为PostgreSQL添加插件

    我目前了解的PG插件大约有两种，一种是利用hook，另一种是建立C函数，然后在数据库中进行关联。PG本身就是最好的老师，大家有兴趣可以看一下contrib目录下的插件。

    下面将对两种方式进行介绍：

    1、利用hook建立插件，hook是PG中可以对PG运行机制进行修改的一种方式，大家可以看一下我之前对PG hook的介绍：

    a.在contrib目录下建立brother目录，建立brother.c文件和Makefile文件。具体内容见下：

brother.c（在用户brother登录验证成功后，显示信息Welcome to the world of PostgreSQL!）:

/*-------------------------------------------------------------------------
 *
 * brother.c
 *	  welcome to the world of PostgreSQL
 *
 *	  contrib/brother/brother.c
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "libpq/auth.h"
#include "miscadmin.h"
#include "utils/guc.h"

PG_MODULE_MAGIC;

void _PG_init(void);
void _PG_fini(void);

static ClientAuthentication_hook_type pre_ClientAuthentication_hook = NULL;

static
void welcome_to_pg(Port *port, int status)
{
	if(status == STATUS_OK && (strcmp(port->user_name, "brother") == 0))
		printf("Welcome to the world of PostgreSQL!\n");
}

void
_PG_init(void)
{
        pre_ClientAuthentication_hook = ClientAuthentication_hook;
        ClientAuthentication_hook = welcome_to_pg;
}

void
_PG_fini(void)
{
        ClientAuthentication_hook = pre_ClientAuthentication_hook;
}

Makefile文件：

# contrib/brother/Makefile

MODULE_big = brother
OBJS = brother.o

# uncomment the following two lines to enable cracklib support
# PG_CPPFLAGS = -DUSE_CRACKLIB '-DCRACKLIB_DICTPATH="/usr/lib/cracklib_dict"'
# SHLIB_LINK = -lcrack

ifdef USE_PGXS
PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)
else
subdir = contrib/brother
top_builddir = ../..
include $(top_builddir)/src/Makefile.global
include $(top_srcdir)/contrib/contrib-global.mk
endif

    b.编译并安装

[postgres@localhost brother]$ make
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -g -O2 -fpic -I. -I. -I../../src/include -D_GNU_SOURCE   -c -o brother.o brother.c
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -g -O2 -fpic -shared -o brother.so brother.o -L../../src/port -L../../src/common -Wl,--as-needed -Wl,-rpath,'/opt/hg3.0/lib',--enable-new-dtags  
[postgres@localhost brother]$ make install
/bin/mkdir -p '/opt/hg3.0/lib/postgresql'
/usr/bin/install -c -m 755  brother.so '/opt/hg3.0/lib/postgresql/brother.so'

    c.文件安装位置

[postgres@localhost postgresql]$ pwd
/opt/hg3.0/lib/postgresql
[postgres@localhost postgresql]$ ll brother.so 
-rwxr-xr-x. 1 postgres postgres 21523 Dec 23 06:27 brother.so

    d.在postgresql.conf中配置

shared_preload_libraries = 'brother'

    e.运行，查看效果

[postgres@localhost bin]$ ./pg_ctl -D ../data start
server starting
[postgres@localhost bin]$ LOG:  database system was shut down at 2015-12-23 06:32:38 PST
LOG:  MultiXact member wraparound protections are now enabled
LOG:  database system is ready to accept connections
LOG:  autovacuum launcher started

[postgres@localhost bin]$ ./psql
psql (9.4.5)
Type "help" for help.

postgres=# create user brother;
CREATE ROLE

[postgres@localhost bin]$ ./psql postgres brother
Welcome to the world of PostgreSQL!
psql (9.4.5)
Type "help" for help.

postgres=>

    2、建立C函数，并建立关联

    a.在contrib下建立目录userid，建立C文件userid.c，建立Makefile文件，建立两个SQL文件，userid--1.0.sql，userid--unpackaged--1.0.sql，以及一个control文件，userid.control。

    (1)C文件和Makefile就不介绍了，内容如下：
userid.c：

/*-------------------------------------------------------------------------
 *
 * userid.c
 *	  display current session user oid
 *
 *	  contrib/userid/userid.c
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "libpq/auth.h"
#include "utils/guc.h"
#include "miscadmin.h"

