diff mbox series

[v3,1/5] branch: move --set-upstream-to behavior to setup_tracking()

Message ID 20211209184928.71413-2-chooglen@google.com (mailing list archive)
State Superseded
Headers show
Series implement branch --recurse-submodules | expand

Commit Message

Glen Choo Dec. 9, 2021, 6:49 p.m. UTC
This refactor is motivated by a desire to add a "dry_run" parameter to
create_branch() that will validate whether or not a branch can be
created without actually creating it - this behavior be used in a
subsequent commit that adds `git branch --recurse-submodules topic`.

Adding "dry_run" is not obvious because create_branch() is also used to
set tracking information without creating a branch, i.e. when using
--set-upstream-to. This appears to be a leftover from 4fc5006676 (Add
branch --set-upstream, 2010-01-18), when --set-upstream would sometimes
create a branch and sometimes update tracking information without
creating a branch. However, we no longer support --set-upstream, so it
makes more sense to set tracking information with another function, like
setup_tracking(), and use create_branch() only to create branches. When
this is done, it will be trivial to add "dry_run".

Do this refactor by moving the branch validation and dwim logic from
create_branch() into a new function, validate_branch_start(), and call
it from setup_tracking(). Now that setup_tracking() can perform dwim and
tracking setup without creating a branch, use it in `git branch
--set-upstream-to` and remove unnecessary behavior from create_branch().

Since there were none, add tests for creating a branch with `--force`.

Signed-off-by: Glen Choo <chooglen@google.com>
---
 branch.c          | 147 +++++++++++++++++++++++++++-------------------
 branch.h          |  13 +++-
 builtin/branch.c  |   7 +--
 t/t3200-branch.sh |  17 ++++++
 4 files changed, 117 insertions(+), 67 deletions(-)

Comments

Jonathan Tan Dec. 9, 2021, 9:19 p.m. UTC | #1
Glen Choo <chooglen@google.com> writes:
> This refactor is motivated by a desire to add a "dry_run" parameter to
> create_branch() that will validate whether or not a branch can be
> created without actually creating it - this behavior be used in a
> subsequent commit that adds `git branch --recurse-submodules topic`.

When I said that the patch was doing too much [1], I meant that for a
patch that is supposed to be about refactoring, there were many lines
that didn't seem to be a straightforward move. I've attached a version
of the patch that I expected - notice that a reviewer can see that the
lines are straightforwardly moved.

[1] https://lore.kernel.org/git/20211123231035.3607109-1-jonathantanmy@google.com/

> +void setup_tracking(const char *new_ref, const char *orig_ref,
> +			   enum branch_track track, int quiet, int expand_orig)
>  {
>  	struct tracking tracking;
>  	struct string_list tracking_srcs = STRING_LIST_INIT_DUP;
>  	int config_flags = quiet ? 0 : BRANCH_CONFIG_VERBOSE;
> +	char *full_orig_ref;
> +	struct object_id unused_oid;
>  
>  	memset(&tracking, 0, sizeof(tracking));
> -	tracking.spec.dst = (char *)orig_ref;
> +	if (expand_orig)
> +		validate_branch_start(the_repository, orig_ref, track, &unused_oid, &full_orig_ref);
> +	else
> +		full_orig_ref = xstrdup(orig_ref);

Having two meanings for a parameter (and which meaning used depending on
another parameter) is quite confusing - I think it's better to call
another function to expand the ref if necessary. See my patch below for
an example.

> diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh
> index 267a624671..18e285a876 100755
> --- a/t/t3200-branch.sh
> +++ b/t/t3200-branch.sh
> @@ -42,6 +42,23 @@ test_expect_success 'git branch abc should create a branch' '
>  	git branch abc && test_path_is_file .git/refs/heads/abc
>  '
>  
> +test_expect_success 'git branch abc should fail when abc exists' '
> +	test_must_fail git branch abc
> +'
> +
> +test_expect_success 'git branch --force abc should fail when abc is checked out' '
> +	test_when_finished git switch main &&
> +	git switch abc &&
> +	test_must_fail git branch --force abc HEAD~1
> +'
> +
> +test_expect_success 'git branch --force abc should succeed when abc exists' '
> +	git rev-parse HEAD~1 >expect &&
> +	git branch --force abc HEAD~1 &&
> +	git rev-parse abc >actual &&
> +	test_cmp expect actual
> +'

This seems like an unrelated test for a refactoring that doesn't seem to
touch "force" (in builtin/branch.c, the line removed hardcodes force to
0).

