🧇 mmap: fix size handling

- Fix nil close on empty files.
- Handle size overflow.
- Handle incomplete mappings on Windows.

ref: https://github.com/golang/go/blob/master/src/cmd/go/internal/mmap/mmap_windows.go

mmap/mmap.go

@@ -7,24 +7,31 @@ import (
 	"unsafe"
 )
 
+var ErrFileTooLarge = errors.New("file too large")
+
 // ReadFile maps the named file into memory for reading.
 // On success, it returns the mapped data as a byte slice or a string,
 // and a function that unmaps the data.
 func ReadFile[T ~[]byte | ~string](name string) (data T, close func() error, err error) {
 	f, err := os.Open(name)
 	if err != nil {
-		return
+		return data, nil, err
 	}
 	defer f.Close()
 
 	fs, err := f.Stat()
 	if err != nil {
-		return
+		return data, nil, err
 	}
 
-	size := fs.Size()
-	if size == 0 {
-		return
+	size64 := fs.Size()
+	if size64 == 0 {
+		return data, func() error { return nil }, nil
+	}
+
+	size := int(size64)
+	if int64(size) != size64 {
+		return data, nil, ErrFileTooLarge
 	}
 
 	addr, close, err := readFile(f, uintptr(size))
@@ -39,7 +46,7 @@ func ReadFile[T ~[]byte | ~string](name string) (data T, close func() error, err
 	return *(*T)(unsafe.Pointer(&b)), close, nil
 }
 
-func readFileFallback[T ~[]byte | ~string](f *os.File, size int64) (data T, close func() error, err error) {
+func readFileFallback[T ~[]byte | ~string](f *os.File, size int) (data T, close func() error, err error) {
 	b := make([]byte, size)
 	if _, err = io.ReadFull(f, b); err != nil {
 		return

mmap/mmap_windows.go

@@ -7,7 +7,7 @@ import (
 	"golang.org/x/sys/windows"
 )
 
-func readFile(f *os.File, _ uintptr) (addr unsafe.Pointer, close func() error, err error) {
+func readFile(f *os.File, size uintptr) (addr unsafe.Pointer, close func() error, err error) {
 	handle, err := windows.CreateFileMapping(windows.Handle(f.Fd()), nil, windows.PAGE_READONLY, 0, 0, nil)
 	if err != nil {
 		return nil, nil, os.NewSyscallError("CreateFileMappingW", err)
@@ -18,6 +18,17 @@ func readFile(f *os.File, _ uintptr) (addr unsafe.Pointer, close func() error, e
 	if err != nil {
 		return nil, nil, os.NewSyscallError("MapViewOfFile", err)
 	}
+
+	var info windows.MemoryBasicInformation
+	if err := windows.VirtualQuery(addrUintptr, &info, unsafe.Sizeof(info)); err != nil {
+		_ = windows.UnmapViewOfFile(addrUintptr)
+		return nil, nil, os.NewSyscallError("VirtualQuery", err)
+	}
+	if info.RegionSize < size {
+		_ = windows.UnmapViewOfFile(addrUintptr)
+		return nil, nil, ErrFileTooLarge
+	}
+
 	return *(*unsafe.Pointer)(unsafe.Pointer(&addrUintptr)), // workaround for unsafeptr check in go vet, see https://github.com/golang/go/issues/58625
 		func() error {
 			return windows.UnmapViewOfFile(addrUintptr)