client: fix overly aggressive project-wide file transfer backoff policy.

Old: if we get more than 3 upload or download failures for a project,
do a backoff for each failure.

Problem: if we start a bunch of transfers (say the N output files of a job)
and they all fail, we back off for too long, e.g.:
try N uploads
back off 2^N minutes
try N uploads
back off 4^N minutes
...

New: on a failure, ignore it if we're already backed off.
So the behavior should be something like:
try N uploads
back off 1 minute
try N uploads
back off 2 minutes
...

client/client_types.cpp

@@ -70,24 +70,37 @@ bool FILE_XFER_BACKOFF::ok_to_transfer() {
     return (dt <= 0);
 }
 
+// A transfer has failed.
+// Back off transfers (project-wide) if needed.
+//
 void FILE_XFER_BACKOFF::file_xfer_failed(PROJECT* p) {
+    // If we're already backed off, ignore this failure.
+    // If we start several transfers at once
+    // (say, N output files of a job) and they all fail,
+    // we don't want to back off N times, which could be hours.
+    //
+    if (gstate.now < next_xfer_time) {
+        return;
+    }
+
     file_xfer_failures++;
     if (file_xfer_failures < FILE_XFER_FAILURE_LIMIT) {
         next_xfer_time = 0;
-    } else {
+        return;
-        double backoff = calculate_exponential_backoff(
-            file_xfer_failures,
-            gstate.pers_retry_delay_min,
-            gstate.pers_retry_delay_max
-        );
-        if (log_flags.file_xfer_debug) {
-            msg_printf(p, MSG_INFO,
-                "[file_xfer] project-wide xfer delay for %f sec",
-                backoff
-            );
-        }
-        next_xfer_time = gstate.now + backoff;
     }
+    double backoff = calculate_exponential_backoff(
+        file_xfer_failures,
+        gstate.pers_retry_delay_min,
+        gstate.pers_retry_delay_max
+    );
+    if (log_flags.file_xfer_debug) {
+        msg_printf(p, MSG_INFO,
+            "[file_xfer] project-wide %s delay for %f sec",
+            is_upload?"upload":"download",
+            backoff
+        );
+    }
+    next_xfer_time = gstate.now + backoff;
 }
 
 void FILE_XFER_BACKOFF::file_xfer_succeeded() {

client/client_types.h

@@ -188,18 +188,25 @@ struct FILE_REF {
     int write(MIOFILE&);
 };
 
-// file xfer backoff state for a project and direction (up/down)
+// File xfer backoff state for a project and direction (up/down).
-// if file_xfer_failures exceeds FILE_XFER_FAILURE_LIMIT,
+// If we get more than FILE_XFER_FAILURE_LIMIT (3) consecutive failures,
 // we switch from a per-file to a project-wide backoff policy
 // (separately for the up/down directions)
+// E.g. if we have 100 files to upload and the first 3 fail,
+// we don't try the other 97 immediately.
+//
 // NOTE: this refers to transient failures, not permanent.
 //
+
 #define FILE_XFER_FAILURE_LIMIT 3
+
 struct FILE_XFER_BACKOFF {
     int file_xfer_failures;
         // count of consecutive failures
     double next_xfer_time;
         // when to start trying again
+    bool is_upload;
+
     bool ok_to_transfer();
     void file_xfer_failed(PROJECT*);
     void file_xfer_succeeded();

client/project.cpp

@@ -125,6 +125,8 @@ void PROJECT::init() {
     gpu_ec = 0;
     gpu_time = 0;
     app_configs.clear();
+    upload_backoff.is_upload = true;
+    download_backoff.is_upload = false;
 
 #ifdef SIM
     idle_time = 0;