Clear SSL Error Queue

The OpenSSL documentation states that the error queue for the current
thread needs to be emptied prior to the next TLS/SSL I/O operation,
or the call to SSL_get_error() will not be reliable.

src/linux_tcp/openssl.cpp

@@ -310,6 +310,19 @@ int SSL_use_certificate_file(SSL *ssl, const char *file, int type)
     return func(ssl, file, type);
 }
 
+/**
+ *  Clear the SSL error queue
+ *  @return void
+ */
+void ERR_clear_error()
+{
+    // create a function
+    static Function<decltype(::ERR_clear_error)> func(handle, "ERR_clear_error");
+
+    // call the openssl function
+    return func();
+}
+
 /**
  *  End of namespace
  */

src/linux_tcp/openssl.h

@@ -18,6 +18,7 @@
  *  Dependencies
  */
 #include <openssl/ssl.h>
+#include <openssl/err.h>
 
 /**
  *  Begin of namespace
@@ -49,6 +50,7 @@ void     SSL_set_connect_state(SSL *ssl);
 void     SSL_CTX_free(SSL_CTX *ctx);
 void     SSL_free(SSL *ssl);
 long     SSL_ctrl(SSL *ssl, int cmd, long larg, void *parg);
+void     ERR_clear_error(void);
 
 /**
  *  End of namespace

src/linux_tcp/sslconnected.h

@@ -270,8 +270,12 @@ private:
             // we may have to repeat the operation on failure
             if (result > 0) continue;
             
+            // Check for error and clear the error queue before the next TLS/SSL I/O operation
+            auto error = OpenSSL::SSL_get_error(_ssl, result);
+            OpenSSL::ERR_clear_error();
+
             // the operation failed, we may have to repeat our call
-            return repeat(monitor, state_sending, OpenSSL::SSL_get_error(_ssl, result));
+            return repeat(monitor, state_sending, error);
         }
         while (_out && !readable);
         
@@ -297,8 +301,14 @@ private:
             auto result = _in.receivefrom(_ssl, _connection->expected());
             
             // if this is a failure, we are going to repeat the operation
-            if (result <= 0) return repeat(monitor, state_receiving, OpenSSL::SSL_get_error(_ssl, result));
-        
+            if (result <= 0)
+            {
+                // Check for error and clear the error queue before the next TLS/SSL I/O operation
+                auto error = OpenSSL::SSL_get_error(_ssl, result);
+                OpenSSL::ERR_clear_error();
+
+                return repeat(monitor, state_receiving, error);
+            }
             // go process the received data
             auto *nextstate = parse(monitor, result);
             
@@ -419,6 +429,9 @@ public:
             {
                 // error was returned, so we must investigate what is going on
                 auto error = OpenSSL::SSL_get_error(_ssl, result);
+
+                // clear the error queue before the next TLS/SSL I/O operation
+                OpenSSL::ERR_clear_error();
                 
                 // get the next state given the error
                 auto *nextstate = repeat(monitor, state_sending, error);