diff --git a/cache/cache.go b/cache/cache.go
index cf8a5ba..b94d6a3 100644
--- a/cache/cache.go
+++ b/cache/cache.go
@@ -30,6 +30,49 @@ type ActionID [HashSize]byte
 // An OutputID is a cache output key, the hash of an output of a computation.
 type OutputID [HashSize]byte
 
+// Cacher is the interface as used by the cmd/go.
+// NOTE: name is changed from upstream's Cache to avoid breaking changes.
+type Cacher interface {
+	// Get returns the cache entry for the provided ActionID.
+	// On miss, the error type should be of type *entryNotFoundError.
+	//
+	// After a successful call to Get, OutputFile(Entry.OutputID) must
+	// exist on disk until Close is called (at the end of the process).
+	Get(ActionID) (Entry, error)
+
+	// Put adds an item to the cache.
+	//
+	// The seeker is only used to seek to the beginning. After a call to Put,
+	// the seek position is not guaranteed to be in any particular state.
+	//
+	// As a special case, if the ReadSeeker is of type noVerifyReadSeeker,
+	// the verification from GODEBUG=goverifycache=1 is skipped.
+	//
+	// After a successful call to Put, OutputFile(OutputID) must
+	// exist on disk until Close is called (at the end of the process).
+	Put(ActionID, io.ReadSeeker) (_ OutputID, size int64, _ error)
+
+	// Close is called at the end of the go process. Implementations can do
+	// cache cleanup work at this phase, or wait for and report any errors from
+	// background cleanup work started earlier. Any cache trimming in one
+	// process should not cause the invariants of this interface to be
+	// violated in another process. Namely, a cache trim from one process should
+	// not delete an OutputID from disk that was recently Get or Put from
+	// another process. As a rule of thumb, don't trim things used in the last
+	// day.
+	Close() error
+
+	// OutputFile returns the path on disk where OutputID is stored.
+	//
+	// It's only called after a successful get or put call so it doesn't need
+	// to return an error; it's assumed that if the previous get or put succeeded,
+	// it's already on disk.
+	OutputFile(OutputID) string
+
+	// FuzzDir returns where fuzz files are stored.
+	FuzzDir() string
+}
+
 // A Cache is a package cache, backed by a file system directory tree.
 type Cache struct {
 	dir string
@@ -266,6 +309,39 @@ func (c *Cache) GetMmap(id ActionID) ([]byte, Entry, error) {
 }
 */
 
+// GetFile looks up the action ID in the cache and returns
+// the name of the corresponding data file.
+func GetFile(c Cacher, id ActionID) (file string, entry Entry, err error) {
+	entry, err = c.Get(id)
+	if err != nil {
+		return "", Entry{}, err
+	}
+	file = c.OutputFile(entry.OutputID)
+	info, err := os.Stat(file)
+	if err != nil {
+		return "", Entry{}, &entryNotFoundError{Err: err}
+	}
+	if info.Size() != entry.Size {
+		return "", Entry{}, &entryNotFoundError{Err: errors.New("file incomplete")}
+	}
+	return file, entry, nil
+}
+
+// GetBytes looks up the action ID in the cache and returns
+// the corresponding output bytes.
+// GetBytes should only be used for data that can be expected to fit in memory.
+func GetBytes(c Cacher, id ActionID) ([]byte, Entry, error) {
+	entry, err := c.Get(id)
+	if err != nil {
+		return nil, entry, err
+	}
+	data, _ := os.ReadFile(c.OutputFile(entry.OutputID))
+	if sha256.Sum256(data) != entry.OutputID {
+		return nil, entry, &entryNotFoundError{Err: errors.New("bad checksum")}
+	}
+	return data, entry, nil
+}
+
 // OutputFile returns the name of the cache file storing output with the given OutputID.
 func (c *Cache) OutputFile(out OutputID) string {
 	file := c.fileName(out, "d")
@@ -308,6 +384,8 @@ func (c *Cache) used(file string) {
 	os.Chtimes(file, c.now(), c.now())
 }
 
+func (c *Cache) Close() error { return c.Trim() }
+
 // Trim removes old cache entries that are likely not to be reused.
 func (c *Cache) Trim() error {
 	now := c.now()
@@ -431,10 +509,21 @@ func (c *Cache) putIndexEntry(id ActionID, out OutputID, size int64, allowVerify
 	return nil
 }
 
+// noVerifyReadSeeker is an io.ReadSeeker wrapper sentinel type
+// that says that Cache.Put should skip the verify check
+// (from GODEBUG=goverifycache=1).
