3 files changed, 296 insertions, 0 deletions
diff --git a/src/cmd/go/internal/renameio/renameio.go b/src/cmd/go/internal/renameio/renameio.go
new file mode 100644
index 0000000..9788171
--- /dev/null
+++ b/src/cmd/go/internal/renameio/renameio.go
@@ -0,0 +1,94 @@
+// Copyright 2018 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 renameio writes files atomically by renaming temporary files.
+package renameio
+
+import (
+	"bytes"
+	"io"
+	"io/fs"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"strconv"
+
+	"cmd/go/internal/robustio"
+)
+
+const patternSuffix = ".tmp"
+
+// Pattern returns a glob pattern that matches the unrenamed temporary files
+// created when writing to filename.
+func Pattern(filename string) string {
+	return filepath.Join(filepath.Dir(filename), filepath.Base(filename)+patternSuffix)
+}
+
+// WriteFile is like os.WriteFile, but first writes data to an arbitrary
+// file in the same directory as filename, then renames it atomically to the
+// final name.
+//
+// That ensures that the final location, if it exists, is always a complete file.
+func WriteFile(filename string, data []byte, perm fs.FileMode) (err error) {
+	return WriteToFile(filename, bytes.NewReader(data), perm)
+}
+
+// WriteToFile is a variant of WriteFile that accepts the data as an io.Reader
+// instead of a slice.
+func WriteToFile(filename string, data io.Reader, perm fs.FileMode) (err error) {
+	f, err := tempFile(filepath.Dir(filename), filepath.Base(filename), perm)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		// Only call os.Remove on f.Name() if we failed to rename it: otherwise,
+		// some other process may have created a new file with the same name after
+		// that.
+		if err != nil {
+			f.Close()
+			os.Remove(f.Name())
+		}
+	}()
+
+	if _, err := io.Copy(f, data); err != nil {
+		return err
+	}
+	// Sync the file before renaming it: otherwise, after a crash the reader may
+	// observe a 0-length file instead of the actual contents.
+	// See https://golang.org/issue/22397#issuecomment-380831736.
+	if err := f.Sync(); err != nil {
+		return err
+	}
+	if err := f.Close(); err != nil {
+		return err
+	}
+
+	return robustio.Rename(f.Name(), filename)
+}
+
+// ReadFile is like os.ReadFile, but on Windows retries spurious errors that
+// may occur if the file is concurrently replaced.
+//
+// Errors are classified heuristically and retries are bounded, so even this
+// function may occasionally return a spurious error on Windows.
+// If so, the error will likely wrap one of:
+// 	- syscall.ERROR_ACCESS_DENIED
+// 	- syscall.ERROR_FILE_NOT_FOUND
+// 	- internal/syscall/windows.ERROR_SHARING_VIOLATION
+func ReadFile(filename string) ([]byte, error) {
+	return robustio.ReadFile(filename)
+}
+
+// tempFile creates a new temporary file with given permission bits.
+func tempFile(dir, prefix string, perm fs.FileMode) (f *os.File, err error) {
+	for i := 0; i < 10000; i++ {
+		name := filepath.Join(dir, prefix+strconv.Itoa(rand.Intn(1000000000))+patternSuffix)
+		f, err = os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, perm)
+		if os.IsExist(err) {
+			continue
+		}
+		break
+	}
+	return
+}
diff --git a/src/cmd/go/internal/renameio/renameio_test.go b/src/cmd/go/internal/renameio/renameio_test.go
new file mode 100644
index 0000000..5b2ed83
--- /dev/null
+++ b/src/cmd/go/internal/renameio/renameio_test.go
@@ -0,0 +1,160 @@
+// 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 !plan9
+
+package renameio
+
+import (
+	"encoding/binary"
+	"errors"
+	"internal/testenv"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"syscall"
+	"testing"
+	"time"
+
+	"cmd/go/internal/robustio"
+)
+
+func TestConcurrentReadsAndWrites(t *testing.T) {
+	if runtime.GOOS == "darwin" && strings.HasSuffix(testenv.Builder(), "-10_14") {
+		testenv.SkipFlaky(t, 33041)
+	}
+
+	dir, err := os.MkdirTemp("", "renameio")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+	path := filepath.Join(dir, "blob.bin")
+
+	const chunkWords = 8 << 10
+	buf := make([]byte, 2*chunkWords*8)
+	for i := uint64(0); i < 2*chunkWords; i++ {
+		binary.LittleEndian.PutUint64(buf[i*8:], i)
+	}
+
+	var attempts int64 = 128
+	if !testing.Short() {
+		attempts *= 16
+	}
+	const parallel = 32
+
+	var sem = make(chan bool, parallel)
