breaking-changes: switch default branch to main

Since 1296cbe (init: document `init.defaultBranch` better,
2020-12-11) "git-init.adoc" has advertised that the default name
of the initial branch may change in the future. The name "main"
is chosen to match the default used by the big Git forge web sites.

The advice printed when init.defaultBranch is not set is updated
to say that the default will change to "main" in Git 3.0. Building
with WITH_BREAKING_CHANGES enabled removes the advice and changes
the default branch name to "main". The code in guess_remote_head()
that looks for "refs/heads/master" is left unchanged as that is only
called when the remote server does not support the symref capability
in the v0 protocol or the symref extension to the ls-refs list in the
v2 protocol. Such an old server is more likely to be using "master"
as the default branch name.

With the exception of the "git-init.adoc" the documentation is left
unchanged. I had hoped to parameterize the name of the default branch
by using an asciidoc attribute. Unfortunately attribute expansion
is inhibited by backticks and we use backticks to mark up ref names
so that idea does not work. As the changes to git-init.adoc show
inserting ifdef's around each instance of the branch name "master"
is cumbersome and makes the documentation sources harder to read.

Apart from "git-init.adoc" there are some other files where "master" is
used as the name of the initial branch rather than as an example of a
branch name such as "user-manual.adoc" and "gitcore-tutorial.adoc". The
name appears a lot in those so updating it with ifdef's is not really
practical. We can update that document in the 3.0 release cycle. The
other documentation where master is used as an example branch name
can be gradually converted over time.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: phillipwood

Documentation/BreakingChanges.adoc

@@ -165,6 +165,12 @@ A prerequisite for this change is that the ecosystem is ready to support the
 "reftable" format. Most importantly, alternative implementations of Git like
 JGit, libgit2 and Gitoxide need to support it.
 
+* In new repositories, the default branch name will be `main`. We have been
+  warning that the default name will change since 675704c74dd (init:
+  provide useful advice about init.defaultBranch, 2020-12-11).  The new name
+  matches the default branch name used in new repositories by many of the
+  big Git forges.
+
 === Removals
 
 * Support for grafting commits has long been superseded by git-replace(1).

Documentation/git-init.adoc

@@ -77,9 +77,15 @@ If this is a reinitialization, the repository will be moved to the specified pat
 `-b <branch-name>`::
 `--initial-branch=<branch-name>`::
 Use _<branch-name>_ for the initial branch in the newly created
-repository.  If not specified, fall back to the default name (currently
-`master`, but this is subject to change in the future; the name can be
-customized via the `init.defaultBranch` configuration variable).
+repository.  If not specified, fall back to the default name
+ifndef::with-breaking-changes[]
+(currently `master`, but this will change to `main` when Git 3.0 is released).
+endif::with-breaking-changes[]
+ifdef::with-breaking-changes[]
+`main`.
+endif::with-breaking-changes[]
+The default name can be customized via the `init.defaultBranch` configuration
+variable.
 
 `--shared[=(false|true|umask|group|all|world|everybody|<perm>)]`::
 

advice.c

@@ -51,7 +51,9 @@ static struct {
 	[ADVICE_AM_WORK_DIR] 				= { "amWorkDir" },
 	[ADVICE_CHECKOUT_AMBIGUOUS_REMOTE_BRANCH_NAME] 	= { "checkoutAmbiguousRemoteBranchName" },
 	[ADVICE_COMMIT_BEFORE_MERGE]			= { "commitBeforeMerge" },
+#ifndef WITH_BREAKING_CHANGES
 	[ADVICE_DEFAULT_BRANCH_NAME]			= { "defaultBranchName" },
+#endif /* WITH_BREAKING_CHANGES */
 	[ADVICE_DETACHED_HEAD]				= { "detachedHead" },
 	[ADVICE_DIVERGING]				= { "diverging" },
 	[ADVICE_FETCH_SET_HEAD_WARN]			= { "fetchRemoteHEADWarn" },

advice.h

@@ -18,7 +18,9 @@ enum advice_type {
 	ADVICE_AM_WORK_DIR,
 	ADVICE_CHECKOUT_AMBIGUOUS_REMOTE_BRANCH_NAME,
 	ADVICE_COMMIT_BEFORE_MERGE,
+#ifndef WITH_BREAKING_CHANGES
 	ADVICE_DEFAULT_BRANCH_NAME,
+#endif /* WITH_BREAKING_CHANGES */
 	ADVICE_DETACHED_HEAD,
 	ADVICE_DIVERGING,
 	ADVICE_FETCH_SET_HEAD_WARN,