+type noVerifyReadSeeker struct {
+	io.ReadSeeker
+}
+
 // Put stores the given output in the cache as the output for the action ID.
 // It may read file twice. The content of file must not change between the two passes.
 func (c *Cache) Put(id ActionID, file io.ReadSeeker) (OutputID, int64, error) {
-	return c.put(id, file, true)
+	wrapper, isNoVerify := file.(noVerifyReadSeeker)
+	if isNoVerify {
+		file = wrapper.ReadSeeker
+	}
+	return c.put(id, file, !isNoVerify)
 }
 
 // PutNoVerify is like Put but disables the verify check
@@ -445,6 +534,14 @@ func (c *Cache) PutNoVerify(id ActionID, file io.ReadSeeker) (OutputID, int64, e
 	return c.put(id, file, false)
 }
 
+// PutNoVerify is like Put but disables the verify check
+// when GODEBUG=goverifycache=1 is set.
+// It is meant for data that is OK to cache but that we expect to vary slightly from run to run,
+// like test output containing times and the like.
+func PutNoVerify(c Cacher, id ActionID, file io.ReadSeeker) (OutputID, int64, error) {
+	return c.Put(id, noVerifyReadSeeker{file})
+}
+
 func (c *Cache) put(id ActionID, file io.ReadSeeker, allowVerify bool) (OutputID, int64, error) {
 	// Compute output ID.
 	h := sha256.New()
@@ -473,6 +570,12 @@ func (c *Cache) PutBytes(id ActionID, data []byte) error {
 	return err
 }
 
+// PutBytes stores the given bytes in the cache as the output for the action ID.
+func PutBytes(c Cacher, id ActionID, data []byte) error {
+	_, _, err := c.Put(id, bytes.NewReader(data))
+	return err
+}
+
 // copyFile copies file into the cache, expecting it to have the given
 // output ID and size, if that file is not present already.
 func (c *Cache) copyFile(file io.ReadSeeker, out OutputID, size int64) error {
diff --git a/cache/cacheprog/cacheprog.go b/cache/cacheprog/cacheprog.go
new file mode 100644
index 0000000..9379636
--- /dev/null
+++ b/cache/cacheprog/cacheprog.go
@@ -0,0 +1,126 @@
+// Copyright 2024 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 cacheprog defines the protocol for a GOCACHEPROG program.
+//
+// By default, the go command manages a build cache stored in the file system
+// itself. GOCACHEPROG can be set to the name of a command (with optional
+// space-separated flags) that implements the go command build cache externally.
+// This permits defining a different cache policy.
+//
+// The go command will start the GOCACHEPROG as a subprocess and communicate
+// with it via JSON messages over stdin/stdout. The subprocess's stderr will be
+// connected to the go command's stderr.
+//
+// The subprocess should immediately send a [Response] with its capabilities.
+// After that, the go command will send a stream of [Request] messages and the
+// subprocess should reply to each [Request] with a [Response] message.
+package cacheprog
+
+import (
+	"io"
+	"time"
+)
+
+// Cmd is a command that can be issued to a child process.
+//
+// If the interface needs to grow, the go command can add new commands or new
+// versioned commands like "get2" in the future. The initial [Response] from
+// the child process indicates which commands it supports.
+type Cmd string
+
+const (
+	// CmdPut tells the cache program to store an object in the cache.
+	//
+	// [Request.ActionID] is the cache key of this object. The cache should
+	// store [Request.OutputID] and [Request.Body] under this key for a
+	// later "get" request. It must also store the Body in a file in the local
+	// file system and return the path to that file in [Response.DiskPath],
+	// which must exist at least until a "close" request.
+	CmdPut = Cmd("put")
+
+	// CmdGet tells the cache program to retrieve an object from the cache.
