Pseudorefs are not stored in the ref database as by definition, they
carry additional metadata that essentially makes them not a ref. As
such, writing pseudorefs via the ref backend does not make any sense
whatsoever as the ref backend wouldn't know how exactly to store the
data.

Restrict writing pseudorefs via the ref backend.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs.c           | 7 +++++++
 t/t5510-fetch.sh | 6 +++---
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/refs.c b/refs.c
index 50d679b7e7..7c3c7465a4 100644
--- a/refs.c
+++ b/refs.c
@@ -1307,6 +1307,13 @@ 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);
 
diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
index 33d34d5ae9..4eb569f4df 100755
--- a/t/t5510-fetch.sh
+++ b/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)"
-- 
2.45.0