os: add FileHandle.SysHandle()

Author: dkegel-fastly

src/os/file.go

@@ -180,6 +180,13 @@ func (f *File) Truncate(size int64) error {
 	return &PathError{"truncate", f.name, ErrNotImplemented}
 }
 
+// SysHandle returns the underlying system filehandle, if any.
+// (Upstream doesn't need this, as it indirects File to system-specific file.)
+// return value should be syscallFd, but that's private.
+func (f *File) SysHandle() uintptr {
+	return f.handle.SysHandle()
+}
+
 // PathError records an error and the operation and file path that caused it.
 // TODO: PathError moved to io/fs in go 1.16 and left an alias in os/errors.go.
 // Do the same once we drop support for go 1.15.

src/os/file_other.go

@@ -50,6 +50,11 @@ func (f stdioFileHandle) Close() error {
 	return ErrUnsupported
 }
 
+// SysHandle returns the underlying system filehandle, if any.
+func (f stdioFileHandle) SysHandle() uintptr {
+	return uintptr(f)
+}
+
 //go:linkname putchar runtime.putchar
 func putchar(c byte)
 

src/os/file_unix.go

@@ -55,3 +55,8 @@ func (f unixFileHandle) ReadAt(b []byte, offset int64) (n int, err error) {
 	}
 	return
 }
+
+// SysHandle returns the underlying system filehandle, if any.
+func (f unixFileHandle) SysHandle() uintptr {
+	return uintptr(f)
+}

src/os/file_windows.go

@@ -57,6 +57,11 @@ func (f unixFileHandle) ReadAt(b []byte, offset int64) (n int, err error) {
 	return -1, ErrNotImplemented
 }
 
+// SysHandle returns the underlying system filehandle, if any.
+func (f unixFileHandle) SysHandle() uintptr {
+	return uintptr(f)
+}
+
 // isWindowsNulName reports whether name is os.DevNull ('NUL') on Windows.
 // True is returned if name is 'NUL' whatever the case.
 func isWindowsNulName(name string) bool {

src/os/filesystem.go

@@ -54,6 +54,9 @@ type FileHandle interface {
 
 	// Close closes the file, making it unusable for further writes.
 	Close() (err error)
+
+	// SysHandle returns the underlying system filehandle, if any.
+	SysHandle() uintptr
 }
 
 // findMount returns the appropriate (mounted) filesystem to use for a given