Below is my diff, as mentioned.

---
diff --git a/branch.c b/branch.c
index 07a46430b3..a6803e9900 100644
--- a/branch.c
+++ b/branch.c
@@ -131,8 +131,8 @@ int install_branch_config(int flag, const char *local, const char *origin, const
  * to infer the settings for branch.<new_ref>.{remote,merge} from the
  * config.
  */
-static void setup_tracking(const char *new_ref, const char *orig_ref,
-			   enum branch_track track, int quiet)
+void setup_tracking(const char *new_ref, const char *orig_ref,
+		    enum branch_track track, int quiet)
 {
 	struct tracking tracking;
 	int config_flags = quiet ? 0 : BRANCH_CONFIG_VERBOSE;
@@ -243,30 +243,12 @@ N_("\n"
 "will track its remote counterpart, you may want to use\n"
 "\"git push -u\" to set the upstream config as you push.");
 
-void create_branch(struct repository *r,
-		   const char *name, const char *start_name,
-		   int force, int clobber_head_ok, int reflog,
-		   int quiet, enum branch_track track)
+void TODO_name(struct repository *r, const char *start_name, int explicit_tracking,
+	       char **out_real_ref, struct commit **out_commit)
 {
 	struct commit *commit;
 	struct object_id oid;
 	char *real_ref;
-	struct strbuf ref = STRBUF_INIT;
-	int forcing = 0;
-	int dont_change_ref = 0;
-	int explicit_tracking = 0;
-
-	if (track == BRANCH_TRACK_EXPLICIT || track == BRANCH_TRACK_OVERRIDE)
-		explicit_tracking = 1;
-
-	if ((track == BRANCH_TRACK_OVERRIDE || clobber_head_ok)
-	    ? validate_branchname(name, &ref)
-	    : validate_new_branchname(name, &ref, force)) {
-		if (!force)
-			dont_change_ref = 1;
-		else
-			forcing = 1;
-	}
 
 	real_ref = NULL;
 	if (get_oid_mb(start_name, &oid)) {
@@ -304,6 +286,38 @@ void create_branch(struct repository *r,
 
 	if ((commit = lookup_commit_reference(r, &oid)) == NULL)
 		die(_("Not a valid branch point: '%s'."), start_name);
+	if (out_real_ref)
+		*out_real_ref = real_ref;
+	if (out_commit)
+		*out_commit = commit;
+}
+
+void create_branch(struct repository *r,
+		   const char *name, const char *start_name,
+		   int force, int clobber_head_ok, int reflog,
+		   int quiet, enum branch_track track)
+{
+	struct commit *commit;
+	struct object_id oid;
+	char *real_ref = NULL;
+	struct strbuf ref = STRBUF_INIT;
+	int forcing = 0;
+	int dont_change_ref = 0;
+	int explicit_tracking = 0;
+
+	if (track == BRANCH_TRACK_EXPLICIT || track == BRANCH_TRACK_OVERRIDE)
+		explicit_tracking = 1;
+
+	if ((track == BRANCH_TRACK_OVERRIDE || clobber_head_ok)
+	    ? validate_branchname(name, &ref)
+	    : validate_new_branchname(name, &ref, force)) {
+		if (!force)
+			dont_change_ref = 1;
+		else
+			forcing = 1;
+	}
+
+	TODO_name(r, start_name, explicit_tracking, &real_ref, &commit);
 	oidcpy(&oid, &commit->object.oid);
 
 	if (reflog)
diff --git a/branch.h b/branch.h
index df0be61506..577483012b 100644
--- a/branch.h
+++ b/branch.h
@@ -1,6 +1,7 @@
 #ifndef BRANCH_H
 #define BRANCH_H
 
+struct commit;
 struct repository;
 struct strbuf;
 
@@ -17,6 +18,11 @@ extern enum branch_track git_branch_track;
 
