src/os/readfrom_linux_test.go - go - Git at Google

 // 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 os_test

 import (
 	"bytes"
 	"internal/poll"
 	"io"
 	"math/rand"
 	"os"
 	. "os"
 	"path/filepath"
 	"strconv"
 	"strings"
 	"syscall"
 	"testing"
 	"time"
 )

 func TestCopyFileRange(t *testing.T) {
 	sizes := []int{
 		1,
 		42,
 		1025,
 		syscall.Getpagesize() + 1,
 		32769,
 	}
 	t.Run("Basic", func(t *testing.T) {
 		for _, size := range sizes {
 			t.Run(strconv.Itoa(size), func(t *testing.T) {
 				testCopyFileRange(t, int64(size), -1)
 			})
 		}
 	})
 	t.Run("Limited", func(t *testing.T) {
 		t.Run("OneLess", func(t *testing.T) {
 			for _, size := range sizes {
 				t.Run(strconv.Itoa(size), func(t *testing.T) {
 					testCopyFileRange(t, int64(size), int64(size)-1)
 				})
 			}
 		})
 		t.Run("Half", func(t *testing.T) {
 			for _, size := range sizes {
 				t.Run(strconv.Itoa(size), func(t *testing.T) {
 					testCopyFileRange(t, int64(size), int64(size)/2)
 				})
 			}
 		})
 		t.Run("More", func(t *testing.T) {
 			for _, size := range sizes {
 				t.Run(strconv.Itoa(size), func(t *testing.T) {
 					testCopyFileRange(t, int64(size), int64(size)+7)
 				})
 			}
 		})
 	})
 	t.Run("DoesntTryInAppendMode", func(t *testing.T) {
 		dst, src, data, hook := newCopyFileRangeTest(t, 42)

 		dst2, err := OpenFile(dst.Name(), O_RDWR|O_APPEND, 0755)
 		if err != nil {
 			t.Fatal(err)
 		}
 		defer dst2.Close()

 		if _, err := io.Copy(dst2, src); err != nil {
 			t.Fatal(err)
 		}
 		if hook.called {
 			t.Fatal("called poll.CopyFileRange for destination in O_APPEND mode")
 		}
 		mustSeekStart(t, dst2)
 		mustContainData(t, dst2, data) // through traditional means
 	})
 	t.Run("CopyFileItself", func(t *testing.T) {
 		hook := hookCopyFileRange(t)

 		f, err := os.CreateTemp("", "file-readfrom-itself-test")
 		if err != nil {
 			t.Fatalf("failed to create tmp file: %v", err)
 		}
 		t.Cleanup(func() {
 			f.Close()
 			os.Remove(f.Name())
 		})

 		data := []byte("hello world!")
 		if _, err := f.Write(data); err != nil {
 			t.Fatalf("failed to create and feed the file: %v", err)
 		}

 		if err := f.Sync(); err != nil {
 			t.Fatalf("failed to save the file: %v", err)
 		}

 		// Rewind it.
 		if _, err := f.Seek(0, io.SeekStart); err != nil {
 			t.Fatalf("failed to rewind the file: %v", err)
 		}

 		// Read data from the file itself.
 		if _, err := io.Copy(f, f); err != nil {
 			t.Fatalf("failed to read from the file: %v", err)
 		}

 		if !hook.called || hook.written != 0 || hook.handled || hook.err != nil {
 			t.Fatalf("poll.CopyFileRange should be called and return the EINVAL error, but got hook.called=%t, hook.err=%v", hook.called, hook.err)
 		}

 		// Rewind it.
 		if _, err := f.Seek(0, io.SeekStart); err != nil {
 			t.Fatalf("failed to rewind the file: %v", err)
 		}

 		data2, err := io.ReadAll(f)
 		if err != nil {
 			t.Fatalf("failed to read from the file: %v", err)
 		}

 		// It should wind up a double of the original data.
 		if strings.Repeat(string(data), 2) != string(data2) {
 			t.Fatalf("data mismatch: %s != %s", string(data), string(data2))
 		}
 	})
 	t.Run("NotRegular", func(t *testing.T) {
 		t.Run("BothPipes", func(t *testing.T) {
 			hook := hookCopyFileRange(t)

 			pr1, pw1, err := Pipe()
 			if err != nil {
 				t.Fatal(err)
 			}
 			defer pr1.Close()
 			defer pw1.Close()

 			pr2, pw2, err := Pipe()
 			if err != nil {
 				t.Fatal(err)
 			}
 			defer pr2.Close()
 			defer pw2.Close()

