Merge branch 'ps/pseudo-ref-terminology' into ps/ref-storage-migration

* ps/pseudo-ref-terminology:
  refs: refuse to write pseudorefs
  ref-filter: properly distinuish pseudo and root refs
  refs: pseudorefs are no refs
  refs: classify HEAD as a root ref
  refs: do not check ref existence in `is_root_ref()`
  refs: rename `is_special_ref()` to `is_pseudo_ref()`
  refs: rename `is_pseudoref()` to `is_root_ref()`
  Documentation/glossary: define root refs as refs
  Documentation/glossary: clarify limitations of pseudorefs
  Documentation/glossary: redefine pseudorefs as special refs

Documentation/glossary-content.txt

@@ -497,20 +497,18 @@ exclude;;
 	unusual refs.
 
 [[def_pseudoref]]pseudoref::
-	Pseudorefs are a class of files under `$GIT_DIR` which behave
-	like refs for the purposes of rev-parse, but which are treated
-	specially by git.  Pseudorefs both have names that are all-caps,
-	and always start with a line consisting of a
-	<<def_SHA1,SHA-1>> followed by whitespace.  So, HEAD is not a
-	pseudoref, because it is sometimes a symbolic ref.  They might
-	optionally contain some additional data.  `MERGE_HEAD` and
-	`CHERRY_PICK_HEAD` are examples.  Unlike
-	<<def_per_worktree_ref,per-worktree refs>>, these files cannot
-	be symbolic refs, and never have reflogs.  They also cannot be
-	updated through the normal ref update machinery.  Instead,
-	they are updated by directly writing to the files.  However,
-	they can be read as if they were refs, so `git rev-parse
-	MERGE_HEAD` will work.
+	A ref that has different semantics than normal refs. These refs can be
+	read via normal Git commands, but cannot be written to by commands like
+	linkgit:git-update-ref[1].
++
+The following pseudorefs are known to Git:
+
+ - `FETCH_HEAD` is written by linkgit:git-fetch[1] or linkgit:git-pull[1]. It
+   may refer to multiple object IDs. Each object ID is annotated with metadata
+   indicating where it was fetched from and its fetch status.
+
+ - `MERGE_HEAD` is written by linkgit:git-merge[1] when resolving merge
+   conflicts. It contains all commit IDs which are being merged.
 
 [[def_pull]]pull::
 	Pulling a <<def_branch,branch>> means to <<def_fetch,fetch>> it and
@@ -552,20 +550,38 @@ exclude;;
 	to the result.
 
 [[def_ref]]ref::
-	A name that begins with `refs/` (e.g. `refs/heads/master`)
-	that points to an <<def_object_name,object name>> or another
-	ref (the latter is called a <<def_symref,symbolic ref>>).
+	A name that that points to an <<def_object_name,object name>> or
+	another ref (the latter is called a <<def_symref,symbolic ref>>).
 	For convenience, a ref can sometimes be abbreviated when used
 	as an argument to a Git command; see linkgit:gitrevisions[7]
 	for details.
 	Refs are stored in the <<def_repository,repository>>.
 +
 The ref namespace is hierarchical.
-Different subhierarchies are used for different purposes (e.g. the
-`refs/heads/` hierarchy is used to represent local branches).
+Ref names must either start with `refs/` or be located in the root of
+the hierarchy. For the latter, their name must follow these rules:
 +
-There are a few special-purpose refs that do not begin with `refs/`.
-The most notable example is `HEAD`.
+ - The name consists of only upper-case characters or underscores.
+
+ - The name ends with "`_HEAD`" or is equal to "`HEAD`".
++
+There are some irregular refs in the root of the hierarchy that do not
+match these rules. The following list is exhaustive and shall not be
+extended in the future:
++
+ - `AUTO_MERGE`
+
+ - `BISECT_EXPECTED_REV`
+
+ - `NOTES_MERGE_PARTIAL`
+
+ - `NOTES_MERGE_REF`
+
+ - `MERGE_AUTOSTASH`
++
+Different subhierarchies are used for different purposes. For example,
+the `refs/heads/` hierarchy is used to represent local branches whereas
+the `refs/tags/` hierarchy is used to represent local tags..
 
