Return error when TSM writer close returns one

The TSM writer uses a bufio.Writer that needs to be flushed before
it's closed.  If the flush fails for some reason, the error is not
handled by the defer and the compactor continues on as if all is good.
This can create files with truncated indexes or zero-length TSM files.

Fixes #5889

tsdb/engine/tsm1/compact.go

@@ -538,7 +538,7 @@ func (c *Compactor) writeNewFiles(generation, sequence int, iter KeyIterator) ([
 	return files, nil
 }
 
-func (c *Compactor) write(path string, iter KeyIterator) error {
+func (c *Compactor) write(path string, iter KeyIterator) (err error) {
 	if _, err := os.Stat(path); !os.IsNotExist(err) {
 		return fmt.Errorf("%v already file exists. aborting", path)
 	}
@@ -553,7 +553,12 @@ func (c *Compactor) write(path string, iter KeyIterator) error {
 	if err != nil {
 		return err
 	}
-	defer w.Close()
+	defer func() {
+		closeErr := w.Close()
+		if err == nil {
+			err = closeErr
+		}
+	}()
 
 	for iter.Next() {
 		select {
@@ -590,7 +595,6 @@ func (c *Compactor) write(path string, iter KeyIterator) error {
 	if err := w.WriteIndex(); err != nil {
 		return err
 	}
-
 	return nil
 }