 			// The pipe is empty, and PIPE_BUF is large enough
 			// for this, by (POSIX) definition, so there is no
 			// need for an additional goroutine.
 			data := []byte("hello")
 			if _, err := pw1.Write(data); err != nil {
 				t.Fatal(err)
 			}
 			pw1.Close()

 			n, err := io.Copy(pw2, pr1)
 			if err != nil {
 				t.Fatal(err)
 			}
 			if n != int64(len(data)) {
 				t.Fatalf("transferred %d, want %d", n, len(data))
 			}
 			if !hook.called {
 				t.Fatalf("should have called poll.CopyFileRange")
 			}
 			pw2.Close()
 			mustContainData(t, pr2, data)
 		})
 		t.Run("DstPipe", func(t *testing.T) {
 			dst, src, data, hook := newCopyFileRangeTest(t, 255)
 			dst.Close()

 			pr, pw, err := Pipe()
 			if err != nil {
 				t.Fatal(err)
 			}
 			defer pr.Close()
 			defer pw.Close()

 			n, err := io.Copy(pw, src)
 			if err != nil {
 				t.Fatal(err)
 			}
 			if n != int64(len(data)) {
 				t.Fatalf("transferred %d, want %d", n, len(data))
 			}
 			if !hook.called {
 				t.Fatalf("should have called poll.CopyFileRange")
 			}
 			pw.Close()
 			mustContainData(t, pr, data)
 		})
 		t.Run("SrcPipe", func(t *testing.T) {
 			dst, src, data, hook := newCopyFileRangeTest(t, 255)
 			src.Close()

 			pr, pw, err := Pipe()
 			if err != nil {
 				t.Fatal(err)
 			}
 			defer pr.Close()
 			defer pw.Close()

 			// The pipe is empty, and PIPE_BUF is large enough
 			// for this, by (POSIX) definition, so there is no
 			// need for an additional goroutine.
 			if _, err := pw.Write(data); err != nil {
 				t.Fatal(err)
 			}
 			pw.Close()

 			n, err := io.Copy(dst, pr)
 			if err != nil {
 				t.Fatal(err)
 			}
 			if n != int64(len(data)) {
 				t.Fatalf("transferred %d, want %d", n, len(data))
 			}
 			if !hook.called {
 				t.Fatalf("should have called poll.CopyFileRange")
 			}
 			mustSeekStart(t, dst)
 			mustContainData(t, dst, data)
 		})
 	})
 	t.Run("Nil", func(t *testing.T) {
 		var nilFile *File
 		anyFile, err := os.CreateTemp("", "")
 		if err != nil {
 			t.Fatal(err)
 		}
 		defer Remove(anyFile.Name())
 		defer anyFile.Close()

 		if _, err := io.Copy(nilFile, nilFile); err != ErrInvalid {
 			t.Errorf("io.Copy(nilFile, nilFile) = %v, want %v", err, ErrInvalid)
 		}
 		if _, err := io.Copy(anyFile, nilFile); err != ErrInvalid {
 			t.Errorf("io.Copy(anyFile, nilFile) = %v, want %v", err, ErrInvalid)
 		}
 		if _, err := io.Copy(nilFile, anyFile); err != ErrInvalid {
 			t.Errorf("io.Copy(nilFile, anyFile) = %v, want %v", err, ErrInvalid)
 		}

 		if _, err := nilFile.ReadFrom(nilFile); err != ErrInvalid {
 			t.Errorf("nilFile.ReadFrom(nilFile) = %v, want %v", err, ErrInvalid)
 		}
 		if _, err := anyFile.ReadFrom(nilFile); err != ErrInvalid {
 			t.Errorf("anyFile.ReadFrom(nilFile) = %v, want %v", err, ErrInvalid)
 		}
 		if _, err := nilFile.ReadFrom(anyFile); err != ErrInvalid {
 			t.Errorf("nilFile.ReadFrom(anyFile) = %v, want %v", err, ErrInvalid)
 		}
 	})
 }

 func testCopyFileRange(t *testing.T, size int64, limit int64) {
 	dst, src, data, hook := newCopyFileRangeTest(t, size)