 [[def_reflog]]reflog::
 	A reflog shows the local "history" of a ref.  In other words,
@@ -639,20 +655,6 @@ The most notable example is `HEAD`.
 	An <<def_object,object>> used to temporarily store the contents of a
 	<<def_dirty,dirty>> working directory and the index for future reuse.
 
-[[def_special_ref]]special ref::
-	A ref that has different semantics than normal refs. These refs can be
-	accessed via normal Git commands but may not behave the same as a
-	normal ref in some cases.
-+
-The following special refs are known to Git:
-
- - "`FETCH_HEAD`" is written by linkgit:git-fetch[1] or linkgit:git-pull[1]. It
-   may refer to multiple object IDs. Each object ID is annotated with metadata
-   indicating where it was fetched from and its fetch status.
-
- - "`MERGE_HEAD`" is written by linkgit:git-merge[1] when resolving merge
-   conflicts. It contains all commit IDs which are being merged.
-
 [[def_submodule]]submodule::
 	A <<def_repository,repository>> that holds the history of a
 	separate project inside another repository (the latter of

builtin/for-each-ref.c

@@ -98,7 +98,7 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
 	}
 
 	if (include_root_refs)
-		flags |= FILTER_REFS_ROOT_REFS;
+		flags |= FILTER_REFS_ROOT_REFS | FILTER_REFS_DETACHED_HEAD;
 
 	filter.match_as_path = 1;
 	filter_and_format_refs(&filter, flags, sorting, &format);

ref-filter.c

@@ -2634,7 +2634,7 @@ static int for_each_fullref_in_pattern(struct ref_filter *filter,
 				       each_ref_fn cb,
 				       void *cb_data)
 {
-	if (filter->kind == FILTER_REFS_KIND_MASK) {
+	if (filter->kind & FILTER_REFS_ROOT_REFS) {
 		/* In this case, we want to print all refs including root refs. */
 		return refs_for_each_include_root_refs(get_main_ref_store(the_repository),
 						       cb, cb_data);
@@ -2764,8 +2764,10 @@ static int ref_kind_from_refname(const char *refname)
 			return ref_kind[i].kind;
 	}
 
-	if (is_pseudoref(get_main_ref_store(the_repository), refname))
+	if (is_pseudo_ref(refname))
 		return FILTER_REFS_PSEUDOREFS;
+	if (is_root_ref(refname))
+		return FILTER_REFS_ROOT_REFS;
 
 	return FILTER_REFS_OTHERS;
 }
@@ -2802,11 +2804,11 @@ static struct ref_array_item *apply_ref_filter(const char *refname, const struct
 	/*
 	 * Generally HEAD refs are printed with special description denoting a rebase,
 	 * detached state and so forth. This is useful when only printing the HEAD ref
-	 * But when it is being printed along with other pseudorefs, it makes sense to
-	 * keep the formatting consistent. So we mask the type to act like a pseudoref.
+	 * But when it is being printed along with other root refs, it makes sense to
+	 * keep the formatting consistent. So we mask the type to act like a root ref.
 	 */
-	if (filter->kind == FILTER_REFS_KIND_MASK && kind == FILTER_REFS_DETACHED_HEAD)
-		kind = FILTER_REFS_PSEUDOREFS;
+	if (filter->kind & FILTER_REFS_ROOT_REFS && kind == FILTER_REFS_DETACHED_HEAD)
+		kind = FILTER_REFS_ROOT_REFS;
 	else if (!(kind & filter->kind))
 		return NULL;
 
