• PostgreSQL的学习心得和知识总结（一百三十九）|深入理解PostgreSQL数据库GUC参数 allow_alter_system 的使用和原理

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    注：提前言明 本文借鉴了以下博主、书籍或网站的内容，其列表如下：

    1、参考书籍：《PostgreSQL数据库内核分析》
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    深入理解PostgreSQL数据库GUC参数 allow_alter_system 的使用和原理

    文章快速说明索引

    学习目标：

    做数据库内核开发久了就会有一种 少年得志，年少轻狂 的错觉，然鹅细细一品觉得自己其实不算特别优秀 远远没有达到自己想要的。也许光鲜的表面掩盖了空洞的内在，每每想到于此，皆有夜半临渊如履薄冰之感。为了睡上几个踏实觉，即日起 暂缓其他基于PostgreSQL数据库的兼容功能开发，近段时间 将着重于学习分享Postgres的基础知识和实践内幕。

    学习内容：（详见目录）

    1、深入理解PostgreSQL数据库GUC参数 allow_alter_system 的使用和原理

    学习时间：

    2024年04月21日 14:44:16

    学习产出：

    1、PostgreSQL数据库基础知识回顾 1个
    2、CSDN 技术博客 1篇
    3、PostgreSQL数据库内核深入学习

    注：下面我们所有的学习环境是Centos8+PostgreSQL master +Oracle19C+MySQL8.0

    postgres=# select version();
                                                  version                                                   
------------------------------------------------------------------------------------------------------------
 PostgreSQL 17devel on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-21), 64-bit
(1 row)

postgres=#

#-----------------------------------------------------------------------------#

SQL> select * from v$version;          

BANNER        Oracle Database 19c EE Extreme Perf Release 19.0.0.0.0 - Production	
BANNER_FULL	  Oracle Database 19c EE Extreme Perf Release 19.0.0.0.0 - Production Version 19.17.0.0.0	
BANNER_LEGACY Oracle Database 19c EE Extreme Perf Release 19.0.0.0.0 - Production	
CON_ID 0


#-----------------------------------------------------------------------------#

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.27    |
+-----------+
1 row in set (0.06 sec)

mysql>

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    参数使用背景说明

    背景

    2024/03/29日，PostgreSQL新增一个GUC参数，如下：

    添加allow_alter_system GUC

    这个被标记为PGC_SIGHUP，所以只能在配置文件中设置，不能在其他地方设置；并且它还被标记为GUC_DISALLOW_IN_AUTO_FILE，因此不能使用ALTER SYSTEM进行设置。当设置为 false 时，不允许使用 ALTER SYSTEM 命令

    人们非常担心这会被误解为安全功能，但事实并非如此，因为坚定的超级用户可以通过多种方式绕过它。因此，我们在文档的措辞上做了很多工作，希望避免任何此类混乱

    官方文档的解释，如下：

    当allow_alter_system设置为off时，执行ALTER SYSTEM命令会返回错误。该参数只能在 postgresql.conf 文件或服务器命令行中设置。默认值是打开的

    请注意，此设置不得视为安全功能。它仅禁用 ALTER SYSTEM 命令。它不会阻止超级用户使用其他 SQL 命令更改配置。超级用户有多种在操作系统级别执行 shell 命令的方法，因此可以修改 postgresql.auto.conf，无论此设置的值如何

    关闭此设置适用于 PostgreSQL 配置由某些外部工具管理的环境。在这种环境中，善意的超级用户可能会错误地使用 ALTER SYSTEM 来更改配置，而不是使用外部工具。这可能会导致意外行为，例如外部工具在稍后更新配置时覆盖更改。将此参数设置为关闭可以帮助避免此类错误

    该参数仅控制 ALTER SYSTEM 的使用。即使allow_alter_system设置为off，存储在postgresql.auto.conf中的设置也会生效

    使用

    接下来看一个简单的使用，示例1如下：

    [postgres@localhost:~/test/bin]$ ./psql 
psql (17devel)
Type "help" for help.

