From a9410281c2bcf7f56e6fd4bb409559ae362fe8c6 Mon Sep 17 00:00:00 2001 From: Ian Lance Taylor Date: Fri, 8 Dec 2017 06:40:43 -0800 Subject: [PATCH] net: calling File disables the SetDeadline methods This essentially applies https://golang.org/cl/81636 to the net package. 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 File 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 socket 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 socket 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 File 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(f.Fd(), true) to change the descriptor, and the original socket, 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 we now have the RawConn approach to frobbing the descriptor, and hopefully most people can use that rather than calling File, I decided to punt. Updates #22934 Fixes #21862 Change-Id: If269da762f6f5a88c334e7b6d6f3998f7e10b11e Reviewed-on: https://go-review.googlesource.com/82915 Reviewed-by: Brad Fitzpatrick --- src/net/net.go | 2 ++ 1 file changed, 2 insertions(+) diff --git a/src/net/net.go b/src/net/net.go index 91ec048e0b..3ad91036e7 100644 --- a/src/net/net.go +++ b/src/net/net.go @@ -288,6 +288,8 @@ func (c *conn) SetWriteBuffer(bytes int) error { // The returned os.File's file descriptor is different from the connection's. // Attempting to change properties of the original using this duplicate // may or may not have the desired effect. +// +// On Unix systems this will cause the SetDeadline methods to stop working. func (c *conn) File() (f *os.File, err error) { f, err = c.fd.dup() if err != nil { -- 2.50.0