PG_MODULE_MAGIC;

PG_FUNCTION_INFO_V1(get_current_user_id);

Datum
get_current_user_id(PG_FUNCTION_ARGS)
{
	PG_RETURN_OID(GetSessionUserId());
}

Makefile：

# contrib/userid/Makefile

MODULE_big = userid
OBJS = userid.o

EXTENSION = userid
DATA = userid--1.0.sql userid--unpackaged--1.0.sql

ifdef USE_PGXS
PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)
else
subdir = contrib/userid
top_builddir = ../..
include $(top_builddir)/src/Makefile.global
include $(top_srcdir)/contrib/contrib-global.mk
endif

    (2)userid--1.0.sql是在create extension时加载的SQL文件，会执行SQL文件中的sql，内容如下：

/* contrib/userid/userid--1.0.sql */

-- complain if script is sourced in psql, rather than via CREATE EXTENSION
\echo Use "CREATE EXTENSION userid" to load this file. \quit

-- Register the function.
CREATE SCHEMA brother;
CREATE FUNCTION get_current_userid()
RETURNS OID
AS 'MODULE_PATHNAME', 'get_current_user_id'
LANGUAGE C;

    (3) userid--unpackaged--1.0.sql是CREATE EXTENSION userid FROM unpackaged时调用的SQL文件，这个文件是在更新9.1之前版本的插件函数时使用的，后面会介绍用法。

/* contrib/userid/userid--unpackaged--1.0.sql */

-- complain if script is sourced in psql, rather than via CREATE EXTENSION
\echo Use "CREATE EXTENSION userid FROM unpackaged" to load this file. \quit

ALTER EXTENSION userid ADD schema brother;
ALTER EXTENSION userid ADD function get_current_userid();

    (4)control文件是在create extension加载的文件，根据这个文件，PG去寻找对应的sql文件，内容如下：

# userid extension
comment = 'display current session user oid'
default_version = '1.0'
module_pathname = '$libdir/userid'
relocatable = true

    b.编译安装

[postgres@localhost userid]$ make
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -g -O2 -fpic -I. -I. -I../../src/include -D_GNU_SOURCE   -c -o userid.o userid.c
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -g -O2 -fpic -shared -o userid.so userid.o -L../../src/port -L../../src/common -Wl,--as-needed -Wl,-rpath,'/opt/hg3.0/lib',--enable-new-dtags  
[postgres@localhost userid]$ make install
/bin/mkdir -p '/opt/hg3.0/lib/postgresql'
/bin/mkdir -p '/opt/hg3.0/share/postgresql/extension'
/bin/mkdir -p '/opt/hg3.0/share/postgresql/extension'
/usr/bin/install -c -m 755  userid.so '/opt/hg3.0/lib/postgresql/userid.so'
/usr/bin/install -c -m 644 userid.control '/opt/hg3.0/share/postgresql/extension/'
/usr/bin/install -c -m 644 userid--1.0.sql userid--unpackaged--1.0.sql '/opt/hg3.0/share/postgresql/extension/'

    c.文件安装位置

[postgres@localhost postgresql]$ pwd
/opt/hg3.0/lib/postgresql
[postgres@localhost postgresql]$ ll userid.so 
-rwxr-xr-x. 1 postgres postgres 18665 Dec 23 06:48 userid.so

[postgres@localhost extension]$ pwd
/opt/hg3.0/share/postgresql/extension
[postgres@localhost extension]$ ll userid*
-rw-r--r--. 1 postgres postgres 329 Dec 23 06:48 userid--1.0.sql
-rw-r--r--. 1 postgres postgres 142 Dec 23 06:48 userid.control
-rw-r--r--. 1 postgres postgres 304 Dec 23 06:48 userid--unpackaged--1.0.sql

    d.配置

[postgres@localhost bin]$ ./psql 
psql (9.4.5)
Type "help" for help.

