Make BearSSL::write() blocking, match axTLS (#4804)

When a message is sent by the app that is larger than the SSL buffer,
it will take multiple TLS fragments to transfer.  Writes will loop
through and not return until either all data is transferred or there
is an error.

Author: earlephilhower

Diff for: libraries/ESP8266WiFi/src/WiFiClientSecureBearSSL.cpp

@@ -235,30 +235,44 @@ uint8_t WiFiClientSecure::connected() {
 }
 
 size_t WiFiClientSecure::_write(const uint8_t *buf, size_t size, bool pmem) {
+  size_t sent_bytes = 0;
+
   if (!connected() || !size || !_handshake_done) {
     return 0;
   }
 
-  if (_run_until(BR_SSL_SENDAPP) < 0) {
-    return 0;
-  }
+  do {
+    // Ensure we yield if we need multiple fragments to avoid WDT
+    if (sent_bytes) {
+      optimistic_yield(1000);
+    }
+
+    // Get BearSSL to a state where we can send
+    if (_run_until(BR_SSL_SENDAPP) < 0) {
+      break;
+    }
 
-  if (br_ssl_engine_current_state(_eng) & BR_SSL_SENDAPP) {
-    size_t sendapp_len;
-    unsigned char *sendapp_buf = br_ssl_engine_sendapp_buf(_eng, &sendapp_len);
-    int to_send = size > sendapp_len ? sendapp_len : size;
-    if (pmem) {
-      memcpy_P(sendapp_buf, buf, to_send);
+    if (br_ssl_engine_current_state(_eng) & BR_SSL_SENDAPP) {
+      size_t sendapp_len;
+      unsigned char *sendapp_buf = br_ssl_engine_sendapp_buf(_eng, &sendapp_len);
+      int to_send = size > sendapp_len ? sendapp_len : size;
+      if (pmem) {
+        memcpy_P(sendapp_buf, buf, to_send);
+      } else {
+        memcpy(sendapp_buf, buf, to_send);
+      }
+      br_ssl_engine_sendapp_ack(_eng, to_send);
+      br_ssl_engine_flush(_eng, 0);
+      flush();
+      buf += to_send;
+      sent_bytes += to_send;
+      size -= to_send;
     } else {
-      memcpy(sendapp_buf, buf, to_send);
+      break;
     }
-    br_ssl_engine_sendapp_ack(_eng, to_send);
-    br_ssl_engine_flush(_eng, 0);
-    flush();
-    return to_send;
-  }
+  } while (size);
 
-  return 0;
+  return sent_bytes;
 }
 
 size_t WiFiClientSecure::write(const uint8_t *buf, size_t size) {