Mainloop.c （1） 函数MainLoop 和PQExpBufferData，_psqlSettings

   上周日重新梳理了下pgxc1.1 Mainloop.c源代码内容。阅读Postgres源码的确能很大程度上提高对PG的认识，在阅读代码的同时，脑中始终有一条线在动，不断探寻如何才能走到这块代码处。

pgxc 1.1

文件Mainloop.c

主循环，把从命令行读出的内容送到后台执行

代码主要包含两部分，正常的SQL语句处理，以及backslash元命令处理。

/* * psql - the PostgreSQL interactive terminal * * Copyright (c) 2000-2011, PostgreSQL Global Development Group * * src/bin/psql/mainloop.c */#include "postgres_fe.h"#include "mainloop.h"#include "command.h"#include "common.h"#include "input.h"#include "settings.h"#include "mb/pg_wchar.h"/* * Main processing loop for reading lines of input *and sending them to the backend. * * This loop is re-entrant. May be called by \i command *which reads input from a file. */intMainLoop(FILE *source){PsqlScanState scan_state;/* lexer working state */volatile PQExpBuffer query_buf;/* buffer for query being accumulated */volatile PQExpBuffer previous_buf;/* if there isn't anything in the new * buffer yet, use this one for \e, * etc. */PQExpBuffer history_buf;/* earlier lines of a multi-line command, not * yet saved to readline history */char   *line;/* current line of input */intadded_nl_pos;boolsuccess;boolline_saved_in_history;volatile int successResult = EXIT_SUCCESS;volatile backslashResult slashCmdStatus = PSQL_CMD_UNKNOWN;volatile promptStatus_t prompt_status = PROMPT_READY;volatile int count_eof = 0;volatile bool die_on_error = false;/* Save the prior command source */FILE   *prev_cmd_source;boolprev_cmd_interactive;uint64prev_lineno;/* Save old settings */prev_cmd_source = pset.cur_cmd_source;prev_cmd_interactive = pset.cur_cmd_interactive;prev_lineno = pset.lineno;/* Establish new source */pset.cur_cmd_source = source;pset.cur_cmd_interactive = ((source == stdin) && !pset.notty);pset.lineno = 0;/* Create working state */scan_state = psql_scan_create();query_buf = createPQExpBuffer();previous_buf = createPQExpBuffer();history_buf = createPQExpBuffer();if (PQExpBufferBroken(query_buf) ||PQExpBufferBroken(previous_buf) ||PQExpBufferBroken(history_buf)){psql_error("out of memory\n");exit(EXIT_FAILURE);}/* main loop to get queries and execute them */while (successResult == EXIT_SUCCESS){/* * Clean up after a previous Control-C */if (cancel_pressed){if (!pset.cur_cmd_interactive){/* * You get here if you stopped a script with Ctrl-C. */successResult = EXIT_USER;break;}cancel_pressed = false;}/* * Establish longjmp destination for exiting from wait-for-input. We * must re-do this each time through the loop for safety, since the * jmpbuf might get changed during command execution. */if (sigsetjmp(sigint_interrupt_jmp, 1) != 0){/* got here with longjmp *//* reset parsing state */psql_scan_finish(scan_state);psql_scan_reset(scan_state);resetPQExpBuffer(query_buf);resetPQExpBuffer(history_buf);count_eof = 0;slashCmdStatus = PSQL_CMD_UNKNOWN;prompt_status = PROMPT_READY;cancel_pressed = false;if (pset.cur_cmd_interactive) // ctrl+c 换行putc('\n', stdout);else{successResult = EXIT_USER;break;}}fflush(stdout);/* * get another line */if (pset.cur_cmd_interactive){/* May need to reset prompt, eg after \r command */if (query_buf->len == 0)prompt_status = PROMPT_READY;line = gets_interactive(get_prompt(prompt_status));}else{line = gets_fromFile(source);if (!line && ferror(source))successResult = EXIT_FAILURE;}/* * query_buf holds query already accumulated.  line is the malloc'd * new line of input (note it must be freed before looping around!) *//* No more input.  Time to quit, or \i done */if (line == NULL){if (pset.cur_cmd_interactive){/* This tries to mimic bash's IGNOREEOF feature. */count_eof++;if (count_eof < GetVariableNum(pset.vars, "IGNOREEOF", 0, 10, false)){if (!pset.quiet)printf(_("Use \"\\q\" to leave %s.\n"), pset.progname);continue;}puts(pset.quiet ? "" : "\\q");}break;}count_eof = 0;pset.lineno++;/* ignore UTF-8 Unicode byte-order mark */if (pset.lineno == 1 && pset.encoding == PG_UTF8 && strncmp(line, "\xef\xbb\xbf", 3) == 0)memmove(line, line + 3, strlen(line + 3) + 1);/* nothing left on line? then ignore */if (line[0] == '\0' && !psql_scan_in_quote(scan_state)){free(line);continue;}/* A request for help? Be friendly and give them some guidance */if (pset.cur_cmd_interactive && query_buf->len == 0 &&pg_strncasecmp(line, "help", 4) == 0 &&(line[4] == '\0' || line[4] == ';' || isspace((unsigned char) line[4]))){free(line);puts(_("You are using psql, the command-line interface to PostgreSQL."));printf(_("Type:  \\copyright for distribution terms\n" "       \\h for help with SQL commands\n" "       \\? for help with psql commands\n"  "       \\g or terminate with semicolon to execute query\n" "       \\q to quit\n"));fflush(stdout);continue;}/* echo back if flag is set */if (pset.echo == PSQL_ECHO_ALL && !pset.cur_cmd_interactive)puts(line);fflush(stdout);/* insert newlines into query buffer between source lines */if (query_buf->len > 0){appendPQExpBufferChar(query_buf, '\n');added_nl_pos = query_buf->len;}elseadded_nl_pos = -1;/* flag we didn't add one *//* Setting this will not have effect until next line. */die_on_error = pset.on_error_stop;/* * Parse line, looking for command separators. */psql_scan_setup(scan_state, line, strlen(line));success = true;line_saved_in_history = false;while (success || !die_on_error){PsqlScanResult scan_result;promptStatus_t prompt_tmp = prompt_status;scan_result = psql_scan(scan_state, query_buf, &prompt_tmp);prompt_status = prompt_tmp;if (PQExpBufferBroken(query_buf)){psql_error("out of memory\n");exit(EXIT_FAILURE);}/* * Send command if semicolon found, or if end of line and we're in * single-line mode. */if (scan_result == PSCAN_SEMICOLON ||(scan_result == PSCAN_EOL && pset.singleline)){/* * Save query in history.  