+	//
+	// [Request.ActionID] specifies the key of the object to get. If the
+	// cache does not contain this object, it should set [Response.Miss] to
+	// true. Otherwise, it should populate the fields of [Response],
+	// including setting [Response.OutputID] to the OutputID of the original
+	// "put" request and [Response.DiskPath] to the path of a local file
+	// containing the Body of the original "put" request. That file must
+	// continue to exist at least until a "close" request.
+	CmdGet = Cmd("get")
+
+	// CmdClose requests that the cache program exit gracefully.
+	//
+	// The cache program should reply to this request and then exit
+	// (thus closing its stdout).
+	CmdClose = Cmd("close")
+)
+
+// Request is the JSON-encoded message that's sent from the go command to
+// the GOCACHEPROG child process over stdin. Each JSON object is on its own
+// line. A ProgRequest of Type "put" with BodySize > 0 will be followed by a
+// line containing a base64-encoded JSON string literal of the body.
+type Request struct {
+	// ID is a unique number per process across all requests.
+	// It must be echoed in the Response from the child.
+	ID int64
+
+	// Command is the type of request.
+	// The go command will only send commands that were declared
+	// as supported by the child.
+	Command Cmd
+
+	// ActionID is the cache key for "put" and "get" requests.
+	ActionID []byte `json:",omitempty"` // or nil if not used
+
+	// OutputID is stored with the body for "put" requests.
+	OutputID []byte `json:",omitempty"` // or nil if not used
+
+	// Body is the body for "put" requests. It's sent after the JSON object
+	// as a base64-encoded JSON string when BodySize is non-zero.
+	// It's sent as a separate JSON value instead of being a struct field
+	// send in this JSON object so large values can be streamed in both directions.
+	// The base64 string body of a Request will always be written
+	// immediately after the JSON object and a newline.
+	Body io.Reader `json:"-"`
+
+	// BodySize is the number of bytes of Body. If zero, the body isn't written.
+	BodySize int64 `json:",omitempty"`
+}
+
+// Response is the JSON response from the child process to the go command.
+//
+// With the exception of the first protocol message that the child writes to its
+// stdout with ID==0 and KnownCommands populated, these are only sent in
+// response to a Request from the go command.
+//
+// Responses can be sent in any order. The ID must match the request they're
+// replying to.
+type Response struct {
+	ID  int64  // that corresponds to Request; they can be answered out of order
+	Err string `json:",omitempty"` // if non-empty, the error
+
+	// KnownCommands is included in the first message that cache helper program
+	// writes to stdout on startup (with ID==0). It includes the
+	// Request.Command types that are supported by the program.
+	//
+	// This lets the go command extend the protocol gracefully over time (adding
+	// "get2", etc), or fail gracefully when needed. It also lets the go command
+	// verify the program wants to be a cache helper.
+	KnownCommands []Cmd `json:",omitempty"`
+
+	// For "get" requests.
+
+	Miss     bool       `json:",omitempty"` // cache miss
+	OutputID []byte     `json:",omitempty"` // the OutputID stored with the body
+	Size     int64      `json:",omitempty"` // body size in bytes
+	Time     *time.Time `json:",omitempty"` // when the object was put in the cache (optional; used for cache expiration)
+
+	// For "get" and "put" requests.
+
+	// DiskPath is the absolute path on disk of the body corresponding to a
+	// "get" (on cache hit) or "put" request's ActionID.
+	DiskPath string `json:",omitempty"`
+}
diff --git a/cache/default.go b/cache/default.go
index a20b33c..fc8f843 100644
--- a/cache/default.go
+++ b/cache/default.go
@@ -14,15 +14,11 @@ import (
 
 // Default returns the default cache to use.
 // It never returns nil.
-func Default() *Cache {
-	defaultOnce.Do(initDefaultCache)
-	return defaultCache
+func Default() Cacher {
+	return initDefaultCacheOnce()
 }
 
-var (
-	defaultOnce  sync.Once
-	defaultCache *Cache
-)
+var initDefaultCacheOnce = sync.OnceValue(initDefaultCache)
 
 // cacheREADME is a message stored in a README in the cache directory.
