ostree-fetcher-curl: add ENV VAR to retry requests

This introduces the OSTREE_CURL_IO_RETRY environment
variable that enables retries on requests that fail
except when they fail with NOT_FOUND. We accomplish
this by returning G_IO_ERROR_TIMED_OUT instead of
G_IO_ERROR_FAILED as error code.

src/libostree/ostree-fetcher-curl.c

@@ -304,6 +304,7 @@ check_multi_info (OstreeFetcher *fetcher)
       const char *eff_url;
       gboolean is_file;
       gboolean continued_request = FALSE;
+      gboolean should_retry;
 
       if (msg->msg != CURLMSG_DONE)
         continue;
@@ -330,7 +331,14 @@ check_multi_info (OstreeFetcher *fetcher)
             }
           else
             {
-              g_task_return_new_error (task, G_IO_ERROR, G_IO_ERROR_FAILED,
+              /* When it is not a file and the OSTREE_CURL_IO_RETRY
+               * environment variable is set, we want to retry the request.
+               * We accomplish that by using G_IO_ERROR_TIMED_OUT.
+               */
+              should_retry = g_getenv ("OSTREE_CURL_IO_RETRY") != NULL;
+              int g_io_error_code
+                  = (is_file || !should_retry) ? G_IO_ERROR_FAILED : G_IO_ERROR_TIMED_OUT;
+              g_task_return_new_error (task, G_IO_ERROR, g_io_error_code,
                                        "While fetching %s: [%u] %s", eff_url, curlres,
                                        curl_easy_strerror (curlres));
               _ostree_fetcher_journal_failure (req->fetcher->remote_name, eff_url,