We use history_buf to accumulate * multi-line queries into a single history entry. */if (pset.cur_cmd_interactive && !line_saved_in_history){pg_append_history(line, history_buf);pg_send_history(history_buf);line_saved_in_history = true;}/* execute query */success = SendQuery(query_buf->data);slashCmdStatus = success ? PSQL_CMD_SEND : PSQL_CMD_ERROR;/* transfer query to previous_buf by pointer-swapping */{PQExpBuffer swap_buf = previous_buf;previous_buf = query_buf;query_buf = swap_buf;}resetPQExpBuffer(query_buf);added_nl_pos = -1;/* we need not do psql_scan_reset() here */}else if (scan_result == PSCAN_BACKSLASH)\\处理反斜杠操作符{/* handle backslash command *//* * If we added a newline to query_buf, and nothing else has * been inserted in query_buf by the lexer, then strip off the * newline again.  This avoids any change to query_buf when a * line contains only a backslash command.Also, in this * situation we force out any previous lines as a separate * history entry; we don't want SQL and backslash commands * intermixed in history if at all possible. */if (query_buf->len == added_nl_pos){query_buf->data[--query_buf->len] = '\0';pg_send_history(history_buf);}added_nl_pos = -1;/* save backslash command in history */if (pset.cur_cmd_interactive && !line_saved_in_history){pg_append_history(line, history_buf);pg_send_history(history_buf);line_saved_in_history = true;}/* execute backslash command */slashCmdStatus = HandleSlashCmds(scan_state, query_buf->len > 0 ? query_buf : previous_buf);success = slashCmdStatus != PSQL_CMD_ERROR;if ((slashCmdStatus == PSQL_CMD_SEND || slashCmdStatus == PSQL_CMD_NEWEDIT) &&query_buf->len == 0){/* copy previous buffer to current for handling */appendPQExpBufferStr(query_buf, previous_buf->data);}if (slashCmdStatus == PSQL_CMD_SEND){success = SendQuery(query_buf->data);/* transfer query to previous_buf by pointer-swapping */{PQExpBuffer swap_buf = previous_buf;previous_buf = query_buf;query_buf = swap_buf;}resetPQExpBuffer(query_buf);/* flush any paren nesting info after forced send */psql_scan_reset(scan_state);}else if (slashCmdStatus == PSQL_CMD_NEWEDIT){/* rescan query_buf as new input */psql_scan_finish(scan_state);free(line);line = pg_strdup(query_buf->data);resetPQExpBuffer(query_buf);/* reset parsing state since we are rescanning whole line */psql_scan_reset(scan_state);psql_scan_setup(scan_state, line, strlen(line));line_saved_in_history = false;prompt_status = PROMPT_READY;}else if (slashCmdStatus == PSQL_CMD_TERMINATE)break;}/* fall out of loop if lexer reached EOL */if (scan_result == PSCAN_INCOMPLETE ||scan_result == PSCAN_EOL)break;}/* Add line to pending history if we didn't execute anything yet */if (pset.cur_cmd_interactive && !line_saved_in_history)pg_append_history(line, history_buf);psql_scan_finish(scan_state);free(line);if (slashCmdStatus == PSQL_CMD_TERMINATE){successResult = EXIT_SUCCESS;break;}if (!pset.cur_cmd_interactive){if (!success && die_on_error)successResult = EXIT_USER;/* Have we lost the db connection? */else if (!pset.db)successResult = EXIT_BADCONN;}}/* while !endoffile/session *//* * Process query at the end of file without a semicolon */if (query_buf->len > 0 && !pset.cur_cmd_interactive &&successResult == EXIT_SUCCESS){/* save query in history */if (pset.cur_cmd_interactive)pg_send_history(history_buf);  //还记得\p,\g吧/* execute query */success = SendQuery(query_buf->data);if (!success && die_on_error)successResult = EXIT_USER;else if (pset.db == NULL)successResult = EXIT_BADCONN;}/* * Let's just make real sure the SIGINT handler won't try to use * sigint_interrupt_jmp after we exit this routine.  If there is an outer * MainLoop instance, it will reset sigint_interrupt_jmp to point to * itself at the top of its loop, before any further interactive input * happens. */sigint_interrupt_enabled = false;destroyPQExpBuffer(query_buf);destroyPQExpBuffer(previous_buf);destroyPQExpBuffer(history_buf);psql_scan_destroy(scan_state);pset.cur_cmd_source = prev_cmd_source;pset.cur_cmd_interactive = prev_cmd_interactive;pset.lineno = prev_lineno;return successResult;}/* MainLoop() *//* * psqlscan.c is #include'd here instead of being compiled on its own. * This is because we need postgres_fe.h to be read before any system * include files, else things tend to break on platforms that have * multiple infrastructures for stdio.h and so on.flex is absolutely * uncooperative about that, so we can't compile psqlscan.c on its own. */#include "psqlscan.c"

