batches: skip empty log output

CHANGELOG.md

@@ -24,6 +24,7 @@ All notable changes to `src-cli` are documented in this file.
 
 - Fix network timeout in `src users clean` occuring in instances with many users  [#901](https://github.com/sourcegraph/src-cli/pull/901)
 - Aligned parsing of spec file parameter of `src batch repos` with other commands. [#919](https://github.com/sourcegraph/src-cli/pull/919)
+- batches: Remove empty log outputs. [#923](https://github.com/sourcegraph/src-cli/pull/923)
 
 ### Removed
 

internal/batches/ui/interval_writer.go

@@ -116,15 +116,17 @@ type prefixedWriter struct {
 	prefix     string
 }
 
-func (w *prefixedWriter) Write(p []byte) (int, error) {
-	var prefixedLines []byte
-	for _, line := range bytes.Split(p, []byte("\n")) {
-		prefixedLine := append([]byte(w.prefix), line...)
-		prefixedLine = append(prefixedLine, []byte("\n")...)
+var newLineByteSlice = []byte("\n")
 
-		prefixedLines = append(prefixedLines, prefixedLine...)
+// Write is only ever called with a single line. That line may or may not end with a newline character.
+// It then writes the content as a single line to the inner writer, regardless of if the provided line
+// had a newline or not. That is, because our encoding requires exactly one line per formatted line.
+func (w *prefixedWriter) Write(p []byte) (int, error) {
+	prefixedLine := append([]byte(w.prefix), p...)
+	if !bytes.HasSuffix(prefixedLine, newLineByteSlice) {
+		prefixedLine = append(prefixedLine, newLineByteSlice...)
 	}
-	w.writes <- prefixedLines
+	w.writes <- prefixedLine
 	<-w.writesDone
 	return len(p), nil
 }

internal/batches/ui/interval_writer_test.go

@@ -50,6 +50,10 @@ func TestIntervalWriter(t *testing.T) {
 	stderrWriter.Write([]byte("4"))
 	stdoutWriter.Write([]byte("5"))
 	stderrWriter.Write([]byte("5"))
+	stdoutWriter.Write([]byte(`Hello world: 1
+`))
+	stderrWriter.Write([]byte(`Hello world: 1
+`))
 
 	select {
 	case <-ch:
@@ -65,10 +69,11 @@ func TestIntervalWriter(t *testing.T) {
 		want := "stdout: 2\nstderr: 2\n" +
 			"stdout: 3\nstderr: 3\n" +
 			"stdout: 4\nstderr: 4\n" +
-			"stdout: 5\nstderr: 5\n"
+			"stdout: 5\nstderr: 5\n" +
+			"stdout: Hello world: 1\nstderr: Hello world: 1\n"
 
 		if d != want {
-			t.Fatalf("wrong data in sink. want")
+			t.Fatalf("wrong data in sink. want=%q, have=%q", want, d)
 		}
 	case <-time.After(1 * time.Second):
 		t.Fatalf("ch has NO data")