[RFC PATCH v2 4/5] branch: introduce dont_fail parameter for create validation

[Kaartic Sivaraam <kaartic.sivaraam@gmail.com>]

This parameter allows the branch create validation function to
optionally return a flag specifying the reason for failure, when
requested. This allows the caller to know why it was about to die.
This allows more useful error messages to be given to the user when
trying to rename a branch.

The flags are specified in the form of an enum and values for success
flags have been assigned explicitly to clearly express that certain
callers rely those values and they cannot be arbitrary.

Only the logic has been added but no caller has been made to use it, yet.
So, no functional changes.

Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
---
 branch.c           | 32 ++++++++++++++++++++++----------
 branch.h           | 23 +++++++++++++++++++++--
 builtin/branch.c   |  2 +-
 builtin/checkout.c |  2 +-
 4 files changed, 45 insertions(+), 14 deletions(-)

diff --git a/branch.c b/branch.c
index db2abeb7e..83305ded6 100644
--- a/branch.c
+++ b/branch.c
@@ -179,27 +179,39 @@ int read_branch_desc(struct strbuf *buf, const char *branch_name)
 }
 
 int validate_branch_creation(const char *name, struct strbuf *ref,
-			     int shouldnt_exist, int clobber_head_ok)
+			     int shouldnt_exist, int clobber_head_ok, unsigned dont_fail)
 {
-	if (strbuf_check_branch_ref(ref, name))
-		die(_("'%s' is not a valid branch name."), name);
+	if (strbuf_check_branch_ref(ref, name)) {
+		if (dont_fail)
+			return INVALID_BRANCH_NAME;
+		else
+			die(_("'%s' is not a valid branch name."), name);
+	}
 
 	if (!ref_exists(ref->buf))
-		return 0;
+		return VALID_BRANCH_NAME;
 
-	if (shouldnt_exist)
-		die(_("A branch named '%s' already exists."), ref->buf + strlen("refs/heads/"));
+	if (shouldnt_exist) {
+		if (dont_fail)
+			return BRANCH_EXISTS;
+		else
+			die(_("A branch named '%s' already exists."), ref->buf + strlen("refs/heads/"));
+	}
 
 	if (!clobber_head_ok) {
 		const char *head;
 		struct object_id oid;
 
 		head = resolve_ref_unsafe("HEAD", 0, oid.hash, NULL);
-		if (!is_bare_repository() && head && !strcmp(head, ref->buf))
-			die(_("Cannot force update the current branch."));
+		if (!is_bare_repository() && head && !strcmp(head, ref->buf)) {
+			if (dont_fail)
+				return CANNOT_FORCE_UPDATE_CURRENT_BRANCH;
+			else
+				die(_("Cannot force update the current branch."));
+		}
 	}
 
-	return 1;
+	return FORCE_UPDATING_BRANCH;
 }
 
 /*
@@ -268,7 +280,7 @@ void create_branch(const char *name, const char *start_name,
 		validate_existing_branch(name, &ref);
 		dont_change_ref = 1;
 	} else {
-		forcing = validate_branch_creation(name, &ref, !force, clobber_head_ok);
+		forcing = validate_branch_creation(name, &ref, !force, clobber_head_ok, 0);
 	}
 
 	real_ref = NULL;
diff --git a/branch.h b/branch.h
index a6dde552c..4b68a789d 100644
--- a/branch.h
+++ b/branch.h
@@ -27,6 +27,16 @@ void create_branch(const char *name, const char *start_name,
 		   int force, int clobber_head_ok,
 		   int reflog, int quiet, enum branch_track track);
 
+enum branch_validation_result {
+	/* Flags that say it's NOT OK to update */
+	BRANCH_EXISTS = -3,
+	CANNOT_FORCE_UPDATE_CURRENT_BRANCH,
+	INVALID_BRANCH_NAME,
+	/* Flags that say it's OK to update */
+	VALID_BRANCH_NAME = 0,
+	FORCE_UPDATING_BRANCH = 1
+};
+
 /*
  * Validates whether the branch with the given name may be updated (created, renamed etc.,)
  * with respect to the given conditions.
@@ -41,10 +51,19 @@ void create_branch(const char *name, const char *start_name,
  *   - clobber_head_ok allows another branch with given branch name to be
  *     the currently checkout branch; with 'shouldnt_exist', it has no effect.
  *
- * A non-zero return value indicates that a branch already exists and can be force updated.
+ * The return values have the following meaning,
+ *
+ *   - If dont_fail is 0, the function dies in case of failure and returns flags of
+ *     'validate_result' that indicate that it's OK to update the branch. The positive
+ *     non-zero flag implies that the branch can be force updated.
+ *
+ *   - If dont_fail is 1, the function doesn't die in case of failure but returns flags
+ *     of 'validate_result' that specify the reason for failure. The behaviour in case of
+ *     success is same as above.
  *
  */
-int validate_branch_creation(const char *name, struct strbuf *ref, int shouldnt_exist, int clobber_head_ok);
+int validate_branch_creation(const char *name, struct strbuf *ref,
+			     int shouldnt_exist, int clobber_head_ok, unsigned dont_fail);
 
 /*
  * Remove information about the state of working on the current
diff --git a/builtin/branch.c b/builtin/branch.c
index aa2f36519..25e3a2f29 100644
--- a/builtin/branch.c
+++ b/builtin/branch.c
@@ -483,7 +483,7 @@ static void rename_branch(const char *oldname, const char *newname, int force)
 	 */
 	clobber_head_ok = !strcmp(oldname, newname);
 
-	validate_branch_creation(newname, &newref, !force, clobber_head_ok);
+	validate_branch_creation(newname, &newref, !force, clobber_head_ok, 0);
 
 	reject_rebase_or_bisect_branch(oldref.buf);
 
diff --git a/builtin/checkout.c b/builtin/checkout.c
index fcbbbb1fa..e9d636c66 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -1285,7 +1285,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		opts.branch_exists =
 			validate_branch_creation(opts.new_branch, &buf,
 						!opts.new_branch_force,
-						!!opts.new_branch_force);
+						!!opts.new_branch_force, 0);
 
 		strbuf_release(&buf);
 	}
-- 
2.14.1.935.ge2b2bcd8a