+
+	var (
+		writeSuccesses, readSuccesses int64 // atomic
+		writeErrnoSeen, readErrnoSeen sync.Map
+	)
+
+	for n := attempts; n > 0; n-- {
+		sem <- true
+		go func() {
+			defer func() { <-sem }()
+
+			time.Sleep(time.Duration(rand.Intn(100)) * time.Microsecond)
+			offset := rand.Intn(chunkWords)
+			chunk := buf[offset*8 : (offset+chunkWords)*8]
+			if err := WriteFile(path, chunk, 0666); err == nil {
+				atomic.AddInt64(&writeSuccesses, 1)
+			} else if robustio.IsEphemeralError(err) {
+				var (
+					errno syscall.Errno
+					dup   bool
+				)
+				if errors.As(err, &errno) {
+					_, dup = writeErrnoSeen.LoadOrStore(errno, true)
+				}
+				if !dup {
+					t.Logf("ephemeral error: %v", err)
+				}
+			} else {
+				t.Errorf("unexpected error: %v", err)
+			}
+
+			time.Sleep(time.Duration(rand.Intn(100)) * time.Microsecond)
+			data, err := ReadFile(path)
+			if err == nil {
+				atomic.AddInt64(&readSuccesses, 1)
+			} else if robustio.IsEphemeralError(err) {
+				var (
+					errno syscall.Errno
+					dup   bool
+				)
+				if errors.As(err, &errno) {
+					_, dup = readErrnoSeen.LoadOrStore(errno, true)
+				}
+				if !dup {
+					t.Logf("ephemeral error: %v", err)
+				}
+				return
+			} else {
+				t.Errorf("unexpected error: %v", err)
+				return
+			}
+
+			if len(data) != 8*chunkWords {
+				t.Errorf("read %d bytes, but each write is a %d-byte file", len(data), 8*chunkWords)
+				return
+			}
+
+			u := binary.LittleEndian.Uint64(data)
+			for i := 1; i < chunkWords; 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
+				}
+				u = next
+			}
+		}()
+	}
+
+	for n := parallel; n > 0; n-- {
+		sem <- true
+	}
+
+	var minWriteSuccesses int64 = attempts
+	if runtime.GOOS == "windows" {
+		// Windows produces frequent "Access is denied" errors under heavy rename load.
+		// As long as those are the only errors and *some* of the writes succeed, we're happy.
+		minWriteSuccesses = attempts / 4
+	}
+
+	if writeSuccesses < minWriteSuccesses {
+		t.Errorf("%d (of %d) writes succeeded; want ≥ %d", writeSuccesses, attempts, minWriteSuccesses)
+	} else {
+		t.Logf("%d (of %d) writes succeeded (ok: ≥ %d)", writeSuccesses, attempts, minWriteSuccesses)
+	}
+
+	var minReadSuccesses int64 = attempts
+
+	switch runtime.GOOS {
+	case "windows":
+		// Windows produces frequent "Access is denied" errors under heavy rename load.
+		// As long as those are the only errors and *some* of the reads succeed, we're happy.
+		minReadSuccesses = attempts / 4
+
+	case "darwin", "ios":
+		// The filesystem on certain versions of macOS (10.14) and iOS (affected
+		// versions TBD) occasionally fail with "no such file or directory" errors.
+		// See https://golang.org/issue/33041 and https://golang.org/issue/42066.
+		// The flake rate is fairly low, so ensure that at least 75% of attempts
+		// succeed.
+		minReadSuccesses = attempts - (attempts / 4)
+	}
+
+	if readSuccesses < minReadSuccesses {
+		t.Errorf("%d (of %d) reads succeeded; want ≥ %d", readSuccesses, attempts, minReadSuccesses)
+	} else {
+		t.Logf("%d (of %d) reads succeeded (ok: ≥ %d)", readSuccesses, attempts, minReadSuccesses)
+	}
+}
diff --git a/src/cmd/go/internal/renameio/umask_test.go b/src/cmd/go/internal/renameio/umask_test.go
new file mode 100644
index 0000000..65e4fa5
--- /dev/null
+++ b/src/cmd/go/internal/renameio/umask_test.go
@@ -0,0 +1,42 @@
+// 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 !plan9,!windows,!js
+
+package renameio
+
+import (
+	"io/fs"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+)
+
+func TestWriteFileModeAppliesUmask(t *testing.T) {
+	dir, err := os.MkdirTemp("", "renameio")
+	if err != nil {
+		t.Fatalf("Failed to create temporary directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	const mode = 0644
+	const umask = 0007
+	defer syscall.Umask(syscall.Umask(umask))
+
+	file := filepath.Join(dir, "testWrite")
+	err = WriteFile(file, []byte("go-build"), mode)
+	if err != nil {
+		t.Fatalf("Failed to write file: %v", err)
+	}
+
+	fi, err := os.Stat(file)
+	if err != nil {
+		t.Fatalf("Stat %q (looking for mode %#o): %s", file, mode, err)
+	}
+
+	if fi.Mode()&fs.ModePerm != 0640 {
+		t.Errorf("Stat %q: mode %#o want %#o", file, fi.Mode()&fs.ModePerm, 0640)
+	}
+}