diff --git a/src/cmd/go/internal/lockedfile/transform_test.go b/src/cmd/go/internal/lockedfile/transform_test.go
new file mode 100644
index 00000000000000..407d48ea4a35f5
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/transform_test.go
@@ -0,0 +1,104 @@
+// Copyright 2019 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.
+
+// js does not support inter-process file locking.
+// +build !js
+
+package lockedfile_test
+
+import (
+	"bytes"
+	"encoding/binary"
+	"math/rand"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"cmd/go/internal/lockedfile"
+)
+
+func isPowerOf2(x int) bool {
+	return x > 0 && x&(x-1) == 0
+}
+
+func roundDownToPowerOf2(x int) int {
+	if x <= 0 {
+		panic("nonpositive x")
+	}
+	bit := 1
+	for x != bit {
+		x = x &^ bit
+		bit <<= 1
+	}
+	return x
+}
+
+func TestTransform(t *testing.T) {
+	dir, remove := mustTempDir(t)
+	defer remove()
+	path := filepath.Join(dir, "blob.bin")
+
+	const maxChunkWords = 8 << 10
+	buf := make([]byte, 2*maxChunkWords*8)
+	for i := uint64(0); i < 2*maxChunkWords; i++ {
+		binary.LittleEndian.PutUint64(buf[i*8:], i)
+	}
+	if err := lockedfile.Write(path, bytes.NewReader(buf[:8]), 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	var attempts int64 = 128
+	if !testing.Short() {
+		attempts *= 16
+	}
+	const parallel = 32
+
+	var sem = make(chan bool, parallel)
+
+	for n := attempts; n > 0; n-- {
+		sem <- true
+		go func() {
+			defer func() { <-sem }()
+
+			time.Sleep(time.Duration(rand.Intn(100)) * time.Microsecond)
+			chunkWords := roundDownToPowerOf2(rand.Intn(maxChunkWords) + 1)
+			offset := rand.Intn(chunkWords)
+
+			err := lockedfile.Transform(path, func(data []byte) (chunk []byte, err error) {
+				chunk = buf[offset*8 : (offset+chunkWords)*8]
+
+				if len(data)&^7 != len(data) {
+					t.Errorf("read %d bytes, but each write is an integer multiple of 8 bytes", len(data))
+					return chunk, nil
+				}
+
+				words := len(data) / 8
+				if !isPowerOf2(words) {
+					t.Errorf("read %d 8-byte words, but each write is a power-of-2 number of words", words)
+					return chunk, nil
+				}
+
+				u := binary.LittleEndian.Uint64(data)
+				for i := 1; i < words; i++ {
+					next := binary.LittleEndian.Uint64(data[i*8:])
+					if next != u+1 {
+						t.Errorf("wrote sequential integers, but read integer out of sequence at offset %d", i)
+						return chunk, nil
+					}
+					u = next
+				}
+
+				return chunk, nil
+			})
+
+			if err != nil {
+				t.Errorf("unexpected error from Transform: %v", err)
+			}
+		}()
+	}
+
+	for n := parallel; n > 0; n-- {
+		sem <- true
+	}
+}