cmd/dist: add -json flag

This enables JSON output for all tests run by dist.

Most the complexity here is that, in order to disambiguate JSON
records from different package variants, we have to rewrite the JSON
stream on the fly to include variant information. We do this by
rewriting the Package field to be pkg:variant so existing CI systems
will naturally pick up the disambiguated test name.

Fixes #37486.

Change-Id: I0094e5e27b3a02ffc108534b8258c699ed8c3b87
Reviewed-on: https://go-review.googlesource.com/c/go/+/494958
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

1 files changed, 86 insertions, 0 deletions

diff --git a/src/cmd/dist/testjson_test.go b/src/cmd/dist/testjson_test.go
new file mode 100644
index 0000000000..2ff7bf61f5
--- /dev/null
+++ b/src/cmd/dist/testjson_test.go
@@ -0,0 +1,86 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestJSONFilterRewritePackage(t *testing.T) {
+	const in = `{"Package":"abc"}
+{"Field1":"1","Package":"abc","Field3":"3"}
+{"Package":123}
+{}
+{"Package":"abc","Unexpected":[null,true,false,99999999999999999999]}
+`
+	want := strings.ReplaceAll(in, `"Package":"abc"`, `"Package":"abc:variant"`)
+
+	checkJSONFilter(t, in, want)
+}
+
+func TestJSONFilterMalformed(t *testing.T) {
+	const in = `unexpected text
+{"Package":"abc"}
+more text
+{"Package":"abc"}trailing text
+{not json}
+`
+	const want = `unexpected text
+{"Package":"abc:variant"}
+more text
+{"Package":"abc:variant"}
+{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)
+}
+
+func TestJSONFilterBoundaries(t *testing.T) {
+	const in = `{"Package":"abc"}
+{"Package":"def"}
+{"Package":"ghi"}
+`
+	want := strings.ReplaceAll(in, `"}`, `:variant"}`)
+
+	// Write one bytes at a time.
+	t.Run("bytes", func(t *testing.T) {
+		checkJSONFilterWith(t, want, func(f *testJSONFilter) {
+			for i := 0; i < len(in); i++ {
+				f.Write([]byte{in[i]})
+			}
+		})
+	})
+	// Write a block containing a whole line bordered by two partial lines.
+	t.Run("bytes", func(t *testing.T) {
+		checkJSONFilterWith(t, want, func(f *testJSONFilter) {
+			const b1 = 5
+			const b2 = len(in) - 5
+			f.Write([]byte(in[:b1]))
+			f.Write([]byte(in[b1:b2]))
+			f.Write([]byte(in[b2:]))
+		})
+	})
+}
+
+func checkJSONFilter(t *testing.T, in, want string) {
+	t.Helper()
+	checkJSONFilterWith(t, want, func(f *testJSONFilter) {
+		f.Write([]byte(in))
+	})
+}
+
+func checkJSONFilterWith(t *testing.T, want string, write func(*testJSONFilter)) {
+	t.Helper()
+
+	out := new(strings.Builder)
+	f := &testJSONFilter{w: out, variant: "variant"}
+	write(f)
+	got := out.String()
+	if want != got {
+		t.Errorf("want:\n%s\ngot:\n%s", want, got)
+	}
+}