Merge branch 'bf/set-head-symref'

When "git fetch $remote" notices that refs/remotes/$remote/HEAD is
missing and discovers what branch the other side points with its
HEAD, refs/remotes/$remote/HEAD is updated to point to it.

* bf/set-head-symref:
  fetch set_head: handle mirrored bare repositories
  fetch: set remote/HEAD if it does not exist
  refs: add create_only option to refs_update_symref_extended
  refs: add TRANSACTION_CREATE_EXISTS error
  remote set-head: better output for --auto
  remote set-head: refactor for readability
  refs: atomically record overwritten ref in update_symref
  refs: standardize output of refs_read_symbolic_ref
  t/t5505-remote: test failure of set-head
  t/t5505-remote: set default branch to main

Author: gitster

builtin/fetch.c

@@ -1574,6 +1574,72 @@ static int backfill_tags(struct display_state *display_state,
 	return retcode;
 }
 
+static const char *strip_refshead(const char *name){
+	skip_prefix(name, "refs/heads/", &name);
+	return name;
+}
+
+static int set_head(const struct ref *remote_refs)
+{
+	int result = 0, is_bare;
+	struct strbuf b_head = STRBUF_INIT, b_remote_head = STRBUF_INIT;
+	const char *remote = gtransport->remote->name;
+	char *head_name = NULL;
+	struct ref *ref, *matches;
+	struct ref *fetch_map = NULL, **fetch_map_tail = &fetch_map;
+	struct refspec_item refspec = {
+		.force = 0,
+		.pattern = 1,
+		.src = (char *) "refs/heads/*",
+		.dst = (char *) "refs/heads/*",
+	};
+	struct string_list heads = STRING_LIST_INIT_DUP;
+	struct ref_store *refs = get_main_ref_store(the_repository);
+
+	get_fetch_map(remote_refs, &refspec, &fetch_map_tail, 0);
+	matches = guess_remote_head(find_ref_by_name(remote_refs, "HEAD"),
+				    fetch_map, 1);
+	for (ref = matches; ref; ref = ref->next) {
+		string_list_append(&heads, strip_refshead(ref->name));
+	}
+
+
+	if (!heads.nr)
+		result = 1;
+	else if (heads.nr > 1)
+		result = 1;
+	else
+		head_name = xstrdup(heads.items[0].string);
+
+	if (!head_name)
+		goto cleanup;
+	is_bare = is_bare_repository();
+	if (is_bare) {
+		strbuf_addstr(&b_head, "HEAD");
+		strbuf_addf(&b_remote_head, "refs/heads/%s", head_name);
+	} else {
+		strbuf_addf(&b_head, "refs/remotes/%s/HEAD", remote);
+		strbuf_addf(&b_remote_head, "refs/remotes/%s/%s", remote, head_name);
+	}
+		/* make sure it's valid */
+	if (!is_bare && !refs_ref_exists(refs, b_remote_head.buf)) {
+		result = 1;
+		goto cleanup;
+	}
+	if (refs_update_symref_extended(refs, b_head.buf, b_remote_head.buf,
+					"fetch", NULL, !is_bare))
+		result = 1;
+
+cleanup:
+	free(head_name);
+	free_refs(fetch_map);
+	free_refs(matches);
+	string_list_clear(&heads, 0);
+	strbuf_release(&b_head);
+	strbuf_release(&b_remote_head);
+	return result;
+}
+
 static int do_fetch(struct transport *transport,
 		    struct refspec *rs,
 		    const struct fetch_config *config)
@@ -1643,6 +1709,8 @@ static int do_fetch(struct transport *transport,
 				    "refs/tags/");
 	}
 
+	strvec_push(&transport_ls_refs_options.ref_prefixes, "HEAD");
+
 	if (must_list_refs) {
 		trace2_region_enter("fetch", "remote_refs", the_repository);
 		remote_refs = transport_get_remote_refs(transport,
@@ -1787,6 +1855,12 @@ static int do_fetch(struct transport *transport,
 				  "you need to specify exactly one branch with the --set-upstream option"));
 		}
 	}
+	if (set_head(remote_refs))
+		;
+		/*
+		 * Way too many cases where this can go wrong
+		 * so let's just fail silently for now.
+		 */
 
 cleanup:
 	if (retcode) {

builtin/remote.c

@@ -1403,12 +1403,41 @@ static int show(int argc, const char **argv, const char *prefix,
 	return result;
 }
 
