gvfs-helper: don't fallback with new config

By default, GVFS Protocol-enabled Scalar clones will fall back to the
origin server if there is a network issue with the cache servers.
However (and especially for the prefetch endpoint) this may be a very
expensive operation for the origin server, leading to the user being
throttled. This shows up later in cases such as 'git push' or other web
operations.

To avoid this, create a new config option, 'gvfs.fallback', which
defaults to true. When set to 'false', pass '--no-fallback' from the
gvfs-helper client to the child gvfs-helper server process.

This will allow users who have hit this problem to avoid it in the
future. In case this becomes a more widespread problem, engineering
systems can enable the config option more broadly.

Enabling the config will of course lead to immediate failures for users,
but at least that will help diagnose the problem when it occurs instead
of later when the throttling shows up and the server load has already
passed, damage done.

Signed-off-by: Derrick Stolee <stolee@gmail.com>

Documentation/config/gvfs.txt

@@ -3,3 +3,8 @@ gvfs.cache-server::
 
 gvfs.sharedcache::
 	TODO
+
+gvfs.fallback::
+	If set to `false`, then never fallback to the origin server when the cache
+	server fails to connect. This will alert users to failures with the cache
+	server, but avoid causing throttling on the origin server.

gvfs-helper-client.c

@@ -14,6 +14,7 @@
 #include "quote.h"
 #include "packfile.h"
 #include "hex.h"
+#include "config.h"
 
 static struct oidset gh_client__oidset_queued = OIDSET_INIT;
 static unsigned long gh_client__oidset_count;
@@ -339,17 +340,25 @@ static struct gh_server__process *gh_client__find_long_running_process(
 	struct gh_server__process *entry;
 	struct strvec argv = STRVEC_INIT;
 	struct strbuf quoted = STRBUF_INIT;
+	int fallback;
 
 	gh_client__choose_odb();
 
 	/*
 	 * TODO decide what defaults we want.
 	 */
 	strvec_push(&argv, "gvfs-helper");
-	strvec_push(&argv, "--fallback");
 	strvec_push(&argv, "--cache-server=trust");
 	strvec_pushf(&argv, "--shared-cache=%s",
 			 gh_client__chosen_odb->path);
+
+	/* If gvfs.fallback=false, then don't add --fallback. */
+	if (!git_config_get_bool("gvfs.fallback", &fallback) &&
+	    !fallback)
+		strvec_push(&argv, "--no-fallback");
+	else
+		strvec_push(&argv, "--fallback");
+
 	strvec_push(&argv, "server");
 
 	sq_quote_argv_pretty(&quoted, argv.v);