safekeeper: correctly handle signals (#7167)

errno is not preserved in the signal handler. This pull request fixes
it. Maybe related: https://github.com/neondatabase/neon/issues/6969, but
does not fix the flaky test problem.

Signed-off-by: Alex Chi Z <chi@neon.tech>

pgxn/neon/walproposer_pg.c

@@ -549,9 +549,10 @@ walprop_pg_init_standalone_sync_safekeepers(void)
 static void
 walprop_sigusr2(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
 	got_SIGUSR2 = true;
-
 	SetLatch(MyLatch);
+	errno = save_errno;
 }
 
 static void