使用自定义库安装 postgresql 扩展的正确方法

Question

TL;DR

One has to compile their custom library as shared library:

gcc -c -fPIC warp_client.c -o warp_client.o
gcc -shared warp_client.o libwarp-client.so 

Include the shared library and additional dependencies of that shared library in the Postgresql Makefile with the flags SHLIB_LINK and PG_LDFLAGS(Here the bachelor_fdw.c is the extension to compile):

EXTENSION = bachelor_fdw
MODULE_big = bachelor_fdw
DATA = bachelor_fdw--0.1.sql
OBJS = bachelor_fdw.o

PG_LIBS = -lpq
SHLIB_LINK = -lwarp_client -lucp
PG_LDFLAGS += -L/usr/lib/warpdrive/ -L/usr/lib/ucx/

PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)

Include the directories of the shared libraries into the environment variable LD_LIBRARY_PATH of Postgresql. For that, one has to add a line to the file 'environment' in the main Postgresql directory and restart Postgresql. Here is mine:

$ cat /etc/postgresql/12/main/environment
# environment variables for postgres processes
# This file has the same syntax as postgresql.conf:
#  VARIABLE = simple_value
#  VARIABLE2 = 'any value!'
# I. e. you need to enclose any value which does not only consist of letters,
# numbers, and '-', '_', '.' in single quotes. Shell commands are not
# evaluated.
LD_LIBRARY_PATH='/usr/include/:/usr/include/ucx/:/usr/lib/:/usr/lib/ucx/'

---

I am trying to create a foreign data wrapper, which uses a custom library from me. The fdw compiles and installs fine, but when using it, symbols to my library are undefined. What is the proper way of using custom c code as library in a postgresql extension and what am i doing wrong? Here are the steps i took:

1. Compile my library (warp_client.c) with flag -fPIC into an object file.

gcc -c -fPIC warp_client.c -o static/warp_client.o

2. Create static library from the object file.

ar -rcs out/libwarp_client.a static/warp_client.o

3. Copy libwarp_client.a and warp_client.h into the postgresql extension project root.
4. Compile postgresql extension with the following makefile.

EXTENSION = bachelor_fdw
MODULE_big = bachelor_fdw
DATA = bachelor_fdw--0.1.sql libwarp_client.a
OBJS = bachelor_fdw.o
HEADERS = warp_client.h

ifdef DEBUG
$(info $(shell echo "debug ist an"))
endif

PG_LIBS = -lpq

PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)

make USE_PGXS=1 install

5. Try to create the extension. The extension makes a call to a library function in it's _PG_INI() function. Error comes up:

CREATE EXTENSION IF NOT EXISTS bachelor_fdw;

psql:only_create.sql:3: ERROR:  could not load library "/usr/lib/postgresql/12/lib/bachelor_fdw.so": /usr/lib/postgresql/12/lib/bachelor_fdw.so: undefined symbol: warpclient_getData

The warp_client.h has the function headers and warp_client.c has the functions. warp_client.c includes "warp_client.h", bachelor_fdw.c (the extension) includes "warp_client.h".

warp_client.h:

#ifndef TEST_FIELD_UCP_WARP_CLIENT_H
#define TEST_FIELD_UCP_WARP_CLIENT_H

#include <ucp/api/ucp.h>

int warpclient_queryServer(char *server_addr_local, int port, int useINet6, char *query);

void *warpclient_getData();

int warpclient_cleanup();

#endif //TEST_FIELD_UCP_WARP_CLIENT_H

Any more desired info? I would be really glad for any help.

EDIT 1

I use the functions from warp_client.h inside of bachelor_fdw.c. Do i still need to export them? I thought only functions, which get called from the postgresql server needs to be exported.

Here is part of bachelor_fdw.c:

#include <warp_client.h>
#include "postgres.h"
#include "foreign/fdwapi.h"
#include "foreign/foreign.h"
#include "nodes/nodes.h"
#include "optimizer/pathnode.h"
#include "optimizer/planmain.h"
...

PG_MODULE_MAGIC;

/*
 * SQL functions
 */
PG_FUNCTION_INFO_V1(bachelor_fdw_handler);
PG_FUNCTION_INFO_V1(bachelor_fdw_validator);

/*
 *  Extension initialization functions
 */
extern void _PG_init(void);
extern void _PG_fini(void);

