pg启动过程中的那些事三：加载GUC参数
1先上个图，看一下函数调用过程梗概，中间细节有略
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????? GUC参数初始化分两步，第一步先读取buildin/compiled-in的GUC参数默认值，这里包括全部的GUC参数，建立GUC参数相关结构变量，第二步读取postgresql.conf配置文件中的参数设置之。从上图中能看出来，这个读取并设置postgresql.conf中参数的过程还是挺复杂的。
2初始化GUC相关数据结构并取hardcode/buildin的参数值。
?????? pg里的GUC参数按设置的值分五种类型，分别是bool、int、real、string、enum，根据这五种类型，定义了五种结构类型，再根据这五种结构，每个类型建一个对应的静态数组，用于存储这些相应类型的GUC参数。这五种类型是config_bool、config_int、config_real、config_string、config_enum，对应的静态数组是ConfigureNamesBool、ConfigureNamesInt、ConfigureNamesReal、ConfigureNamesString、ConfigureNamesEnum。具体结构和数组定义见下面。
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五个结构定义：
struct config_bool
{
??? struct config_generic gen;
??? /* these fields must be set correctly in initial value: */
??? /* (all but reset_val are constants) */
??? bool??? ?? *variable;
??? bool??????? boot_val;
??? GucIntCheckHookcheck_hook;
??? GucBoolAssignHookassign_hook;
??? GucShowHookshow_hook;
??? /* variable fields, initialized at runtime: */
??? bool??????? reset_val;
??? void??????? * reset_extra
};
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struct config_int
{
??? struct config_generic gen;
/*constant fields, must be set correctly in initial value: */
??? int???? ?? *variable;
??? int???????? boot_val;
??? int???????? min;
??? int???????? max;
??? GucIntCheckHookcheck_hook;
??? GucIntAssignHookassign_hook;
??? GucShowHookshow_hook;
??? /* variable fields, initialized at runtime: */
??? int???????? reset_val;
??? void??????? * reset_extra
};
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struct config_real
{
??? struct config_generic gen;
/* constantfields, must be set correctly in initial value: */
??? double? ?? *variable;
??? double????? boot_val;
??? double????? min;
??? double????? max;
??? GucIntCheckHookcheck_hook;
??? GucRealAssignHookassign_hook;
??? GucShowHookshow_hook;
??? /* variable fields, initialized at runtime: */
??? double????? reset_val;
??? void??????? * reset_extra
};
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struct config_string
{
??? struct config_generic gen;
/* constant fields, must beset correctly in initial value: */
??? char??? ? **variable;
??? const char *boot_val;
??? GucIntCheckHookcheck_hook;
??? GucStringAssignHookassign_hook;
??? GucShowHookshow_hook;
??? /* variable fields, initialized at runtime: */
??? char??? ?? *reset_val;
void??????? * reset_extra
};
struct config_enum
{
??? struct config_generic gen;
/* constant fields, must beset correctly in initial value: */
??? int ? *variable;
??? int ??? boot_val;
??? GucIntCheckHookcheck_hook;
??? GucStringAssignHookassign_hook;
??? GucShowHookshow_hook;
??? /* variable fields, initialized at runtime: */
??? int ?? reset_val;
void??????? * reset_extra
};
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和结构类型对应的五个静态数组：
static struct config_boolConfigureNamesBool[] =
{
?????? {
????????????? {"enable_seqscan",PGC_USERSET, QUERY_TUNING_METHOD,
???????????????????? gettext_noop("Enablesthe planner's use of sequential-scan plans."),
???????????????????? NULL
????????????? },
????????????? &enable_seqscan,
????????????? true,
????????????? NULL,NULL, NULL
?????? },
……
?????? /*End-of-list marker */
?????? {
????????????? {NULL,0, 0, NULL, NULL}, NULL, false, NULL, NULL, NULL
?????? }
};
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static struct config_int ConfigureNamesInt[]=
{
?????? {
????????????? {"archive_timeout",PGC_SIGHUP, WAL_ARCHIVING,
???????????????????? gettext_noop("Forcesa switch to the next xlog file if a "
????????????????????????????????????????? ?"new file has not been started within Nseconds."),
???????????????????? NULL,
???????????????????? GUC_UNIT_S
????????????? },
????????????? &XLogArchiveTimeout,
????????????? 0,0, INT_MAX,
????????????? NULL,NULL, NULL
?????? },
……
?????? /*End-of-list marker */
?????? {
????????????? {NULL,0, 0, NULL, NULL}, NULL, 0, 0, 0, NULL, NULL, NULL
?????? }
};
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static struct config_realConfigureNamesReal[] =
{
?????? {
????????????? {"seq_page_cost",PGC_USERSET, QUERY_TUNING_COST,
???????????????????? gettext_noop("Setsthe planner's estimate of the cost of a "
????????????????????????????????????????? ?"sequentially fetched disk page."),
???????????????????? NULL
????????????? },
????????????? &seq_page_cost,
????????????? DEFAULT_SEQ_PAGE_COST,0, DBL_MAX,
????????????? NULL,NULL, NULL
?????? },
?????? /*End-of-list marker */
?????? {
????????????? {NULL,0, 0, NULL, NULL}, NULL, 0.0, 0.0, 0.0, NULL, NULL, NULL
?????? }
};
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static struct config_stringConfigureNamesString[] =
{
?????? {
????????????? {"archive_command",PGC_SIGHUP, WAL_ARCHIVING,
???????????????????? gettext_noop("Setsthe shell command that will be called to archive a WAL file."),
???????????????????? NULL