 // Because the cache lives outside the normal Go trees, we leave the
@@ -35,7 +31,7 @@ See golang.org to learn more about Go.
 
 // initDefaultCache does the work of finding the default cache
 // the first time Default is called.
-func initDefaultCache() {
+func initDefaultCache() Cacher {
 	dir := DefaultDir()
 	if dir == "off" {
 		if defaultDirErr != nil {
@@ -51,11 +47,18 @@ func initDefaultCache() {
 		os.WriteFile(filepath.Join(dir, "README"), []byte(cacheREADME), 0666)
 	}
 
-	c, err := Open(dir)
+	diskCache, err := Open(dir)
 	if err != nil {
 		log.Fatalf("failed to initialize build cache at %s: %s\n", dir, err)
 	}
-	defaultCache = c
+
+	// NOTE: changed from upstream's cfg.Getenv, so it will ignore "go env -w".
+	// Consider calling "go env" or copying the cfg package instead.
+	if cacheprog := os.Getenv("GOCACHEPROG"); cacheprog != "" {
+		return startCacheProg(cacheprog, diskCache)
+	}
+
+	return diskCache
 }
 
 var (
diff --git a/cache/prog.go b/cache/prog.go
new file mode 100644
index 0000000..268ba84
--- /dev/null
+++ b/cache/prog.go
@@ -0,0 +1,374 @@
+// 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 cache
+
+import (
+	"bufio"
+	"context"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"os/exec"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"github.com/rogpeppe/go-internal/cache/cacheprog"
+	"github.com/rogpeppe/go-internal/internal/quoted"
+)
+
+// ProgCache implements Cache via JSON messages over stdin/stdout to a child
+// helper process which can then implement whatever caching policy/mechanism it
+// wants.
+//
+// See https://github.com/golang/go/issues/59719
+type ProgCache struct {
+	cmd    *exec.Cmd
+	stdout io.ReadCloser  // from the child process
+	stdin  io.WriteCloser // to the child process
+	bw     *bufio.Writer  // to stdin
+	jenc   *json.Encoder  // to bw
+
+	// can are the commands that the child process declared that it supports.
+	// This is effectively the versioning mechanism.
+	can map[cacheprog.Cmd]bool
+
+	// fuzzDirCache is another Cache implementation to use for the FuzzDir
+	// method. In practice this is the default GOCACHE disk-based
+	// implementation.
+	//
+	// TODO(bradfitz): maybe this isn't ideal. But we'd need to extend the Cache
+	// interface and the fuzzing callers to be less disk-y to do more here.
+	fuzzDirCache Cacher
+
+	closing      atomic.Bool
+	ctx          context.Context    // valid until Close via ctxClose
+	ctxCancel    context.CancelFunc // called on Close
+	readLoopDone chan struct{}      // closed when readLoop returns
+
+	mu         sync.Mutex // guards following fields
+	nextID     int64
+	inFlight   map[int64]chan<- *cacheprog.Response
+	outputFile map[OutputID]string // object => abs path on disk
+
+	// writeMu serializes writing to the child process.
+	// It must never be held at the same time as mu.
+	writeMu sync.Mutex
+}
+
+// startCacheProg starts the prog binary (with optional space-separated flags)
+// and returns a Cache implementation that talks to it.
+//
+// It blocks a few seconds to wait for the child process to successfully start
+// and advertise its capabilities.
+func startCacheProg(progAndArgs string, fuzzDirCache Cacher) Cacher {
+	if fuzzDirCache == nil {
+		panic("missing fuzzDirCache")
+	}
+	args, err := quoted.Split(progAndArgs)
+	if err != nil {
+		log.Fatalf("GOCACHEPROG args: %v", err)
+	}
+	var prog string
+	if len(args) > 0 {
+		prog = args[0]
+		args = args[1:]
+	}
+
+	ctx, ctxCancel := context.WithCancel(context.Background())
+
+	cmd := exec.CommandContext(ctx, prog, args...)
+	out, err := cmd.StdoutPipe()
+	if err != nil {
+		log.Fatalf("StdoutPipe to GOCACHEPROG: %v", err)
+	}
+	in, err := cmd.StdinPipe()
+	if err != nil {
+		log.Fatalf("StdinPipe to GOCACHEPROG: %v", err)
+	}
+	cmd.Stderr = os.Stderr
+	// On close, we cancel the context. Rather than killing the helper,
+	// close its stdin.
