net: Conn.Read() returns Error with .Timeout()==true on KeepAlive failure

[networkimprov]

net Docs say:

A zero value for [deadline] means I/O operations will not time out.

A zero value for [read deadline] means Read will not time out.

KeepAlive specifies the keep-alive period for an active network connection.
If zero, keep-alives are enabled if supported by the protocol and operating system.
Network protocols or operating systems that do not support keep-alives ignore this field.
If negative, keep-alives are disabled.

For a TLS connection that's been severed, Conn.Read() returns a net.Error with .Timeout()==true after ~18 minutes, apparently due to KeepAlive failure. (Go 1.12, Linux, amd64)

The Error should probably give .Timeout()==false to comply with the docs. Code that checks for .Timeout()==true would generally assume that an explicit deadline had passed.

The .Error() value should mention keepalive. It's currently:
"read tcp n.n.n.n:p->n.n.n.n:p: read: connection timed out"

Since 1.11, net.Dialer.KeepAlive gets 15*time.Second if unset/zero. This isn't documented in package net.

cc @bradfitz

[networkimprov]

Also filed #31490 to report that tls.DialWithDialer() doesn't respect .KeepAlive.

[CAFxX]

Since 1.11, net.Dialer.KeepAlive gets 15*time.Second if unset/zero. This isn't documented in package net.

It is documented that we enable keep-alives if that field is unset/zero. The choice of not specifying 15s in the docs was deliberate and is actually documented in the commit message: 5bd7e9c