Enforce a specific order for probing library loadability in pg_upgrade.

pg_upgrade checks whether all the shared libraries used in the old cluster
are also available in the new one by issuing LOAD for each library name.
Previously, it cared not what order it did the LOADs in.  Ideally it
should not have to care, but currently the transform modules in contrib
fail unless both the language and datatype modules they depend on are
loaded first.  A backend-side solution for that looks possible but
probably not back-patchable, so as a stopgap measure, let's do the LOAD
tests in order by library name length.  That should fix the problem for
reasonably-named transform modules, eg "hstore_plpython" will be loaded
after both "hstore" and "plpython".  (Yeah, it's a hack.)

In a larger sense, having a predictable order of these probes is a good
thing, since it will make upgrades predictably work or not work in the
face of inter-library dependencies.  Also, this patch replaces O(N^2)
de-duplication logic with O(N log N) logic, which could matter in
installations with very many databases.  So I don't foresee reverting this
even after we have a proper fix for the library-dependency problem.

In passing, improve a couple of SQL queries used here.

Per complaint from Andrew Dunstan that pg_upgrade'ing the transform contrib
modules failed.  Back-patch to 9.5 where transform modules were introduced.

Discussion: <f7ac29f3-515c-2a44-21c5-ec925053265f@dunslane.net>

src/bin/pg_upgrade/function.c

@@ -12,6 +12,31 @@
 #include "pg_upgrade.h"
 
 #include "access/transam.h"
+#include "catalog/pg_language.h"
+
+
+/*
+ * qsort comparator for pointers to library names
+ *
+ * We sort first by name length, then alphabetically for names of the same
+ * length.  This is to ensure that, eg, "hstore_plpython" sorts after both
+ * "hstore" and "plpython"; otherwise transform modules will probably fail
+ * their LOAD tests.  (The backend ought to cope with that consideration,
+ * but it doesn't yet, and even when it does it'll still be a good idea
+ * to have a predictable order of probing here.)
+ */
+static int
+library_name_compare(const void *p1, const void *p2)
+{
+	const char *str1 = *(const char *const *) p1;
+	const char *str2 = *(const char *const *) p2;
+	int			slen1 = strlen(str1);
+	int			slen2 = strlen(str2);
+
+	if (slen1 != slen2)
+		return slen1 - slen2;
+	return strcmp(str1, str2);
+}
 
 
 /*
@@ -38,17 +63,15 @@ get_loadable_libraries(void)
 		PGconn	   *conn = connectToServer(&old_cluster, active_db->db_name);
 
 		/*
-		 * Fetch all libraries referenced in this DB.  We can't exclude the
-		 * "pg_catalog" schema because, while such functions are not
-		 * explicitly dumped by pg_dump, they do reference implicit objects
-		 * that pg_dump does dump, e.g. CREATE LANGUAGE plperl.
+		 * Fetch all libraries containing non-built-in C functions in this DB.
 		 */
 		ress[dbnum] = executeQueryOrDie(conn,
 										"SELECT DISTINCT probin "
 										"FROM pg_catalog.pg_proc "
-										"WHERE	prolang = 13 /* C */ AND "
+										"WHERE prolang = %u AND "
 										"probin IS NOT NULL AND "
 										"oid >= %u;",
+										ClanguageId,
 										FirstNormalObjectId);
 		totaltups += PQntuples(ress[dbnum]);
 
@@ -69,13 +92,15 @@ get_loadable_libraries(void)
 
 			res = executeQueryOrDie(conn,
 									"SELECT 1 "
-						   "FROM	pg_catalog.pg_proc JOIN pg_namespace "
-							 "		ON pronamespace = pg_namespace.oid "
+									"FROM pg_catalog.pg_proc p "
+									"    JOIN pg_catalog.pg_namespace n "
+									"    ON pronamespace = n.oid "
 							   "WHERE proname = 'plpython_call_handler' AND "
 									"nspname = 'public' AND "
-									"prolang = 13 /* C */ AND "
+									"prolang = %u AND "
 									"probin = '$libdir/plpython' AND "
-									"pg_proc.oid >= %u;",
+									"p.oid >= %u;",
+									ClanguageId,
 									FirstNormalObjectId);
 			if (PQntuples(res) > 0)
 			{
@@ -112,13 +137,18 @@ get_loadable_libraries(void)
 	if (found_public_plpython_handler)
 		pg_fatal("Remove the problem functions from the old cluster to continue.\n");
 
-	/* Allocate what's certainly enough space */
-	os_info.libraries = (char **) pg_malloc(totaltups * sizeof(char *));
-
 	/*
-	 * Now remove duplicates across DBs.  This is pretty inefficient code, but
-	 * there probably aren't enough entries to matter.
+	 * Now we want to remove duplicates across DBs and sort the library names
+	 * into order.  This avoids multiple probes of the same library, and
+	 * ensures that libraries are probed in a consistent order, which is
+	 * important for reproducible behavior if one library depends on another.
+	 *
+	 * First transfer all the names into one array, then sort, then remove
+	 * duplicates.  Note: we strdup each name in the first loop so that we can
+	 * safely clear the PGresults in the same loop.  This is a bit wasteful
+	 * but it's unlikely there are enough names to matter.
 	 */
+	os_info.libraries = (char **) pg_malloc(totaltups * sizeof(char *));
 	totaltups = 0;
 
 	for (dbnum = 0; dbnum < old_cluster.dbarr.ndbs; dbnum++)
@@ -131,27 +161,34 @@ get_loadable_libraries(void)
 		for (rowno = 0; rowno < ntups; rowno++)
 		{
 			char	   *lib = PQgetvalue(res, rowno, 0);
-			bool		dup = false;
-			int			n;
 
-			for (n = 0; n < totaltups; n++)
-			{
-				if (strcmp(lib, os_info.libraries[n]) == 0)
-				{
-					dup = true;
-					break;
-				}
-			}
-			if (!dup)
 			os_info.libraries[totaltups++] = pg_strdup(lib);
 		}
-
 		PQclear(res);
 	}
 
-	os_info.num_libraries = totaltups;
-
 	pg_free(ress);
+
+	if (totaltups > 1)
+	{
+		int			i,
+					lastnondup;
+
+		qsort((void *) os_info.libraries, totaltups, sizeof(char *),
+			  library_name_compare);
+
+		for (i = 1, lastnondup = 0; i < totaltups; i++)
+		{
+			if (strcmp(os_info.libraries[i],
+					   os_info.libraries[lastnondup]) != 0)
+				os_info.libraries[++lastnondup] = os_info.libraries[i];
+			else
+				pg_free(os_info.libraries[i]);
+		}
+		totaltups = lastnondup + 1;
+	}
+
+	os_info.num_libraries = totaltups;
 }