postgres=# create extension userid;
CREATE EXTENSION
postgres=# select * from pg_extension ;
      extname       | extowner | extnamespace | extrelocatable | extversion | ex
tconfig | extcondition 
--------------------+----------+--------------+----------------+------------+---
--------+--------------
 plpgsql            |       10 |           11 | f              | 1.0        |   
        | 
 pgfincore          |       10 |         2200 | t              | 1.1.1      |   
        | 
 pg_stat_statements |       10 |         2200 | t              | 1.2        |   
        | 
 pg_freespacemap    |       10 |         2200 | t              | 1.0        |   
        | 
 pgstattuple        |       10 |         2200 | t              | 1.2        |   
        | 
 userid             |       10 |         2200 | t              | 1.0        |   
        | 
(6 rows)

    e.运行，查看效果 

postgres=# select SESSION_USER;
 session_user 
--------------
 postgres
(1 row)

postgres=# select oid from pg_authid where rolname = 'postgres';
 oid 
-----
  10
(1 row)

postgres=# select get_current_userid();
 get_current_userid 
--------------------
                 10
(1 row)

postgres=# \dn
  List of schemas
  Name   |  Owner   
---------+----------
 brother | postgres
 public  | postgres
(2 rows)

postgres=#

    f.使用unpackaged.sql，这里是更新9.1之前的插件使用的，现在来模拟一下：

postgres=# create schema brother;
CREATE SCHEMA
postgres=# CREATE FUNCTION get_current_userid()
RETURNS OID
AS '$libdir/userid', 'get_current_user_id'
LANGUAGE C;
CREATE FUNCTION
postgres=# create extension userid with schema public from unpackaged;
CREATE EXTENSION

    因为在9.1之前都是利用SQL安装的插件，没有和extension进行关联，so，在这里可以关联一下。那什么是关联呢？

    g.extension关联的意思，首先大家需要知道PG的表，pg_depend。这张表是当我们进行drop时，添加参数CASCADE会调用查询的一张表，这张表主要存储对象oid依赖的对象oid等等。下面来看一下：

postgres=# select oid from pg_proc where proname = 'get_current_userid';
  oid  
-------
 24715
(1 row)

postgres=# select oid from pg_namespace where nspname = 'brother'; 
  oid  
-------
 24714
(1 row)

postgres=# select oid from pg_extension where extname = 'userid';
  oid  
-------
 24716
(1 row)

postgres=# select * from pg_depend where refobjid = 24716;
 classid | objid | objsubid | refclassid | refobjid | refobjsubid | deptype 
---------+-------+----------+------------+----------+-------------+---------
    2615 | 24714 |        0 |       3079 |    24716 |           0 | e
    1255 | 24715 |        0 |       3079 |    24716 |           0 | e
(2 rows)

    这个地方就是当你drop extension时，同时会删除你插件中SQL文件建立的对象的原因。那当create extension时，又是什么地方去写pg_depend信息的呢？

    h.当你进行create extension时，下面这段代码就是自动建立关联的代码：

/*
 * If we are executing a CREATE EXTENSION operation, mark the given object
 * as being a member of the extension.  Otherwise, do nothing.
 *
 * This must be called during creation of any user-definable object type
 * that could be a member of an extension.
 *
 * If isReplace is true, the object already existed (or might have already
 * existed), so we must check for a pre-existing extension membership entry.
 * Passing false is a guarantee that the object is newly created, and so
 * could not already be a member of any extension.
 */
void
recordDependencyOnCurrentExtension(const ObjectAddress *object,
								   bool isReplace)
{
	/* Only whole objects can be extension members */
	Assert(object->objectSubId == 0);

	if (creating_extension)
	{
		ObjectAddress extension;

		/* Only need to check for existing membership if isReplace */
		if (isReplace)
		{
			Oid			oldext;

			oldext = getExtensionOfObject(object->classId, object->objectId);
			if (OidIsValid(oldext))
			{
				/* If already a member of this extension, nothing to do */
				if (oldext == CurrentExtensionObject)
					return;
				/* Already a member of some other extension, so reject */
				ereport(ERROR,
						(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
						 errmsg("%s is already a member of extension \"%s\"",
								getObjectDescription(object),
								get_extension_name(oldext))));
			}
		}

		/* OK, record it as a member of CurrentExtensionObject */
		extension.classId = ExtensionRelationId;
		extension.objectId = CurrentExtensionObject;
		extension.objectSubId = 0;

		recordDependencyOn(object, &extension, DEPENDENCY_EXTENSION);
	}
}

    至此，就是我在学习PG源码中学习到的知识。这里介绍的还比较简单，请大家多多关注，我后续还会继续整理的。