Merge branch 'bf/set-head-symref' into seen

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.

Comments?

* bf/set-head-symref:
  fetch: set remote/HEAD if it does not exist
  refs_update_symref: add create_only option
  transaction: add TRANSACTION_CREATE_EXISTS error
  set-head: better output for --auto
  set-head: add new variable for readability
  refs_update_symref: atomically record overwritten ref

Author: gitster

builtin/branch.c

@@ -559,7 +559,7 @@ static int replace_each_worktree_head_symref(struct worktree **worktrees,
 			continue;
 
 		refs = get_worktree_ref_store(worktrees[i]);
-		if (refs_update_symref(refs, "HEAD", newref, logmsg))
+		if (refs_update_symref(refs, "HEAD", newref, logmsg, NULL, 0))
 			ret = error(_("HEAD of working tree %s is not updated"),
 				    worktrees[i]->path);
 	}

builtin/checkout.c

@@ -1015,7 +1015,8 @@ static void update_refs_for_switch(const struct checkout_opts *opts,
 			describe_detached_head(_("HEAD is now at"), new_branch_info->commit);
 		}
 	} else if (new_branch_info->path) {	/* Switch branches. */
-		if (refs_update_symref(get_main_ref_store(the_repository), "HEAD", new_branch_info->path, msg.buf) < 0)
+		if (refs_update_symref(get_main_ref_store(the_repository), "HEAD", new_branch_info->path,
+					msg.buf, NULL, 0) < 0)
 			die(_("unable to update HEAD"));
 		if (!opts->quiet) {
 			if (old_branch_info->path && !strcmp(new_branch_info->path, old_branch_info->path)) {
@@ -1479,7 +1480,7 @@ static int switch_unborn_to_new_branch(const struct checkout_opts *opts)
 		die(_("You are on a branch yet to be born"));
 	strbuf_addf(&branch_ref, "refs/heads/%s", opts->new_branch);
 	status = refs_update_symref(get_main_ref_store(the_repository),
-				    "HEAD", branch_ref.buf, "checkout -b");
+				    "HEAD", branch_ref.buf, "checkout -b", NULL, 0);
 	strbuf_release(&branch_ref);
 	if (!opts->quiet)
 		fprintf(stderr, _("Switched to a new branch '%s'\n"),

builtin/clone.c

@@ -661,7 +661,7 @@ static void update_remote_refs(const struct ref *refs,
 		strbuf_addstr(&head_ref, "HEAD");
 		if (refs_update_symref(get_main_ref_store(the_repository), head_ref.buf,
 				       remote_head_points_at->peer_ref->name,
-				       msg) < 0)
+				       msg, NULL, 0) < 0)
 			die(_("unable to update %s"), head_ref.buf);
 		strbuf_release(&head_ref);
 	}
@@ -673,7 +673,8 @@ static void update_head(const struct ref *our, const struct ref *remote,
 	const char *head;
 	if (our && skip_prefix(our->name, "refs/heads/", &head)) {
 		/* Local default branch link */
-		if (refs_update_symref(get_main_ref_store(the_repository), "HEAD", our->name, NULL) < 0)
+		if (refs_update_symref(get_main_ref_store(the_repository), "HEAD", our->name,
+					NULL, NULL, 0) < 0)
 			die(_("unable to update HEAD"));
 		if (!option_bare) {
 			refs_update_ref(get_main_ref_store(the_repository),
@@ -702,7 +703,8 @@ static void update_head(const struct ref *our, const struct ref *remote,
 		 * Unborn head from remote; same as "our" case above except
 		 * that we have no ref to update.
 		 */
-		if (refs_update_symref(get_main_ref_store(the_repository), "HEAD", unborn, NULL) < 0)
+		if (refs_update_symref(get_main_ref_store(the_repository), "HEAD", unborn,
+					NULL, NULL, 0) < 0)
 			die(_("unable to update HEAD"));
 		if (!option_bare)
 			install_branch_config(0, head, remote_name, unborn);

builtin/fetch.c

@@ -1578,6 +1578,80 @@ static int backfill_tags(struct display_state *display_state,
 	return retcode;
 }
 
