Measure delivery output and version costs

internal/gotool/performance_test.go

@@ -0,0 +1,38 @@
+package gotool
+
+import (
+	"bytes"
+	"io"
+	"testing"
+)
+
+const performanceDiagnosticSize = 8 << 20
+
+func BenchmarkPerformanceCombinedOutput(b *testing.B) {
+	for _, testCase := range []struct {
+		name string
+		size int
+	}{
+		{name: "modest", size: 4 << 10},
+		{name: "diagnostic-8MiB", size: performanceDiagnosticSize},
+	} {
+		input := bytes.Repeat([]byte("go tool diagnostic output\n"), (testCase.size+25)/26)
+		input = input[:testCase.size]
+		b.Run(testCase.name+"/discard-forwarding", func(b *testing.B) {
+			b.ReportAllocs()
+			b.SetBytes(int64(len(input)))
+			b.ResetTimer()
+			b.ReportMetric(float64(len(input)), "bytes/op")
+			for i := 0; i < b.N; i++ {
+				writer := &combinedWriter{output: io.Discard}
+				if written, err := writer.Write(input); err != nil || written != len(input) {
+					b.Fatalf("write captured output = %d, err %v; want %d bytes", written, err, len(input))
+				}
+				captured := writer.Bytes()
+				if len(captured) != len(input) {
+					b.Fatalf("captured output = %d bytes, want %d", len(captured), len(input))
+				}
+			}
+		})
+	}
+}