/*
 * FDW callback routines
 */
static void bachelorBeginForeignScan(ForeignScanState *node, int eflags);
static TupleTableSlot *bachelorIterateForeignScan(ForeignScanState *node);
static void bachelorReScanForeignScan(ForeignScanState *node);
static void bachelorEndForeignScan(ForeignScanState *node);
static void bachelorGetForeignRelSize(PlannerInfo *root, RelOptInfo *baserel, Oid foreigntableid);
static void bachelorGetForeignPaths(PlannerInfo *root, RelOptInfo *baserel, Oid foreigntableid);
static ForeignScan* bachelorGetForeignPlan(PlannerInfo *root, RelOptInfo *baserel, Oid foreigntableid, ForeignPath *best_path, List *tlist, List *scan_clauses, Plan *outer_plan);


void _PG_init(void){
    int ret = 0;
    void *data;
    ret = warpclient_queryServer(NULL, -1, 0, "SELECT TEST FROM TEST;");
    elog_debug("Testquery for server. Return code (%d)...\n", ret);
    while(NULL != (data = warpclient_getData())){
        elog_debug("Data received as fdw: %s\n", data);
    }
    elog_debug("Finished receiving data.\n");

    /* Call cleanup */
    ret = warpclient_cleanup();
    elog_debug("Warpclient cleanup (%d)...\n", ret);
}

And here is part of warp_client.c:

#include "warp_client.h"

...

int warpclient_cleanup(){
    int ret = 0;

    //free buffers
    free(recvbuffer->buffer);
    free(recvbuffer);

    /* Close the endpoint to the server */
    debugmsg("Close endpoint.\n");
    ep_close();

    /* releasing UCX ressources */
    ucp_worker_destroy(ucp_worker);
    ucp_cleanup(ucp_context);

    return ret;
}

int warpclient_queryServer(char *server_addr_local, int port, int useINet6, char *query){
    /*
     * Initialize important connection variables
     */
    debugmsg("Initializing connection variables...\n");
    if(NULL != server_addr_local) server_addr = server_addr_local;
    if((port >= 0) && (port <= UINT16_MAX)) server_port = port;
    if(useINet6) ai_family = AF_INET6;

    int ret;

    /* Initialize the UCX required objects worker and context*/
    debugmsg("Initializing context and worker...\n");
    ret = init_context_and_worker();
    if (ret != 0) {
        fprintf(stderr, "Initializing worker or context failed! Exiting..\n");
        return -2;
    }

    /*
     * UCP objects: client_ep as communication endpoint for the worker.
     *              status for function error code check.
     */
    ucs_status_t status;

    /* ep initialization and exchange with server over sockets */
    debugmsg("Creating Client endpoint.\n");
    status = create_client_endpoint();
    if (status != UCS_OK) {
        fprintf(stderr, "failed to start client (%s)\n", ucs_status_string(status));
        return -1;
    }

    ret = send_query(query);
    if(ret!=0){
        debugmsg("Failed to connect to Server.\n");
    }

    return ret;
}

EDIT 2

I managed to get a good step forward thanks to Laurenz Albe. But i still have a problem with a shared library used in my shared library. Do I also need to link to shared libraries used in my own shared library, even though i linked that as i compiled my shared library before distribution?

what I did:

I added SHLIB_LINK = -lwarp_client to the Makefile and also needed the line PG_LDFLAGS += -L. for the linker to find libwarp_client.so. I also managed to include the environment variable LD_LIBRARY_PATH for the postgres service, so that it can find my library in the standard places. And removed the library from the DATA flag in the Makefile.

New Makefile:

EXTENSION = bachelor_fdw
MODULE_big = bachelor_fdw
DATA = bachelor_fdw--0.1.sql
OBJS = bachelor_fdw.o

ifdef DEBUG
$(info $(shell echo "debug ist an"))
endif

PG_LIBS = -lpq
SHLIB_LINK = -lwarp_client
PG_LDFLAGS += -L.

PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)

Enrivonment variables:

/proc/1551/environ | xargs -0 -n 1 echo
LD_LIBRARY_PATH=/usr/include/:/usr/include/ucx/:/usr/lib/:/usr/lib/ucx/
...

When using CREATE on the extension, my library gets used but postgres complains about another shared library, which my library uses.

psql:only_create.sql:3: ERROR:  could not load library "/usr/lib/postgresql/12/lib/bachelor_fdw.so": /usr/lib/warpdrive/libwarp_client.so: undefined symbol: ucp_ep_create

The error clearly says, it uses my shared library from a subdirectory "warpdrive" in the included standard directory. The shared library from UCP is also in that standard directory:

ls /usr/lib/ucx
cmake            libjucx.so.0.0.0  libucp.a         libucs.la        libuct.so
jucx-1.12.1.jar  libucm.a          libucp.la        libucs.so        libuct.so.0
libjucx.a        libucm.la         libucp.so        libucs.so.0      libuct.so.0.0.0
libjucx.la       libucm.so         libucp.so.0      libucs.so.0.0.0  pkgconfig
libjucx.so       libucm.so.0       libucp.so.0.0.0  libuct.a         ucx
libjucx.so.0     libucm.so.0.0.0   libucs.a         libuct.la

Answer 1

That looks like warpclient_getData gets used in your code, but you didn't link your shared object with the library that provides the function. Add the library to the SHLIB_LINK variable:

SHLIB_LINK = -lwarp

(That example assumes a library called libwarp.so .)