cmd/dist: tweaks to -json mode

These are some follow-up tweaks to CL 494958. This CL drops a stale
and unnecessary check and passes through trailing non-JSON output.

Updates #37486.

Change-Id: I7cdb73a103f9cd49767d5491812d5ad011ee5c14
Reviewed-on: https://go-review.googlesource.com/c/go/+/496297
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Austin Clements <austin@google.com>

src/cmd/dist/test.go

@@ -349,9 +349,7 @@ func (opts *goTest) bgCommand(t *tester, stdout, stderr io.Writer) *exec.Cmd {
 		// Rewrite Package in the JSON output to be pkg:variant. For sharded
 		// variants, pkg.TestName is already unambiguous, so we don't need to
 		// rewrite the Package field.
-		if len(opts.pkgs) != 0 {
-			panic("cannot combine multiple packages with variants")
-		}
+		//
 		// We only want to process JSON on the child's stdout. Ideally if
 		// stdout==stderr, we would also use the same testJSONFilter for
 		// cmd.Stdout and cmd.Stderr in order to keep the underlying

src/cmd/dist/testjson.go

@@ -92,7 +92,10 @@ func (f *testJSONFilter) process(line []byte) {
 					// Should never happen.
 					panic(fmt.Sprintf("failed to round-trip JSON %q: %s", string(line), err))
 				}
-				data = append(data, '\n')
+				// Copy any trailing text. We expect at most a "\n" here, but
+				// there could be other text and we want to feed that through.
+				extra, _ := io.ReadAll(dec.Buffered())
+				data = append(data, extra...)
 				f.w.Write(data)
 				return
 			}

src/cmd/dist/testjson_test.go

@@ -31,11 +31,9 @@ more text
 	const want = `unexpected text
 {"Package":"abc:variant"}
 more text
-{"Package":"abc:variant"}
+{"Package":"abc:variant"}trailing text
 {not json}
 `
-	// Note that currently we won't round-trip trailing text after a valid JSON
-	// line. That might be a mistake.
 	checkJSONFilter(t, in, want)
 }