postgres=# show timezone;
      TimeZone       
---------------------
 America/Los_Angeles
(1 row)

postgres=# set timezone = 'PRC';
SET
postgres=# show timezone;
 TimeZone 
----------
 PRC
(1 row)

postgres=# select now();
              now              
-------------------------------
 2024-04-18 15:35:49.411729+08
(1 row)

postgres=# reset timezone;
RESET
postgres=# show timezone;
      TimeZone       
---------------------
 America/Los_Angeles
(1 row)

postgres=# select now();
              now              
-------------------------------
 2024-04-18 00:36:06.505543-07
(1 row)

postgres=# alter system set timezone = 'PRC';
2024-04-18 00:36:23.026 PDT [30697] ERROR:  ALTER SYSTEM is not allowed in this environment
2024-04-18 00:36:23.026 PDT [30697] STATEMENT:  alter system set timezone = 'PRC';
ERROR:  ALTER SYSTEM is not allowed in this environment
postgres=# 
postgres=# show timezone;
      TimeZone       
---------------------
 America/Los_Angeles
(1 row)

postgres=# show allow_alter_system;
 allow_alter_system 
--------------------
 off
(1 row)

postgres=# alter system set allow_alter_system = on;
2024-04-18 00:40:03.694 PDT [30697] ERROR:  ALTER SYSTEM is not allowed in this environment
2024-04-18 00:40:03.694 PDT [30697] STATEMENT:  alter system set allow_alter_system = on;
ERROR:  ALTER SYSTEM is not allowed in this environment
postgres=#

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    [postgres@localhost:~/test/bin]$ cat test/postgresql.conf | grep allow_alter_system
allow_alter_system = off
[postgres@localhost:~/test/bin]$

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    示例2，如下：

    [postgres@localhost:~/test/bin]$ cat test/postgresql.conf | grep allow_alter_system
allow_alter_system = off
[postgres@localhost:~/test/bin]$ 
[postgres@localhost:~/test/bin]$ vim test/postgresql.auto.conf 
[postgres@localhost:~/test/bin]$ 
[postgres@localhost:~/test/bin]$ cat test/postgresql.auto.conf 
# Do not edit this file manually!
# It will be overwritten by the ALTER SYSTEM command.
allow_alter_system = on
[postgres@localhost:~/test/bin]$ 
[postgres@localhost:~/test/bin]$ ./pg_ctl restart -D test/
waiting for server to shut down....2024-04-24 23:05:34.217 PDT [30683] LOG:  received fast shutdown request
2024-04-24 23:05:34.219 PDT [30683] LOG:  aborting any active transactions
2024-04-24 23:05:34.220 PDT [30683] LOG:  background worker "logical replication launcher" (PID 30689) exited with exit code 1
2024-04-24 23:05:34.221 PDT [30684] LOG:  shutting down
2024-04-24 23:05:34.240 PDT [30683] LOG:  database system is shut down
 done
server stopped
waiting for server to start....2024-04-24 23:05:34.361 PDT [50234] LOG:  starting PostgreSQL 17devel on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-21), 64-bit
2024-04-24 23:05:34.361 PDT [50234] LOG:  listening on IPv6 address "::1", port 5432
2024-04-24 23:05:34.361 PDT [50234] LOG:  listening on IPv4 address "127.0.0.1", port 5432
2024-04-24 23:05:34.363 PDT [50234] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2024-04-24 23:05:34.366 PDT [50237] LOG:  database system was shut down at 2024-04-24 23:05:34 PDT
2024-04-24 23:05:34.370 PDT [50234] LOG:  database system is ready to accept connections
 done
server started
[postgres@localhost:~/test/bin]$ ./psql 
psql (17devel)
Type "help" for help.

postgres=# show allow_alter_system;
 allow_alter_system 
--------------------
 on
(1 row)

postgres=# show timezone;
      TimeZone       
---------------------
 America/Los_Angeles
(1 row)

postgres=# alter system set timezone = 'PRC';
ALTER SYSTEM
postgres=# alter system set allow_alter_system = off;
2024-04-24 23:06:26.063 PDT [50248] ERROR:  parameter "allow_alter_system" cannot be changed
2024-04-24 23:06:26.063 PDT [50248] STATEMENT:  alter system set allow_alter_system = off;
ERROR:  parameter "allow_alter_system" cannot be changed
postgres=#

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    参数实现源码解析

    guc定义，如下：

    // src\backend\utils\misc\guc_tables.c

	{
		/*
		 * This setting itself cannot be set by ALTER SYSTEM to avoid an
		 * operator turning this setting off by using ALTER SYSTEM, without a
		 * way to turn it back on.
		 *  
		 * 此设置本身无法通过 ALTER SYSTEM 设置，以避免操作员使用 ALTER SYSTEM 关闭此设置
		 * 而无法将其重新打开
		 */
		{"allow_alter_system", PGC_SIGHUP, COMPAT_OPTIONS_OTHER,
			gettext_noop("Allows running the ALTER SYSTEM command."),
			gettext_noop("Can be set to off for environments where global configuration "
						 "changes should be made using a different method."),
			GUC_DISALLOW_IN_AUTO_FILE
		},
		&AllowAlterSystem,
		true,
		NULL, NULL, NULL
	},

