cmd/uplink: better error handling for parallel transfer

Few improvements were made to how we are handling errors
while doing parallel upload/download for single object:
* unhide error under 'context canceled' which was shown in most of
cases
* add part number to error message
* don't try to commit if any error occurs while operation
* combine errors into more readable form, example:

---
failed to download part 3: uplink: eestream: failed to download stripe 0:
error retrieving piece 00: ecclient: piecestore: rpc: tcp connector failed: rpc: dial tcp 97.119.158.36:28967: i/o timeout
...
error retrieving piece 89: ecclient: piecestore: rpc: tcp connector failed: rpc: dial tcp 161.129.152.194:28967: i/o timeout
failed to download part 1: uplink: eestream: failed to download stripe 0:
error retrieving piece 01: io: read/write on closed pipe
...
error retrieving piece 97: io: read/write on closed pipe
failed to download part 2: uplink: eestream: failed to download stripe 0:
error retrieving piece 00: io: read/write on closed pipe
...
error retrieving piece 01: ecclient: piecestore: rpc: tcp connector failed: rpc: dial tcp 180.183.132.234:28967: operation was canceled
error retrieving piece 96: io: read/write on closed pipe
	main.(*cmdCp).parallelCopy:418
	main.(*cmdCp).copyFile:262
	main.(*cmdCp).Execute:156
	main.(*external).Wrap:123
	github.com/zeebo/clingy.(*Environment).dispatchDesc:126
	github.com/zeebo/clingy.(*Environment).dispatch:53
	github.com/zeebo/clingy.Environment.Run:34
	main.main:26
	runtime.main:250
---

Change-Id: I9bb70b3f754567761fa8d17bef8ef59b0709e33b

cmd/uplink/cmd_cp.go

@@ -259,7 +259,7 @@ func (c *cmdCp) copyFile(ctx clingy.Context, fs ulfs.Filesystem, source, dest ul
 		return err
 	}
 
-	return errs.Wrap(parallelCopy(
+	return errs.Wrap(c.parallelCopy(
 		ctx,
 		mwh, mrh,
 		c.parallelism, partSize,
@@ -308,7 +308,7 @@ func joinDestWith(dest ulloc.Location, suffix string) ulloc.Location {
 	return dest
 }
 
-func parallelCopy(
+func (c *cmdCp) parallelCopy(
 	clctx clingy.Context,
 	dst ulfs.MultiWriteHandle,
 	src ulfs.MultiReadHandle,
@@ -396,26 +396,25 @@ func parallelCopy(
 				// to have concurrent safe API with that regards.
 				//
 				// Also, we may want to check that it actually helps, before implementing it.
-			}
-
-			mu.Lock()
-			defer mu.Unlock()
 
-			es.Add(err)
+				mu.Lock()
+				es.Add(errs.New("failed to %s part %d: %v", copyVerb(c.source, c.dest), i, err))
+				mu.Unlock()
+			}
 		})
 		if !ok {
-			mu.Lock()
-			fmt.Fprintln(clctx.Stderr(), "Failed to start part copy", i)
-			mu.Unlock()
-			return ctx.Err()
+			break
 		}
 	}
 
 	limiter.Wait()
 
-	es.Add(dst.Commit(ctx))
+	// don't try to commit if any error occur
+	if len(es) == 0 {
+		es.Add(dst.Commit(ctx))
+	}
 
-	return es.Err()
+	return errs.Wrap(combineErrs(es))
 }
 
 func parseRange(r string) (offset, length int64, err error) {
@@ -466,3 +465,17 @@ func parseRange(r string) (offset, length int64, err error) {
 		return starti, endi - starti + 1, nil
 	}
 }
+
+// combineErrs makes a more readable error message from the errors group.
+func combineErrs(group errs.Group) error {
+	if len(group) == 0 {
+		return nil
+	}
+
+	errstrings := make([]string, len(group))
+	for i, err := range group {
+		errstrings[i] = err.Error()
+	}
+
+	return fmt.Errorf("%s", strings.Join(errstrings, "\n"))
+}