Restructure index access method API to hide most of it at the C level.

This patch reduces pg_am to just two columns, a name and a handler
function.  All the data formerly obtained from pg_am is now provided
in a C struct returned by the handler function.  This is similar to
the designs we've adopted for FDWs and tablesample methods.  There
are multiple advantages.  For one, the index AM's support functions
are now simple C functions, making them faster to call and much less
error-prone, since the C compiler can now check function signatures.
For another, this will make it far more practical to define index access
methods in installable extensions.

A disadvantage is that SQL-level code can no longer see attributes
of index AMs; in particular, some of the crosschecks in the opr_sanity
regression test are no longer possible from SQL.  We've addressed that
by adding a facility for the index AM to perform such checks instead.
(Much more could be done in that line, but for now we're content if the
amvalidate functions more or less replace what opr_sanity used to do.)
We might also want to expose some sort of reporting functionality, but
this patch doesn't do that.

Alexander Korotkov, reviewed by Petr Jelínek, and rather heavily
editorialized on by me.

src/backend/access/index/amapi.c

@@ -0,0 +1,114 @@
+/*-------------------------------------------------------------------------
+ *
+ * amapi.c
+ *	  Support routines for API for Postgres index access methods.
+ *
+ * Copyright (c) 2015-2016, PostgreSQL Global Development Group
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/access/index/amapi.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/amapi.h"
+#include "access/htup_details.h"
+#include "catalog/pg_am.h"
+#include "catalog/pg_opclass.h"
+#include "utils/syscache.h"
+
+
+/*
+ * GetIndexAmRoutine - call the specified access method handler routine to get
+ * its IndexAmRoutine struct, which will be palloc'd in the caller's context.
+ *
+ * Note that if the amhandler function is built-in, this will not involve
+ * any catalog access.  It's therefore safe to use this while bootstrapping
+ * indexes for the system catalogs.  relcache.c relies on that.
+ */
+IndexAmRoutine *
+GetIndexAmRoutine(Oid amhandler)
+{
+	Datum		datum;
+	IndexAmRoutine *routine;
+
+	datum = OidFunctionCall0(amhandler);
+	routine = (IndexAmRoutine *) DatumGetPointer(datum);
+
+	if (routine == NULL || !IsA(routine, IndexAmRoutine))
+		elog(ERROR, "index access method handler function %u did not return an IndexAmRoutine struct",
+			 amhandler);
+
+	return routine;
+}
+
+/*
+ * GetIndexAmRoutineByAmId - look up the handler of the index access method
+ * with the given OID, and get its IndexAmRoutine struct.
+ */
+IndexAmRoutine *
+GetIndexAmRoutineByAmId(Oid amoid)
+{
+	HeapTuple	tuple;
+	Form_pg_am	amform;
+	regproc		amhandler;
+
+	/* Get handler function OID for the access method */
+	tuple = SearchSysCache1(AMOID, ObjectIdGetDatum(amoid));
+	if (!HeapTupleIsValid(tuple))
+		elog(ERROR, "cache lookup failed for access method %u",
+			 amoid);
+	amform = (Form_pg_am) GETSTRUCT(tuple);
+
+	amhandler = amform->amhandler;
+
+	/* Complain if handler OID is invalid */
+	if (!RegProcedureIsValid(amhandler))
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("index access method \"%s\" does not have a handler",
+						NameStr(amform->amname))));
+
+	ReleaseSysCache(tuple);
+
+	/* And finally, call the handler function to get the API struct. */
+	return GetIndexAmRoutine(amhandler);
+}
+
+
+/*
+ * Ask appropriate access method to validate the specified opclass.
+ */
+Datum
+amvalidate(PG_FUNCTION_ARGS)
+{
+	Oid			opclassoid = PG_GETARG_OID(0);
+	bool		result;
+	HeapTuple	classtup;
+	Form_pg_opclass classform;
+	Oid			amoid;
+	IndexAmRoutine *amroutine;
+
+	classtup = SearchSysCache1(CLAOID, ObjectIdGetDatum(opclassoid));
+	if (!HeapTupleIsValid(classtup))
+		elog(ERROR, "cache lookup failed for operator class %u", opclassoid);
+	classform = (Form_pg_opclass) GETSTRUCT(classtup);
+
+	amoid = classform->opcmethod;
+
+	ReleaseSysCache(classtup);
+
+	amroutine = GetIndexAmRoutineByAmId(amoid);
+
+	if (amroutine->amvalidate == NULL)
+		elog(ERROR, "function amvalidate is not defined for index access method %u",
+			 amoid);
+
+	result = amroutine->amvalidate(opclassoid);
+
+	pfree(amroutine);
+
+	PG_RETURN_BOOL(result);
+}