os: calling Fd disables the SetDeadline methods

The full truth seems too complicated to write in this method's doc, so
I'm going with a simple half truth.

The full truth is that Fd returns the descriptor in blocking mode,
because that is historically how it worked, and existing programs
would be surprised if the descriptor is suddenly non-blocking. On Unix
systems whether a file is non-blocking or not is a property of the
underlying file description, not of a particular file descriptor, so
changing the returned descriptor to blocking mode also changes the
existing File to blocking mode. Blocking mode works fine, althoug I/O
operations now take up a thread. SetDeadline and friends rely on the
runtime poller, and the runtime poller only works if the descriptor is
non-blocking. So it's correct that calling Fd disables SetDeadline.
The other half of the truth is that if the program is willing to work
with a non-blocking descriptor, it could call
syscall.SetNonblock(descriptor, true) to change the descriptor, and
the original File, to non-blocking mode. At that point SetDeadline
would start working again. I tried to write that in a way that is
short and comprehensible but failed. Since deadlines mostly work on
pipes, and there isn't much reason to call Fd on a pipe, and few
people use SetDeadline, I decided to punt.

Fixes #22934

Change-Id: I2e49e036f0bcf71f5365193831696f9e4120527c
Reviewed-on: https://go-review.googlesource.com/81636
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

diff --git a/src/os/file_plan9.go b/src/os/file_plan9.go
index 74c377127dd9ac866283816e5ce2a16482eb24b1..e4f8fd987bcdcfef11ba2f5b7d1a0c18a1f1004b 100644 (file)
--- a/src/os/file_plan9.go
+++ b/src/os/file_plan9.go
@@ -29,6 +29,7 @@ type file struct {
 
 // 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.
+// On Unix systems this will cause the SetDeadline methods to stop working.
 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 84a2bb5f00faf626843400616ff3722ac4ff1a41..b834f525890fa11eaeee5f5fba0ea4d0c0134517 100644 (file)
--- a/src/os/file_unix.go
+++ b/src/os/file_unix.go
@@ -54,6 +54,7 @@ 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.
+// On Unix systems this will cause the SetDeadline methods to stop working.
 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 c8307a6d22676670c989ac0855c8d3ca4a250b61..81a0ab73467a14691e441137d276191e8c81713b 100644 (file)
--- a/src/os/file_windows.go
+++ b/src/os/file_windows.go
@@ -25,6 +25,7 @@ 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.
+// On Unix systems this will cause the SetDeadline methods to stop working.
 func (file *File) Fd() uintptr {
        if file == nil {
                return uintptr(syscall.InvalidHandle)