Fix error when trying to convert text message partial payload to UTF-8

[SemyonBaryshev]

There was an issue with "not valid UTF-8 data" error being returned sometimes for incoming text messages that were split into multiple frames. If incoming text message consists of multiple frames, buffered partial payload may not necessarily represent a valid UTF-8 string, only a whole received message must represent a valid UTF-8 string. So we have to check if buffered payload of text message can be converted to a UTF-8 string only after receiving all frames of incoming text message, and only then try to convert to UTF-8 string and return error if it was not valid.

[daltoniam]

Thanks! Makes sense. Appreciate the fix.

[sjmueller]

Can this same error happen with the native engine using URLSession.webSocketTask? I have a scenario using the native websocket that sometimes results in this error receiving a large payload:

2020-07-09 16:19:35.522322-0700 [] nw_protocol_copy_ws_definition_block_invoke [C2.1:5] received invalid utf-8 sequence
2020-07-09 16:19:35.523424-0700 [] nw_read_request_report [C2] Receive failed with error "Bad address"
2020-07-09 16:19:35.524531-0700 Connection 2: received failure notification
2020-07-09 16:19:35.525757-0700 [] nw_read_request_report [C2] Receive failed with error "Socket is not connected"
2020-07-09 16:19:35.526124-0700 [] nw_content_context_copy_protocol_metadata called with null context
2020-07-09 16:19:35.526408-0700 [] nw_content_context_copy_protocol_metadata called with null context, dumping backtrace:
        [arm64] libnetcore-1880.120.4
    0   libnetwork.dylib                    0x00000001b03fbe80 __nw_create_backtrace_string + 120
    1   libnetwork.dylib                    0x00000001affe93d8 nw_content_context_copy_protocol_metadata + 548
    2   CFNetwork                           0x00000001b11b30b0 _CFHTTPServerResponseEnqueue + 29872
    3   libdispatch.dylib                   0x00000001065a9d10 _dispatch_call_block_and_release + 32
    4   libdispatch.dylib                   0x00000001065ab18c _dispatch_client_callout + 20
    5   libdispatch.dylib                   0x00000001065b2968 _dispatch_lane_serial_drain + 724
    6   libdispatch.dylib                   0x00000001065b35b4 _dispatch_lane_invoke + 492
    7   libdispatch.dylib                   0x00000001065bf0f0 _dispatch_workloop_worker_thread + 1344
    8   libsystem_pthread.dylib             0x00000001ada4b714 _pthread_wqthread + 276
    9   libsystem_pthread.dylib             0x00000001ada519c8 start_wqthread + 8
2020-07-09 16:19:35.527667-0700 Blink[32470:8174263] [websocket] Read completed with an error Socket is not connected