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    该参数真实使用的逻辑，如下：

    // src\backend\utils\misc\guc.c

/*
 * Execute ALTER SYSTEM statement.
 *
 * Read the old PG_AUTOCONF_FILENAME file, merge in the new variable value,
 * and write out an updated file.  If the command is ALTER SYSTEM RESET ALL,
 * we can skip reading the old file and just write an empty file.
 * 读取旧的 PG_AUTOCONF_FILENAME 文件，合并新的变量值，并写出更新的文件
 * 如果命令是 ALTER SYSTEM RESET ALL，我们可以跳过读取旧文件，只写入一个空文件
 *
 * An LWLock is used to serialize updates of the configuration file.
 * LWLock 用于序列化配置文件的更新
 *
 * In case of an error, we leave the original automatic
 * configuration file (PG_AUTOCONF_FILENAME) intact.
 * 如果出现错误，我们会保留原始自动配置文件（PG_AUTOCONF_FILENAME）不变
 */
void
AlterSystemSetConfigFile(AlterSystemStmt *altersysstmt)
{
	char	   *name;
	char	   *value;
	bool		resetall = false;
	ConfigVariable *head = NULL;
	ConfigVariable *tail = NULL;
	volatile int Tmpfd;
	char		AutoConfFileName[MAXPGPATH];
	char		AutoConfTmpFileName[MAXPGPATH];

	/*
	 * Extract statement arguments
	 */
	name = altersysstmt->setstmt->name;

	if (!AllowAlterSystem)
		ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("ALTER SYSTEM is not allowed in this environment")));

	switch (altersysstmt->setstmt->kind)
	{
		case VAR_SET_VALUE:
			value = ExtractSetVariableArgs(altersysstmt->setstmt);
			break;

		case VAR_SET_DEFAULT:
		case VAR_RESET:
			value = NULL;
			break;

		case VAR_RESET_ALL:
			value = NULL;
			resetall = true;
			break;

		default:
			elog(ERROR, "unrecognized alter system stmt type: %d",
				 altersysstmt->setstmt->kind);
			break;
	}

	/*
	 * Check permission to run ALTER SYSTEM on the target variable
	 */
	if (!superuser())
	{
		if (resetall)
			ereport(ERROR,
					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
					 errmsg("permission denied to perform ALTER SYSTEM RESET ALL")));
		else
		{
			AclResult	aclresult;

			aclresult = pg_parameter_aclcheck(name, GetUserId(),
											  ACL_ALTER_SYSTEM);
			if (aclresult != ACLCHECK_OK)
				ereport(ERROR,
						(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
						 errmsg("permission denied to set parameter \"%s\"",
								name)));
		}
	}

	/*
	 * Unless it's RESET_ALL, validate the target variable and value
	 */
	if (!resetall)
	{
		struct config_generic *record;

		/* We don't want to create a placeholder if there's not one already */
		record = find_option(name, false, true, DEBUG5);
		if (record != NULL)
		{
			/*
			 * Don't allow parameters that can't be set in configuration files
			 * to be set in PG_AUTOCONF_FILENAME file.
			 */
			if ((record->context == PGC_INTERNAL) ||
				(record->flags & GUC_DISALLOW_IN_FILE) ||
				(record->flags & GUC_DISALLOW_IN_AUTO_FILE))
				ereport(ERROR,
						(errcode(ERRCODE_CANT_CHANGE_RUNTIME_PARAM),
						 errmsg("parameter \"%s\" cannot be changed",
								name)));

			/*
			 * If a value is specified, verify that it's sane.
			 */
			if (value)
			{
				union config_var_val newval;
				void	   *newextra = NULL;

				if (!parse_and_validate_value(record, name, value,
											  PGC_S_FILE, ERROR,
											  &newval, &newextra))
					ereport(ERROR,
							(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
							 errmsg("invalid value for parameter \"%s\": \"%s\"",
									name, value)));

				if (record->vartype == PGC_STRING && newval.stringval != NULL)
					guc_free(newval.stringval);
				guc_free(newextra);
			}
		}
		else
		{
			/*
			 * Variable not known; check we'd be allowed to create it.  (We
			 * cannot validate the value, but that's fine.  A non-core GUC in
			 * the config file cannot cause postmaster start to fail, so we
			 * don't have to be too tense about possibly installing a bad
			 * value.)
			 */
			(void) assignable_custom_variable_name(name, false, ERROR);
		}

