Print a better error message

This is still missing the dlsym wrappers.

src/linux_tcp/sslconnected.h

@@ -167,7 +167,7 @@ private:
             _state = state_error;
 
             // report an error to user-space
-            _parent->onError(this, "ssl protocol error");
+            _parent->onError(this, strerror(error).data());
 
             // ssl level error, we have to tear down the tcp connection
             return false;
@@ -305,6 +305,73 @@ private:
         return _out && isWritable() ? write(monitor) : proceed();
     }
 
+    /**
+     *  Get the error description of the returnvalue of SSL_get_error
+     *  @param[in]  returnvalue  The return value of SSL_get_error
+     *  @param      bp           Pointer to a BIO in case the error is SSL_ERROR_SSL
+     *  @return     A static string describing the error, or nullptr.
+     *              In the case of a nullptr, the passed-in BIO describes the error.
+     */
+    static const char *strerror(int returnvalue, ::BIO *bp)
+    {
+        // check the return value of the SSL_get_error function, which has a very unfortunate name.
+        switch (returnvalue)
+        {
+            // It can be a syscall error.
+            case SSL_ERROR_SYSCALL:
+
+                // The SSL_ERROR_SYSCALL with errno value of 0 indicates unexpected
+                // EOF from the peer. This will be properly reported as SSL_ERROR_SSL
+                // with reason code SSL_R_UNEXPECTED_EOF_WHILE_READING in the
+                // OpenSSL 3.0 release because it is truly a TLS protocol error to
+                // terminate the connection without a SSL_shutdown().
+                if (errno == 0) return "SSL_R_UNEXPECTED_EOF_WHILE_READING";
+
+                // Otherwise we ask the OS for a description of the error.
+                return ::strerror(errno);
+
+            // It can be an error in OpenSSL. In that case the error stack contains
+            // more information. The documentation notes: if this error occurs then
+            // no further I/O operations should be performed on the connection and
+            // SSL_shutdown() must not be called.
+            case SSL_ERROR_SSL:
+
+                // invoke the convenience function to extract the whole error stack
+                ::ERR_print_errors(bp);
+
+                // we return nullptr because the error is described by the bio
+                return nullptr;
+
+            default:
+                // we don't know what kind of error this is
+                return "unknown ssl error";
+        }
+    }
+
+    /**
+     *  Get a human-readable string from the return value of SSL_get_error in case it's an error.
+     *  @param[in]  returnvalue  The return value. Must not be a non-error.
+     *  @return     std::string describing the error.
+     */
+    static std::string strerror(int returnvalue)
+    {
+        // create a BIO so we can call our strerror overload
+        std::unique_ptr<::BIO, decltype(&::BIO_free)> bio(::BIO_new(::BIO_s_mem()), &::BIO_free);
+
+        // deduce the error message, and return it if it's not null
+        if (const char *message = strerror(returnvalue, bio.get())) return {message};
+
+        // the error is described in the BIO
+        // get the buffer memory structure
+        ::BUF_MEM *bufmem;
+
+        // get it from the bio
+        ::BIO_get_mem_ptr(bio.get(), &bufmem);
+
+        // ensure it's null-terminated by copying it to std::string
+        return {bufmem->data, bufmem->length};
+    }
+
     
 public:
     /**