 /* Functions for acting on the information about branches. */
 
+void setup_tracking(const char *new_ref, const char *orig_ref,
+		    enum branch_track track, int quiet);
+void TODO_name(struct repository *r, const char *start_name, int explicit_tracking,
+	       char **out_real_ref, struct commit **out_commit);
+
 /*
  * Creates a new branch, where:
  *
diff --git a/builtin/branch.c b/builtin/branch.c
index 81b5c111cb..78f5c1d17e 100644
--- a/builtin/branch.c
+++ b/builtin/branch.c
@@ -806,6 +806,7 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 			die(_("too many arguments for a rename operation"));
 	} else if (new_upstream) {
 		struct branch *branch = branch_get(argv[0]);
+		char *real_ref;
 
 		if (argc > 1)
 			die(_("too many arguments to set new upstream"));
@@ -821,12 +822,9 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 		if (!ref_exists(branch->refname))
 			die(_("branch '%s' does not exist"), branch->name);
 
-		/*
-		 * create_branch takes care of setting up the tracking
-		 * info and making sure new_upstream is correct
-		 */
-		create_branch(the_repository, branch->name, new_upstream,
-			      0, 0, 0, quiet, BRANCH_TRACK_OVERRIDE);
+		TODO_name(the_repository, new_upstream, 1, &real_ref, NULL);
+		setup_tracking(branch->name, real_ref, BRANCH_TRACK_OVERRIDE, quiet);
+		free(real_ref);
 	} else if (unset_upstream) {
 		struct branch *branch = branch_get(argv[0]);
 		struct strbuf buf = STRBUF_INIT;
Glen Choo Dec. 9, 2021, 10:16 p.m. UTC | #2
Jonathan Tan <jonathantanmy@google.com> writes:

>> This refactor is motivated by a desire to add a "dry_run" parameter to
>> create_branch() that will validate whether or not a branch can be
>> created without actually creating it - this behavior be used in a
>> subsequent commit that adds `git branch --recurse-submodules topic`.
>
> When I said that the patch was doing too much [1], I meant that for a
> patch that is supposed to be about refactoring, there were many lines
> that didn't seem to be a straightforward move. I've attached a version
> of the patch that I expected - notice that a reviewer can see that the
> lines are straightforwardly moved.
>
> [1] https://lore.kernel.org/git/20211123231035.3607109-1-jonathantanmy@google.com/

Thanks, the patch gave me a lot of think about.

Comparing yours and mine side-by-side makes it obvious that what I
thought was a 'simple' refactor is not so obvious to readers. I think I
can incorporate most of your patch.

>> +void setup_tracking(const char *new_ref, const char *orig_ref,
>> +			   enum branch_track track, int quiet, int expand_orig)
>>  {
>>  	struct tracking tracking;
>>  	struct string_list tracking_srcs = STRING_LIST_INIT_DUP;
>>  	int config_flags = quiet ? 0 : BRANCH_CONFIG_VERBOSE;
>> +	char *full_orig_ref;
>> +	struct object_id unused_oid;
>>  
>>  	memset(&tracking, 0, sizeof(tracking));
>> -	tracking.spec.dst = (char *)orig_ref;
>> +	if (expand_orig)
>> +		validate_branch_start(the_repository, orig_ref, track, &unused_oid, &full_orig_ref);
>> +	else
>> +		full_orig_ref = xstrdup(orig_ref);
>
> Having two meanings for a parameter (and which meaning used depending on
> another parameter) is quite confusing - I think it's better to call
> another function to expand the ref if necessary. See my patch below for
> an example.

Since you and Junio have both commented on this inconsistency, I'll drop
the extra parameter.

I wonder if you've considered another alternative [1], which is to
_always_ expand the ref, instead of _never_ expanding the ref. Always
expanding the ref wastes cycles, but it avoids creating an implicit
contract between setup_tracking() and
validate_branch_start_or_TODO_name().

>> diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh
>> index 267a624671..18e285a876 100755
>> --- a/t/t3200-branch.sh
>> +++ b/t/t3200-branch.sh
>> @@ -42,6 +42,23 @@ test_expect_success 'git branch abc should create a branch' '
>>  	git branch abc && test_path_is_file .git/refs/heads/abc
>>  '
>>  
>> +test_expect_success 'git branch abc should fail when abc exists' '
>> +	test_must_fail git branch abc
>> +'
>> +
>> +test_expect_success 'git branch --force abc should fail when abc is checked out' '
>> +	test_when_finished git switch main &&
>> +	git switch abc &&
>> +	test_must_fail git branch --force abc HEAD~1
>> +'
>> +
>> +test_expect_success 'git branch --force abc should succeed when abc exists' '
>> +	git rev-parse HEAD~1 >expect &&
>> +	git branch --force abc HEAD~1 &&
>> +	git rev-parse abc >actual &&
>> +	test_cmp expect actual
>> +'
>
> This seems like an unrelated test for a refactoring that doesn't seem to
> touch "force" (in builtin/branch.c, the line removed hardcodes force to
> 0).

