From d29c656d4f2b362a0cf993e51acb6956825801a6 Mon Sep 17 00:00:00 2001
From: Juliusz Sosinowicz <juliusz@wolfssl.com>
Date: Wed, 1 Jun 2022 16:48:57 +0200
Subject: [PATCH] SendAlert: clear output buffer to try and send the alert now

---
 src/internal.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/internal.c b/src/internal.c
index f78299161..286c26c6f 100644
--- a/src/internal.c
+++ b/src/internal.c
@@ -20640,8 +20640,25 @@ int SendAlert(WOLFSSL* ssl, int severity, int type)
 
     /* check for available size */
     outputSz = ALERT_SIZE + MAX_MSG_EXTRA + dtlsExtra;
-    if ((ret = CheckAvailableSize(ssl, outputSz)) != 0)
+    if ((ret = CheckAvailableSize(ssl, outputSz)) != 0) {
+#ifdef WOLFSSL_DTLS
+        /* If CheckAvailableSize returned WANT_WRITE due to a blocking write
+         * then discard pending output and just send the alert. */
+        if (ssl->options.dtls) {
+            if (ret != WANT_WRITE || severity != alert_fatal)
+                return ret;
+            ShrinkOutputBuffer(ssl);
+            if ((ret = CheckAvailableSize(ssl, outputSz)) != 0) {
+                return ret;
+            }
+        }
+        else {
+            return ret;
+        }
+#else
         return ret;
+#endif
+    }
 
     /* Check output buffer */
     if (ssl->buffers.outputBuffer.buffer == NULL)