		/*
		 * We must also reject values containing newlines, because the grammar
		 * for config files doesn't support embedded newlines in string
		 * literals.
		 */
		if (value && strchr(value, '\n'))
			ereport(ERROR,
					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
					 errmsg("parameter value for ALTER SYSTEM must not contain a newline")));
	}

	/*
	 * PG_AUTOCONF_FILENAME and its corresponding temporary file are always in
	 * the data directory, so we can reference them by simple relative paths.
	 */
	snprintf(AutoConfFileName, sizeof(AutoConfFileName), "%s",
			 PG_AUTOCONF_FILENAME);
	snprintf(AutoConfTmpFileName, sizeof(AutoConfTmpFileName), "%s.%s",
			 AutoConfFileName,
			 "tmp");

	/*
	 * Only one backend is allowed to operate on PG_AUTOCONF_FILENAME at a
	 * time.  Use AutoFileLock to ensure that.  We must hold the lock while
	 * reading the old file contents.
	 */
	LWLockAcquire(AutoFileLock, LW_EXCLUSIVE);

	/*
	 * If we're going to reset everything, then no need to open or parse the
	 * old file.  We'll just write out an empty list.
	 */
	if (!resetall)
	{
		struct stat st;

		if (stat(AutoConfFileName, &st) == 0)
		{
			/* open old file PG_AUTOCONF_FILENAME */
			FILE	   *infile;

			infile = AllocateFile(AutoConfFileName, "r");
			if (infile == NULL)
				ereport(ERROR,
						(errcode_for_file_access(),
						 errmsg("could not open file \"%s\": %m",
								AutoConfFileName)));

			/* parse it */
			if (!ParseConfigFp(infile, AutoConfFileName, CONF_FILE_START_DEPTH,
							   LOG, &head, &tail))
				ereport(ERROR,
						(errcode(ERRCODE_CONFIG_FILE_ERROR),
						 errmsg("could not parse contents of file \"%s\"",
								AutoConfFileName)));

			FreeFile(infile);
		}

		/*
		 * Now, replace any existing entry with the new value, or add it if
		 * not present.
		 */
		replace_auto_config_value(&head, &tail, name, value);
	}

	/*
	 * Invoke the post-alter hook for setting this GUC variable.  GUCs
	 * typically do not have corresponding entries in pg_parameter_acl, so we
	 * call the hook using the name rather than a potentially-non-existent
	 * OID.  Nonetheless, we pass ParameterAclRelationId so that this call
	 * context can be distinguished from others.  (Note that "name" will be
	 * NULL in the RESET ALL case.)
	 *
	 * We do this here rather than at the end, because ALTER SYSTEM is not
	 * transactional.  If the hook aborts our transaction, it will be cleaner
	 * to do so before we touch any files.
	 */
	InvokeObjectPostAlterHookArgStr(ParameterAclRelationId, name,
									ACL_ALTER_SYSTEM,
									altersysstmt->setstmt->kind,
									false);

	/*
	 * To ensure crash safety, first write the new file data to a temp file,
	 * then atomically rename it into place.
	 *
	 * If there is a temp file left over due to a previous crash, it's okay to
	 * truncate and reuse it.
	 */
	Tmpfd = BasicOpenFile(AutoConfTmpFileName,
						  O_CREAT | O_RDWR | O_TRUNC);
	if (Tmpfd < 0)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not open file \"%s\": %m",
						AutoConfTmpFileName)));

	/*
	 * Use a TRY block to clean up the file if we fail.  Since we need a TRY
	 * block anyway, OK to use BasicOpenFile rather than OpenTransientFile.
	 */
	PG_TRY();
	{
		/* Write and sync the new contents to the temporary file */
		write_auto_conf_file(Tmpfd, AutoConfTmpFileName, head);

		/* Close before renaming; may be required on some platforms */
		close(Tmpfd);
		Tmpfd = -1;

		/*
		 * As the rename is atomic operation, if any problem occurs after this
		 * at worst it can lose the parameters set by last ALTER SYSTEM
		 * command.
		 */
		durable_rename(AutoConfTmpFileName, AutoConfFileName, ERROR);
	}
	PG_CATCH();
	{
		/* Close file first, else unlink might fail on some platforms */
		if (Tmpfd >= 0)
			close(Tmpfd);

		/* Unlink, but ignore any error */
		(void) unlink(AutoConfTmpFileName);

		PG_RE_THROW();
	}
	PG_END_TRY();

	FreeConfigVariables(head);

	LWLockRelease(AutoFileLock);
}

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  • 原文地址：https://blog.csdn.net/weixin_43949535/article/details/138125430