Check error on Close() and Flush() in Backup.

We are probably doing a bad job of checking these errors in a lot of
places, but it's particularly important to get it right for backups.

We don't worry about Close() on the Reader side.

When using an AllErrorRecorder, we don't have to worry about hiding the
Write error with a Close error. But we do need to make sure not to call
Close as part of saving the arguments of a defer statement.

go/vt/mysqlctl/backup.go

@@ -297,8 +297,7 @@ func backup(mysqld MysqlDaemon, logger logutil.Logger, bh backupstorage.BackupHa
 	return nil
 }
 
-func backupFiles(mysqld MysqlDaemon, logger logutil.Logger, bh backupstorage.BackupHandle, fes []FileEntry, replicationPosition proto.ReplicationPosition, backupConcurrency int) error {
-
+func backupFiles(mysqld MysqlDaemon, logger logutil.Logger, bh backupstorage.BackupHandle, fes []FileEntry, replicationPosition proto.ReplicationPosition, backupConcurrency int) (err error) {
 	sema := sync2.NewSemaphore(backupConcurrency, 0)
 	rec := concurrency.AllErrorRecorder{}
 	wg := sync.WaitGroup{}
@@ -330,7 +329,7 @@ func backupFiles(mysqld MysqlDaemon, logger logutil.Logger, bh backupstorage.Bac
 				rec.RecordError(fmt.Errorf("cannot add file: %v", err))
 				return
 			}
-			defer wc.Close()
+			defer func() { rec.RecordError(wc.Close()) }()
 			dst := bufio.NewWriterSize(wc, 2*1024*1024)
 
 			// create the hasher and the tee on top
@@ -358,7 +357,7 @@ func backupFiles(mysqld MysqlDaemon, logger logutil.Logger, bh backupstorage.Bac
 			}
 
 			// flush the buffer to finish writing, save the hash
-			dst.Flush()
+			rec.RecordError(dst.Flush())
 			fes[i].Hash = hasher.HashString()
 		}(i, fe)
 	}
@@ -373,7 +372,11 @@ func backupFiles(mysqld MysqlDaemon, logger logutil.Logger, bh backupstorage.Bac
 	if err != nil {
 		return fmt.Errorf("cannot add %v to backup: %v", backupManifest, err)
 	}
-	defer wc.Close()
+	defer func() {
+		if closeErr := wc.Close(); err == nil {
+			err = closeErr
+		}
+	}()
 
 	// JSON-encode and write the MANIFEST
 	bm := &BackupManifest{
@@ -450,7 +453,7 @@ func restoreFiles(cnf *Mycnf, bh backupstorage.BackupHandle, fes []FileEntry, re
 				rec.RecordError(err)
 				return
 			}
-			defer dstFile.Close()
+			defer func() { rec.RecordError(dstFile.Close()) }()
 
 			// create a buffering output
 			dst := bufio.NewWriterSize(dstFile, 2*1024*1024)
@@ -468,7 +471,7 @@ func restoreFiles(cnf *Mycnf, bh backupstorage.BackupHandle, fes []FileEntry, re
 				rec.RecordError(err)
 				return
 			}
-			defer gz.Close()
+			defer func() { rec.RecordError(gz.Close()) }()
 
 			// copy the data. Will also write to the hasher
 			if _, err = io.Copy(dst, gz); err != nil {
@@ -484,7 +487,7 @@ func restoreFiles(cnf *Mycnf, bh backupstorage.BackupHandle, fes []FileEntry, re
 			}
 
 			// flush the buffer
-			dst.Flush()
+			rec.RecordError(dst.Flush())
 		}(i, fe)
 	}
 	wg.Wait()