Handle EPERM in pg_numa_init

When running in Docker, the container may not have privileges needed by
get_mempolicy(). This is called by numa_available() in libnuma, but
versions prior to 2.0.19 did not expect that. The numa_available() call
seemingly succeeds, but then we get unexpected failures when trying to
query status of pages:

  postgres =# select * from pg_shmem_allocations_numa;
  ERROR:  XX000: failed NUMA pages inquiry status: Operation not
          permitted
  LOCATION:  pg_get_shmem_allocations_numa, shmem.c:691

The best solution is to call get_mempolicy() first, and proceed to
numa_available() only when it does not fail with EPERM. Otherwise we'd
need to treat older libnuma versions as insufficient, which seems a bit
too harsh, as this only affects containerized systems.

Fix by me, based on suggestions by Christoph. Backpatch to 18, where the
NUMA functions were introduced.

Reported-by: Christoph Berg <myon@debian.org>
Reviewed-by: Christoph Berg <myon@debian.org>
Discussion: https://postgr.es/m/aPDZOxjrmEo_1JRG@msg.df7cb.de
Backpatch-through: 18

src/port/pg_numa.c

@@ -47,7 +47,17 @@
 int
 pg_numa_init(void)
 {
-	int			r = numa_available();
+	int			r;
+
+	/*
+	 * XXX libnuma versions before 2.0.19 don't handle EPERM by disabling
+	 * NUMA, which then leads to unexpected failures later. This affects
+	 * containers that disable get_mempolicy by a seccomp profile.
+	 */
+	if (get_mempolicy(NULL, NULL, 0, 0, 0) < 0 && (errno == EPERM))
+		r = -1;
+	else
+		r = numa_available();
 
 	return r;
 }