????????????? },
????????????? &XLogArchiveCommand,
????????????? "",
????????????? NULL,NULL, show_archive_command
?????? },
……
?????? /*End-of-list marker */
?????? {
????????????? {NULL,0, 0, NULL, NULL}, NULL, NULL, NULL, NULL, NULL
?????? }
};
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static struct config_enumConfigureNamesEnum[] =
{
?????? {
????????????? {"backslash_quote",PGC_USERSET, COMPAT_OPTIONS_PREVIOUS,
???????????????????? gettext_noop("Setswhether \"\\'\" is allowed in string literals."),
???????????????????? NULL
????????????? },
????????????? &backslash_quote,
????????????? BACKSLASH_QUOTE_SAFE_ENCODING,backslash_quote_options,
????????????? NULL,NULL, NULL
?????? },
……
?????? /*End-of-list marker */
?????? {
????????????? {NULL,0, 0, NULL, NULL}, NULL, 0, NULL, NULL, NULL, NULL
?????? }
};
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上面五个结构定义中，每个结构的第一个成员变量都是一个config_generic结构的gen成员，下面是config_generic的结构定义：
struct config_generic
{
?????? /* constantfields, must be set correctly in initial value: */
?????? const char *name;???????????????? /* name of variable - MUST BE FIRST */
?????? GucContext?????? context;????????????? /* context required to set the variable */
?????? enumconfig_group group;?? /* to help organize variables by function */
?????? const char *short_desc;???????? /* short desc. of this variable's purpose */
?????? const char *long_desc;????????? /* long desc. of this variable's purpose */
?????? int???????????????? flags;?????????????????? /* flag bits, see below*/
?????? /* variablefields, initialized at runtime: */
?????? enumconfig_type vartype;? /* type of variable (set only at startup) */
?????? int???????????????? status;???????????????? /* status bits, see below*/
?????? GucSource? reset_source;????? /* source of thereset_value */
?????? GucSource? source;??????????????? /*source of the current actual value */
?????? GucStack??*stack;?????????????? /* stacked outside-of-transaction states */
?????? void?????? ??*extra;?????????????????? /*"extra" pointer for current actual value */
?????? char????? ?? *sourcefile;?????????? /* filecurrent setting is from (NULL if not
??????????????????????????????? ?* file) */
?????? int???????????????? sourceline;????????? /* line in source file */
};
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然后，定义一个config_generic **类型的静态变量数组guc_variables，再计算参数总数，所有参数以config_generic*类型计算所需内存空间，冗余25%内存后malloc分配内存空间。把guc_variables每一个元素指向ConfigureNamesBool、ConfigureNamesInt、ConfigureNamesReal、ConfigureNamesString、ConfigureNamesEnum这五个数组的config_generic类型成员gen的地址，然后按照参数名称把所有元素做了快速排序。这个过程中还设置了一些GUC参数的默认值。
static struct config_generic **guc_variables;
??? 后面查询GUC参数都是在guc_variables这个已排序的数组里找。这样GUC参数的数据结构就搭建完成了，下面看看GUC参数相关的数据结构图吧。
??? 先把涉及到的结构的图分别列出，再画个这些结构的组织关系示意图。
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3加载postgresql.conf参数配置文件里的参数设置
?????? 从main->PostmasterMain->SelectConfigFiles->ProcessConfigFile开始处理参数配置文件postgresql.conf，读取postgresql.conf配置文件的调用过程是ProcessConfigFile -> ParseConfigFile -> AllocateFile->fopen，最后用fopen打开文件，其中ProcessConfigFile、ParseConfigFile在文件src\backend\utils\misc\guc-file.l中，AllocateFile在文件src\backend\storage\file\fd.c中。
??? pg使用 flex 去处理 conf 文件。在ParseConfigFile中把配置文件中的配置项组织成一个链表，调用set_config_option检查这些值是否有效，若可以设置就调用set_config_option设置这些值。
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这里以"max_connections"做例子，从配置文件读取"max_connections"，然后从guc_variables数组中找元素" max_connections "，比较参数结构的GucContext （枚举类参数能被设置的时机。定义见下面）枚举类型成员context和当前时间，看是否可以此刻修改。接着比较参数结构的GucSource（枚举了当前GUC参数设置的来源。除非参数新值的来源等级不小于原参数的来源等级时，新设置才能生效。例如，修改配置文件不能覆盖postmaster command line的设置。定义见下面）枚举类型成员source和新参数值的来源，看是否可以修改。如果可以，把config_generic结构类型的元素" max_connections "类型转换为config_int类型，修改variable成员为新值，修改该参数的来源source为当前来源PGC_S_FILE，如果元素" max_connections "的reset_source <= source，修改reset_val成员为新值，修改该参数的reset_source为当前来源PGC_S_FILE。
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typedef enum
{
??? PGC_INTERNAL,
??? PGC_POSTMASTER,
??? PGC_SIGHUP,
??? PGC_BACKEND,
??? PGC_SUSET,
??? PGC_USERSET
} GucContext;
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typedef enum
{
??? PGC_S_DEFAULT,????????????? /*wired-in default */
??? PGC_S_ENV_VAR,????????????? /*postmaster environment variable */
??? PGC_S_FILE,???????????????? /*postgresql.conf */
??? PGC_S_ARGV,???????????????? /*postmaster command line */
??? PGC_S_DATABASE,???????????? /*per-database setting */
??? PGC_S_USER,???????????????? /*per-user setting */
??? PGC_S_CLIENT,?????????????? /* fromclient connection request */
??? PGC_S_OVERRIDE,???????????? /*special case to forcibly set default */
??? PGC_S_INTERACTIVE,????????? /* dividingline for error reporting */
??? PGC_S_TEST,???????????????? /*test per-database or per-user setting */
??? PGC_S_SESSION?????????????? /* SETcommand */
} GucSource
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