+	cmd.Cancel = in.Close
+
+	if err := cmd.Start(); err != nil {
+		log.Fatalf("error starting GOCACHEPROG program %q: %v", prog, err)
+	}
+
+	pc := &ProgCache{
+		ctx:          ctx,
+		ctxCancel:    ctxCancel,
+		fuzzDirCache: fuzzDirCache,
+		cmd:          cmd,
+		stdout:       out,
+		stdin:        in,
+		bw:           bufio.NewWriter(in),
+		inFlight:     make(map[int64]chan<- *cacheprog.Response),
+		outputFile:   make(map[OutputID]string),
+		readLoopDone: make(chan struct{}),
+	}
+
+	// Register our interest in the initial protocol message from the child to
+	// us, saying what it can do.
+	capResc := make(chan *cacheprog.Response, 1)
+	pc.inFlight[0] = capResc
+
+	pc.jenc = json.NewEncoder(pc.bw)
+	go pc.readLoop(pc.readLoopDone)
+
+	// Give the child process a few seconds to report its capabilities. This
+	// should be instant and not require any slow work by the program.
+	timer := time.NewTicker(5 * time.Second)
+	defer timer.Stop()
+	for {
+		select {
+		case <-timer.C:
+			log.Printf("# still waiting for GOCACHEPROG %v ...", prog)
+		case capRes := <-capResc:
+			can := map[cacheprog.Cmd]bool{}
+			for _, cmd := range capRes.KnownCommands {
+				can[cmd] = true
+			}
+			if len(can) == 0 {
+				log.Fatalf("GOCACHEPROG %v declared no supported commands", prog)
+			}
+			pc.can = can
+			return pc
+		}
+	}
+}
+
+func (c *ProgCache) readLoop(readLoopDone chan<- struct{}) {
+	defer close(readLoopDone)
+	jd := json.NewDecoder(c.stdout)
+	for {
+		res := new(cacheprog.Response)
+		if err := jd.Decode(res); err != nil {
+			if c.closing.Load() {
+				c.mu.Lock()
+				for _, ch := range c.inFlight {
+					close(ch)
+				}
+				c.inFlight = nil
+				c.mu.Unlock()
+				return // quietly
+			}
+			if err == io.EOF {
+				c.mu.Lock()
+				inFlight := len(c.inFlight)
+				c.mu.Unlock()
+				log.Fatalf("GOCACHEPROG exited pre-Close with %v pending requests", inFlight)
+			}
+			log.Fatalf("error reading JSON from GOCACHEPROG: %v", err)
+		}
+		c.mu.Lock()
+		ch, ok := c.inFlight[res.ID]
+		delete(c.inFlight, res.ID)
+		c.mu.Unlock()
+		if ok {
+			ch <- res
+		} else {
+			log.Fatalf("GOCACHEPROG sent response for unknown request ID %v", res.ID)
+		}
+	}
+}
+
+var errCacheprogClosed = errors.New("GOCACHEPROG program closed unexpectedly")
+
+func (c *ProgCache) send(ctx context.Context, req *cacheprog.Request) (*cacheprog.Response, error) {
+	resc := make(chan *cacheprog.Response, 1)
+	if err := c.writeToChild(req, resc); err != nil {
+		return nil, err
+	}
+	select {
+	case res := <-resc:
+		if res == nil {
+			return nil, errCacheprogClosed
+		}
+		if res.Err != "" {
+			return nil, errors.New(res.Err)
+		}
+		return res, nil
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	}
+}
+
+func (c *ProgCache) writeToChild(req *cacheprog.Request, resc chan<- *cacheprog.Response) (err error) {
+	c.mu.Lock()
+	if c.inFlight == nil {
+		return errCacheprogClosed
+	}
+	c.nextID++
+	req.ID = c.nextID
+	c.inFlight[req.ID] = resc
+	c.mu.Unlock()
+
+	defer func() {
+		if err != nil {
+			c.mu.Lock()
+			if c.inFlight != nil {
+				delete(c.inFlight, req.ID)
+			}
+			c.mu.Unlock()
+		}
+	}()
+
+	c.writeMu.Lock()
+	defer c.writeMu.Unlock()
+
+	if err := c.jenc.Encode(req); err != nil {
+		return err
+	}
+	if err := c.bw.WriteByte('\n'); err != nil {
+		return err
+	}
+	if req.Body != nil && req.BodySize > 0 {
