cmd/dist: fix JSON processing of trailing bytes

The testJSONFilter.process was implicitly depending
on the exact internal buffer size of the json.Decoder,
which happened to be large enough in v1 for most lines
that cmd/dist cared about.

However, the re-implementation of v1 using encoding/json/v2
changes the implicit buffer size used, which results in
some trailing bytes accidentally being discarded.

The correct use of json.Decoder.Buffered requires also
consulting the input io.Reader for additional trailing data.

Fixes #79498

Change-Id: If4bdac12b638d78e0870808f2285fb1731b0fae6
Reviewed-on: https://go-review.googlesource.com/c/go/+/779980
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/dist/testjson.go

@@ -80,7 +80,8 @@ func (f *testJSONFilter) process(line []byte) {
 		// struct, or other additions outside of it. If humans are ever looking
 		// at the output, it's really nice to keep field order because it
 		// preserves a lot of regularity in the output.
-		dec := json.NewDecoder(bytes.NewBuffer(line))
+		lineBuf := bytes.NewBuffer(line)
+		dec := json.NewDecoder(lineBuf)
 		dec.UseNumber()
 		val, err := decodeJSONValue(dec)
 		if err == nil && val.atom == json.Delim('{') {
@@ -105,6 +106,7 @@ func (f *testJSONFilter) process(line []byte) {
 				// Copy any trailing text. We expect at most a "\n" here, but
 				// there could be other text and we want to feed that through.
 				io.Copy(f.w, dec.Buffered())
+				io.Copy(f.w, lineBuf)
 				return
 			}
 		}