Add test for early backend startup errors

The new test tests the libpq fallback behavior on an early error,
which was fixed in the previous commit.

This adds an IS_INJECTION_POINT_ATTACHED() macro, to allow writing
injected test code alongside the normal source code. In principle, the
new test could've been implemented by an extra test module with a
callback that sets the FrontendProtocol global variable, but I think
it's more clear to have the test code right where the injection point
is, because it has pretty intimate knowledge of the surrounding
context it runs in.

Reviewed-by: Michael Paquier
Discussion: https://www.postgresql.org/message-id/CAOYmi%2Bnwvu21mJ4DYKUa98HdfM_KZJi7B1MhyXtnsyOO-PB6Ww%40mail.gmail.com

src/backend/tcop/backend_startup.c

@@ -33,6 +33,7 @@
 #include "tcop/backend_startup.h"
 #include "tcop/tcopprot.h"
 #include "utils/builtins.h"
+#include "utils/injection_point.h"
 #include "utils/memutils.h"
 #include "utils/ps_status.h"
 #include "utils/timeout.h"
@@ -213,6 +214,21 @@ BackendInitialize(ClientSocket *client_sock, CAC_state cac)
 							remote_host)));
 	}
 
+	/* For testing client error handling */
+#ifdef USE_INJECTION_POINTS
+	INJECTION_POINT("backend-initialize");
+	if (IS_INJECTION_POINT_ATTACHED("backend-initialize-v2-error"))
+	{
+		/*
+		 * This simulates an early error from a pre-v14 server, which used the
+		 * version 2 protocol for any errors that occurred before processing
+		 * the startup packet.
+		 */
+		FrontendProtocol = PG_PROTOCOL(2, 0);
+		elog(FATAL, "protocol version 2 error triggered");
+	}
+#endif
+
 	/*
 	 * If we did a reverse lookup to name, we might as well save the results
 	 * rather than possibly repeating the lookup during authentication.