@@ -3086,7 +3088,7 @@ static int do_filter_refs(struct ref_filter *filter, unsigned int type, each_ref
 		 * When printing all ref types, HEAD is already included,
 		 * so we don't want to print HEAD again.
 		 */
-		if (!ret && (filter->kind != FILTER_REFS_KIND_MASK) &&
+		if (!ret && !(filter->kind & FILTER_REFS_ROOT_REFS) &&
 		    (filter->kind & FILTER_REFS_DETACHED_HEAD))
 			refs_head_ref(get_main_ref_store(the_repository), fn,
 				      cb_data);

ref-filter.h

@@ -23,9 +23,9 @@
 				    FILTER_REFS_REMOTES | FILTER_REFS_OTHERS)
 #define FILTER_REFS_DETACHED_HEAD  0x0020
 #define FILTER_REFS_PSEUDOREFS     0x0040
-#define FILTER_REFS_ROOT_REFS      (FILTER_REFS_DETACHED_HEAD | FILTER_REFS_PSEUDOREFS)
+#define FILTER_REFS_ROOT_REFS      0x0080
 #define FILTER_REFS_KIND_MASK      (FILTER_REFS_REGULAR | FILTER_REFS_DETACHED_HEAD | \
-				    FILTER_REFS_PSEUDOREFS)
+				    FILTER_REFS_PSEUDOREFS | FILTER_REFS_ROOT_REFS)
 
 struct atom_value;
 struct ref_sorting;

refs.c

@@ -782,7 +782,22 @@ int is_per_worktree_ref(const char *refname)
 	       starts_with(refname, "refs/rewritten/");
 }
 
-static int is_pseudoref_syntax(const char *refname)
+int is_pseudo_ref(const char *refname)
+{
+	static const char * const pseudo_refs[] = {
+		"FETCH_HEAD",
+		"MERGE_HEAD",
+	};
+	size_t i;
+
+	for (i = 0; i < ARRAY_SIZE(pseudo_refs); i++)
+		if (!strcmp(refname, pseudo_refs[i]))
+			return 1;
+
+	return 0;
+}
+
+static int is_root_ref_syntax(const char *refname)
 {
 	const char *c;
 
@@ -791,56 +806,37 @@ static int is_pseudoref_syntax(const char *refname)
 			return 0;
 	}
 
-	/*
-	 * HEAD is not a pseudoref, but it certainly uses the
-	 * pseudoref syntax.
-	 */
 	return 1;
 }
 
-int is_pseudoref(struct ref_store *refs, const char *refname)
+int is_root_ref(const char *refname)
 {
-	static const char *const irregular_pseudorefs[] = {
+	static const char *const irregular_root_refs[] = {
+		"HEAD",
 		"AUTO_MERGE",
 		"BISECT_EXPECTED_REV",
 		"NOTES_MERGE_PARTIAL",
 		"NOTES_MERGE_REF",
 		"MERGE_AUTOSTASH",
 	};
-	struct object_id oid;
 	size_t i;
 
-	if (!is_pseudoref_syntax(refname))
+	if (!is_root_ref_syntax(refname) ||
+	    is_pseudo_ref(refname))
 		return 0;
 
-	if (ends_with(refname, "_HEAD")) {
-		refs_resolve_ref_unsafe(refs, refname,
-					RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
-					&oid, NULL);
-		return !is_null_oid(&oid);
-	}
+	if (ends_with(refname, "_HEAD"))
+		return 1;
 
-	for (i = 0; i < ARRAY_SIZE(irregular_pseudorefs); i++)
-		if (!strcmp(refname, irregular_pseudorefs[i])) {
-			refs_resolve_ref_unsafe(refs, refname,
-						RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
-						&oid, NULL);
-			return !is_null_oid(&oid);
-		}
-
-	return 0;
-}
-
-int is_headref(struct ref_store *refs, const char *refname)
-{
-	if (!strcmp(refname, "HEAD"))
-		return refs_ref_exists(refs, refname);
+	for (i = 0; i < ARRAY_SIZE(irregular_root_refs); i++)
+		if (!strcmp(refname, irregular_root_refs[i]))
+			return 1;
 
 	return 0;
 }
 
 static int is_current_worktree_ref(const char *ref) {
-	return is_pseudoref_syntax(ref) || is_per_worktree_ref(ref);
+	return is_root_ref_syntax(ref) || is_per_worktree_ref(ref);
 }
 
 enum ref_worktree_type parse_worktree_ref(const char *maybe_worktree_ref,
@@ -1206,6 +1202,13 @@ int ref_transaction_update(struct ref_transaction *transaction,
 		return -1;
 	}
 
