branch: move --set-upstream-to behavior to dwim_and_setup_tracking()

This commit is preparation for a future commit that will simplify
create_branch() so that it always creates a branch. This will allow
create_branch() to accept a dry_run parameter (which is needed for "git
branch --recurse-submodules").

create_branch() used to always create a branch, but 4fc5006676 (Add
branch --set-upstream, 2010-01-18) changed it to also be able to set
tracking information without creating a branch.

Refactor the code that sets tracking information into its own functions
dwim_branch_start() and dwim_and_setup_tracking(). Also change an
invocation of create_branch() in cmd_branch() in builtin/branch.c to use
dwim_and_setup_tracking(), since that invocation is only for setting
tracking information (in "git branch --set-upstream-to").

As of this commit, create_branch() is no longer invoked in a way that
does not create branches.

Helped-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Glen Choo <chooglen@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -218,9 +218,11 @@ static int inherit_tracking(struct tracking *tracking, const char *orig_ref)
 }
 
 /*
- * 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.
+ * Used internally to set the branch.<new_ref>.{remote,merge} config
+ * settings so that branch 'new_ref' tracks 'orig_ref'. Unlike
+ * dwim_and_setup_tracking(), this does not do DWIM, i.e. "origin/main"
+ * will not be expanded to "refs/remotes/origin/main", so it is not safe
+ * for 'orig_ref' to be raw user input.
  */
 static void setup_tracking(const char *new_ref, const char *orig_ref,
 			   enum branch_track track, int quiet)
@@ -341,31 +343,37 @@ 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)
+/**
+ * DWIMs a user-provided ref to determine the starting point for a
+ * branch and validates it, 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 out_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)
+ *
+ *   - out_oid is an out parameter containing the object_id of start_name
+ *
+ *   - out_real_ref is an out parameter containing the full, 'real' form
+ *     of start_name e.g. refs/heads/main instead of main
+ *
+ */
+static void dwim_branch_start(struct repository *r, const char *start_name,
+			   enum branch_track track, char **out_real_ref,
+			   struct object_id *out_oid)
 {
 	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 (explicit_tracking) {
@@ -402,7 +410,37 @@ void create_branch(struct repository *r,
 
 	if ((commit = lookup_commit_reference(r, &oid)) == NULL)
 		die(_("Not a valid branch point: '%s'."), start_name);
-	oidcpy(&oid, &commit->object.oid);
+	if (out_real_ref) {
+		*out_real_ref = real_ref;
+		real_ref = NULL;
+	}
+	if (out_oid)
+		oidcpy(out_oid, &commit->object.oid);
+
+	FREE_AND_NULL(real_ref);
+}
+
+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;
+	int dont_change_ref = 0;
+
+	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;
+	}
+
+	dwim_branch_start(r, start_name, track, &real_ref, &oid);
 
 	if (reflog)
 		log_all_ref_updates = LOG_REFS_NORMAL;
@@ -436,6 +474,15 @@ void create_branch(struct repository *r,
 	free(real_ref);
 }
 
+void dwim_and_setup_tracking(struct repository *r, const char *new_ref,
+			     const char *orig_ref, enum branch_track track,
+			     int quiet)
+{
+	char *real_orig_ref;
+	dwim_branch_start(r, orig_ref, track, &real_orig_ref, NULL);
+	setup_tracking(new_ref, real_orig_ref, track, quiet);
+}
+
 void remove_merge_branch_state(struct repository *r)
 {
 	unlink(git_path_merge_head(r));

branch.h

@@ -18,6 +18,28 @@ extern enum branch_track git_branch_track;
 
 /* Functions for acting on the information about branches. */
 
+/**
+ * Sets branch.<new_ref>.{remote,merge} config settings such that
+ * new_ref tracks orig_ref according to the specified tracking mode.
+ *
+ *   - new_ref is the name of the branch that we are setting tracking
+ *     for.
+ *
+ *   - orig_ref is the name of the ref that is 'upstream' of new_ref.
+ *     orig_ref will be expanded with DWIM so that the config settings
+ *     are in the correct format e.g. "refs/remotes/origin/main" instead
+ *     of "origin/main".
+ *
+ *   - track is the tracking mode e.g. BRANCH_TRACK_REMOTE causes
+ *     new_ref to track orig_ref directly, whereas BRANCH_TRACK_INHERIT
+ *     causes new_ref to track whatever orig_ref tracks.
+ *
+ *   - quiet suppresses tracking information.
+ */
+void dwim_and_setup_tracking(struct repository *r, const char *new_ref,
+			     const char *orig_ref, enum branch_track track,
+			     int quiet);
+
 /*
  * Creates a new branch, where:
  *

builtin/branch.c

@@ -823,12 +823,9 @@ 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);
+		dwim_and_setup_tracking(the_repository, branch->name,
+					new_upstream, BRANCH_TRACK_OVERRIDE,
+					quiet);
 	} else if (unset_upstream) {
 		struct branch *branch = branch_get(argv[0]);
 		struct strbuf buf = STRBUF_INIT;