 	// If we have a limit, wrap the reader.
 	var (
 		realsrc io.Reader
 		lr      *io.LimitedReader
 	)
 	if limit >= 0 {
 		lr = &io.LimitedReader{N: limit, R: src}
 		realsrc = lr
 		if limit < int64(len(data)) {
 			data = data[:limit]
 		}
 	} else {
 		realsrc = src
 	}

 	// Now call ReadFrom (through io.Copy), which will hopefully call
 	// poll.CopyFileRange.
 	n, err := io.Copy(dst, realsrc)
 	if err != nil {
 		t.Fatal(err)
 	}

 	// If we didn't have a limit, we should have called poll.CopyFileRange
 	// with the right file descriptor arguments.
 	if limit > 0 && !hook.called {
 		t.Fatal("never called poll.CopyFileRange")
 	}
 	if hook.called && hook.dstfd != int(dst.Fd()) {
 		t.Fatalf("wrong destination file descriptor: got %d, want %d", hook.dstfd, dst.Fd())
 	}
 	if hook.called && hook.srcfd != int(src.Fd()) {
 		t.Fatalf("wrong source file descriptor: got %d, want %d", hook.srcfd, src.Fd())
 	}

 	// Check that the offsets after the transfer make sense, that the size
 	// of the transfer was reported correctly, and that the destination
 	// file contains exactly the bytes we expect it to contain.
 	dstoff, err := dst.Seek(0, io.SeekCurrent)
 	if err != nil {
 		t.Fatal(err)
 	}
 	srcoff, err := src.Seek(0, io.SeekCurrent)
 	if err != nil {
 		t.Fatal(err)
 	}
 	if dstoff != srcoff {
 		t.Errorf("offsets differ: dstoff = %d, srcoff = %d", dstoff, srcoff)
 	}
 	if dstoff != int64(len(data)) {
 		t.Errorf("dstoff = %d, want %d", dstoff, len(data))
 	}
 	if n != int64(len(data)) {
 		t.Errorf("short ReadFrom: wrote %d bytes, want %d", n, len(data))
 	}
 	mustSeekStart(t, dst)
 	mustContainData(t, dst, data)

 	// If we had a limit, check that it was updated.
 	if lr != nil {
 		if want := limit - n; lr.N != want {
 			t.Fatalf("didn't update limit correctly: got %d, want %d", lr.N, want)
 		}
 	}
 }

 // newCopyFileRangeTest initializes a new test for copy_file_range.
 //
 // It creates source and destination files, and populates the source file
 // with random data of the specified size. It also hooks package os' call
 // to poll.CopyFileRange and returns the hook so it can be inspected.
 func newCopyFileRangeTest(t *testing.T, size int64) (dst, src *File, data []byte, hook *copyFileRangeHook) {
 	t.Helper()

 	hook = hookCopyFileRange(t)
 	tmp := t.TempDir()

 	src, err := Create(filepath.Join(tmp, "src"))
 	if err != nil {
 		t.Fatal(err)
 	}
 	t.Cleanup(func() { src.Close() })

 	dst, err = Create(filepath.Join(tmp, "dst"))
 	if err != nil {
 		t.Fatal(err)
 	}
 	t.Cleanup(func() { dst.Close() })

 	// Populate the source file with data, then rewind it, so it can be
 	// consumed by copy_file_range(2).
 	prng := rand.New(rand.NewSource(time.Now().Unix()))
 	data = make([]byte, size)
 	prng.Read(data)
 	if _, err := src.Write(data); err != nil {
 		t.Fatal(err)
 	}
 	if _, err := src.Seek(0, io.SeekStart); err != nil {
 		t.Fatal(err)
 	}

 	return dst, src, data, hook
 }

 // mustContainData ensures that the specified file contains exactly the
 // specified data.
 func mustContainData(t *testing.T, f *File, data []byte) {
 	t.Helper()

