Subject: [PATCH v3] fetch: add configuration for set_head behaviour

From: Bence Ferdinandy <[email-masked]>
Date: Fri, 29 Nov 2024 00:09:38 +0100

In the current implementation, if refs/remotes/$remote/HEAD does not
exist, running fetch will create it, but if it does exist it will not do
anything, which is a somewhat safe and minimal approach. Unfortunately,
for users who wish to NOT have refs/remotes/$remote/HEAD set for any
reason (e.g. so that `git rev-parse origin` doesn't accidentally point
them somewhere they do not want to), there is no way to remove this
behaviour. On the other side of the spectrum, users may want fetch to
automatically update HEAD or at least give them a warning if something
changed on the remote.

Introduce a new setting, remote.$remote.followRemoteHEAD with four
options:

    - "never": do not ever do anything, not even create
    - "create": the current behaviour, now the default behaviour
    - "warn": print a message if remote and local HEAD is different
    - "always": silently update HEAD on every change

Signed-off-by: Bence Ferdinandy <[email-masked]>
---

Notes:
    This patch is built off v15 of the set-head/fetch updates series:
    https://lore.kernel.org/git/[email-masked]/T/#m13a0c3e8919872188ef07fd9fd984c78e8aa35ba
    
    v2:
        - magic numbers converted to enum
        - since the enum is descriptive (well, along with the documentation)
          I felt there is no need for the comment explaining the values
    
    v3: missed a magic number in v2

 Documentation/config/remote.txt |  11 ++++
 builtin/fetch.c                 |  46 ++++++++++++--
 remote.c                        |   9 +++
 remote.h                        |   9 +++
 t/t5510-fetch.sh                | 102 ++++++++++++++++++++++++++++++++
 5 files changed, 171 insertions(+), 6 deletions(-)

diff --git a/Documentation/config/remote.txt b/Documentation/config/remote.txt
index 6d8b7d6c63..024f92befc 100644
--- a/Documentation/config/remote.txt
+++ b/Documentation/config/remote.txt
@@ -101,6 +101,17 @@ remote.<name>.serverOption::
 	The default set of server options used when fetching from this remote.
 	These server options can be overridden by the `--server-option=` command
 	line arguments.
+
+remote.<name>.followRemoteHEAD::
+	How linkgit:git-fetch[1] should handle updates to `remotes/<name>/HEAD`.
+	The default value is "create", which will create `remotes/<name>/HEAD`
+	if it exists on the remote, but not locally, but will not touch an
+	already existing local reference.  Setting to "warn" will print
+	a message if the remote has a different value, than the local one and
+	in case there is no local reference, it behaves like "create". Setting
+	to "always" will silently update it to the value on the remote.
+	Finally, setting it to "never" will never change or create the local
+	reference.
 +
 This is a multi-valued variable, and an empty value can be used in a higher
 priority configuration file (e.g. `.git/config` in a repository) to clear
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 2f416cf867..88c5c5d781 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1579,10 +1579,35 @@ static const char *strip_refshead(const char *name){
 	return name;
 }
 
-static int set_head(const struct ref *remote_refs)
+static void report_set_head(const char *remote, const char *head_name,
+			struct strbuf *buf_prev, int updateres) {
+	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' is '%s', but we have '%s' locally.\n",
+			remote, head_name, prev_head);
+		printf("Run 'git remote set-head %s %s' to follow the change.\n",
+			remote, head_name);
+	}
+	else if (updateres && buf_prev->len) {
+		printf("'HEAD' at '%s' is '%s', "
+			"but we have a detached HEAD pointing to '%s' locally.\n",
+			remote, head_name, buf_prev->buf);
+		printf("Run 'git remote set-head %s %s' to follow the change.\n",
+			remote, head_name);
+	}
+	strbuf_release(&buf_prefix);
+}
+
+static int set_head(const struct ref *remote_refs, int follow_remote_head)
 {
-	int result = 0, is_bare;
-	struct strbuf b_head = STRBUF_INIT, b_remote_head = STRBUF_INIT;
+	int result = 0, create_only, is_bare, was_detached;
+	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;
@@ -1603,6 +1628,8 @@ static int set_head(const struct ref *remote_refs)
 		string_list_append(&heads, strip_refshead(ref->name));
 	}
 