+		if err := c.bw.WriteByte('"'); err != nil {
+			return err
+		}
+		e := base64.NewEncoder(base64.StdEncoding, c.bw)
+		wrote, err := io.Copy(e, req.Body)
+		if err != nil {
+			return err
+		}
+		if err := e.Close(); err != nil {
+			return nil
+		}
+		if wrote != req.BodySize {
+			return fmt.Errorf("short write writing body to GOCACHEPROG for action %x, output %x: wrote %v; expected %v",
+				req.ActionID, req.OutputID, wrote, req.BodySize)
+		}
+		if _, err := c.bw.WriteString("\"\n"); err != nil {
+			return err
+		}
+	}
+	if err := c.bw.Flush(); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (c *ProgCache) Get(a ActionID) (Entry, error) {
+	if !c.can[cacheprog.CmdGet] {
+		// They can't do a "get". Maybe they're a write-only cache.
+		//
+		// TODO(bradfitz,bcmills): figure out the proper error type here. Maybe
+		// errors.ErrUnsupported? Is entryNotFoundError even appropriate? There
+		// might be places where we rely on the fact that a recent Put can be
+		// read through a corresponding Get. Audit callers and check, and document
+		// error types on the Cache interface.
+		return Entry{}, &entryNotFoundError{}
+	}
+	res, err := c.send(c.ctx, &cacheprog.Request{
+		Command:  cacheprog.CmdGet,
+		ActionID: a[:],
+	})
+	if err != nil {
+		return Entry{}, err // TODO(bradfitz): or entryNotFoundError? Audit callers.
+	}
+	if res.Miss {
+		return Entry{}, &entryNotFoundError{}
+	}
+	e := Entry{
+		Size: res.Size,
+	}
+	if res.Time != nil {
+		e.Time = *res.Time
+	} else {
+		e.Time = time.Now()
+	}
+	if res.DiskPath == "" {
+		return Entry{}, &entryNotFoundError{errors.New("GOCACHEPROG didn't populate DiskPath on get hit")}
+	}
+	if copy(e.OutputID[:], res.OutputID) != len(res.OutputID) {
+		return Entry{}, &entryNotFoundError{errors.New("incomplete ProgResponse OutputID")}
+	}
+	c.noteOutputFile(e.OutputID, res.DiskPath)
+	return e, nil
+}
+
+func (c *ProgCache) noteOutputFile(o OutputID, diskPath string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.outputFile[o] = diskPath
+}
+
+func (c *ProgCache) OutputFile(o OutputID) string {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.outputFile[o]
+}
+
+func (c *ProgCache) Put(a ActionID, file io.ReadSeeker) (_ OutputID, size int64, _ error) {
+	// Compute output ID.
+	h := sha256.New()
+	if _, err := file.Seek(0, 0); err != nil {
+		return OutputID{}, 0, err
+	}
+	size, err := io.Copy(h, file)
+	if err != nil {
+		return OutputID{}, 0, err
+	}
+	var out OutputID
+	h.Sum(out[:0])
+
+	if _, err := file.Seek(0, 0); err != nil {
+		return OutputID{}, 0, err
+	}
+
+	if !c.can[cacheprog.CmdPut] {
+		// Child is a read-only cache. Do nothing.
+		return out, size, nil
+	}
+
+	res, err := c.send(c.ctx, &cacheprog.Request{
+		Command:  cacheprog.CmdPut,
+		ActionID: a[:],
+		OutputID: out[:],
+		Body:     file,
+		BodySize: size,
+	})
+	if err != nil {
+		return OutputID{}, 0, err
+	}
+	if res.DiskPath == "" {
+		return OutputID{}, 0, errors.New("GOCACHEPROG didn't return DiskPath in put response")
+	}
+	c.noteOutputFile(out, res.DiskPath)
+	return out, size, err
+}
+
+func (c *ProgCache) Close() error {
+	c.closing.Store(true)
+	var err error
+
+	// First write a "close" message to the child so it can exit nicely
+	// and clean up if it wants. Only after that exchange do we cancel
+	// the context that kills the process.
