cmd/link: fix size calculation for file space preallocation on darwin

On darwin, we preallocate file storage space with fcntl
F_ALLOCATEALL in F_PEOFPOSMODE mode. This is specified as
allocating from the physical end of the file. So the size we give
it should be the increment, instead of the total size.

Fixes #39044.

Change-Id: I10c7ee8d51f237b4a7604233ac7abc6f91dcd602
Reviewed-on: https://go-review.googlesource.com/c/go/+/234481
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

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

@@ -0,0 +1,50 @@
+// 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.
+
+// +build darwin linux
+
+package ld
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+)
+
+func TestFallocate(t *testing.T) {
+	dir, err := ioutil.TempDir("", "TestFallocate")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+	filename := filepath.Join(dir, "a.out")
+	out := NewOutBuf(nil)
+	err = out.Open(filename)
+	if err != nil {
+		t.Fatalf("Open file failed: %v", err)
+	}
+	defer out.Close()
+
+	// 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} {
+		err = out.Mmap(uint64(sz))
+		if err != nil {
+			t.Fatalf("Mmap failed: %v", err)
+		}
+		stat, err := os.Stat(filename)
+		if err != nil {
+			t.Fatalf("Stat failed: %v", err)
+		}
+		if got := stat.Size(); got != sz {
+			t.Errorf("unexpected file size: got %d, want %d", got, sz)
+		}
+		if got, want := stat.Sys().(*syscall.Stat_t).Blocks, (sz+511)/512; got != want {
+			t.Errorf("unexpected disk usage: got %d blocks, want %d", got, want)
+		}
+		out.munmap()
+	}
+}

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

@@ -10,16 +10,25 @@ import (
 )
 
 func (out *OutBuf) fallocate(size uint64) error {
+	stat, err := out.f.Stat()
+	if err != nil {
+		return err
+	}
+	cursize := uint64(stat.Size())
+	if size <= cursize {
+		return nil
+	}
+
 	store := &syscall.Fstore_t{
 		Flags:   syscall.F_ALLOCATEALL,
 		Posmode: syscall.F_PEOFPOSMODE,
 		Offset:  0,
-		Length:  int64(size),
+		Length:  int64(size - cursize), // F_PEOFPOSMODE allocates from the end of the file, so we want the size difference here
 	}
 
-	_, _, err := syscall.Syscall(syscall.SYS_FCNTL, uintptr(out.f.Fd()), syscall.F_PREALLOCATE, uintptr(unsafe.Pointer(store)))
-	if err != 0 {
-		return err
+	_, _, errno := syscall.Syscall(syscall.SYS_FCNTL, uintptr(out.f.Fd()), syscall.F_PREALLOCATE, uintptr(unsafe.Pointer(store)))
+	if errno != 0 {
+		return errno
 	}
 
 	return nil