fetch: do not override partial clone filter

When a fetch with the --filter argument is made, the configured default
filter is set even if one already exists. This change was made in
5e46139 ("builtin/fetch: remove unique promisor remote limitation",
2019-06-25) - in particular, changing from:

 * If this is the FIRST partial-fetch request, we enable partial
 * on this repo and remember the given filter-spec as the default
 * for subsequent fetches to this remote.

to:

 * If this is a partial-fetch request, we enable partial on
 * this repo if not already enabled and remember the given
 * filter-spec as the default for subsequent fetches to this
 * remote.

(The given filter-spec is "remembered" even if there is already an
existing one.)

This is problematic whenever a lazy fetch is made, because lazy fetches
are made using "git fetch --filter=blob:none", but this will also happen
if the user invokes "git fetch --filter=<filter>" manually. Therefore,
restore the behavior prior to 5e46139, which writes a filter-spec
only if the current fetch request is the first partial-fetch one (for
that remote).

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -1677,7 +1677,7 @@ static inline void fetch_one_setup_partial(struct remote *remote)
 	 * If this is a partial-fetch request, we enable partial on
 	 * this repo if not already enabled and remember the given
 	 * filter-spec as the default for subsequent fetches to this
-	 * remote.
+	 * remote if there is currently no default filter-spec.
 	 */
 	if (filter_options.choice) {
 		partial_clone_register(remote->name, &filter_options);

list-objects-filter-options.c

@@ -344,11 +344,19 @@ void partial_clone_register(
 	const char *remote,
 	struct list_objects_filter_options *filter_options)
 {
+	struct promisor_remote *promisor_remote;
 	char *cfg_name;
 	char *filter_name;
 
 	/* Check if it is already registered */
-	if (!promisor_remote_find(remote)) {
+	if ((promisor_remote = promisor_remote_find(remote))) {
+		if (promisor_remote->partial_clone_filter)
+			/*
+			 * Remote is already registered and a filter is already
+			 * set, so we don't need to do anything here.
+			 */
+			return;
+	} else {
 		if (upgrade_repository_format(1) < 0)
 			die(_("unable to upgrade repository format to support partial clone"));
 

t/t5601-clone.sh

@@ -669,7 +669,8 @@ test_expect_success 'partial clone' '
 
 test_expect_success 'partial clone with -o' '
 	partial_clone_server server &&
-	git clone -o blah --filter=blob:limit=0 "file://$(pwd)/server" client
+	git clone -o blah --filter=blob:limit=0 "file://$(pwd)/server" client &&
+	test_cmp_config -C client "blob:limit=0" --get-all remote.blah.partialclonefilter
 '
 
 test_expect_success 'partial clone: warn if server does not support object filtering' '