Better exception on COPY API confusion

If a user calls one of the COPY APIs but supplies a server-side file
instead of STDIN, a proper exception is thrown.

Fixes #1673

(cherry picked from commit 2a5029e)

src/Npgsql/NpgsqlBinaryExporter.cs

@@ -72,11 +72,24 @@ internal NpgsqlBinaryExporter(NpgsqlConnector connector, string copyToCommand)
             {
                 _connector.SendQuery(copyToCommand);
 
-                // TODO: Failure will break the connection (e.g. if we get CopyOutResponse), handle more gracefully
-                var copyOutResponse = _connector.ReadExpecting<CopyOutResponseMessage>();
-                if (!copyOutResponse.IsBinary) {
-                    throw new ArgumentException("copyToCommand triggered a text transfer, only binary is allowed", nameof(copyToCommand));
+                CopyOutResponseMessage copyOutResponse;
+                var msg = _connector.ReadMessage();
+                switch (msg.Code)
+                {
+                case BackendMessageCode.CopyOutResponse:
+                    copyOutResponse = (CopyOutResponseMessage)msg;
+                    if (!copyOutResponse.IsBinary)
+                        throw new ArgumentException("copyToCommand triggered a text transfer, only binary is allowed", nameof(copyToCommand));
+                    break;
+                case BackendMessageCode.CompletedResponse:
+                    throw new InvalidOperationException(
+                        "This API only supports import/export from the client, i.e. COPY commands containing TO/FROM STDIN. " +
+                        "To import/export with files on your PostgreSQL machine, simply execute the command with ExecuteNonQuery. " +
+                        "Note that your data has been successfully imported/exported.");
+                default:
+                    throw _connector.UnexpectedMessageReceived(msg.Code);
                 }
+
                 NumColumns = copyOutResponse.NumColumns;
                 ReadHeader();
             }

src/Npgsql/NpgsqlBinaryImporter.cs

@@ -89,10 +89,24 @@ internal NpgsqlBinaryImporter(NpgsqlConnector connector, string copyFromCommand)
             {
                 _connector.SendQuery(copyFromCommand);
 
-                // TODO: Failure will break the connection (e.g. if we get CopyOutResponse), handle more gracefully
-                var copyInResponse = _connector.ReadExpecting<CopyInResponseMessage>();
-                if (!copyInResponse.IsBinary)
-                    throw new ArgumentException("copyFromCommand triggered a text transfer, only binary is allowed", nameof(copyFromCommand));
+                CopyInResponseMessage copyInResponse;
+                var msg = _connector.ReadMessage();
+                switch (msg.Code)
+                {
+                case BackendMessageCode.CopyInResponse:
+                    copyInResponse = (CopyInResponseMessage)msg;
+                    if (!copyInResponse.IsBinary)
+                        throw new ArgumentException("copyFromCommand triggered a text transfer, only binary is allowed", nameof(copyFromCommand));
+                    break;
+                case BackendMessageCode.CompletedResponse:
+                    throw new InvalidOperationException(
+                        "This API only supports import/export from the client, i.e. COPY commands containing TO/FROM STDIN. " +
+                        "To import/export with files on your PostgreSQL machine, simply execute the command with ExecuteNonQuery. " +
+                        "Note that your data has been successfully imported/exported.");
+                default:
+                    throw _connector.UnexpectedMessageReceived(msg.Code);
+                }
+
                 NumColumns = copyInResponse.NumColumns;
                 _buf.StartCopyMode();
                 WriteHeader();

src/Npgsql/NpgsqlRawCopyStream.cs

@@ -96,6 +96,11 @@ internal NpgsqlRawCopyStream(NpgsqlConnector connector, string copyCommand)
                 IsBinary = copyOutResponse.IsBinary;
                 _canRead = true;
                 break;
+            case BackendMessageCode.CompletedResponse:
+                throw new InvalidOperationException(
+                    "This API only supports import/export from the client, i.e. COPY commands containing TO/FROM STDIN. " +
+                    "To import/export with files on your PostgreSQL machine, simply execute the command with ExecuteNonQuery. " +
+                    "Note that your data has been successfully imported/exported.");
             default:
                 throw _connector.UnexpectedMessageReceived(msg.Code);
             }