sync,march: check the cancel context on every channel send and receive

This fixes a deadlock on sync when all the copying channels receive a
Fatal Error.

fs/march/march.go

@@ -146,7 +146,12 @@ func (m *March) Run() {
 							// Now we have traversed this directory, send these
 							// jobs off for traversal in the background
 							for _, newJob := range jobs {
-								in <- newJob
+								select {
+								case <-m.ctx.Done():
+									// discard job if finishing
+									traversing.Done()
+								case in <- newJob:
+								}
 							}
 						}()
 					}
@@ -164,6 +169,13 @@ func (m *March) Run() {
 		dstRemote: m.dir,
 		dstDepth:  dstDepth - 1,
 	}
+	go func() {
+		// when the context is cancelled discard the remaining jobs
+		<-m.ctx.Done()
+		for range in {
+			traversing.Done()
+		}
+	}()
 	traversing.Wait()
 	close(in)
 	wg.Wait()