I added this test because I was trying to simplify create_branch() by
moving around/deleting forcing and clobber_head_ok. It turns out that I
had actually broken the behavior of --force, but the test suite didn't
catch it.

I think this test is a step in the right direction, but it probably
falls into the category of 'incidental fix'. I am not sure where it
belongs if not here.

> Below is my diff, as mentioned.
>
> ---
> diff --git a/branch.c b/branch.c
> index 07a46430b3..a6803e9900 100644
> --- a/branch.c
> +++ b/branch.c
> @@ -131,8 +131,8 @@ int install_branch_config(int flag, const char *local, const char *origin, const
>   * to infer the settings for branch.<new_ref>.{remote,merge} from the
>   * config.
>   */
> -static void setup_tracking(const char *new_ref, const char *orig_ref,
> -			   enum branch_track track, int quiet)
> +void setup_tracking(const char *new_ref, const char *orig_ref,
> +		    enum branch_track track, int quiet)
>  {
>  	struct tracking tracking;
>  	int config_flags = quiet ? 0 : BRANCH_CONFIG_VERBOSE;
> @@ -243,30 +243,12 @@ N_("\n"
>  "will track its remote counterpart, you may want to use\n"
>  "\"git push -u\" to set the upstream config as you push.");
>  
> -void create_branch(struct repository *r,
> -		   const char *name, const char *start_name,
> -		   int force, int clobber_head_ok, int reflog,
> -		   int quiet, enum branch_track track)
> +void TODO_name(struct repository *r, const char *start_name, int explicit_tracking,
> +	       char **out_real_ref, struct commit **out_commit)
>  {
>  	struct commit *commit;
>  	struct object_id oid;
>  	char *real_ref;
> -	struct strbuf ref = STRBUF_INIT;
> -	int forcing = 0;
> -	int dont_change_ref = 0;
> -	int explicit_tracking = 0;
> -
> -	if (track == BRANCH_TRACK_EXPLICIT || track == BRANCH_TRACK_OVERRIDE)
> -		explicit_tracking = 1;
> -
> -	if ((track == BRANCH_TRACK_OVERRIDE || clobber_head_ok)
> -	    ? validate_branchname(name, &ref)
> -	    : validate_new_branchname(name, &ref, force)) {
> -		if (!force)
> -			dont_change_ref = 1;
> -		else
> -			forcing = 1;
> -	}
>  
>  	real_ref = NULL;
>  	if (get_oid_mb(start_name, &oid)) {
> @@ -304,6 +286,38 @@ void create_branch(struct repository *r,
>  
>  	if ((commit = lookup_commit_reference(r, &oid)) == NULL)
>  		die(_("Not a valid branch point: '%s'."), start_name);
> +	if (out_real_ref)
> +		*out_real_ref = real_ref;
> +	if (out_commit)
> +		*out_commit = commit;
> +}

Using a separate out_real_ref makes a lot more sense than reusing
real_ref. I'll borrow this :)

> +
> +void create_branch(struct repository *r,
> +		   const char *name, const char *start_name,
> +		   int force, int clobber_head_ok, int reflog,
> +		   int quiet, enum branch_track track)
> +{
> +	struct commit *commit;
> +	struct object_id oid;
> +	char *real_ref = NULL;
> +	struct strbuf ref = STRBUF_INIT;
> +	int forcing = 0;
> +	int dont_change_ref = 0;
> +	int explicit_tracking = 0;
> +
> +	if (track == BRANCH_TRACK_EXPLICIT || track == BRANCH_TRACK_OVERRIDE)
> +		explicit_tracking = 1;
> +
> +	if ((track == BRANCH_TRACK_OVERRIDE || clobber_head_ok)
> +	    ? validate_branchname(name, &ref)
> +	    : validate_new_branchname(name, &ref, force)) {
> +		if (!force)
> +			dont_change_ref = 1;
> +		else
> +			forcing = 1;
> +	}

This patch just moves things around without performing internal
simplification (like removing dont_change_ref). The simplification would
probably be done in a later patch.