+static void report_set_head_auto(const char *remote, const char *head_name,
+			struct strbuf *b_local_head, int was_detached) {
+	struct strbuf buf_prefix = STRBUF_INIT;
+	const char *prev_head = NULL;
+
+	strbuf_addf(&buf_prefix, "refs/remotes/%s/", remote);
+	skip_prefix(b_local_head->buf, buf_prefix.buf, &prev_head);
+
+	if (prev_head && !strcmp(prev_head, head_name))
+		printf(_("'%s/HEAD' is unchanged and points to '%s'\n"),
+			remote, head_name);
+	else if (prev_head)
+		printf(_("'%s/HEAD' has changed from '%s' and now points to '%s'\n"),
+			remote, prev_head, head_name);
+	else if (!b_local_head->len)
+		printf(_("'%s/HEAD' is now created and points to '%s'\n"),
+			remote, head_name);
+	else if (was_detached && b_local_head->len)
+		printf(_("'%s/HEAD' was detached at '%s' and now points to '%s'\n"),
+			remote, b_local_head->buf, head_name);
+	else
+		printf(_("'%s/HEAD' used to point to '%s' "
+			"(which is not a remote branch), but now points to '%s'\n"),
+			remote, b_local_head->buf, head_name);
+	strbuf_release(&buf_prefix);
+}
+
 static int set_head(int argc, const char **argv, const char *prefix,
 		    struct repository *repo UNUSED)
 {
-	int i, opt_a = 0, opt_d = 0, result = 0;
-	struct strbuf buf = STRBUF_INIT, buf2 = STRBUF_INIT;
+	int i, opt_a = 0, opt_d = 0, result = 0, was_detached;
+	struct strbuf b_head = STRBUF_INIT, b_remote_head = STRBUF_INIT,
+		b_local_head = STRBUF_INIT;
 	char *head_name = NULL;
+	struct ref_store *refs = get_main_ref_store(the_repository);
 
 	struct option options[] = {
 		OPT_BOOL('a', "auto", &opt_a,
@@ -1420,7 +1449,7 @@ static int set_head(int argc, const char **argv, const char *prefix,
 	argc = parse_options(argc, argv, prefix, options,
 			     builtin_remote_sethead_usage, 0);
 	if (argc)
-		strbuf_addf(&buf, "refs/remotes/%s/HEAD", argv[0]);
+		strbuf_addf(&b_head, "refs/remotes/%s/HEAD", argv[0]);
 
 	if (!opt_a && !opt_d && argc == 2) {
 		head_name = xstrdup(argv[1]);
@@ -1439,25 +1468,32 @@ static int set_head(int argc, const char **argv, const char *prefix,
 			head_name = xstrdup(states.heads.items[0].string);
 		free_remote_ref_states(&states);
 	} else if (opt_d && !opt_a && argc == 1) {
-		if (refs_delete_ref(get_main_ref_store(the_repository), NULL, buf.buf, NULL, REF_NO_DEREF))
-			result |= error(_("Could not delete %s"), buf.buf);
+		if (refs_delete_ref(refs, NULL, b_head.buf, NULL, REF_NO_DEREF))
+			result |= error(_("Could not delete %s"), b_head.buf);
 	} else
 		usage_with_options(builtin_remote_sethead_usage, options);
 