+static void report_set_head(const char *remote, const char *head_name,
+			struct strbuf *buf_prev) {
+	struct strbuf buf_prefix = STRBUF_INIT;
+	const char *prev_head = NULL;
+
+	strbuf_addf(&buf_prefix, "refs/remotes/%s/", remote);
+	skip_prefix(buf_prev->buf, buf_prefix.buf, &prev_head);
+
+	if (prev_head && strcmp(prev_head, head_name)) {
+		printf("'HEAD' at '%s' has changed from '%s' to '%s'\n",
+			remote, prev_head, head_name);
+		printf("Run 'git remote set-head %s %s' to follow the change.\n",
+			remote, head_name);
+	}
+}
+
+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;
+	struct strbuf b_head = STRBUF_INIT, b_remote_head = STRBUF_INIT,
+		b_local_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) {
+		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 (!refs_ref_exists(refs, b_remote_head.buf))
+			result = 1;
+		else if (refs_update_symref(refs, b_head.buf, b_remote_head.buf,
+					"remote set-head", &b_local_head, 1))
+			result = 1;
+		else
+			report_set_head(remote, head_name, &b_local_head);
+
+		free(head_name);
+	}
+
+	strbuf_release(&b_head);
+	strbuf_release(&b_local_head);
+	strbuf_release(&b_remote_head);
+	return result;
+}
+
 static int do_fetch(struct transport *transport,
 		    struct refspec *rs,
 		    const struct fetch_config *config)
@@ -1647,6 +1721,8 @@ 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,
@@ -1791,6 +1867,12 @@ 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/notes.c

@@ -980,7 +980,8 @@ static int merge(int argc, const char **argv, const char *prefix)
 			die(_("a notes merge into %s is already in-progress at %s"),
 			    notes_ref, wt->path);
 		free_worktrees(worktrees);
-		if (refs_update_symref(get_main_ref_store(the_repository), "NOTES_MERGE_REF", notes_ref, NULL))
+		if (refs_update_symref(get_main_ref_store(the_repository), "NOTES_MERGE_REF", notes_ref,
+					NULL, NULL, 0))
 			die(_("failed to store link to current notes ref (%s)"),
 			    notes_ref);
 		fprintf(stderr, _("Automatic notes merge failed. Fix conflicts in %s "

builtin/remote.c

@@ -244,7 +244,8 @@ static int add(int argc, const char **argv, const char *prefix)
 		strbuf_reset(&buf2);
 		strbuf_addf(&buf2, "refs/remotes/%s/%s", name, master);
 
-		if (refs_update_symref(get_main_ref_store(the_repository), buf.buf, buf2.buf, "remote add"))
+		if (refs_update_symref(get_main_ref_store(the_repository), buf.buf, buf2.buf,
+					"remote add", NULL, 0))
 			result = error(_("Could not setup master '%s'"), master);
 	}
 
@@ -864,7 +865,8 @@ static int mv(int argc, const char **argv, const char *prefix)
 		strbuf_reset(&buf3);
 		strbuf_addf(&buf3, "remote: renamed %s to %s",
 				item->string, buf.buf);
-		if (refs_update_symref(get_main_ref_store(the_repository), buf.buf, buf2.buf, buf3.buf))
+		if (refs_update_symref(get_main_ref_store(the_repository), buf.buf, buf2.buf,
+					buf3.buf, NULL, 0))
 			die(_("creating '%s' failed"), buf.buf);
 		display_progress(progress, ++refs_renamed_nr);
 	}
@@ -1399,11 +1401,36 @@ static int show(int argc, const char **argv, const char *prefix)
 	return result;
 }
 