 	got := make([]byte, len(data))
 	if _, err := io.ReadFull(f, got); err != nil {
 		t.Fatal(err)
 	}
 	if !bytes.Equal(got, data) {
 		t.Fatalf("didn't get the same data back from %s", f.Name())
 	}
 	if _, err := f.Read(make([]byte, 1)); err != io.EOF {
 		t.Fatalf("not at EOF")
 	}
 }

 func mustSeekStart(t *testing.T, f *File) {
 	if _, err := f.Seek(0, io.SeekStart); err != nil {
 		t.Fatal(err)
 	}
 }

 func hookCopyFileRange(t *testing.T) *copyFileRangeHook {
 	h := new(copyFileRangeHook)
 	h.install()
 	t.Cleanup(h.uninstall)
 	return h
 }

 type copyFileRangeHook struct {
 	called bool
 	dstfd  int
 	srcfd  int
 	remain int64

 	written int64
 	handled bool
 	err     error

 	original func(dst, src *poll.FD, remain int64) (int64, bool, error)
 }

 func (h *copyFileRangeHook) install() {
 	h.original = *PollCopyFileRangeP
 	*PollCopyFileRangeP = func(dst, src *poll.FD, remain int64) (int64, bool, error) {
 		h.called = true
 		h.dstfd = dst.Sysfd
 		h.srcfd = src.Sysfd
 		h.remain = remain
 		h.written, h.handled, h.err = h.original(dst, src, remain)
 		return h.written, h.handled, h.err
 	}
 }

 func (h *copyFileRangeHook) uninstall() {
 	*PollCopyFileRangeP = h.original
 }

 // On some kernels copy_file_range fails on files in /proc.
 func TestProcCopy(t *testing.T) {
 	t.Parallel()

 	const cmdlineFile = "/proc/self/cmdline"
 	cmdline, err := os.ReadFile(cmdlineFile)
 	if err != nil {
 		t.Skipf("can't read /proc file: %v", err)
 	}
 	in, err := os.Open(cmdlineFile)
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer in.Close()
 	outFile := filepath.Join(t.TempDir(), "cmdline")
 	out, err := os.Create(outFile)
 	if err != nil {
 		t.Fatal(err)
 	}
 	if _, err := io.Copy(out, in); err != nil {
 		t.Fatal(err)
 	}
 	if err := out.Close(); err != nil {
 		t.Fatal(err)
 	}
 	copy, err := os.ReadFile(outFile)
 	if err != nil {
 		t.Fatal(err)
 	}
 	if !bytes.Equal(cmdline, copy) {
 		t.Errorf("copy of %q got %q want %q\n", cmdlineFile, copy, cmdline)
 	}
 }
	// 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 os_test

	import (
	"bytes"
	"internal/poll"
	"io"
	"math/rand"
	"os"
	. "os"
	"path/filepath"
	"strconv"
	"strings"
	"syscall"
	"testing"
	"time"
	)

	func TestCopyFileRange(t *testing.T) {
	sizes := []int{
	1,
	42,
	1025,
	syscall.Getpagesize() + 1,
	32769,
	}
	t.Run("Basic", func(t *testing.T) {
	for _, size := range sizes {
	t.Run(strconv.Itoa(size), func(t *testing.T) {
	testCopyFileRange(t, int64(size), -1)
	})
	}
	})
	t.Run("Limited", func(t *testing.T) {
	t.Run("OneLess", func(t *testing.T) {
	for _, size := range sizes {
	t.Run(strconv.Itoa(size), func(t *testing.T) {
	testCopyFileRange(t, int64(size), int64(size)-1)
	})
	}
	})
	t.Run("Half", func(t *testing.T) {
	for _, size := range sizes {
	t.Run(strconv.Itoa(size), func(t *testing.T) {
	testCopyFileRange(t, int64(size), int64(size)/2)
	})
	}
	})
	t.Run("More", func(t *testing.T) {
	for _, size := range sizes {
	t.Run(strconv.Itoa(size), func(t *testing.T) {
	testCopyFileRange(t, int64(size), int64(size)+7)
	})
	}
	})
	})
	t.Run("DoesntTryInAppendMode", func(t *testing.T) {
	dst, src, data, hook := newCopyFileRangeTest(t, 42)

