ADBDEV-2976: Handle multi_perform curl errors immediately (#24)

The internal buffer will be filled during the first pxfprotocol_import call.
If there is chunk corruption, we fail here without trying to interpret the
received message during check_response. But if a corrupted chunk is read
during the next filling of the buffer, this chunk will be written in the
internal buffer. Then if trailer CRLF sequence is not faced, CHUNKE_BAD_CHUNK
(with code 3) will be returned and connection must be interrupted as far as
application isn't able to find next chunk start position. But current PXF
implementation doesn't check error buffer in case of internal buffer isn't
empty (we've already placed corrupted chunk here) and tries to interpret it
with gpdbwritableformatter_import. If we are lucky and memory corruption
isn't happened, we receive error during next buffer filling.

To solve this problem, we should check multi_perform result, similar to
curl_easy_perform libcurl function.

external-table/src/libchurl.c

@@ -473,7 +473,6 @@ churl_read_check_connectivity(CHURL_HANDLE handle)
 	Assert(!context->upload);
 
 	fill_internal_buffer(context, 1);
-	check_response(context);
 }
 
 /*
@@ -626,6 +625,8 @@ multi_perform(churl_context *context)
 	if (curl_error != CURLM_OK)
 		elog(ERROR, "internal error: curl_multi_perform failed (%d - %s)",
 			 curl_error, curl_easy_strerror(curl_error));
+
+	check_response(context);
 }
 
 static bool
@@ -653,8 +654,6 @@ flush_internal_buffer(churl_context *context)
 		multi_perform(context);
 	}
 
-	check_response(context);
-
 	if ((context->curl_still_running == 0) &&
 		((context_buffer->top - context_buffer->bot) > 0))
 		elog(ERROR, "failed sending to remote component %s", get_dest_address(context->curl_handle));
@@ -709,8 +708,6 @@ finish_upload(churl_context *context)
 	 */
 	while (context->curl_still_running != 0)
 		multi_perform(context);
-
-	check_response(context);
 }
 
 static void

fdw/libchurl.c

@@ -457,7 +457,6 @@ churl_read_check_connectivity(CHURL_HANDLE handle)
 	Assert(!context->upload);
 
 	fill_internal_buffer(context, 1);
-	check_response(context);
 }
 
 /*
@@ -610,6 +609,8 @@ multi_perform(churl_context *context)
 	if (curl_error != CURLM_OK)
 		elog(ERROR, "internal error: curl_multi_perform failed (%d - %s)",
 			 curl_error, curl_easy_strerror(curl_error));
+
+	check_response(context);
 }
 
 bool
@@ -641,8 +642,6 @@ flush_internal_buffer(churl_context *context)
 		((context_buffer->top - context_buffer->bot) > 0))
 		elog(ERROR, "failed sending to remote component %s", get_dest_address(context->curl_handle));
 
-	check_response(context);
-
 	context_buffer->top = 0;
 	context_buffer->bot = 0;
 }
@@ -694,8 +693,6 @@ finish_upload(churl_context *context)
 	 */
 	while (context->curl_still_running != 0)
 		multi_perform(context);
-
-	check_response(context);
 }
 
 void