cmd/link: skip fallocate test if not supported, and adjust allocation size on darwin

On Linux, the linker uses fallocate to preallocate the output
file storage. The underlying file system may not support
fallocate, causing the test to fail. Skip the test in this case.

On darwin, apparently F_PREALLOCATE allocates from the end of the
allocation instead of the logical end of the file. Adjust the
size calculation.

Fixes #39905.

Change-Id: I01e676737fd2619ebbdba05c7cf7f424ec27de35
Reviewed-on: https://go-review.googlesource.com/c/go/+/240618
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: cherrymui

src/cmd/link/internal/ld/fallocate_test.go

@@ -28,6 +28,21 @@ func TestFallocate(t *testing.T) {
 	}
 	defer out.Close()
 
+	// Try fallocate first.
+	for {
+		err = out.fallocate(1 << 10)
+		if err == syscall.EOPNOTSUPP { // The underlying file system may not support fallocate
+			t.Skip("fallocate is not supported")
+		}
+		if err == syscall.EINTR {
+			continue // try again
+		}
+		if err != nil {
+			t.Fatalf("fallocate failed: %v", err)
+		}
+		break
+	}
+
 	// Mmap 1 MiB initially, and grow to 2 and 3 MiB.
 	// Check if the file size and disk usage is expected.
 	for _, sz := range []int64{1 << 20, 2 << 20, 3 << 20} {

src/cmd/link/internal/ld/outbuf_darwin.go

@@ -14,7 +14,10 @@ func (out *OutBuf) fallocate(size uint64) error {
 	if err != nil {
 		return err
 	}
-	cursize := uint64(stat.Size())
+	// F_PEOFPOSMODE allocates from the end of the file, so we want the size difference.
+	// Apparently, it uses the end of the allocation, instead of the logical end of the
+	// the file.
+	cursize := uint64(stat.Sys().(*syscall.Stat_t).Blocks * 512) // allocated size
 	if size <= cursize {
 		return nil
 	}
@@ -23,7 +26,7 @@ func (out *OutBuf) fallocate(size uint64) error {
 		Flags:   syscall.F_ALLOCATEALL,
 		Posmode: syscall.F_PEOFPOSMODE,
 		Offset:  0,
-		Length:  int64(size - cursize), // F_PEOFPOSMODE allocates from the end of the file, so we want the size difference here
+		Length:  int64(size - cursize),
 	}
 
 	_, _, errno := syscall.Syscall(syscall.SYS_FCNTL, uintptr(out.f.Fd()), syscall.F_PREALLOCATE, uintptr(unsafe.Pointer(store)))