net: socket: Wait new buffers when sending

If we run out of network buffers and cannot send data, and if
we have a blocking socket, then wait until new buffers are
available before returning. As this might lead to deadlock,
wait only max 10 seconds and return ENOMEM if we cannot get
buffers in a reasonable amount of time.

Fixes #28216

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

subsys/net/lib/sockets/sockets.c

@@ -575,15 +575,21 @@ static inline int z_vrfy_zsock_accept(int sock, struct sockaddr *addr,
 #include <syscalls/zsock_accept_mrsh.c>
 #endif /* CONFIG_USERSPACE */
 
+#define WAIT_BUFS K_MSEC(100)
+#define MAX_WAIT_BUFS K_SECONDS(10)
+
 ssize_t zsock_sendto_ctx(struct net_context *ctx, const void *buf, size_t len,
 			 int flags,
 			 const struct sockaddr *dest_addr, socklen_t addrlen)
 {
 	k_timeout_t timeout = K_FOREVER;
+	uint64_t buf_timeout = 0;
 	int status;
 
 	if ((flags & ZSOCK_MSG_DONTWAIT) || sock_is_nonblock(ctx)) {
 		timeout = K_NO_WAIT;
+	} else {
+		buf_timeout = z_timeout_end_calc(MAX_WAIT_BUFS);
 	}
 
 	/* Register the callback before sending in order to receive the response
@@ -596,18 +602,39 @@ ssize_t zsock_sendto_ctx(struct net_context *ctx, const void *buf, size_t len,
 		return -1;
 	}
 
-	if (dest_addr) {
-		status = net_context_sendto(ctx, buf, len, dest_addr,
-					    addrlen, NULL, timeout,
-					    ctx->user_data);
-	} else {
-		status = net_context_send(ctx, buf, len, NULL, timeout,
-					  ctx->user_data);
-	}
+	while (1) {
+		if (dest_addr) {
+			status = net_context_sendto(ctx, buf, len, dest_addr,
+						    addrlen, NULL, timeout,
+						    ctx->user_data);
+		} else {
+			status = net_context_send(ctx, buf, len, NULL, timeout,
+						  ctx->user_data);
+		}
 
-	if (status < 0) {
-		errno = -status;
-		return -1;
+		if (status < 0) {
+			if (status == -ENOBUFS &&
+			    K_TIMEOUT_EQ(timeout, K_FOREVER)) {
+				/* If we cannot get any buffers in reasonable
+				 * amount of time, then do not wait forever as
+				 * there might be some bigger issue.
+				 */
+				int64_t remaining = buf_timeout - z_tick_get();
+
+				if (remaining <= 0) {
+					errno = ENOMEM;
+					return -1;
+				}
+
+				k_sleep(WAIT_BUFS);
+				continue;
+			} else {
+				errno = -status;
+				return -1;
+			}
+		}
+
+		break;
 	}
 
 	return status;