bpo-37811: FreeBSD, OSX: fix poll(2) usage in sockets module (GH-15202)

FreeBSD implementation of poll(2) restricts the timeout argument to be
either zero, or positive, or equal to INFTIM (-1).

Unless otherwise overridden, socket timeout defaults to -1. This value
is then converted to milliseconds (-1000) and used as argument to the
poll syscall. poll returns EINVAL (22), and the connection fails.

This bug was discovered during the EINTR handling testing, and the
reproduction code can be found in
https://bugs.python.org/issue23618 (see connect_eintr.py,
attached). On GNU/Linux, the example runs as expected.

This change is trivial:
If the supplied timeout value is negative, truncate it to -1.

Misc/ACKS

@@ -837,6 +837,7 @@ Lawrence Kesteloot
 Garvit Khatri
 Vivek Khera
 Dhiru Kholia
+Artem Khramov
 Akshit Khurana
 Sanyam Khurana
 Mads Kiilerich

Misc/NEWS.d/next/Library/2019-08-14-21-41-07.bpo-37811.d1xYj7.rst

@@ -0,0 +1,4 @@
+Fix ``socket`` module's ``socket.connect(address)`` function being unable to
+establish connection in case of interrupted system call. The problem was
+observed on all OSes which ``poll(2)`` system call can take only
+non-negative integers and -1 as a timeout value.

Modules/socketmodule.c

@@ -789,6 +789,17 @@ internal_select(PySocketSockObject *s, int writing, _PyTime_t interval,
     ms = _PyTime_AsMilliseconds(interval, _PyTime_ROUND_CEILING);
     assert(ms <= INT_MAX);
 
+    /* On some OSes, typically BSD-based ones, the timeout parameter of the
+       poll() syscall, when negative, must be exactly INFTIM, where defined,
+       or -1. See issue 37811. */
+    if (ms < 0) {
+#ifdef INFTIM
+        ms = INFTIM;
+#else
+        ms = -1;
+#endif
+    }
+
     Py_BEGIN_ALLOW_THREADS;
     n = poll(&pollfd, 1, (int)ms);
     Py_END_ALLOW_THREADS;