+	if (!(flags & REF_SKIP_REFNAME_VERIFICATION) &&
+	    is_pseudo_ref(refname)) {
+		strbuf_addf(err, _("refusing to update pseudoref '%s'"),
+			    refname);
+		return -1;
+	}
+
 	if (flags & ~REF_TRANSACTION_UPDATE_ALLOWED_FLAGS)
 		BUG("illegal flags 0x%x passed to ref_transaction_update()", flags);
 
@@ -1737,43 +1740,12 @@ done:
 	return result;
 }
 
-static int is_special_ref(const char *refname)
-{
-	/*
-	 * Special references are refs that have different semantics compared
-	 * to "normal" refs. These refs can thus not be stored in the ref
-	 * backend, but must always be accessed via the filesystem. The
-	 * following refs are special:
-	 *
-	 * - FETCH_HEAD may contain multiple object IDs, and each one of them
-	 *   carries additional metadata like where it came from.
-	 *
-	 * - MERGE_HEAD may contain multiple object IDs when merging multiple
-	 *   heads.
-	 *
-	 * Reading, writing or deleting references must consistently go either
-	 * through the filesystem (special refs) or through the reference
-	 * backend (normal ones).
-	 */
-	static const char * const special_refs[] = {
-		"FETCH_HEAD",
-		"MERGE_HEAD",
-	};
-	size_t i;
-
-	for (i = 0; i < ARRAY_SIZE(special_refs); i++)
-		if (!strcmp(refname, special_refs[i]))
-			return 1;
-
-	return 0;
-}
-
 int refs_read_raw_ref(struct ref_store *ref_store, const char *refname,
 		      struct object_id *oid, struct strbuf *referent,
 		      unsigned int *type, int *failure_errno)
 {
 	assert(failure_errno);
-	if (is_special_ref(refname))
+	if (is_pseudo_ref(refname))
 		return refs_read_special_head(ref_store, refname, oid, referent,
 					      type, failure_errno);
 

refs.h

@@ -1006,8 +1006,52 @@ extern struct ref_namespace_info ref_namespace[NAMESPACE__COUNT];
  */
 void update_ref_namespace(enum ref_namespace namespace, char *ref);
 
-int is_pseudoref(struct ref_store *refs, const char *refname);
-int is_headref(struct ref_store *refs, const char *refname);
+/*
+ * Check whether the provided name names a root reference. This function only
+ * performs a syntactic check.
+ *
+ * A root ref is a reference that lives in the root of the reference hierarchy.
+ * These references must conform to special syntax:
+ *
+ *   - Their name must be all-uppercase or underscores ("_").
+ *
+ *   - Their name must end with "_HEAD". As a special rule, "HEAD" is a root
+ *     ref, as well.
+ *
+ *   - Their name may not contain a slash.
+ *
+ * There is a special set of irregular root refs that exist due to historic
+ * reasons, only. This list shall not be expanded in the future:
+ *
+ *   - AUTO_MERGE
+ *
+ *   - BISECT_EXPECTED_REV
+ *
+ *   - NOTES_MERGE_PARTIAL
+ *
+ *   - NOTES_MERGE_REF
+ *
+ *   - MERGE_AUTOSTASH
+ */
+int is_root_ref(const char *refname);
+
+/*
+ * Pseudorefs are refs that have different semantics compared to
+ * "normal" refs. These refs can thus not be stored in the ref backend,
+ * but must always be accessed via the filesystem. The following refs
+ * are pseudorefs:
+ *
+ * - FETCH_HEAD may contain multiple object IDs, and each one of them
+ *   carries additional metadata like where it came from.
+ *
+ * - MERGE_HEAD may contain multiple object IDs when merging multiple
+ *   heads.
+ *
+ * Reading, writing or deleting references must consistently go either
+ * through the filesystem (pseudorefs) or through the reference
+ * backend (normal ones).
+ */
+int is_pseudo_ref(const char *refname);
 