ci/run-build-and-tests.sh

@@ -9,7 +9,6 @@ run_tests=t
 
 case "$jobname" in
 linux-breaking-changes)
-	export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 	export WITH_BREAKING_CHANGES=YesPlease
 	;;
 linux-TEST-vars)

refs.c

@@ -627,10 +627,12 @@ void expand_ref_prefix(struct strvec *prefixes, const char *prefix)
 		strvec_pushf(prefixes, *p, len, prefix);
 }
 
+#ifndef WITH_BREAKING_CHANGES
 static const char default_branch_name_advice[] = N_(
 "Using '%s' as the name for the initial branch. This default branch name\n"
-"is subject to change. To configure the initial branch name to use in all\n"
-"of your new repositories, which will suppress this warning, call:\n"
+"will change to \"main\" in Git 3.0. To configure the initial branch name\n"
+"to use in all of your new repositories, which will suppress this warning,\n"
+"call:\n"
 "\n"
 "\tgit config --global init.defaultBranch <name>\n"
 "\n"
@@ -639,8 +641,9 @@ static const char default_branch_name_advice[] = N_(
 "\n"
 "\tgit branch -m <name>\n"
 );
+#endif /* WITH_BREAKING_CHANGES */
 
-char *repo_default_branch_name(struct repository *r, int quiet)
+char *repo_default_branch_name(struct repository *r, MAYBE_UNUSED int quiet)
 {
 	const char *config_key = "init.defaultbranch";
 	const char *config_display_key = "init.defaultBranch";
@@ -649,14 +652,18 @@ char *repo_default_branch_name(struct repository *r, int quiet)
 
 	if (env && *env)
 		ret = xstrdup(env);
-	else if (repo_config_get_string(r, config_key, &ret) < 0)
+	if (!ret && repo_config_get_string(r, config_key, &ret) < 0)
 		die(_("could not retrieve `%s`"), config_display_key);
 
 	if (!ret) {
+#ifdef WITH_BREAKING_CHANGES
+		ret = xstrdup("main");
+#else
 		ret = xstrdup("master");
 		if (!quiet)
 			advise_if_enabled(ADVICE_DEFAULT_BRANCH_NAME,
 					  _(default_branch_name_advice), ret);
+#endif /* WITH_BREAKING_CHANGES */
 	}
 
 	full_ref = xstrfmt("refs/heads/%s", ret);

t/t0001-init.sh

@@ -868,7 +868,7 @@ test_expect_success 'overridden default initial branch name (config)' '
 	grep nmb actual
 '
 
-test_expect_success 'advice on unconfigured init.defaultBranch' '
+test_expect_success !WITH_BREAKING_CHANGES 'advice on unconfigured init.defaultBranch' '
 	GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME= git -c color.advice=always \
 		init unconfigured-default-branch-name 2>err &&
 	test_decode_color <err >decoded &&
@@ -883,6 +883,22 @@ test_expect_success 'advice on unconfigured init.defaultBranch disabled' '
 	test_grep ! "hint: " err
 '
 
+test_expect_success 'default branch name' '
+	if test_have_prereq WITH_BREAKING_CHANGES
+	then
+		expect=main
+	else
+		expect=master
+	fi &&
+	echo "refs/heads/$expect" >expect &&
+	(
+		sane_unset GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME &&
+		git init default-initial-branch-name
+	) &&
+	git -C default-initial-branch-name symbolic-ref HEAD >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'overridden default main branch name (env)' '
 	test_config_global init.defaultBranch nmb &&
 	GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=env git init main-branch-env &&

t/test-lib.sh

@@ -127,10 +127,13 @@ then
 	export GIT_TEST_DISALLOW_ABBREVIATED_OPTIONS
 fi
 
-# Explicitly set the default branch name for testing, to avoid the
-# transitory "git init" warning under --verbose.
-: ${GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME:=master}
-export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+if test -z "$WITH_BREAKING_CHANGES"
+then
+	# Explicitly set the default branch name for testing, to avoid the
+	# transitory "git init" warning under --verbose.
+	: ${GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME:=master}
+	export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+fi
 
 ################################################################
 # It appears that people try to run tests without building...