	dst2, err := OpenFile(dst.Name(), O_RDWR\|O_APPEND, 0755)
	if err != nil {
	t.Fatal(err)
	}
	defer dst2.Close()

	if _, err := io.Copy(dst2, src); err != nil {
	t.Fatal(err)
	}
	if hook.called {
	t.Fatal("called poll.CopyFileRange for destination in O_APPEND mode")
	}
	mustSeekStart(t, dst2)
	mustContainData(t, dst2, data) // through traditional means
	})
	t.Run("CopyFileItself", func(t *testing.T) {
	hook := hookCopyFileRange(t)

	f, err := os.CreateTemp("", "file-readfrom-itself-test")
	if err != nil {
	t.Fatalf("failed to create tmp file: %v", err)
	}
	t.Cleanup(func() {
	f.Close()
	os.Remove(f.Name())
	})

	data := []byte("hello world!")
	if _, err := f.Write(data); err != nil {
	t.Fatalf("failed to create and feed the file: %v", err)
	}

	if err := f.Sync(); err != nil {
	t.Fatalf("failed to save the file: %v", err)
	}

	// Rewind it.
	if _, err := f.Seek(0, io.SeekStart); err != nil {
	t.Fatalf("failed to rewind the file: %v", err)
	}

	// Read data from the file itself.
	if _, err := io.Copy(f, f); err != nil {
	t.Fatalf("failed to read from the file: %v", err)
	}

	if !hook.called \|\| hook.written != 0 \|\| hook.handled \|\| hook.err != nil {
	t.Fatalf("poll.CopyFileRange should be called and return the EINVAL error, but got hook.called=%t, hook.err=%v", hook.called, hook.err)
	}

	// Rewind it.
	if _, err := f.Seek(0, io.SeekStart); err != nil {
	t.Fatalf("failed to rewind the file: %v", err)
	}

	data2, err := io.ReadAll(f)
	if err != nil {
	t.Fatalf("failed to read from the file: %v", err)
	}

	// It should wind up a double of the original data.
	if strings.Repeat(string(data), 2) != string(data2) {
	t.Fatalf("data mismatch: %s != %s", string(data), string(data2))
	}
	})
	t.Run("NotRegular", func(t *testing.T) {
	t.Run("BothPipes", func(t *testing.T) {
	hook := hookCopyFileRange(t)

	pr1, pw1, err := Pipe()
	if err != nil {
	t.Fatal(err)
	}
	defer pr1.Close()
	defer pw1.Close()

	pr2, pw2, err := Pipe()
	if err != nil {
	t.Fatal(err)
	}
	defer pr2.Close()
	defer pw2.Close()

	// The pipe is empty, and PIPE_BUF is large enough
	// for this, by (POSIX) definition, so there is no
	// need for an additional goroutine.
	data := []byte("hello")
	if _, err := pw1.Write(data); err != nil {
	t.Fatal(err)
	}
	pw1.Close()

	n, err := io.Copy(pw2, pr1)
	if err != nil {
	t.Fatal(err)
	}
	if n != int64(len(data)) {
	t.Fatalf("transferred %d, want %d", n, len(data))
	}
	if !hook.called {
	t.Fatalf("should have called poll.CopyFileRange")
	}
	pw2.Close()
	mustContainData(t, pr2, data)
	})
	t.Run("DstPipe", func(t *testing.T) {
	dst, src, data, hook := newCopyFileRangeTest(t, 255)
	dst.Close()

	pr, pw, err := Pipe()
	if err != nil {
	t.Fatal(err)
	}
	defer pr.Close()
	defer pw.Close()

	n, err := io.Copy(pw, src)
	if err != nil {
	t.Fatal(err)
	}
	if n != int64(len(data)) {
	t.Fatalf("transferred %d, want %d", n, len(data))
	}
	if !hook.called {
	t.Fatalf("should have called poll.CopyFileRange")
	}
	pw.Close()
	mustContainData(t, pr, data)
	})
	t.Run("SrcPipe", func(t *testing.T) {
	dst, src, data, hook := newCopyFileRangeTest(t, 255)
	src.Close()

	pr, pw, err := Pipe()
	if err != nil {
	t.Fatal(err)
	}
	defer pr.Close()
	defer pw.Close()

