Re: [PATCH v3 2/2] merge-tree: add a new --dry-run flag

> From: Elijah Newren <newren@gmail.com>
> 
> +--dry-run::
> +	Disable all output from the program.  Useful when you are only
> +	interested in the exit status.  Allows merge-tree to exit
> +	early when it finds a conflict, and allows it to avoid writing
> +	most objects created by merges.

>   --allow-unrelated-histories::
>   	merge-tree will by default error out if the two branches specified
>   	share no common history.  This flag can be given to override that
> diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c
> index 4aafa73c6155..273ec171e988 100644
> --- a/builtin/merge-tree.c
> +++ b/builtin/merge-tree.c
> @@ -490,6 +490,9 @@ static int real_merge(struct merge_tree_options *o,
>   	if (result.clean < 0)
>   		die(_("failure to merge"));
>   
> +	if (o->merge_options.mergeability_only)
> +		goto cleanup;
> +
>   	if (show_messages == -1)
>   		show_messages = !result.clean;
>   
> @@ -522,6 +525,8 @@ static int real_merge(struct merge_tree_options *o,
>   	}
>   	if (o->use_stdin)
>   		putchar(line_termination);
> +
> +cleanup:
>   	merge_finalize(&opt, &result);
>   	clear_merge_options(&opt);
>   	return !result.clean; /* result.clean < 0 handled above */
> @@ -538,6 +543,7 @@ int cmd_merge_tree(int argc,
>   	int original_argc;
>   	const char *merge_base = NULL;
>   	int ret;
> +	int dry_run = 0;
>   
>   	const char * const merge_tree_usage[] = {
>   		N_("git merge-tree [--write-tree] [<options>] <branch1> <branch2>"),
> @@ -552,6 +558,10 @@ int cmd_merge_tree(int argc,
>   			    N_("do a trivial merge only"), MODE_TRIVIAL),
>   		OPT_BOOL(0, "messages", &o.show_messages,
>   			 N_("also show informational/conflict messages")),
> +		OPT_BOOL_F(0, "dry-run",
> +			   &dry_run,
> +			   N_("suppress all output; only exit status wanted"),
> +			   PARSE_OPT_NONEG),
>   		OPT_SET_INT('z', NULL, &line_termination,
>   			    N_("separate paths with the NUL character"), '\0'),
>   		OPT_BOOL_F(0, "name-only",
> @@ -583,6 +593,18 @@ int cmd_merge_tree(int argc,
>   	argc = parse_options(argc, argv, prefix, mt_options,
>   			     merge_tree_usage, PARSE_OPT_STOP_AT_NON_OPTION);
>   
> +	if (dry_run && o.show_messages == -1)
> +		o.show_messages = 0;
> +	o.merge_options.mergeability_only = dry_run;
> +	die_for_incompatible_opt2(dry_run, "--dry-run",
> +				  o.show_messages, "--messages");
> +	die_for_incompatible_opt2(dry_run, "--dry-run",
> +				  o.name_only, "--name-only");
> +	die_for_incompatible_opt2(dry_run, "--dry-run",
> +				  o.use_stdin, "--stdin");
> +	die_for_incompatible_opt2(dry_run, "--dry-run",
> +				  !line_termination, "-z");
> +
>   	if (xopts.nr && o.mode == MODE_TRIVIAL)
>   		die(_("--trivial-merge is incompatible with all other options"));
>   	for (size_t x = 0; x < xopts.nr; x++)
> diff --git a/t/t4301-merge-tree-write-tree.sh b/t/t4301-merge-tree-write-tree.sh
> index f9c5883a7f7c..566a2b4ec737 100755
> --- a/t/t4301-merge-tree-write-tree.sh
> +++ b/t/t4301-merge-tree-write-tree.sh
> @@ -54,6 +54,25 @@ test_expect_success setup '
>   	git commit -m first-commit
>   '
>   
> +test_expect_success '--dry-run on clean merge' '
> +	# Get rid of loose objects to start with
> +	git gc &&
> +	echo "0 objects, 0 kilobytes" >expect &&
> +	git count-objects >actual &&
> +	test_cmp expect actual &&
> +
> +	# Ensure merge is successful (exit code of 0)
> +	git merge-tree --write-tree --dry-run side1 side3 >output &&
> +
> +	# Ensure there is no output
> +	test_must_be_empty output &&
> +
> +	# Ensure no loose objects written (all new objects written would have
> +	# been in "outer layer" of the merge)
> +	git count-objects >actual &&
> +	test_cmp expect actual
> +'
> +
>   test_expect_success 'Clean merge' '
>   	TREE_OID=$(git merge-tree --write-tree side1 side3) &&
>   	q_to_tab <<-EOF >expect &&
> @@ -72,6 +91,25 @@ test_expect_success 'Failed merge without rename detection' '
>   	grep "CONFLICT (modify/delete): numbers deleted" out
>   '
>   
> +test_expect_success  '--dry-run on conflicted merge' '
> +	# Get rid of loose objects to start with
> +	git gc &&
> +	echo "0 objects, 0 kilobytes" >expect &&
> +	git count-objects >actual &&
> +	test_cmp expect actual &&
> +
> +	# Ensure merge has conflict
> +	test_expect_code 1 git merge-tree --write-tree --dry-run side1 side2 >output &&
> +
> +	# Ensure there is no output
> +	test_must_be_empty output &&
> +
> +	# Ensure no loose objects written (all new objects written would have
> +	# been in "outer layer" of the merge)
> +	git count-objects >actual &&
> +	test_cmp expect actual
> +'
> +
>   test_expect_success 'Content merge and a few conflicts' '
>   	git checkout side1^0 &&
>   	test_must_fail git merge side2 &&