shippable.c

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/*-------------------------------------------------------------------------
 *
 * shippable.c
 *    Determine which database objects are shippable to a remote server.
 *
 * We need to determine whether particular functions, operators, and indeed
 * data types are shippable to a remote server for execution --- that is,
 * do they exist and have the same behavior remotely as they do locally?
 * Built-in objects are generally considered shippable.  Other objects can
 * be shipped if they are declared as such by the user.
 *
 * Note: there are additional filter rules that prevent shipping mutable
 * functions or functions using nonportable collations.  Those considerations
 * need not be accounted for here.
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    contrib/postgres_fdw/shippable.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/transam.h"
#include "catalog/dependency.h"
#include "postgres_fdw.h"
#include "utils/hsearch.h"
#include "utils/inval.h"
#include "utils/syscache.h"

/* Hash table for caching the results of shippability lookups */
static HTAB *ShippableCacheHash = NULL;

/*
 * Hash key for shippability lookups.  We include the FDW server OID because
 * decisions may differ per-server.  Otherwise, objects are identified by
 * their (local!) OID and catalog OID.
 */
typedef struct
{
    /* XXX we assume this struct contains no padding bytes */
    Oid         objid;          /* function/operator/type OID */
    Oid         classid;        /* OID of its catalog (pg_proc, etc) */
    Oid         serverid;       /* FDW server we are concerned with */
} ShippableCacheKey;

typedef struct
{
    ShippableCacheKey key;      /* hash key - must be first */
    bool        shippable;
} ShippableCacheEntry;


/*
 * Flush cache entries when pg_foreign_server is updated.
 *
 * We do this because of the possibility of ALTER SERVER being used to change
 * a server's extensions option.  We do not currently bother to check whether
 * objects' extension membership changes once a shippability decision has been
 * made for them, however.
 */
static void
InvalidateShippableCacheCallback(Datum arg, int cacheid, uint32 hashvalue)
{
    HASH_SEQ_STATUS status;
    ShippableCacheEntry *entry;

    /*
     * In principle we could flush only cache entries relating to the
     * pg_foreign_server entry being outdated; but that would be more
     * complicated, and it's probably not worth the trouble.  So for now, just
     * flush all entries.
     */
    hash_seq_init(&status, ShippableCacheHash);
    while ((entry = (ShippableCacheEntry *) hash_seq_search(&status)) != NULL)
    {
        if (hash_search(ShippableCacheHash,
                        (void *) &entry->key,
                        HASH_REMOVE,
                        NULL) == NULL)
            elog(ERROR, "hash table corrupted");
    }
}

/*
 * Initialize the backend-lifespan cache of shippability decisions.
 */
static void
InitializeShippableCache(void)
{
    HASHCTL     ctl;

    /* Create the hash table. */
    ctl.keysize = sizeof(ShippableCacheKey);
    ctl.entrysize = sizeof(ShippableCacheEntry);
    ShippableCacheHash =
        hash_create("Shippability cache", 256, &ctl, HASH_ELEM | HASH_BLOBS);

    /* Set up invalidation callback on pg_foreign_server. */
    CacheRegisterSyscacheCallback(FOREIGNSERVEROID,
                                  InvalidateShippableCacheCallback,
                                  (Datum) 0);
}

/*
 * Returns true if given object (operator/function/type) is shippable
 * according to the server options.
 *
 * Right now "shippability" is exclusively a function of whether the object
 * belongs to an extension declared by the user.  In the future we could
 * additionally have a list of functions/operators declared one at a time.
 */
static bool
lookup_shippable(Oid objectId, Oid classId, PgFdwRelationInfo *fpinfo)
{
    Oid         extensionOid;

    /*
     * Is object a member of some extension?  (Note: this is a fairly
     * expensive lookup, which is why we try to cache the results.)
     */
    extensionOid = getExtensionOfObject(classId, objectId);

    /* If so, is that extension in fpinfo->shippable_extensions? */
    if (OidIsValid(extensionOid) &&
        list_member_oid(fpinfo->shippable_extensions, extensionOid))
        return true;

    return false;
}

/*
 * Return true if given object is one of PostgreSQL's built-in objects.
 *
 * We use FirstGenbkiObjectId as the cutoff, so that we only consider
 * objects with hand-assigned OIDs to be "built in", not for instance any
 * function or type defined in the information_schema.
 *
 * Our constraints for dealing with types are tighter than they are for
 * functions or operators: we want to accept only types that are in pg_catalog,
 * else deparse_type_name might incorrectly fail to schema-qualify their names.
 * Thus we must exclude information_schema types.
 *
 * XXX there is a problem with this, which is that the set of built-in
 * objects expands over time.  Something that is built-in to us might not
 * be known to the remote server, if it's of an older version.  But keeping
 * track of that would be a huge exercise.
 */
bool
is_builtin(Oid objectId)
{
    return (objectId < FirstGenbkiObjectId);
}

/*
 * is_shippable
 *     Is this object (function/operator/type) shippable to foreign server?
 */
bool
is_shippable(Oid objectId, Oid classId, PgFdwRelationInfo *fpinfo)
{
    ShippableCacheKey key;
    ShippableCacheEntry *entry;

    /* Built-in objects are presumed shippable. */
    if (is_builtin(objectId))
        return true;

    /* Otherwise, give up if user hasn't specified any shippable extensions. */
    if (fpinfo->shippable_extensions == NIL)
        return false;

    /* Initialize cache if first time through. */
    if (!ShippableCacheHash)
        InitializeShippableCache();

    /* Set up cache hash key */
    key.objid = objectId;
    key.classid = classId;
    key.serverid = fpinfo->server->serverid;

    /* See if we already cached the result. */
    entry = (ShippableCacheEntry *)
        hash_search(ShippableCacheHash,
                    (void *) &key,
                    HASH_FIND,
                    NULL);

    if (!entry)
    {
        /* Not found in cache, so perform shippability lookup. */
        bool        shippable = lookup_shippable(objectId, classId, fpinfo);

        /*
         * Don't create a new hash entry until *after* we have the shippable
         * result in hand, as the underlying catalog lookups might trigger a
         * cache invalidation.
         */
        entry = (ShippableCacheEntry *)
            hash_search(ShippableCacheHash,
                        (void *) &key,
                        HASH_ENTER,
                        NULL);

        entry->shippable = shippable;
    }

    return entry->shippable;
}