+	if c.can[cacheprog.CmdClose] {
+		_, err = c.send(c.ctx, &cacheprog.Request{Command: cacheprog.CmdClose})
+		if errors.Is(err, errCacheprogClosed) {
+			// Allow the child to quit without responding to close.
+			err = nil
+		}
+	}
+	// Cancel the context, which will close the helper's stdin.
+	c.ctxCancel()
+	// Wait until the helper closes its stdout.
+	<-c.readLoopDone
+	return err
+}
+
+func (c *ProgCache) FuzzDir() string {
+	// TODO(bradfitz): figure out what to do here. For now just use the
+	// disk-based default.
+	return c.fuzzDirCache.FuzzDir()
+}
diff --git a/internal/quoted/quoted.go b/internal/quoted/quoted.go
new file mode 100644
index 0000000..a812275
--- /dev/null
+++ b/internal/quoted/quoted.go
@@ -0,0 +1,129 @@
+// Copyright 2017 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 quoted provides string manipulation utilities.
+package quoted
+
+import (
+	"flag"
+	"fmt"
+	"strings"
+	"unicode"
+)
+
+func isSpaceByte(c byte) bool {
+	return c == ' ' || c == '\t' || c == '\n' || c == '\r'
+}
+
+// Split splits s into a list of fields,
+// allowing single or double quotes around elements.
+// There is no unescaping or other processing within
+// quoted fields.
+//
+// Keep in sync with cmd/dist/quoted.go
+func Split(s string) ([]string, error) {
+	// Split fields allowing '' or "" around elements.
+	// Quotes further inside the string do not count.
+	var f []string
+	for len(s) > 0 {
+		for len(s) > 0 && isSpaceByte(s[0]) {
+			s = s[1:]
+		}
+		if len(s) == 0 {
+			break
+		}
+		// Accepted quoted string. No unescaping inside.
+		if s[0] == '"' || s[0] == '\'' {
+			quote := s[0]
+			s = s[1:]
+			i := 0
+			for i < len(s) && s[i] != quote {
+				i++
+			}
+			if i >= len(s) {
+				return nil, fmt.Errorf("unterminated %c string", quote)
+			}
+			f = append(f, s[:i])
+			s = s[i+1:]
+			continue
+		}
+		i := 0
+		for i < len(s) && !isSpaceByte(s[i]) {
+			i++
+		}
+		f = append(f, s[:i])
+		s = s[i:]
+	}
+	return f, nil
+}
+
+// Join joins a list of arguments into a string that can be parsed
+// with Split. Arguments are quoted only if necessary; arguments
+// without spaces or quotes are kept as-is. No argument may contain both
+// single and double quotes.
+func Join(args []string) (string, error) {
+	var buf []byte
+	for i, arg := range args {
+		if i > 0 {
+			buf = append(buf, ' ')
+		}
+		var sawSpace, sawSingleQuote, sawDoubleQuote bool
+		for _, c := range arg {
+			switch {
+			case c > unicode.MaxASCII:
+				continue
+			case isSpaceByte(byte(c)):
+				sawSpace = true
+			case c == '\'':
+				sawSingleQuote = true
+			case c == '"':
+				sawDoubleQuote = true
+			}
+		}
+		switch {
+		case !sawSpace && !sawSingleQuote && !sawDoubleQuote:
+			buf = append(buf, arg...)
+
+		case !sawSingleQuote:
+			buf = append(buf, '\'')
+			buf = append(buf, arg...)
+			buf = append(buf, '\'')
+
+		case !sawDoubleQuote:
+			buf = append(buf, '"')
+			buf = append(buf, arg...)
+			buf = append(buf, '"')
+
+		default:
+			return "", fmt.Errorf("argument %q contains both single and double quotes and cannot be quoted", arg)
+		}
+	}
+	return string(buf), nil
+}
+
+// A Flag parses a list of string arguments encoded with Join.
