fix(piethernet): bound the EAGAIN retry loop in writeDevice by writeTimeout

The loop may run in a thread that belongs to another device (the picloud
dispatcher forwards data from one connection's read thread to another
connection's socket), so isThreadedReadStopping() alone cannot stop it:
it sees the wrong device's flag. Stalling for at most writeTimeout()
without progress makes every write finite; the receiver resynchronizes
on the next packet signature.

libs/main/io_devices/piethernet.cpp

@@ -388,7 +388,7 @@ void PIEthernet::applyBuffers() {
 
 void PIEthernet::applyTimeout(int fd, int opt, PISystemTime tm) {
 	if (fd == 0) return;
-	// piCoutObj << "setReadIsBlocking" << yes;
+		// piCoutObj << "setReadIsBlocking" << yes;
 #ifdef WINDOWS
 	DWORD _tm = tm.toMilliseconds();
 #else
@@ -851,13 +851,17 @@ ssize_t PIEthernet::writeDevice(const void * data, ssize_t max_size) {
 		} else {
 			ssize_t remain_size      = max_size;
 			const char * remain_data = (const char *)data;
+			// the socket is non-blocking, so EAGAIN means the send buffer is full
+			// and the peer is not draining it. The write may be issued from a thread
+			// that belongs to another device (forwarding in the picloud dispatcher),
+			// so this loop must be bounded by itself: stall for at most
+			// writeTimeout() without progress, then give up. The receiver
+			// resynchronizes on the next packet signature, and any concurrent
+			// stopAndWait() on the writing thread's device becomes finite.
+			PITimeMeasurer stalled_tm;
 			while (remain_size > 0) {
-				if (isThreadedReadStopping()) {
-					// socket is non-blocking and peer may not drain the send buffer,
-					// without this check the loop below would spin forever and the
-					// thread could never be stopped by stop()/stopAndWait()
-					break;
-				}
+				if (isThreadedReadStopping()) break;
+				if (stalled_tm.elapsed_s() > writeTimeout().toSeconds()) break;
 				int sr = ::send(sock, remain_data, remain_size, 0);
 				if (sr < 0) {
 					int err = ethErrorCore();
@@ -877,8 +881,12 @@ ssize_t PIEthernet::writeDevice(const void * data, ssize_t max_size) {
 				ret += sr;
 				remain_data += sr;
 				remain_size -= sr;
+				stalled_tm.reset();
+			}
+			if (remain_size > 0) {
+				piCoutObj << "Error: write stalled," << remain_size << "bytes not sent";
+				return -1;
 			}
-			if (remain_size > 0) return -1;
 		}
 		return ret;
 	}