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)
+	}
+}