Makes sense, I see how this is easier to review.

> @@ -821,12 +822,9 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
>  		if (!ref_exists(branch->refname))
>  			die(_("branch '%s' does not exist"), branch->name);
>  
> -		/*
> -		 * create_branch takes care of setting up the tracking
> -		 * info and making sure new_upstream is correct
> -		 */
> -		create_branch(the_repository, branch->name, new_upstream,
> -			      0, 0, 0, quiet, BRANCH_TRACK_OVERRIDE);
> +		TODO_name(the_repository, new_upstream, 1, &real_ref, NULL);
> +		setup_tracking(branch->name, real_ref, BRANCH_TRACK_OVERRIDE, quiet);
> +		free(real_ref);
>  	} else if (unset_upstream) {
>  		struct branch *branch = branch_get(argv[0]);
>  		struct strbuf buf = STRBUF_INIT;

See my previous comment and [1] about avoiding the implicit contract of
TODO_name() + setup_tracking(), but of course, this is still more
consistent than what what I proposed.

[1] https://lore.kernel.org/git/kl6l5yrzaq5z.fsf@chooglen-macbookpro.roam.corp.google.com

Thanks!
diff mbox series

Patch

diff --git a/branch.c b/branch.c
index 29e835d367..9429936734 100644
--- a/branch.c
+++ b/branch.c
@@ -209,20 +209,26 @@  static int inherit_tracking(struct tracking *tracking, const char *orig_ref)
 	return 0;
 }
 
-/*
- * This is called when new_ref is branched off of orig_ref, and tries
- * to infer the settings for branch.<new_ref>.{remote,merge} from the
- * config.
- */
-static void setup_tracking(const char *new_ref, const char *orig_ref,
-			   enum branch_track track, int quiet)
+static void validate_branch_start(struct repository *r, const char *start_name,
+				  enum branch_track track,
+				  struct object_id *oid, char **real_ref);
+
+void setup_tracking(const char *new_ref, const char *orig_ref,
+			   enum branch_track track, int quiet, int expand_orig)
 {
 	struct tracking tracking;
 	struct string_list tracking_srcs = STRING_LIST_INIT_DUP;
 	int config_flags = quiet ? 0 : BRANCH_CONFIG_VERBOSE;
+	char *full_orig_ref;
+	struct object_id unused_oid;
 
 	memset(&tracking, 0, sizeof(tracking));
-	tracking.spec.dst = (char *)orig_ref;
+	if (expand_orig)
+		validate_branch_start(the_repository, orig_ref, track, &unused_oid, &full_orig_ref);
+	else
+		full_orig_ref = xstrdup(orig_ref);
+
+	tracking.spec.dst = full_orig_ref;
 	tracking.srcs = &tracking_srcs;
 	if (track != BRANCH_TRACK_INHERIT)
 		for_each_remote(find_tracked_branch, &tracking);
@@ -236,20 +242,22 @@  static void setup_tracking(const char *new_ref, const char *orig_ref,
 		case BRANCH_TRACK_OVERRIDE:
 			break;
 		default:
-			return;
+			goto cleanup;
 		}
 
 	if (tracking.matches > 1)
 		die(_("Not tracking: ambiguous information for ref %s"),
-		    orig_ref);
+		    full_orig_ref);
 
 	if (tracking.srcs->nr < 1)
-		string_list_append(tracking.srcs, orig_ref);
+		string_list_append(tracking.srcs, full_orig_ref);
 	if (install_branch_config_multiple_remotes(config_flags, new_ref, tracking.remote,
 			      tracking.srcs) < 0)
 		exit(-1);
 
+cleanup:
 	string_list_clear(tracking.srcs, 0);
+	free(full_orig_ref);
 }
 
 int read_branch_desc(struct strbuf *buf, const char *branch_name)
@@ -332,33 +340,35 @@  N_("\n"
 "will track its remote counterpart, you may want to use\n"
 "\"git push -u\" to set the upstream config as you push.");
 
