gvfs-helper: show network advice for transient service errors (#880)

When gvfs_advice_on_retry() fires after exhausting retries, it
unconditionally suggests checking disk space or deleting the shared
object cache. This is misleading when the failures are transient network
or service errors (e.g. CURLE_RECV_ERROR, HTTP 429, HTTP 503) rather
than local filesystem problems like index-pack failures.

The current advice (delete your scalar cache and retry) can be even
worse under a service outage, such as recently occurred when the India
region's cache servers were unable to respond to incoming requests. The
users who followed this advice were not able to redownload data and were
stuck without an ability to work.

Teach gvfs_advice_on_retry() to distinguish between network/service
errors and local errors by inspecting the error code. When the error is
a curl error, HTTP 429, or HTTP 503, advise the user about potential
service outages or network connectivity issues instead. The existing
disk/cache advice is preserved for local failures such as index-pack
errors (GH__ERROR_CODE__INDEX_PACK_FAILED).

* [X] This change only applies to interactions with Azure DevOps and the
      GVFS Protocol.

Author: dscho

gvfs-helper.c

@@ -328,12 +328,15 @@ enum gh__error_code {
 	GH__ERROR_CODE__HTTP_503 = 6,
 	GH__ERROR_CODE__HTTP_OTHER = 7,
 	GH__ERROR_CODE__UNEXPECTED_CONTENT_TYPE = 8,
-	GH__ERROR_CODE__COULD_NOT_CREATE_TEMPFILE = 8,
-	GH__ERROR_CODE__COULD_NOT_INSTALL_LOOSE = 10,
-	GH__ERROR_CODE__COULD_NOT_INSTALL_PACKFILE = 11,
-	GH__ERROR_CODE__SUBPROCESS_SYNTAX = 12,
-	GH__ERROR_CODE__INDEX_PACK_FAILED = 13,
-	GH__ERROR_CODE__COULD_NOT_INSTALL_PREFETCH = 14,
+
+	GH__ERROR_CODE__HTTP_ERROR_LIMIT = 9,
+
+	GH__ERROR_CODE__COULD_NOT_CREATE_TEMPFILE = 10,
+	GH__ERROR_CODE__COULD_NOT_INSTALL_LOOSE = 11,
+	GH__ERROR_CODE__COULD_NOT_INSTALL_PACKFILE = 12,
+	GH__ERROR_CODE__SUBPROCESS_SYNTAX = 13,
+	GH__ERROR_CODE__INDEX_PACK_FAILED = 14,
+	GH__ERROR_CODE__COULD_NOT_INSTALL_PREFETCH = 15,
 };
 
 enum gh__cache_server_mode {
@@ -3142,14 +3145,23 @@ static int compute_transient_delay(int attempt)
 	return v;
 }
 
-static void gvfs_advice_on_retry(void)
+static void gvfs_advice_on_retry(enum gh__error_code ec)
 {
 	static int advice_given = 0;
 
 	if (advice_given)
 		return;
 	advice_given = 1;
 
+	if (ec < GH__ERROR_CODE__HTTP_ERROR_LIMIT) {
+		advise_if_enabled(ADVICE_GVFS_HELPER_TRANSIENT_RETRY,
+				  "GVFS Protocol network requests are failing. This is\n"
+				  "likely caused by a service outage or unstable network\n"
+				  "connection. Check your local network or contact your\n"
+				  "engineering systems team for assistance.");
+		return;
+	}
+
 	if (gvfs_shared_cache_pathname.len) {
 		advise_if_enabled(ADVICE_GVFS_HELPER_TRANSIENT_RETRY,
 				  "These retries may hint towards issues with your disk or\n"
@@ -3213,7 +3225,7 @@ static void do_req__with_robust_retry(const char *url_base,
 			/*
 			 * Give advice for common reasons this could happen:
 			 */
-			gvfs_advice_on_retry();
+			gvfs_advice_on_retry(status->ec);
 			params->k_transient_delay_sec =
 				compute_transient_delay(params->k_attempt);
 			continue;