negotiator: unknown fetch.negotiationAlgorithm should error out

Change the handling of fetch.negotiationAlgorithm=<str> to error out
on unknown strings, i.e. everything except "default" or "skipping".

This changes the behavior added in 42cc7485a2 ("negotiator/skipping:
skip commits during fetch", 2018-07-16) which would ignore all unknown
values and silently fall back to the "default" value.

For a feature like this it's much better to produce an error than
proceed. We don't want users to debug some amazingly slow fetch that
should benefit from "skipping", only to find that they'd forgotten to
deploy the new git version on that particular machine.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1496,9 +1496,10 @@ fetch.negotiationAlgorithm::
 	sent when negotiating the contents of the packfile to be sent by the
 	server. Set to "skipping" to use an algorithm that skips commits in an
 	effort to converge faster, but may result in a larger-than-necessary
-	packfile; any other value instructs Git to use the default algorithm
+	packfile; The default is "default" which instructs Git to use the default algorithm
 	that never skips commits (unless the server has acknowledged it or one
 	of its descendants).
+	Unknown values will cause 'git fetch' to error out.
 
 format.attach::
 	Enable multipart/mixed attachments as the default for

fetch-negotiator.c

@@ -6,9 +6,15 @@
 void fetch_negotiator_init(struct fetch_negotiator *negotiator,
 			   const char *algorithm)
 {
-	if (algorithm && !strcmp(algorithm, "skipping")) {
+	if (algorithm) {
+		if (!strcmp(algorithm, "skipping")) {
 			skipping_negotiator_init(negotiator);
 			return;
+		} else if (!strcmp(algorithm, "default")) {
+			/* Fall through to default initialization */
+		} else {
+			die("unknown fetch negotiation algorithm '%s'", algorithm);
+		}
 	}
 	default_negotiator_init(negotiator);
 }

t/t5552-skipping-fetch-negotiator.sh

@@ -47,6 +47,29 @@ test_expect_success 'commits with no parents are sent regardless of skip distanc
 	have_not_sent c6 c4 c3
 '
 
+test_expect_success 'unknown fetch.negotiationAlgorithm values error out' '
+	rm -rf server client trace &&
+	git init server &&
+	test_commit -C server to_fetch &&
+
+	git init client &&
+	test_commit -C client on_client &&
+	git -C client checkout on_client &&
+
+	test_config -C client fetch.negotiationAlgorithm invalid &&
+	test_must_fail git -C client fetch "$(pwd)/server" 2>err &&
+	test_i18ngrep "unknown fetch negotiation algorithm" err &&
+
+	# Explicit "default" value
+	test_config -C client fetch.negotiationAlgorithm default &&
+	git -C client -c fetch.negotiationAlgorithm=default fetch "$(pwd)/server" &&
+
+	# Implementation detail: If there is nothing to fetch, we will not error out
+	test_config -C client fetch.negotiationAlgorithm invalid &&
+	git -C client fetch "$(pwd)/server" 2>err &&
+	test_i18ngrep ! "unknown fetch negotiation algorithm" err
+'
+
 test_expect_success 'when two skips collide, favor the larger one' '
 	rm -rf server client trace &&
 	git init server &&