branch: add a --copy (-c) option to go with --move (-m)

Add the ability to --copy a branch and its reflog and configuration,
this uses the same underlying machinery as the --move (-m) option
except the reflog and configuration is copied instead of being moved.

This is useful for e.g. copying a topic branch to a new version,
e.g. work to work-2 after submitting the work topic to the list, while
preserving all the tracking info and other configuration that goes
with the branch, and unlike --move keeping the other already-submitted
branch around for reference.

Like --move, when the source branch is the currently checked out
branch the HEAD is moved to the destination branch. In the case of
--move we don't really have a choice (other than remaining on a
detached HEAD) and in order to keep the functionality consistent, we
are doing it in similar way for --copy too.

The most common usage of this feature is expected to be moving to a
new topic branch which is a copy of the current one, in that case
moving to the target branch is what the user wants, and doesn't
unexpectedly behave differently than --move would.

One outstanding caveat of this implementation is that:

    git checkout maint &&
    git checkout master &&
    git branch -c topic &&
    git checkout -

Will check out 'maint' instead of 'master'. This is because the @{-N}
feature (or its -1 shorthand "-") relies on HEAD reflogs created by
the checkout command, so in this case we'll checkout maint instead of
master, as the user might expect. What to do about that is left to a
future change.

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Sahil Dua <sahildua2305@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 38 insertions, 8 deletions

diff --git a/refs/files-backend.c b/refs/files-backend.c
index d8b3f7314..fcaf4f719 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1733,9 +1733,9 @@ static int commit_ref_update(struct files_ref_store *refs,
 			     const struct object_id *oid, const char *logmsg,
 			     struct strbuf *err);
 
-static int files_rename_ref(struct ref_store *ref_store,
+static int files_copy_or_rename_ref(struct ref_store *ref_store,
 			    const char *oldrefname, const char *newrefname,
-			    const char *logmsg)
+			    const char *logmsg, int copy)
 {
 	struct files_ref_store *refs =
 		files_downcast(ref_store, REF_STORE_WRITE, "rename_ref");
@@ -1767,8 +1767,12 @@ static int files_rename_ref(struct ref_store *ref_store,
 	}
 
 	if (flag & REF_ISSYMREF) {
-		ret = error("refname %s is a symbolic ref, renaming it is not supported",
-			    oldrefname);
+		if (copy)
+			ret = error("refname %s is a symbolic ref, copying it is not supported",
+				    oldrefname);
+		else
+			ret = error("refname %s is a symbolic ref, renaming it is not supported",
+				    oldrefname);
 		goto out;
 	}
 	if (!refs_rename_ref_available(&refs->base, oldrefname, newrefname)) {
@@ -1776,13 +1780,19 @@ static int files_rename_ref(struct ref_store *ref_store,
 		goto out;
 	}
 
-	if (log && rename(sb_oldref.buf, tmp_renamed_log.buf)) {
+	if (!copy && log && rename(sb_oldref.buf, tmp_renamed_log.buf)) {
 		ret = error("unable to move logfile logs/%s to logs/"TMP_RENAMED_LOG": %s",
 			    oldrefname, strerror(errno));
 		goto out;
 	}
 
-	if (refs_delete_ref(&refs->base, logmsg, oldrefname,
+	if (copy && log && copy_file(tmp_renamed_log.buf, sb_oldref.buf, 0644)) {
+		ret = error("unable to copy logfile logs/%s to logs/"TMP_RENAMED_LOG": %s",
+			    oldrefname, strerror(errno));
+		goto out;
+	}
+
+	if (!copy && refs_delete_ref(&refs->base, logmsg, oldrefname,
 			    orig_oid.hash, REF_NODEREF)) {
 		error("unable to delete old %s", oldrefname);
 		goto rollback;
@@ -1795,7 +1805,7 @@ static int files_rename_ref(struct ref_store *ref_store,
 	 * the safety anyway; we want to delete the reference whatever
 	 * its current value.
 	 */
-	if (!refs_read_ref_full(&refs->base, newrefname,
+	if (!copy && !refs_read_ref_full(&refs->base, newrefname,
 				RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
 				oid.hash, NULL) &&
 	    refs_delete_ref(&refs->base, NULL, newrefname,
@@ -1826,7 +1836,10 @@ static int files_rename_ref(struct ref_store *ref_store,
 	lock = lock_ref_sha1_basic(refs, newrefname, NULL, NULL, NULL,
 				   REF_NODEREF, NULL, &err);
 	if (!lock) {
-		error("unable to rename '%s' to '%s': %s", oldrefname, newrefname, err.buf);
+		if (copy)
+			error("unable to copy '%s' to '%s': %s", oldrefname, newrefname, err.buf);
+		else
+			error("unable to rename '%s' to '%s': %s", oldrefname, newrefname, err.buf);
 		strbuf_release(&err);
 		goto rollback;
 	}
@@ -1877,6 +1890,22 @@ static int files_rename_ref(struct ref_store *ref_store,
 	return ret;
 }
 
+static int files_rename_ref(struct ref_store *ref_store,
+			    const char *oldrefname, const char *newrefname,
+			    const char *logmsg)
+{
+	return files_copy_or_rename_ref(ref_store, oldrefname,
+				 newrefname, logmsg, 0);
+}
+
+static int files_copy_ref(struct ref_store *ref_store,
+			    const char *oldrefname, const char *newrefname,
+			    const char *logmsg)
+{
+	return files_copy_or_rename_ref(ref_store, oldrefname,
+				 newrefname, logmsg, 1);
+}
+
 static int close_ref(struct ref_lock *lock)
 {
 	if (close_lock_file(lock->lk))
@@ -3383,6 +3412,7 @@ struct ref_storage_be refs_be_files = {
 	files_create_symref,
 	files_delete_refs,
 	files_rename_ref,
+	files_copy_ref,
 
 	files_ref_iterator_begin,
 	files_read_raw_ref,