[PATCH v13 2/6] fetch: extract collect_upstream_from_remote() helper

["Harald Nordgren via GitGitGadget" <gitgitgadget@gmail.com>]

From: Harald Nordgren <haraldnordgren@gmail.com>

When a fetch has no refspec to work from, we decide what to fetch from
two sources: the remote's configured fetch refspec, and the current
branch's upstream, if it lives on that remote. For example, after

    git clone --single-branch <url>
    git switch -c work
    git push -u origin work

the remote's refspec only covers "main", but a refspec-less "git fetch
origin" still fetches "work" into FETCH_HEAD, so that a plain "git
pull" finds the branch to merge. See 49ca2fba39 (fetch: add
branch.*.merge to default ref-prefix extension, 2022-09-08).

That "does this branch build on this remote" check was inlined for
just the current branch. Move it into its own function,
collect_upstream_from_remote(), so the same logic can be reused for
branches other than just the current one.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
---
 builtin/fetch.c | 18 ++++++++----------
 remote.c        | 15 +++++++++++++++
 remote.h        |  9 +++++++++
 3 files changed, 32 insertions(+), 10 deletions(-)

diff --git a/builtin/fetch.c b/builtin/fetch.c
index 305934f5f4..31ec963db7 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1985,8 +1985,6 @@ static int do_fetch(struct transport *transport,
 	if (rs->nr) {
 		refspec_ref_prefixes(rs, &transport_ls_refs_options.ref_prefixes);
 	} else {
-		struct branch *branch = branch_get(NULL);
-
 		if (transport->remote->fetch.nr) {
 			refspec_ref_prefixes(&transport->remote->fetch,
 					     &transport_ls_refs_options.ref_prefixes);
@@ -2001,14 +1999,14 @@ static int do_fetch(struct transport *transport,
 			else
 				follow_remote_head = BUILTIN_FOLLOW_REMOTE_HEAD_DFLT;
 		}
-		if (branch && branch_has_merge_config(branch) &&
-		    !strcmp(branch->remote_name, transport->remote->name)) {
-			int i;
-			for (i = 0; i < branch->merge_nr; i++) {
-				strvec_push(&transport_ls_refs_options.ref_prefixes,
-					    branch->merge[i]->src);
-			}
-		}
+
+		/*
+		 * The configured refspec may not cover the current
+		 * branch's upstream (e.g. a narrowed -t refspec), so
+		 * make sure we can still fetch it regardless.
+		 */
+		collect_upstream_from_remote(the_repository, transport->remote,
+					      NULL, &transport_ls_refs_options.ref_prefixes);
 
 		/*
 		 * If there are no refs specified to fetch, then we just
diff --git a/remote.c b/remote.c
index 9acab2e01a..d4259fe7f3 100644
--- a/remote.c
+++ b/remote.c
@@ -1899,6 +1899,21 @@ int branch_merge_matches(struct branch *branch,
 	return refname_match(branch->merge[i]->src, refname);
 }
 
+void collect_upstream_from_remote(struct repository *repo,
+				   const struct remote *remote,
+				   const char *refname,
+				   struct strvec *tracked)
+{
+	struct branch *branch = repo_branch_get(repo, refname);
+
+	if (!branch_has_merge_config(branch) ||
+	    strcmp(branch->remote_name, remote->name))
+		return;
+
+	for (int i = 0; i < branch->merge_nr; i++)
+		strvec_push(tracked, branch->merge[i]->src);
+}
+
 __attribute__((format (printf,2,3)))
 static char *error_buf(struct strbuf *err, const char *fmt, ...)
 {
diff --git a/remote.h b/remote.h
index ac485a584d..003c8fd180 100644
--- a/remote.h
+++ b/remote.h
@@ -359,6 +359,15 @@ int branch_has_merge_config(struct branch *branch);
 
 int branch_merge_matches(struct branch *, int n, const char *);
 
+/*
+ * If refname's branch builds on remote, add its upstream on remote to
+ * tracked. A NULL refname means the current branch.
+ */
+void collect_upstream_from_remote(struct repository *repo,
+				   const struct remote *remote,
+				   const char *refname,
+				   struct strvec *tracked);
+
 /* list of the remote in a group as configured */
 struct remote_group_data {
 	const char *name;
-- 
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