Merge branch 'ps/symlink-symref-deprecation' into seen

* ps/symlink-symref-deprecation:
  refs/files: deprecate writing symrefs as symbolic links

Author: gitster

Documentation/BreakingChanges.adoc

@@ -295,6 +295,24 @@ The command will be removed.
 +
 cf. <xmqqa59i45wc.fsf@gitster.g>
 
+* Support for `core.preferSymlinkRefs=true` has been deprecated and will be
+  removed in Git 3.0. If set, symbolic refs like "HEAD" would be written as
+  symbolic links instead of as a plain file using the symref format.
++
+Symbolic references were initially always stored as a symbolic link. This was
+changed in 9b143c6e15 (Teach update-ref about a symbolic ref stored in a
+textfile., 2005-09-25), where a new symref format was introduced to store those
+symbolic refs in a plain file. In 9f0bb90d16 (core.prefersymlinkrefs: use
+symlinks for .git/HEAD, 2006-05-02), the Git project switched the default to use
+the symref format in favor of symbolic links.
++
+The migration away from symbolic links has happened almost 20 years ago by now,
+and there is no known reason why one should prefer them nowadays. Furthermore,
+symbolic links are not supported on some platforms.
++
+Note that for now, only the writing side for such symbolic links is deprecated.
+Reading such symbolic links is still supported for now.
+
 == Superseded features that will not be deprecated
 
 Some features have gained newer replacements that aim to improve the design in

Documentation/config/core.adoc

@@ -290,6 +290,9 @@ core.preferSymlinkRefs::
 	and other symbolic reference files, use symbolic links.
 	This is sometimes needed to work with old scripts that
 	expect HEAD to be a symbolic link.
++
+This configuration is deprecated and will be removed in Git 3.0. Writing
+symbolic links for symrefs will not be supported anymore.
 
 core.alternateRefsCommand::
 	When advertising tips of available history from an alternate, use the shell to

refs/files-backend.c

@@ -2098,14 +2098,28 @@ static int commit_ref_update(struct files_ref_store *refs,
 	return 0;
 }
 
-#ifdef NO_SYMLINK_HEAD
+#if defined(NO_SYMLINK_HEAD) || defined(WITH_BREAKING_CHANGES)
 #define create_ref_symlink(a, b) (-1)
 #else
 static int create_ref_symlink(struct ref_lock *lock, const char *target)
 {
+	static int warn_once = 1;
+	char *ref_path;
 	int ret = -1;
 
-	char *ref_path = get_locked_file_path(&lock->lk);
+	if (warn_once)
+		warning(_("'core.preferSymlinkRefs=true' is nominated for removal.\n"
+			  "hint: The use of symbolic links for symbolic refs is deprecated\n"
+			  "hint: and will be removed in Git 3.0. The configuration that\n"
+			  "hint: tells Git to use them is thus going away. You can unset\n"
+			  "hint: it with:\n"
+			  "hint:\n"
+			  "hint:\tgit config unset core.preferSymlinkRefs\n"
+			  "hint:\n"
+			  "hint: Git will then use the symref format instead."));
+	warn_once = 0;
+
+	ref_path = get_locked_file_path(&lock->lk);
 	unlink(ref_path);
 	ret = symlink(target, ref_path);
 	free(ref_path);

t/t0600-reffiles-backend.sh

@@ -477,9 +477,29 @@ test_expect_success SYMLINKS 'symref transaction supports symlinks' '
 	prepare
 	commit
 	EOF
-	git update-ref --no-deref --stdin <stdin &&
-	test_path_is_symlink .git/TEST_SYMREF_HEAD &&
-	test "$(test_readlink .git/TEST_SYMREF_HEAD)" = refs/heads/new
+	git update-ref --no-deref --stdin <stdin 2>err &&
+	if test_have_prereq WITH_BREAKING_CHANGES
+	then
+		test_path_is_file .git/TEST_SYMREF_HEAD &&
+		echo "ref: refs/heads/new" >expect &&
+		test_cmp expect .git/TEST_SYMREF_HEAD &&
+		test_must_be_empty err
+	else
+		test_path_is_symlink .git/TEST_SYMREF_HEAD &&
+		test "$(test_readlink .git/TEST_SYMREF_HEAD)" = refs/heads/new &&
+		cat >expect <<-EOF &&
+		warning: ${SQ}core.preferSymlinkRefs=true${SQ} is nominated for removal.
+		hint: The use of symbolic links for symbolic refs is deprecated
+		hint: and will be removed in Git 3.0. The configuration that
+		hint: tells Git to use them is thus going away. You can unset
+		hint: it with:
+		hint:
+		hint:	git config unset core.preferSymlinkRefs
+		hint:
+		hint: Git will then use the symref format instead.
+		EOF
+		test_cmp expect err
+	fi
 '
 
 test_expect_success 'symref transaction supports false symlink config' '