os: test that copying to append-only files doesn't fail on Linux

Before CL 494915, this test would fail on Linux in io.Copy with error
"write /dev/stdout: copy_file_range: bad file descriptor" because the
copy_file_range syscall doesn't support destination files opened with
O_APPEND, see
https://man7.org/linux/man-pages/man2/copy_file_range.2.html#ERRORS

For #60181

Change-Id: I2eb4bcac71175121821e0033eb2297a2bc4ec759
Reviewed-on: https://go-review.googlesource.com/c/go/+/517755
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>

diff --git a/src/os/os_test.go b/src/os/os_test.go
index 94c3ad04f352ac7a6bda338dd41f3f4181df8750..28256ee213ec4804b53e2e8d3e6bca46a3031db3 100644 (file)
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -3270,3 +3270,59 @@ func TestPipeCloseRace(t *testing.T) {
                t.Errorf("got nils %d errs %d, want 2 2", nils, errs)
        }
 }
+
+// Test that copying to files opened with O_APPEND works and
+// the copy_file_range syscall isn't used on Linux.
+//
+// Regression test for go.dev/issue/60181
+func TestIssue60181(t *testing.T) {
+       defer chtmpdir(t)()
+
+       want := "hello gopher"
+
+       a, err := CreateTemp("", "a")
+       if err != nil {
+               t.Fatal(err)
+       }
+       a.WriteString(want[:5])
+       a.Close()
+
+       b, err := CreateTemp("", "b")
+       if err != nil {
+               t.Fatal(err)
+       }
+       b.WriteString(want[5:])
+       b.Close()
+
+       afd, err := syscall.Open(a.Name(), syscall.O_RDWR|syscall.O_APPEND, 0)
+       if err != nil {
+               t.Fatal(err)
+       }
+
+       bfd, err := syscall.Open(b.Name(), syscall.O_RDONLY, 0)
+       if err != nil {
+               t.Fatal(err)
+       }
+
+       aa := NewFile(uintptr(afd), a.Name())
+       defer aa.Close()
+       bb := NewFile(uintptr(bfd), b.Name())
+       defer bb.Close()
+
+       // This would fail on Linux in case the copy_file_range syscall was used because it doesn't
+       // support destination files opened with O_APPEND, see
+       // https://man7.org/linux/man-pages/man2/copy_file_range.2.html#ERRORS
+       _, err = io.Copy(aa, bb)
+       if err != nil {
+               t.Fatal(err)
+       }
+
+       buf, err := ReadFile(aa.Name())
+       if err != nil {
+               t.Fatal(err)
+       }
+
+       if got := string(buf); got != want {
+               t.Errorf("files not concatenated: got %q, want %q", got, want)
+       }
+}