Measure output capture allocations for chunked writes

internal/gotool/performance_test.go

@@ -10,23 +10,40 @@ const performanceDiagnosticSize = 8 << 20
 
 func BenchmarkPerformanceCombinedOutput(b *testing.B) {
 	for _, testCase := range []struct {
-		name string
-		size int
+		name      string
+		size      int
+		chunkSize int
 	}{
-		{name: "modest", size: 4 << 10},
-		{name: "diagnostic-8MiB", size: performanceDiagnosticSize},
+		{name: "modest", size: 4 << 10, chunkSize: 4 << 10},
+		{name: "diagnostic-8MiB", size: performanceDiagnosticSize, chunkSize: performanceDiagnosticSize},
+		{name: "diagnostic-8MiB-32KiB-chunks", size: performanceDiagnosticSize, chunkSize: 32 << 10},
 	} {
 		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) {
+			// Verify capture and forwarding for this workload outside the measurement.
+			var forwarded bytes.Buffer
+			verification := &combinedWriter{output: &forwarded}
+			for offset := 0; offset < len(input); offset += testCase.chunkSize {
+				chunk := input[offset:min(offset+testCase.chunkSize, len(input))]
+				if written, err := verification.Write(chunk); err != nil || written != len(chunk) {
+					b.Fatalf("verify output write = %d, err %v; want %d bytes", written, err, len(chunk))
+				}
+			}
+			if !bytes.Equal(verification.take(), input) || !bytes.Equal(forwarded.Bytes(), input) {
+				b.Fatal("captured and forwarded output must equal the complete input")
+			}
 			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))
+				for offset := 0; offset < len(input); offset += testCase.chunkSize {
+					chunk := input[offset:min(offset+testCase.chunkSize, len(input))]
+					if written, err := writer.Write(chunk); err != nil || written != len(chunk) {
+						b.Fatalf("write captured output = %d, err %v; want %d bytes", written, err, len(chunk))
+					}
 				}
 				captured := writer.take()
 				if len(captured) != len(input) {