-	if (head_name) {
-		strbuf_addf(&buf2, "refs/remotes/%s/%s", argv[0], head_name);
-		/* make sure it's valid */
-		if (!refs_ref_exists(get_main_ref_store(the_repository), buf2.buf))
-			result |= error(_("Not a valid ref: %s"), buf2.buf);
-		else if (refs_update_symref(get_main_ref_store(the_repository), buf.buf, buf2.buf, "remote set-head"))
-			result |= error(_("Could not setup %s"), buf.buf);
-		else if (opt_a)
-			printf("%s/HEAD set to %s\n", argv[0], head_name);
-		free(head_name);
+	if (!head_name)
+		goto cleanup;
+	strbuf_addf(&b_remote_head, "refs/remotes/%s/%s", argv[0], head_name);
+	if (!refs_ref_exists(refs, b_remote_head.buf)) {
+		result |= error(_("Not a valid ref: %s"), b_remote_head.buf);
+		goto cleanup;
 	}
-
-	strbuf_release(&buf);
-	strbuf_release(&buf2);
+	was_detached = refs_update_symref_extended(refs, b_head.buf, b_remote_head.buf,
+			"remote set-head", &b_local_head, 0);
+	if (was_detached == -1) {
+		result |= error(_("Could not set up %s"), b_head.buf);
+		goto cleanup;
+	}
+	if (opt_a)
+		report_set_head_auto(argv[0], head_name, &b_local_head, was_detached);
+
+cleanup:
+	free(head_name);
+	strbuf_release(&b_head);
+	strbuf_release(&b_remote_head);
+	strbuf_release(&b_local_head);
 	return result;
 }
 

refs.c

@@ -2166,20 +2166,54 @@ int peel_iterated_oid(struct repository *r, const struct object_id *base, struct
 
 int refs_update_symref(struct ref_store *refs, const char *ref,
 		       const char *target, const char *logmsg)
+{
+	return refs_update_symref_extended(refs, ref, target, logmsg, NULL, 0);
+}
+
+int refs_update_symref_extended(struct ref_store *refs, const char *ref,
+		       const char *target, const char *logmsg,
+		       struct strbuf *referent, int create_only)
 {
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
-	int ret = 0;
+	int ret = 0, prepret = 0;
 
 	transaction = ref_store_transaction_begin(refs, 0, &err);
-	if (!transaction ||
-	    ref_transaction_update(transaction, ref, NULL, NULL,
-				   target, NULL, REF_NO_DEREF,
-				   logmsg, &err) ||
-	    ref_transaction_commit(transaction, &err)) {
+	if (!transaction) {
+	error_return:
 		ret = error("%s", err.buf);
+		goto cleanup;
+	}
+	if (create_only) {
+		if (ref_transaction_create(transaction, ref, NULL, target,
+					   REF_NO_DEREF, logmsg, &err))
+			goto error_return;
+		prepret = ref_transaction_prepare(transaction, &err);
+		if (prepret && prepret != TRANSACTION_CREATE_EXISTS)
+			goto error_return;
+	} else {
+		if (ref_transaction_update(transaction, ref, NULL, NULL,
+					   target, NULL, REF_NO_DEREF,
+					   logmsg, &err) ||
+			ref_transaction_prepare(transaction, &err))
+			goto error_return;
+	}
+
+	if (referent && refs_read_symbolic_ref(refs, ref, referent) == NOT_A_SYMREF) {
+		struct object_id oid;
+		if (!refs_read_ref(refs, ref, &oid)) {
+			strbuf_addstr(referent, oid_to_hex(&oid));
+			ret = NOT_A_SYMREF;
+		}
 	}
 
+	if (prepret == TRANSACTION_CREATE_EXISTS)
+		goto cleanup;
+
+	if (ref_transaction_commit(transaction, &err))
+		goto error_return;
+
+cleanup:
 	strbuf_release(&err);
 	if (transaction)
 		ref_transaction_free(transaction);
@@ -2993,4 +3027,3 @@ int ref_update_expects_existing_old_ref(struct ref_update *update)
 	return (update->flags & REF_HAVE_OLD) &&
 		(!is_null_oid(&update->old_oid) || update->old_target);
 }
-

refs.h

@@ -83,6 +83,17 @@ int refs_read_ref_full(struct ref_store *refs, const char *refname,
 
 int refs_read_ref(struct ref_store *refs, const char *refname, struct object_id *oid);
 
+#define NOT_A_SYMREF -2
+
+/*
+ * Read the symbolic ref named "refname" and write its immediate referent into
+ * the provided buffer. Referent is left empty if "refname" is not a symbolic
+ * ref. It does not resolve the symbolic reference recursively in case the
+ * target is also a symbolic ref.
+ *
+ * Returns 0 on success, -2 if the "refname" is not a symbolic ref,
+ * -1 otherwise.
+ */
 int refs_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
 			   struct strbuf *referent);
 