+static void report_auto(const char *remote, const char *head_name,
+			struct strbuf *buf_prev) {
+	struct strbuf buf_prefix = STRBUF_INIT;
+	const char *prev_head = NULL;
+
+	strbuf_addf(&buf_prefix, "refs/remotes/%s/", remote);
+	skip_prefix(buf_prev->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 (buf_prev->len == 0)
+		printf("'%s/HEAD' is now created and points to '%s'\n",
+			remote, head_name);
+	else
+		printf("'%s/HEAD' used to point to '%s' "
+			"(which is unusual), but now points to '%s'\n",
+			remote, buf_prev->buf, head_name);
+}
+
 static int set_head(int argc, const char **argv, const char *prefix)
 {
 	int i, opt_a = 0, opt_d = 0, result = 0;
-	struct strbuf buf = STRBUF_INIT, buf2 = STRBUF_INIT;
+	struct strbuf buf = STRBUF_INIT, buf2 = STRBUF_INIT,
+		buf_prev = 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,
@@ -1434,25 +1461,28 @@ 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))
+		if (refs_delete_ref(refs, NULL, buf.buf, NULL, REF_NO_DEREF))
 			result |= error(_("Could not delete %s"), buf.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))
+		if (!refs_ref_exists(refs, 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"))
+		else if (refs_update_symref(refs, buf.buf, buf2.buf,
+					"remote set-head", &buf_prev, 0))
 			result |= error(_("Could not setup %s"), buf.buf);
-		else if (opt_a)
-			printf("%s/HEAD set to %s\n", argv[0], head_name);
+		else if (opt_a) {
+			report_auto(argv[0], head_name, &buf_prev);
+		}
 		free(head_name);
 	}
 
 	strbuf_release(&buf);
 	strbuf_release(&buf2);
+	strbuf_release(&buf_prev);
 	return result;
 }
 

builtin/symbolic-ref.c

@@ -88,7 +88,7 @@ int cmd_symbolic_ref(int argc,
 		if (check_refname_format(argv[1], REFNAME_ALLOW_ONELEVEL) < 0)
 			die("Refusing to set '%s' to invalid ref '%s'", argv[0], argv[1]);
 		ret = !!refs_update_symref(get_main_ref_store(the_repository),
-					   argv[0], argv[1], msg);
+					   argv[0], argv[1], msg, NULL, 0);
 		break;
 	default:
 		usage_with_options(git_symbolic_ref_usage, options);

builtin/worktree.c

@@ -517,7 +517,7 @@ static int add_worktree(const char *path, const char *refname,
 		ret = refs_update_ref(wt_refs, NULL, "HEAD", &commit->object.oid,
 				      NULL, 0, UPDATE_REFS_MSG_ON_ERR);
 	else
-		ret = refs_update_symref(wt_refs, "HEAD", symref.buf, NULL);
+		ret = refs_update_symref(wt_refs, "HEAD", symref.buf, NULL, NULL, 0);
 	if (ret)
 		goto done;
 

refs.c

@@ -2114,25 +2114,44 @@ 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)
+		       const char *target, const char *logmsg,
+		       struct strbuf *before_target, int create_only)
 {
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
 	int ret = 0;
 
 	transaction = ref_store_transaction_begin(refs, &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) {
+		int create_ret = 0;
+		if (ref_transaction_create(transaction, ref, NULL, target,
+					   REF_NO_DEREF, logmsg, &err))
+			goto error_return;
+		create_ret = ref_transaction_commit(transaction, &err);
+		if (create_ret && create_ret != TRANSACTION_CREATE_EXISTS)
+			ret = error("%s", err.buf);
+
+	} else {
+		if (ref_transaction_update(transaction, ref, NULL, NULL,
+					   target, NULL, REF_NO_DEREF,
+					   logmsg, &err) ||
+		    ref_transaction_commit(transaction, &err))
+			goto error_return;
 	}
 
+cleanup:
 	strbuf_release(&err);
-	if (transaction)
-		ref_transaction_free(transaction);
 
+	if (transaction) {
+		if (before_target && transaction->updates[0]->before_target)
+			strbuf_addstr(before_target, transaction->updates[0]->before_target);
+		ref_transaction_free(transaction);
+	}
 	return ret;
 }
 
@@ -2948,4 +2967,3 @@ 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

@@ -571,7 +571,8 @@ int refs_copy_existing_ref(struct ref_store *refs, const char *oldref,
 		    const char *newref, const char *logmsg);
 
 int refs_update_symref(struct ref_store *refs, const char *refname,
-		       const char *target, const char *logmsg);
+		       const char *target, const char *logmsg,
+		       struct strbuf *before_target, int create_only);
 
 enum action_on_err {
 	UPDATE_REFS_MSG_ON_ERR,
@@ -758,8 +759,10 @@ 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