-void create_branch(struct repository *r,
-		   const char *name, const char *start_name,
-		   int force, int clobber_head_ok, int reflog,
-		   int quiet, enum branch_track track)
+/**
+ * Validates whether a ref is a valid starting point for a branch, where:
+ *
+ *   - r is the repository to validate the branch for
+ *
+ *   - start_name is the ref that we would like to test. This is
+ *     expanded with DWIM and assigned to real_ref.
+ *
+ *   - track is the tracking mode of the new branch. If tracking is
+ *     explicitly requested, start_name must be a branch (because
+ *     otherwise start_name cannot be tracked)
+ *
+ *   - oid is an out parameter containing the object_id of start_name
+ *
+ *   - real_ref is an out parameter containing the full, 'real' form of
+ *     start_name e.g. refs/heads/main instead of main
+ *
+ */
+static void validate_branch_start(struct repository *r, const char *start_name,
+				  enum branch_track track,
+				  struct object_id *oid, char **real_ref)
 {
 	struct commit *commit;
-	struct object_id oid;
-	char *real_ref;
-	struct strbuf ref = STRBUF_INIT;
-	int forcing = 0;
-	int dont_change_ref = 0;
 	int explicit_tracking = 0;
 
 	if (track == BRANCH_TRACK_EXPLICIT || track == BRANCH_TRACK_OVERRIDE)
 		explicit_tracking = 1;
 
-	if ((track == BRANCH_TRACK_OVERRIDE || clobber_head_ok)
-	    ? validate_branchname(name, &ref)
-	    : validate_new_branchname(name, &ref, force)) {
-		if (!force)
-			dont_change_ref = 1;
-		else
-			forcing = 1;
-	}
-
-	real_ref = NULL;
-	if (get_oid_mb(start_name, &oid)) {
+	if (repo_get_oid_mb(r, start_name, oid)) {
 		if (explicit_tracking) {
 			if (advice_enabled(ADVICE_SET_UPSTREAM_FAILURE)) {
 				error(_(upstream_missing), start_name);
@@ -370,7 +380,8 @@  void create_branch(struct repository *r,
 		die(_("Not a valid object name: '%s'."), start_name);
 	}
 
-	switch (dwim_ref(start_name, strlen(start_name), &oid, &real_ref, 0)) {
+	switch (repo_dwim_ref(r, start_name, strlen(start_name), oid, real_ref,
+			      0)) {
 	case 0:
 		/* Not branching from any existing branch */
 		if (explicit_tracking)
@@ -378,12 +389,12 @@  void create_branch(struct repository *r,
 		break;
 	case 1:
 		/* Unique completion -- good, only if it is a real branch */
-		if (!starts_with(real_ref, "refs/heads/") &&
-		    validate_remote_tracking_branch(real_ref)) {
+		if (!starts_with(*real_ref, "refs/heads/") &&
+		    validate_remote_tracking_branch(*real_ref)) {
 			if (explicit_tracking)
 				die(_(upstream_not_branch), start_name);
 			else
-				FREE_AND_NULL(real_ref);
+				FREE_AND_NULL(*real_ref);
 		}
 		break;
 	default:
@@ -391,37 +402,55 @@  void create_branch(struct repository *r,
 		break;
 	}
 
-	if ((commit = lookup_commit_reference(r, &oid)) == NULL)
+	if ((commit = lookup_commit_reference(r, oid)) == NULL)
 		die(_("Not a valid branch point: '%s'."), start_name);
-	oidcpy(&oid, &commit->object.oid);
+	oidcpy(oid, &commit->object.oid);
+}
+
+void create_branch(struct repository *r, const char *name,
+		   const char *start_name, int force, int clobber_head_ok,
+		   int reflog, int quiet, enum branch_track track)
+{
+	struct object_id oid;
+	char *real_ref;
+	struct strbuf ref = STRBUF_INIT;
+	int forcing = 0;
+	struct ref_transaction *transaction;
+	struct strbuf err = STRBUF_INIT;
+	char *msg;
+
+	if (clobber_head_ok && !force)
+		BUG("'clobber_head_ok' can only be used with 'force'");
+
+	if (clobber_head_ok ?
+			  validate_branchname(name, &ref) :
+			  validate_new_branchname(name, &ref, force)) {
+		forcing = 1;
+	}
+
+	validate_branch_start(r, start_name, track, &oid, &real_ref);
 
 	if (reflog)
 		log_all_ref_updates = LOG_REFS_NORMAL;
 
