Fix tests by only flushing gzip writer once

Closing the gzip writer causes a flush, and doesn't actually close the
underlying file, so we can switch to closing the gzip writer before
closing the underlying file, instead of using Flush(). The previous
approach of Flushing and then Closing caused us to flush twice, which
had the side-effect of causing the underlying file to have an additional 5
bytes of data that was not expected in our tests.

Signed-off-by: Chance Zibolski <chance.zibolski@gmail.com>

lumberjack.go

@@ -507,8 +507,12 @@ func compressLogFile(src, dst string) (err error) {
 		return err
 	}
 
-	// Flush the gz writer before Sync()ing
-	if err := gz.Flush(); err != nil {
+	// Close the gzip writer.
+	// Closing also triggers a Flush to the underlying
+	// io.Writer, and doesnot close the underlying io.Writer.
+	// We must Close() or Flush() the gz writer before Sync()ing otherwise we may
+	// see partially written data and a corrupt gzip archive.
+	if err := gz.Close(); err != nil {
 		return err
 	}
 
@@ -517,11 +521,6 @@ func compressLogFile(src, dst string) (err error) {
 		return err
 	}
 
-	// Close the gzip writer
-	if err := gz.Close(); err != nil {
-		return err
-	}
-
 	// close the underlying gzip file
 	if err := gzf.Close(); err != nil {
 		return err