+	if (follow_remote_head == FOLLOW_REMOTE_NEVER)
+		goto cleanup;
 
 	if (!heads.nr)
 		result = 1;
@@ -1614,6 +1641,7 @@ static int set_head(const struct ref *remote_refs)
 	if (!head_name)
 		goto cleanup;
 	is_bare = is_bare_repository();
+	create_only = follow_remote_head == FOLLOW_REMOTE_ALWAYS ? 0 : !is_bare;
 	if (is_bare) {
 		strbuf_addstr(&b_head, "HEAD");
 		strbuf_addf(&b_remote_head, "refs/heads/%s", head_name);
@@ -1626,9 +1654,14 @@ static int set_head(const struct ref *remote_refs)
 		result = 1;
 		goto cleanup;
 	}
-	if (refs_update_symref_extended(refs, b_head.buf, b_remote_head.buf,
-					"fetch", NULL, !is_bare))
+	was_detached = refs_update_symref_extended(refs, b_head.buf, b_remote_head.buf,
+					"fetch", &b_local_head, create_only);
+	if (was_detached == -1) {
 		result = 1;
+		goto cleanup;
+	}
+	if (follow_remote_head == FOLLOW_REMOTE_WARN && verbosity >= 0)
+		report_set_head(remote, head_name, &b_local_head, was_detached);
 
 cleanup:
 	free(head_name);
@@ -1636,6 +1669,7 @@ static int set_head(const struct ref *remote_refs)
 	free_refs(matches);
 	string_list_clear(&heads, 0);
 	strbuf_release(&b_head);
+	strbuf_release(&b_local_head);
 	strbuf_release(&b_remote_head);
 	return result;
 }
@@ -1855,7 +1889,7 @@ static int do_fetch(struct transport *transport,
 				  "you need to specify exactly one branch with the --set-upstream option"));
 		}
 	}
-	if (set_head(remote_refs))
+	if (set_head(remote_refs, transport->remote->follow_remote_head))
 		;
 		/*
 		 * Way too many cases where this can go wrong
diff --git a/remote.c b/remote.c
index 10104d11e3..0b18840d43 100644
--- a/remote.c
+++ b/remote.c
@@ -514,6 +514,15 @@ static int handle_config(const char *key, const char *value,
 	} else if (!strcmp(subkey, "serveroption")) {
 		return parse_transport_option(key, value,
 					      &remote->server_options);
+	} else if (!strcmp(subkey, "followremotehead")) {
+		if (!strcmp(value, "never"))
+			remote->follow_remote_head = FOLLOW_REMOTE_NEVER;
+		else if (!strcmp(value, "create"))
+			remote->follow_remote_head = FOLLOW_REMOTE_CREATE;
+		else if (!strcmp(value, "warn"))
+			remote->follow_remote_head = FOLLOW_REMOTE_WARN;
+		else if (!strcmp(value, "always"))
+			remote->follow_remote_head = FOLLOW_REMOTE_ALWAYS;
 	}
 	return 0;
 }
diff --git a/remote.h b/remote.h
index a7e5c4e07c..184b35653d 100644
--- a/remote.h
+++ b/remote.h
@@ -59,6 +59,13 @@ struct remote_state {
 void remote_state_clear(struct remote_state *remote_state);
 struct remote_state *remote_state_new(void);
 
+	enum follow_remote_head_settings {
+		FOLLOW_REMOTE_NEVER = -1,
+		FOLLOW_REMOTE_CREATE = 0,
+		FOLLOW_REMOTE_WARN = 1,
+		FOLLOW_REMOTE_ALWAYS = 2,
+	};
+
 struct remote {
 	struct hashmap_entry ent;
 
@@ -107,6 +114,8 @@ struct remote {
 	char *http_proxy_authmethod;
 
 	struct string_list server_options;
+
+	enum follow_remote_head_settings follow_remote_head;
 };
 
 /**
diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
index 87698341f5..2467027d34 100755
--- a/t/t5510-fetch.sh
+++ b/t/t5510-fetch.sh
@@ -99,6 +99,108 @@ test_expect_success "fetch test remote HEAD change" '
 	branch=$(git rev-parse refs/remotes/origin/other) &&
 	test "z$head" = "z$branch"'
 
