proto: factor ff_network_wait_fd and use it on udp

Support the URL_FLAG_NONBLOCK semantic and uniform the protocol.
The quick retry loop is already part of retry_transfer_wrapper.

The polling routine is common to the network protocols:
udp, tcp and, once merged, sctp.

libavformat/network.h

@@ -55,6 +55,10 @@ static inline int ff_neterrno() {
 #include <arpa/inet.h>
 #endif
 
+#if HAVE_POLL_H
+#include <poll.h>
+#endif
+
 int ff_socket_nonblock(int socket, int enable);
 
 static inline int ff_network_init(void)
@@ -67,6 +71,15 @@ static inline int ff_network_init(void)
     return 1;
 }
 
+static inline int ff_network_wait_fd(int fd, int write)
+{
+    int ev = write ? POLLOUT : POLLIN;
+    struct pollfd p = { .fd = fd, .events = ev, .revents = 0 };
+    int ret;
+    ret = poll(&p, 1, 100);
+    return ret < 0 ? ff_neterrno() : p.revents & ev ? 0 : AVERROR(EAGAIN);
+}
+
 static inline void ff_network_close(void)
 {
 #if HAVE_WINSOCK2_H

libavformat/tcp.c

@@ -131,23 +131,13 @@ static int tcp_open(URLContext *h, const char *uri, int flags)
     return ret;
 }
 
-static int tcp_wait_fd(int fd, int write)
-{
-    int ev = write ? POLLOUT : POLLIN;
-    struct pollfd p = { .fd = fd, .events = ev, .revents = 0 };
-    int ret;
-
-    ret = poll(&p, 1, 100);
-    return ret < 0 ? ff_neterrno() : p.revents & ev ? 0 : AVERROR(EAGAIN);
-}
-
 static int tcp_read(URLContext *h, uint8_t *buf, int size)
 {
     TCPContext *s = h->priv_data;
     int ret;
 
     if (!(h->flags & URL_FLAG_NONBLOCK)) {
-        ret = tcp_wait_fd(s->fd, 0);
+        ret = ff_network_wait_fd(s->fd, 0);
         if (ret < 0)
             return ret;
     }
@@ -161,7 +151,7 @@ static int tcp_write(URLContext *h, const uint8_t *buf, int size)
     int ret;
 
     if (!(h->flags & URL_FLAG_NONBLOCK)) {
-        ret = tcp_wait_fd(s->fd, 1);
+        ret = ff_network_wait_fd(s->fd, 1);
         if (ret < 0)
             return ret;
     }

libavformat/udp.c

@@ -33,9 +33,6 @@
 #include "internal.h"
 #include "network.h"
 #include "os_support.h"
-#if HAVE_POLL_H
-#include <poll.h>
-#endif
 #include <sys/time.h>
 
 #ifndef IPV6_ADD_MEMBERSHIP
@@ -447,31 +444,15 @@ static int udp_open(URLContext *h, const char *uri, int flags)
 static int udp_read(URLContext *h, uint8_t *buf, int size)
 {
     UDPContext *s = h->priv_data;
-    struct pollfd p = {s->udp_fd, POLLIN, 0};
-    int len;
     int ret;
 
-    for(;;) {
-        if (url_interrupt_cb())
-            return AVERROR_EXIT;
-        ret = poll(&p, 1, 100);
-        if (ret < 0) {
-            if (ff_neterrno() == AVERROR(EINTR))
-                continue;
-            return AVERROR(EIO);
-        }
-        if (!(ret == 1 && p.revents & POLLIN))
-            continue;
-        len = recv(s->udp_fd, buf, size, 0);
-        if (len < 0) {
-            if (ff_neterrno() != AVERROR(EAGAIN) &&
-                ff_neterrno() != AVERROR(EINTR))
-                return AVERROR(EIO);
-        } else {
-            break;
-        }
+    if (!(h->flags & URL_FLAG_NONBLOCK)) {
+        ret = ff_network_wait_fd(s->udp_fd, 0);
+        if (ret < 0)
+            return ret;
     }
-    return len;
+    ret = recv(s->udp_fd, buf, size, 0);
+    return ret < 0 ? ff_neterrno() : ret;
 }
 
 static int udp_write(URLContext *h, const uint8_t *buf, int size)
@@ -479,22 +460,20 @@ static int udp_write(URLContext *h, const uint8_t *buf, int size)
     UDPContext *s = h->priv_data;
     int ret;
 
-    for(;;) {
+    if (!(h->flags & URL_FLAG_NONBLOCK)) {
+        ret = ff_network_wait_fd(s->udp_fd, 1);
+        if (ret < 0)
+            return ret;
+    }
+
     if (!s->is_connected) {
         ret = sendto (s->udp_fd, buf, size, 0,
                       (struct sockaddr *) &s->dest_addr,
                       s->dest_addr_len);
     } else
         ret = send(s->udp_fd, buf, size, 0);
-        if (ret < 0) {
-            if (ff_neterrno() != AVERROR(EINTR) &&
-                ff_neterrno() != AVERROR(EAGAIN))
-                return ff_neterrno();
-        } else {
-            break;
-        }
-    }
-    return size;
+
+    return ret < 0 ? ff_neterrno() : ret;
 }
 
 static int udp_close(URLContext *h)