	// The pipe is empty, and PIPE_BUF is large enough
	// for this, by (POSIX) definition, so there is no
	// need for an additional goroutine.
	if _, err := pw.Write(data); err != nil {
	t.Fatal(err)
	}
	pw.Close()

	n, err := io.Copy(dst, pr)
	if err != nil {
	t.Fatal(err)
	}
	if n != int64(len(data)) {
	t.Fatalf("transferred %d, want %d", n, len(data))
	}
	if !hook.called {
	t.Fatalf("should have called poll.CopyFileRange")
	}
	mustSeekStart(t, dst)
	mustContainData(t, dst, data)
	})
	})
	t.Run("Nil", func(t *testing.T) {
	var nilFile *File
	anyFile, err := os.CreateTemp("", "")
	if err != nil {
	t.Fatal(err)
	}
	defer Remove(anyFile.Name())
	defer anyFile.Close()

	if _, err := io.Copy(nilFile, nilFile); err != ErrInvalid {
	t.Errorf("io.Copy(nilFile, nilFile) = %v, want %v", err, ErrInvalid)
	}
	if _, err := io.Copy(anyFile, nilFile); err != ErrInvalid {
	t.Errorf("io.Copy(anyFile, nilFile) = %v, want %v", err, ErrInvalid)
	}
	if _, err := io.Copy(nilFile, anyFile); err != ErrInvalid {
	t.Errorf("io.Copy(nilFile, anyFile) = %v, want %v", err, ErrInvalid)
	}

	if _, err := nilFile.ReadFrom(nilFile); err != ErrInvalid {
	t.Errorf("nilFile.ReadFrom(nilFile) = %v, want %v", err, ErrInvalid)
	}
	if _, err := anyFile.ReadFrom(nilFile); err != ErrInvalid {
	t.Errorf("anyFile.ReadFrom(nilFile) = %v, want %v", err, ErrInvalid)
	}
	if _, err := nilFile.ReadFrom(anyFile); err != ErrInvalid {
	t.Errorf("nilFile.ReadFrom(anyFile) = %v, want %v", err, ErrInvalid)
	}
	})
	}

	func testCopyFileRange(t *testing.T, size int64, limit int64) {
	dst, src, data, hook := newCopyFileRangeTest(t, size)

	// If we have a limit, wrap the reader.
	var (
	realsrc io.Reader
	lr *io.LimitedReader
	)
	if limit >= 0 {
	lr = &io.LimitedReader{N: limit, R: src}
	realsrc = lr
	if limit < int64(len(data)) {
	data = data[:limit]
	}
	} else {
	realsrc = src
	}

	// Now call ReadFrom (through io.Copy), which will hopefully call
	// poll.CopyFileRange.
	n, err := io.Copy(dst, realsrc)
	if err != nil {
	t.Fatal(err)
	}

	// If we didn't have a limit, we should have called poll.CopyFileRange
	// with the right file descriptor arguments.
	if limit > 0 && !hook.called {
	t.Fatal("never called poll.CopyFileRange")
	}
	if hook.called && hook.dstfd != int(dst.Fd()) {
	t.Fatalf("wrong destination file descriptor: got %d, want %d", hook.dstfd, dst.Fd())
	}
	if hook.called && hook.srcfd != int(src.Fd()) {
	t.Fatalf("wrong source file descriptor: got %d, want %d", hook.srcfd, src.Fd())
	}

	// Check that the offsets after the transfer make sense, that the size
	// of the transfer was reported correctly, and that the destination
	// file contains exactly the bytes we expect it to contain.
	dstoff, err := dst.Seek(0, io.SeekCurrent)
	if err != nil {
	t.Fatal(err)
	}
	srcoff, err := src.Seek(0, io.SeekCurrent)
	if err != nil {
	t.Fatal(err)
	}
	if dstoff != srcoff {
	t.Errorf("offsets differ: dstoff = %d, srcoff = %d", dstoff, srcoff)
	}
	if dstoff != int64(len(data)) {
	t.Errorf("dstoff = %d, want %d", dstoff, len(data))
	}
	if n != int64(len(data)) {
	t.Errorf("short ReadFrom: wrote %d bytes, want %d", n, len(data))
	}
	mustSeekStart(t, dst)
	mustContainData(t, dst, data)

