os: document that users of Fd should keep f alive

Fixes issue 9046.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/162680043

3 files changed, 4 insertions, 1 deletions

diff --git a/src/os/file_plan9.go b/src/os/file_plan9.go
index 5efc2a4f1..132594eed 100644
--- a/src/os/file_plan9.go
+++ b/src/os/file_plan9.go
@@ -25,7 +25,8 @@ type file struct {
 	dirinfo *dirInfo // nil unless directory being read
 }
 
-// Fd returns the integer Unix file descriptor referencing the open file.
+// Fd returns the integer Plan 9 file descriptor referencing the open file.
+// The file descriptor is valid only until f.Close is called or f is garbage collected.
 func (f *File) Fd() uintptr {
 	if f == nil {
 		return ^(uintptr(0))
diff --git a/src/os/file_unix.go b/src/os/file_unix.go
index f59d563e6..ff4fc7d12 100644
--- a/src/os/file_unix.go
+++ b/src/os/file_unix.go
@@ -29,6 +29,7 @@ type file struct {
 }
 
 // Fd returns the integer Unix file descriptor referencing the open file.
+// The file descriptor is valid only until f.Close is called or f is garbage collected.
 func (f *File) Fd() uintptr {
 	if f == nil {
 		return ^(uintptr(0))
diff --git a/src/os/file_windows.go b/src/os/file_windows.go
index 3b5519390..2a90a5055 100644
--- a/src/os/file_windows.go
+++ b/src/os/file_windows.go
@@ -36,6 +36,7 @@ type file struct {
 }
 
 // Fd returns the Windows handle referencing the open file.
+// The handle is valid only until f.Close is called or f is garbage collected.
 func (file *File) Fd() uintptr {
 	if file == nil {
 		return uintptr(syscall.InvalidHandle)