Fix macOS osAllocate.

Mozilla is just wrong.
https://searchfox.org/mozilla-central/source/xpcom/glue/FileUtils.cpp

vfs/os_darwin.go

@@ -39,12 +39,11 @@ func osAllocate(file *os.File, size int64) error {
 		return nil
 	}
 
-	// https://stackoverflow.com/a/11497568/867786
 	store := unix.Fstore_t{
-		Flags:   unix.F_ALLOCATECONTIG,
+		Flags:   unix.F_ALLOCATEALL | unix.F_ALLOCATECONTIG,
 		Posmode: unix.F_PEOFPOSMODE,
 		Offset:  0,
-		Length:  size,
+		Length:  size - off,
 	}
 
 	// Try to get a continuous chunk of disk space.

vfs/os_unix_test.go

@@ -0,0 +1,52 @@
+//go:build unix && !sqlite3_flock && !sqlite3_nosys
+
+package vfs
+
+import (
+	"crypto/rand"
+	"io"
+	"os"
+	"runtime"
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func Test_osAllocate(t *testing.T) {
+	if runtime.GOOS != "linux" && runtime.GOOS != "darwin" {
+		t.Skip()
+	}
+
+	f, err := os.CreateTemp(t.TempDir(), "file")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = io.CopyN(f, rand.Reader, 1024*1024)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	n, err := f.Seek(0, io.SeekEnd)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 1024*1024 {
+		t.Fatalf("got %d, want %d", n, 1024*1024)
+	}
+
+	err = osAllocate(f, 16*1024*1024)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var stat unix.Stat_t
+	err = unix.Stat(f.Name(), &stat)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if stat.Blocks*512 != 16*1024*1024 {
+		t.Fatalf("got %d, want %d", stat.Blocks*512, 16*1024*1024)
+	}
+}