	// If we had a limit, check that it was updated.
	if lr != nil {
	if want := limit - n; lr.N != want {
	t.Fatalf("didn't update limit correctly: got %d, want %d", lr.N, want)
	}
	}
	}

	// newCopyFileRangeTest initializes a new test for copy_file_range.
	//
	// It creates source and destination files, and populates the source file
	// with random data of the specified size. It also hooks package os' call
	// to poll.CopyFileRange and returns the hook so it can be inspected.
	func newCopyFileRangeTest(t testing.T, size int64) (dst, src File, data []byte, hook *copyFileRangeHook) {
	t.Helper()

	hook = hookCopyFileRange(t)
	tmp := t.TempDir()

	src, err := Create(filepath.Join(tmp, "src"))
	if err != nil {
	t.Fatal(err)
	}
	t.Cleanup(func() { src.Close() })

	dst, err = Create(filepath.Join(tmp, "dst"))
	if err != nil {
	t.Fatal(err)
	}
	t.Cleanup(func() { dst.Close() })

	// Populate the source file with data, then rewind it, so it can be
	// consumed by copy_file_range(2).
	prng := rand.New(rand.NewSource(time.Now().Unix()))
	data = make([]byte, size)
	prng.Read(data)
	if _, err := src.Write(data); err != nil {
	t.Fatal(err)
	}
	if _, err := src.Seek(0, io.SeekStart); err != nil {
	t.Fatal(err)
	}

	return dst, src, data, hook
	}

	// mustContainData ensures that the specified file contains exactly the
	// specified data.
	func mustContainData(t testing.T, f File, data []byte) {
	t.Helper()

	got := make([]byte, len(data))
	if _, err := io.ReadFull(f, got); err != nil {
	t.Fatal(err)
	}
	if !bytes.Equal(got, data) {
	t.Fatalf("didn't get the same data back from %s", f.Name())
	}
	if _, err := f.Read(make([]byte, 1)); err != io.EOF {
	t.Fatalf("not at EOF")
	}
	}

	func mustSeekStart(t testing.T, f File) {
	if _, err := f.Seek(0, io.SeekStart); err != nil {
	t.Fatal(err)
	}
	}

	func hookCopyFileRange(t testing.T) copyFileRangeHook {
	h := new(copyFileRangeHook)
	h.install()
	t.Cleanup(h.uninstall)
	return h
	}

	type copyFileRangeHook struct {
	called bool
	dstfd int
	srcfd int
	remain int64

	written int64
	handled bool
	err error

	original func(dst, src *poll.FD, remain int64) (int64, bool, error)
	}

	func (h *copyFileRangeHook) install() {
	h.original = *PollCopyFileRangeP
	PollCopyFileRangeP = func(dst, src poll.FD, remain int64) (int64, bool, error) {
	h.called = true
	h.dstfd = dst.Sysfd
	h.srcfd = src.Sysfd
	h.remain = remain
	h.written, h.handled, h.err = h.original(dst, src, remain)
	return h.written, h.handled, h.err
	}
	}

	func (h *copyFileRangeHook) uninstall() {
	*PollCopyFileRangeP = h.original
	}

	// On some kernels copy_file_range fails on files in /proc.
	func TestProcCopy(t *testing.T) {
	t.Parallel()

	const cmdlineFile = "/proc/self/cmdline"
	cmdline, err := os.ReadFile(cmdlineFile)
	if err != nil {
	t.Skipf("can't read /proc file: %v", err)
	}
	in, err := os.Open(cmdlineFile)
	if err != nil {
	t.Fatal(err)
	}
	defer in.Close()
	outFile := filepath.Join(t.TempDir(), "cmdline")
	out, err := os.Create(outFile)
	if err != nil {
	t.Fatal(err)
	}
	if _, err := io.Copy(out, in); err != nil {
	t.Fatal(err)
	}
	if err := out.Close(); err != nil {
	t.Fatal(err)
	}
	copy, err := os.ReadFile(outFile)
	if err != nil {
	t.Fatal(err)
	}
	if !bytes.Equal(cmdline, copy) {
	t.Errorf("copy of %q got %q want %q\n", cmdlineFile, copy, cmdline)
	}
	}