[PATCH v2] promisor-remote: fix segfault when remote URL is missing

[Christian Couder <christian.couder@gmail.com>]

Using strvec_push() to push `NULL` into a 'strvec' results in a
segfault, because `xstrdup(NULL)` crashes.

So when an URL is missing from the config, let's push an empty string
instead of `NULL` into the 'strvec' that stores URLs.

We could have modified strvec_push() to behave like
strvec_push_nodup() and accept `NULL`, but it's not clear that it's
the right thing to do for the strvec API. 'strvec' is a kind of NULL
terminated array that is designed to be compatible with 'argv'
variables used on the command line. So we might want to disallow
pushing any `NULL` in it instead.

It's also not clear if `xstrdup(NULL)` should crash or BUG or just
return NULL.

For all these reasons, let's just focus on fixing the issue in
"promisor-remote.c" and let's leave improving the strvec API and/or
xstrdup() for a future effort.

While at it let's warn and reject the remote, in the 'KnownUrl'
case, when no URL is advertised by the server or no URL is
configured on the client for a remote name advertised by the server
and configured on the client. This is on par with a warning already
emitted when URLs are different in the same case.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
---
 promisor-remote.c                     | 20 +++++++++++---
 t/t5710-promisor-remote-capability.sh | 39 +++++++++++++++++++++++++++
 2 files changed, 56 insertions(+), 3 deletions(-)

diff --git a/promisor-remote.c b/promisor-remote.c
index 6a0a61382f..92786d72b4 100644
--- a/promisor-remote.c
+++ b/promisor-remote.c
@@ -323,11 +323,15 @@ static void promisor_info_vecs(struct repository *repo,
 	promisor_remote_init(repo);
 
 	for (r = repo->promisor_remote_config->promisors; r; r = r->next) {
-		char *url;
+		char *url = NULL;
+		const char *url_pushed = "";
 		char *url_key = xstrfmt("remote.%s.url", r->name);
 
+		if (!git_config_get_string(url_key, &url) && url)
+			url_pushed = url;
+
 		strvec_push(names, r->name);
-		strvec_push(urls, git_config_get_string(url_key, &url) ? NULL : url);
+		strvec_push(urls, url_pushed);
 
 		free(url);
 		free(url_key);
@@ -356,7 +360,7 @@ char *promisor_remote_info(struct repository *repo)
 			strbuf_addch(&sb, ';');
 		strbuf_addstr(&sb, "name=");
 		strbuf_addstr_urlencode(&sb, names.v[i], allow_unsanitized);
-		if (urls.v[i]) {
+		if (*urls.v[i]) {
 			strbuf_addstr(&sb, ",url=");
 			strbuf_addstr_urlencode(&sb, urls.v[i], allow_unsanitized);
 		}
@@ -409,6 +413,16 @@ static int should_accept_remote(enum accept_promisor accept,
 	if (accept != ACCEPT_KNOWN_URL)
 		BUG("Unhandled 'enum accept_promisor' value '%d'", accept);
 
+	if (!remote_url) {
+		warning(_("no URL advertised for remote '%s'"), remote_name);
+		return 0;
+	}
+
+	if (!*urls->v[i]) {
+		warning(_("no URL configured for remote '%s'"), remote_name);
+		return 0;
+	}
+
 	if (!strcmp(urls->v[i], remote_url))
 		return 1;
 
diff --git a/t/t5710-promisor-remote-capability.sh b/t/t5710-promisor-remote-capability.sh
index d2cc69a17e..23203814d6 100755
--- a/t/t5710-promisor-remote-capability.sh
+++ b/t/t5710-promisor-remote-capability.sh
@@ -193,6 +193,25 @@ test_expect_success "clone with 'KnownName' and different remote names" '
 	initialize_server 1 "$oid"
 '
 
+test_expect_success "clone with 'KnownName' and missing URL in the config" '
+	git -C server config promisor.advertise true &&
+
+	# Clone from server to create a client
+	# Lazy fetching by the client from the LOP will fail because of the
+	# missing URL in the client config, so the server will have to lazy
+	# fetch from the LOP.
+	GIT_NO_LAZY_FETCH=0 git clone -c remote.lop.promisor=true \
+		-c promisor.acceptfromserver=KnownName \
+		--no-local --filter="blob:limit=5k" server client &&
+	test_when_finished "rm -rf client" &&
+
+	# Check that the largest object is not missing on the server
+	check_missing_objects server 0 "" &&
+
+	# Reinitialize server so that the largest object is missing again
+	initialize_server 1 "$oid"
+'
+
 test_expect_success "clone with promisor.acceptfromserver set to 'KnownUrl'" '
 	git -C server config promisor.advertise true &&
 
@@ -228,6 +247,26 @@ test_expect_success "clone with 'KnownUrl' and different remote urls" '
 	initialize_server 1 "$oid"
 '
 
+test_expect_success "clone with 'KnownUrl' and url not advertised" '
+	git -C server config promisor.advertise true &&
+
+	git -C server config unset remote.lop.url &&
+	test_when_finished "git -C server config set remote.lop.url \"file://$(pwd)/lop\"" &&
+
+	# Clone from server to create a client
+	# It should fail because the client will reject the LOP
+	# as URLs are different, and the server cannot lazy fetch as
+	# the LOP URL is missing.
+	GIT_NO_LAZY_FETCH=0 test_must_fail git clone -c remote.lop.promisor=true \
+		-c remote.lop.fetch="+refs/heads/*:refs/remotes/lop/*" \
+		-c remote.lop.url="file://$(pwd)/lop" \
+		-c promisor.acceptfromserver=KnownUrl \
+		--no-local --filter="blob:limit=5k" server client &&
+
+	# Check that the largest object is still missing on the server
+	check_missing_objects server 1 "$oid"
+'
+
 test_expect_success "clone with promisor.advertise set to 'true' but don't delete the client" '
 	git -C server config promisor.advertise true &&
 
-- 
2.49.0.rc2.1.gb8a6f6852f