1 files changed, 91 insertions, 0 deletions
diff --git a/src/cmd/go/internal/robustio/robustio_flaky.go b/src/cmd/go/internal/robustio/robustio_flaky.go
new file mode 100644
index 0000000..5bd44bd
--- /dev/null
+++ b/src/cmd/go/internal/robustio/robustio_flaky.go
@@ -0,0 +1,91 @@
+// 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.
+
+// +build windows darwin
+
+package robustio
+
+import (
+	"errors"
+	"math/rand"
+	"os"
+	"syscall"
+	"time"
+)
+
+const arbitraryTimeout = 2000 * time.Millisecond
+
+// retry retries ephemeral errors from f up to an arbitrary timeout
+// to work around filesystem flakiness on Windows and Darwin.
+func retry(f func() (err error, mayRetry bool)) error {
+	var (
+		bestErr     error
+		lowestErrno syscall.Errno
+		start       time.Time
+		nextSleep   time.Duration = 1 * time.Millisecond
+	)
+	for {
+		err, mayRetry := f()
+		if err == nil || !mayRetry {
+			return err
+		}
+
+		var errno syscall.Errno
+		if errors.As(err, &errno) && (lowestErrno == 0 || errno < lowestErrno) {
+			bestErr = err
+			lowestErrno = errno
+		} else if bestErr == nil {
+			bestErr = err
+		}
+
+		if start.IsZero() {
+			start = time.Now()
+		} else if d := time.Since(start) + nextSleep; d >= arbitraryTimeout {
+			break
+		}
+		time.Sleep(nextSleep)
+		nextSleep += time.Duration(rand.Int63n(int64(nextSleep)))
+	}
+
+	return bestErr
+}
+
+// rename is like os.Rename, but retries ephemeral errors.
+//
+// On Windows it wraps os.Rename, which (as of 2019-06-04) uses MoveFileEx with
+// MOVEFILE_REPLACE_EXISTING.
+//
+// Windows also provides a different system call, ReplaceFile,
+// that provides similar semantics, but perhaps preserves more metadata. (The
+// documentation on the differences between the two is very sparse.)
+//
+// Empirical error rates with MoveFileEx are lower under modest concurrency, so
+// for now we're sticking with what the os package already provides.
+func rename(oldpath, newpath string) (err error) {
+	return retry(func() (err error, mayRetry bool) {
+		err = os.Rename(oldpath, newpath)
+		return err, isEphemeralError(err)
+	})
+}
+
+// readFile is like os.ReadFile, but retries ephemeral errors.
+func readFile(filename string) ([]byte, error) {
+	var b []byte
+	err := retry(func() (err error, mayRetry bool) {
+		b, err = os.ReadFile(filename)
+
+		// Unlike in rename, we do not retry errFileNotFound here: it can occur
+		// as a spurious error, but the file may also genuinely not exist, so the
+		// increase in robustness is probably not worth the extra latency.
+		return err, isEphemeralError(err) && !errors.Is(err, errFileNotFound)
+	})
+	return b, err
+}
+
+func removeAll(path string) error {
+	return retry(func() (err error, mayRetry bool) {
+		err = os.RemoveAll(path)
+		return err, isEphemeralError(err)
+	})
+}