-	if (!dont_change_ref) {
-		struct ref_transaction *transaction;
-		struct strbuf err = STRBUF_INIT;
-		char *msg;
-
-		if (forcing)
-			msg = xstrfmt("branch: Reset to %s", start_name);
-		else
-			msg = xstrfmt("branch: Created from %s", start_name);
-
-		transaction = ref_transaction_begin(&err);
-		if (!transaction ||
-		    ref_transaction_update(transaction, ref.buf,
-					   &oid, forcing ? NULL : null_oid(),
-					   0, msg, &err) ||
-		    ref_transaction_commit(transaction, &err))
-			die("%s", err.buf);
-		ref_transaction_free(transaction);
-		strbuf_release(&err);
-		free(msg);
-	}
+	if (forcing)
+		msg = xstrfmt("branch: Reset to %s", start_name);
+	else
+		msg = xstrfmt("branch: Created from %s", start_name);
+
+	transaction = ref_transaction_begin(&err);
+	if (!transaction ||
+		ref_transaction_update(transaction, ref.buf,
+					&oid, forcing ? NULL : null_oid(),
+					0, msg, &err) ||
+		ref_transaction_commit(transaction, &err))
+		die("%s", err.buf);
+	ref_transaction_free(transaction);
+	strbuf_release(&err);
+	free(msg);
 
 	if (real_ref && track)
-		setup_tracking(ref.buf + 11, real_ref, track, quiet);
+		setup_tracking(ref.buf + 11, real_ref, track, quiet, 0);
 
 	strbuf_release(&ref);
 	free(real_ref);
diff --git a/branch.h b/branch.h
index 6484bda8a2..46951c446b 100644
--- a/branch.h
+++ b/branch.h
@@ -18,6 +18,15 @@  extern enum branch_track git_branch_track;
 
 /* Functions for acting on the information about branches. */
 
+/*
+ * This sets the branch.<new_ref>.{remote,merge} config settings so that
+ * branch 'new_ref' tracks 'orig_ref'. This is called when branches are
+ * created, or when branch configs are updated (e.g. with
+ * git branch --set-upstream-to).
+ */
+void setup_tracking(const char *new_ref, const char *orig_ref,
+		    enum branch_track track, int quiet, int expand_orig);
+
 /*
  * Creates a new branch, where:
  *
@@ -30,8 +39,8 @@  extern enum branch_track git_branch_track;
  *
  *   - force enables overwriting an existing (non-head) branch
  *
- *   - clobber_head_ok allows the currently checked out (hence existing)
- *     branch to be overwritten; without 'force', it has no effect.
+ *   - clobber_head_ok, when enabled with 'force', allows the currently
+ *     checked out (head) branch to be overwritten
  *
  *   - reflog creates a reflog for the branch
  *
diff --git a/builtin/branch.c b/builtin/branch.c
index 81a29edb4a..e19aab5356 100644
--- a/builtin/branch.c
+++ b/builtin/branch.c
@@ -823,12 +823,7 @@  int cmd_branch(int argc, const char **argv, const char *prefix)
 		if (!ref_exists(branch->refname))
 			die(_("branch '%s' does not exist"), branch->name);
 
-		/*
-		 * create_branch takes care of setting up the tracking
-		 * info and making sure new_upstream is correct
-		 */
-		create_branch(the_repository, branch->name, new_upstream,
-			      0, 0, 0, quiet, BRANCH_TRACK_OVERRIDE);
+		setup_tracking(branch->name, new_upstream, BRANCH_TRACK_OVERRIDE, quiet, 1);
 	} else if (unset_upstream) {
 		struct branch *branch = branch_get(argv[0]);
 		struct strbuf buf = STRBUF_INIT;
diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh
index 267a624671..18e285a876 100755
--- a/t/t3200-branch.sh
+++ b/t/t3200-branch.sh
@@ -42,6 +42,23 @@  test_expect_success 'git branch abc should create a branch' '
 	git branch abc && test_path_is_file .git/refs/heads/abc
 '
 
+test_expect_success 'git branch abc should fail when abc exists' '
+	test_must_fail git branch abc
+'
+
+test_expect_success 'git branch --force abc should fail when abc is checked out' '
+	test_when_finished git switch main &&
+	git switch abc &&
+	test_must_fail git branch --force abc HEAD~1
+'
+
+test_expect_success 'git branch --force abc should succeed when abc exists' '
+	git rev-parse HEAD~1 >expect &&
+	git branch --force abc HEAD~1 &&
+	git rev-parse abc >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'git branch a/b/c should create a branch' '
 	git branch a/b/c && test_path_is_file .git/refs/heads/a/b/c
 '