 /*
  * The following functions have been removed in Git v2.45 in favor of functions

refs/files-backend.c

@@ -359,8 +359,7 @@ static void add_pseudoref_and_head_entries(struct ref_store *ref_store,
 		strbuf_addstr(&refname, de->d_name);
 
 		dtype = get_dtype(de, &path, 1);
-		if (dtype == DT_REG && (is_pseudoref(ref_store, de->d_name) ||
-								is_headref(ref_store, de->d_name)))
+		if (dtype == DT_REG && is_root_ref(de->d_name))
 			loose_fill_ref_dir_regular_file(refs, refname.buf, dir);
 
 		strbuf_setlen(&refname, dirnamelen);

refs/reftable-backend.c

@@ -375,8 +375,7 @@ static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
 		 */
 		if (!starts_with(iter->ref.refname, "refs/") &&
 		    !(iter->flags & DO_FOR_EACH_INCLUDE_ROOT_REFS &&
-		     (is_pseudoref(&iter->refs->base, iter->ref.refname) ||
-		      is_headref(&iter->refs->base, iter->ref.refname)))) {
+		      is_root_ref(iter->ref.refname))) {
 			continue;
 		}
 

t/t5510-fetch.sh

@@ -518,7 +518,7 @@ test_expect_success 'fetch with a non-applying branch.<name>.merge' '
 test_expect_success 'fetch from GIT URL with a non-applying branch.<name>.merge [1]' '
 	one_head=$(cd one && git rev-parse HEAD) &&
 	this_head=$(git rev-parse HEAD) &&
-	git update-ref -d FETCH_HEAD &&
+	rm .git/FETCH_HEAD &&
 	git fetch one &&
 	test $one_head = "$(git rev-parse --verify FETCH_HEAD)" &&
 	test $this_head = "$(git rev-parse --verify HEAD)"
@@ -530,7 +530,7 @@ test_expect_success 'fetch from GIT URL with a non-applying branch.<name>.merge
 	one_ref=$(cd one && git symbolic-ref HEAD) &&
 	git config branch.main.remote blub &&
 	git config branch.main.merge "$one_ref" &&
-	git update-ref -d FETCH_HEAD &&
+	rm .git/FETCH_HEAD &&
 	git fetch one &&
 	test $one_head = "$(git rev-parse --verify FETCH_HEAD)" &&
 	test $this_head = "$(git rev-parse --verify HEAD)"
@@ -540,7 +540,7 @@ test_expect_success 'fetch from GIT URL with a non-applying branch.<name>.merge
 # the merge spec does not match the branch the remote HEAD points to
 test_expect_success 'fetch from GIT URL with a non-applying branch.<name>.merge [3]' '
 	git config branch.main.merge "${one_ref}_not" &&
-	git update-ref -d FETCH_HEAD &&
+	rm .git/FETCH_HEAD &&
 	git fetch one &&
 	test $one_head = "$(git rev-parse --verify FETCH_HEAD)" &&
 	test $this_head = "$(git rev-parse --verify HEAD)"

t/t6302-for-each-ref-filter.sh

@@ -52,6 +52,23 @@ test_expect_success '--include-root-refs pattern prints pseudorefs' '
 	test_cmp expect actual
 '
 
+test_expect_success '--include-root-refs pattern does not print special refs' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		git rev-parse HEAD >.git/MERGE_HEAD &&
+		git for-each-ref --format="%(refname)" --include-root-refs >actual &&
+		cat >expect <<-EOF &&
+		HEAD
+		$(git symbolic-ref HEAD)
+		refs/tags/initial
+		EOF
+		test_cmp expect actual
+	)
+'
+
 test_expect_success '--include-root-refs with other patterns' '
 	cat >expect <<-\EOF &&
 	HEAD
@@ -62,6 +79,23 @@ test_expect_success '--include-root-refs with other patterns' '
 	test_cmp expect actual
 '
 
+test_expect_success '--include-root-refs omits dangling symrefs' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		git symbolic-ref DANGLING_HEAD refs/heads/missing &&
+		cat >expect <<-EOF &&
+		HEAD
+		$(git symbolic-ref HEAD)
+		refs/tags/initial
+		EOF
+		git for-each-ref --format="%(refname)" --include-root-refs >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success 'filtering with --points-at' '
 	cat >expect <<-\EOF &&
 	refs/heads/main