@@ -604,6 +615,10 @@ int refs_copy_existing_ref(struct ref_store *refs, const char *oldref,
 int refs_update_symref(struct ref_store *refs, const char *refname,
 		       const char *target, const char *logmsg);
 
+int refs_update_symref_extended(struct ref_store *refs, const char *refname,
+		       const char *target, const char *logmsg,
+		       struct strbuf *referent, int create_only);
+
 enum action_on_err {
 	UPDATE_REFS_MSG_ON_ERR,
 	UPDATE_REFS_DIE_ON_ERR,
@@ -805,8 +820,10 @@ int ref_transaction_verify(struct ref_transaction *transaction,
 
 /* Naming conflict (for example, the ref names A and A/B conflict). */
 #define TRANSACTION_NAME_CONFLICT -1
+/* When only creation was requested, but the ref already exists. */
+#define TRANSACTION_CREATE_EXISTS -2
 /* All other errors. */
-#define TRANSACTION_GENERIC_ERROR -2
+#define TRANSACTION_GENERIC_ERROR -3
 
 /*
  * Perform the preparatory stages of committing `transaction`. Acquire

refs/files-backend.c

@@ -598,10 +598,9 @@ static int files_read_symbolic_ref(struct ref_store *ref_store, const char *refn
 	unsigned int type;
 
 	ret = read_ref_internal(ref_store, refname, &oid, referent, &type, &failure_errno, 1);
-	if (ret)
-		return ret;
-
-	return !(type & REF_ISSYMREF);
+	if (!ret && !(type & REF_ISSYMREF))
+		return NOT_A_SYMREF;
+	return ret;
 }
 
 int parse_loose_ref_contents(const struct git_hash_algo *algop,
@@ -2509,14 +2508,18 @@ static int split_symref_update(struct ref_update *update,
 static int check_old_oid(struct ref_update *update, struct object_id *oid,
 			 struct strbuf *err)
 {
+	int ret = TRANSACTION_GENERIC_ERROR;
+
 	if (!(update->flags & REF_HAVE_OLD) ||
 		   oideq(oid, &update->old_oid))
 		return 0;
 
-	if (is_null_oid(&update->old_oid))
+	if (is_null_oid(&update->old_oid)) {
 		strbuf_addf(err, "cannot lock ref '%s': "
 			    "reference already exists",
 			    ref_update_original_update_refname(update));
+		ret = TRANSACTION_CREATE_EXISTS;
+	}
 	else if (is_null_oid(oid))
 		strbuf_addf(err, "cannot lock ref '%s': "
 			    "reference is missing but expected %s",
@@ -2529,7 +2532,7 @@ static int check_old_oid(struct ref_update *update, struct object_id *oid,
 			    oid_to_hex(oid),
 			    oid_to_hex(&update->old_oid));
 
-	return -1;
+	return ret;
 }
 
 /*
@@ -2609,9 +2612,11 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 					ret = TRANSACTION_GENERIC_ERROR;
 					goto out;
 				}
-			} else if  (check_old_oid(update, &lock->old_oid, err)) {
-				ret = TRANSACTION_GENERIC_ERROR;
-				goto out;
+			} else {
+				ret = check_old_oid(update, &lock->old_oid, err);
+				if  (ret) {
+					goto out;
+				}
 			}
 		} else {
 			/*
@@ -2642,9 +2647,11 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 				    update->old_target);
 			ret = TRANSACTION_GENERIC_ERROR;
 			goto out;
-		} else if  (check_old_oid(update, &lock->old_oid, err)) {
-			ret = TRANSACTION_GENERIC_ERROR;
-			goto out;
+		} else {
+			ret = check_old_oid(update, &lock->old_oid, err);
+			if  (ret) {
+				goto out;
+			}
 		}
 
 		/*

refs/refs-internal.h

@@ -674,6 +674,11 @@ struct ref_storage_be {
 
 	ref_iterator_begin_fn *iterator_begin;
 	read_raw_ref_fn *read_raw_ref;
+
+	/*
+	 * Please refer to `refs_read_symbolic_ref()` for the expected
+	 * behaviour.
+	 */
 	read_symbolic_ref_fn *read_symbolic_ref;
 
 	reflog_iterator_begin_fn *reflog_iterator_begin;