+// It is useful for flags like cmd/link's -extldflags.
+type Flag []string
+
+var _ flag.Value = (*Flag)(nil)
+
+func (f *Flag) Set(v string) error {
+	fs, err := Split(v)
+	if err != nil {
+		return err
+	}
+	*f = fs[:len(fs):len(fs)]
+	return nil
+}
+
+func (f *Flag) String() string {
+	if f == nil {
+		return ""
+	}
+	s, err := Join(*f)
+	if err != nil {
+		return strings.Join(*f, " ")
+	}
+	return s
+}
diff --git a/internal/quoted/quoted_test.go b/internal/quoted/quoted_test.go
new file mode 100644
index 0000000..d76270c
--- /dev/null
+++ b/internal/quoted/quoted_test.go
@@ -0,0 +1,88 @@
+// Copyright 2020 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 quoted
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestSplit(t *testing.T) {
+	for _, test := range []struct {
+		name    string
+		value   string
+		want    []string
+		wantErr string
+	}{
+		{name: "empty", value: "", want: nil},
+		{name: "space", value: " ", want: nil},
+		{name: "one", value: "a", want: []string{"a"}},
+		{name: "leading_space", value: " a", want: []string{"a"}},
+		{name: "trailing_space", value: "a ", want: []string{"a"}},
+		{name: "two", value: "a b", want: []string{"a", "b"}},
+		{name: "two_multi_space", value: "a  b", want: []string{"a", "b"}},
+		{name: "two_tab", value: "a\tb", want: []string{"a", "b"}},
+		{name: "two_newline", value: "a\nb", want: []string{"a", "b"}},
+		{name: "quote_single", value: `'a b'`, want: []string{"a b"}},
+		{name: "quote_double", value: `"a b"`, want: []string{"a b"}},
+		{name: "quote_both", value: `'a '"b "`, want: []string{"a ", "b "}},
+		{name: "quote_contains", value: `'a "'"'b"`, want: []string{`a "`, `'b`}},
+		{name: "escape", value: `\'`, want: []string{`\'`}},
+		{name: "quote_unclosed", value: `'a`, wantErr: "unterminated ' string"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			got, err := Split(test.value)
+			if err != nil {
+				if test.wantErr == "" {
+					t.Fatalf("unexpected error: %v", err)
+				} else if errMsg := err.Error(); !strings.Contains(errMsg, test.wantErr) {
+					t.Fatalf("error %q does not contain %q", errMsg, test.wantErr)
+				}
+				return
+			}
+			if test.wantErr != "" {
+				t.Fatalf("unexpected success; wanted error containing %q", test.wantErr)
+			}
+			if !reflect.DeepEqual(got, test.want) {
+				t.Errorf("got %q; want %q", got, test.want)
+			}
+		})
+	}
+}
+
+func TestJoin(t *testing.T) {
+	for _, test := range []struct {
+		name          string
+		args          []string
+		want, wantErr string
+	}{
+		{name: "empty", args: nil, want: ""},
+		{name: "one", args: []string{"a"}, want: "a"},
+		{name: "two", args: []string{"a", "b"}, want: "a b"},
+		{name: "space", args: []string{"a ", "b"}, want: "'a ' b"},
+		{name: "newline", args: []string{"a\n", "b"}, want: "'a\n' b"},
+		{name: "quote", args: []string{`'a `, "b"}, want: `"'a " b`},
+		{name: "unquoteable", args: []string{`'"`}, wantErr: "contains both single and double quotes and cannot be quoted"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			got, err := Join(test.args)
+			if err != nil {
+				if test.wantErr == "" {
+					t.Fatalf("unexpected error: %v", err)
+				} else if errMsg := err.Error(); !strings.Contains(errMsg, test.wantErr) {
+					t.Fatalf("error %q does not contain %q", errMsg, test.wantErr)
+				}
+				return
+			}
+			if test.wantErr != "" {
+				t.Fatalf("unexpected success; wanted error containing %q", test.wantErr)
+			}
+			if got != test.want {
+				t.Errorf("got %s; want %s", got, test.want)
+			}
+		})
+	}
+}