+test_expect_success "fetch test followRemoteHEAD never" '
+	test_when_finished "git config unset remote.origin.followRemoteHEAD" &&
+	(
+		cd "$D" &&
+		cd two &&
+		git update-ref --no-deref -d refs/remotes/origin/HEAD &&
+		git config set remote.origin.followRemoteHEAD "never" &&
+		git fetch &&
+		test_must_fail git rev-parse --verify refs/remotes/origin/HEAD
+	)
+'
+
+test_expect_success "fetch test followRemoteHEAD warn no change" '
+	test_when_finished "git config unset remote.origin.followRemoteHEAD" &&
+	(
+		cd "$D" &&
+		cd two &&
+		git rev-parse --verify refs/remotes/origin/other &&
+		git remote set-head origin other &&
+		git rev-parse --verify refs/remotes/origin/HEAD &&
+		git rev-parse --verify refs/remotes/origin/main &&
+		git config set remote.origin.followRemoteHEAD "warn" &&
+		git fetch >output &&
+		echo "${SQ}HEAD${SQ} at ${SQ}origin${SQ} is ${SQ}main${SQ}," \
+			"but we have ${SQ}other${SQ} locally." >expect &&
+		echo "Run ${SQ}git remote set-head origin main${SQ} to follow the change." >>expect &&
+		test_cmp expect output &&
+		head=$(git rev-parse refs/remotes/origin/HEAD) &&
+		branch=$(git rev-parse refs/remotes/origin/other) &&
+		test "z$head" = "z$branch"
+	)
+'
+
+test_expect_success "fetch test followRemoteHEAD warn create" '
+	test_when_finished "git config unset remote.origin.followRemoteHEAD" &&
+	(
+		cd "$D" &&
+		cd two &&
+		git update-ref --no-deref -d refs/remotes/origin/HEAD &&
+		git config set remote.origin.followRemoteHEAD "warn" &&
+		git rev-parse --verify refs/remotes/origin/main &&
+		output=$(git fetch) &&
+		test "z" = "z$output" &&
+		head=$(git rev-parse refs/remotes/origin/HEAD) &&
+		branch=$(git rev-parse refs/remotes/origin/main) &&
+		test "z$head" = "z$branch"
+	)
+'
+
+test_expect_success "fetch test followRemoteHEAD warn detached" '
+	test_when_finished "git config unset remote.origin.followRemoteHEAD" &&
+	(
+		cd "$D" &&
+		cd two &&
+		git update-ref --no-deref -d refs/remotes/origin/HEAD &&
+		git update-ref refs/remotes/origin/HEAD HEAD &&
+		HEAD=$(git log --pretty="%H") &&
+		git config set remote.origin.followRemoteHEAD "warn" &&
+		git fetch >output &&
+		echo "${SQ}HEAD${SQ} at ${SQ}origin${SQ} is ${SQ}main${SQ}," \
+			"but we have a detached HEAD pointing to" \
+			"${SQ}${HEAD}${SQ} locally." >expect &&
+		echo "Run ${SQ}git remote set-head origin main${SQ} to follow the change." >>expect &&
+		test_cmp expect output
+	)
+'
+
+test_expect_success "fetch test followRemoteHEAD warn quiet" '
+	test_when_finished "git config unset remote.origin.followRemoteHEAD" &&
+	(
+		cd "$D" &&
+		cd two &&
+		git rev-parse --verify refs/remotes/origin/other &&
+		git remote set-head origin other &&
+		git rev-parse --verify refs/remotes/origin/HEAD &&
+		git rev-parse --verify refs/remotes/origin/main &&
+		git config set remote.origin.followRemoteHEAD "warn" &&
+		output=$(git fetch --quiet) &&
+		test "z" = "z$output" &&
+		head=$(git rev-parse refs/remotes/origin/HEAD) &&
+		branch=$(git rev-parse refs/remotes/origin/other) &&
+		test "z$head" = "z$branch"
+	)
+'
+
+test_expect_success "fetch test followRemoteHEAD always" '
+	test_when_finished "git config unset remote.origin.followRemoteHEAD" &&
+	(
+		cd "$D" &&
+		cd two &&
+		git rev-parse --verify refs/remotes/origin/other &&
+		git remote set-head origin other &&
+		git rev-parse --verify refs/remotes/origin/HEAD &&
+		git rev-parse --verify refs/remotes/origin/main &&
+		git config set remote.origin.followRemoteHEAD "always" &&
+		git fetch &&
+		head=$(git rev-parse refs/remotes/origin/HEAD) &&
+		branch=$(git rev-parse refs/remotes/origin/main) &&
+		test "z$head" = "z$branch"
+	)
+'
+
 test_expect_success 'fetch --prune on its own works as expected' '
 	cd "$D" &&
 	git clone . prune &&

Range-diff against v2:
1:  60986e890e ! 1:  4e9ac5524a fetch: add configuration for set_head behaviour
    @@ Notes
             - since the enum is descriptive (well, along with the documentation)
               I felt there is no need for the comment explaining the values
     
    +    v3: missed a magic number in v2
    +
      ## Documentation/config/remote.txt ##
     @@ Documentation/config/remote.txt: remote.<name>.serverOption::
      	The default set of server options used when fetching from this remote.
    @@ builtin/fetch.c: static int set_head(const struct ref *remote_refs)
      	if (!head_name)
      		goto cleanup;
      	is_bare = is_bare_repository();
    -+	create_only = follow_remote_head == 2 ? 0 : !is_bare;
    ++	create_only = follow_remote_head == FOLLOW_REMOTE_ALWAYS ? 0 : !is_bare;
      	if (is_bare) {
      		strbuf_addstr(&b_head, "HEAD");
      		strbuf_addf(&b_remote_head, "refs/heads/%s", head_name);
-- 
2.47.1.295.g66bad26ef4