9 +1961,12 @@ test_expect_success '--delete-merged keeps the upstream of a surviving branch' '
 		git checkout -b topic feature --track &&
 		git commit --allow-empty -m "topic work" &&
 
-		git branch --delete-merged origin/next 2>err &&
+		git branch --dry-run --delete-merged origin/next >out &&
+		test_grep ! "feature" out &&
 
+		git branch --delete-merged origin/next 2>err &&
 		test_must_be_empty err &&
+
 		check_branches <<-\EOF &&
 		feature
 		main
@@ -1978,6 +1994,21 @@ test_expect_success '--delete-merged keeps the upstream chain of a surviving bra
 		git checkout -b tip mid --track &&
 		git commit --allow-empty -m "tip work" &&
 
+		git branch --dry-run --delete-merged origin/next \
+			--delete-merged lower >actual 2>&1 &&
+		test_must_be_empty actual &&
+
+		git config --local --get-regexp "branch\\.(lower|mid|tip)\\.(merge|remote)" >actual &&
+		cat >expect <<-\EOF &&
+		branch.lower.remote origin
+		branch.lower.merge refs/heads/next
+		branch.mid.remote .
+		branch.mid.merge refs/heads/lower
+		branch.tip.remote .
+		branch.tip.merge refs/heads/mid
+		EOF
+		test_cmp expect actual &&
+
 		git branch --delete-merged origin/next \
 			--delete-merged lower >actual 2>&1 &&
 		test_must_be_empty actual &&
@@ -2074,4 +2105,9 @@ test_expect_success "branch -d still deletes a deleteMerged=false branch" '
 	)
 '
 
+test_expect_success '--dry-run without --delete-merged is rejected' '
+	test_must_fail git -C forked branch --dry-run 2>err &&
+	test_grep "requires --delete-merged" err
+'
+
 test_done