Lock principal keys in RAM

We just lock the hash table entry. Although it will lock the whole mem
page we ok with that potential waste. The number of principal keys
should be less than internal and allocated in the shared memory once
for all backends (unlike internal keys in the TopMemoryContext of every
backend). Plus hash table (via dsa) allocates in 4kb pages and
hopefully, principal keys end up compactly placed.

src/catalog/tde_principal_key.c

@@ -26,6 +26,7 @@
 #include "utils/builtins.h"
 #include "pg_tde.h"
 #include "access/pg_tde_xlog.h"
+#include <sys/mman.h>
 #include <sys/time.h>
 
 #include "access/pg_tde_tdemap.h"
@@ -662,6 +663,10 @@ push_principal_key_to_cache(TDEPrincipalKey *principalKey)
     if (!found)
         memcpy(cacheEntry, principalKey, sizeof(TDEPrincipalKey));
     dshash_release_lock(get_principal_key_Hash(), cacheEntry);
+
+    /* we don't want principal keys to end up paged to the swap */
+    if (mlock(cacheEntry, sizeof(TDEPrincipalKey)) == -1)
+        elog(ERROR, "could not mlock principal key cache entry: %m");
 }
 
 /*