主要结构体说明：

PQExpBuffer  可以看做一个扩充的buffer，通过函数createPQExpBuffer 来分配空间，创建指定长度256字节。

typedef struct PQExpBufferData
{
char   *data;
size_t len;
size_t maxlen;
} PQExpBufferData;
typedef PQExpBufferData *PQExpBuffer;

/*
 * createPQExpBuffer
 *
 * Create an empty 'PQExpBufferData' & return a pointer to it.
 */
PQExpBuffer
createPQExpBuffer(void)
{
PQExpBuffer res;


res = (PQExpBuffer) malloc(sizeof(PQExpBufferData));
if (res != NULL)
initPQExpBuffer(res);


return res;
}

进入MainLoop函数，首先声明下面3个结构体变量：

volatile PQExpBuffer query_buf;/* buffer for query being accumulated */
volatile PQExpBuffer previous_buf;/* if there isn't anything in the new
* buffer yet, use this one for \e,
* etc. */
PQExpBuffer history_buf;/* earlier lines of a multi-line command

其中history_buf 用来保存历史操作，previous_buf 用来保存当前操作，当命令发送时，将previous_buf 赋值给query_buf。

get_prompt(prompt_status) 用来输出提示符，(gdb) p get_prompt(prompt_status) 

$32 = 0x80a2860 "postgres=# "

gets_interactive   函数用来与服务器后台进行交互，line = gets_interactive(get_prompt(prompt_status));(gdb) p line
$34 = 0x80bf168 "select 1;"

每次SendQuery(query_buf->data) 后都会 previous_buf = query_buf;resetPQExpBuffer(query_buf);

如果交互输入的不是SQL语句，比如blackslash, 则query_buf 内容为空。

(gdb) p *scan_state
$28 = {buffer_stack = 0x0, scanbufhandle = 0x0, scanbuf = 0x0, 
  scanline = 0x80bf168 "select 1;", encoding = 6, safe_encoding = 1 '\001', 
  curline = 0x80bf178 "", refline = 0x80bf168 "select 1;", start_state = 0, 
  paren_depth = 0, xcdepth = 0, dolqstart = 0x0}

MainLoop通过函数 psql_scan 来PsqlSettings pset 各个属性来控制交互流程，PsqlSetting结构体如下：

typedef struct _psqlSettings{PGconn   *db;/* connection to backend */intencoding;/* client_encoding */FILE   *queryFout;/* where to send the query results */boolqueryFoutPipe;/* queryFout is from a popen() */printQueryOpt popt;char   *gfname;/* one-shot file output argument for \g */boolnotty;/* stdin or stdout is not a tty (as determined * on startup) */enum trivalue getPassword;/* prompt the user for a username and password */FILE   *cur_cmd_source; /* describe the status of the current main * loop */boolcur_cmd_interactive;intsversion;/* backend server version */const char *progname;/* in case you renamed psql */char   *inputfile;/* for error reporting */char   *dirname;/* current directory for \s display */uint64lineno;/* also for error reporting */booltiming;/* enable timing of all queries */FILE   *logfile;/* session log file handle */VariableSpace vars;/* "shell variable" repository *//* * The remaining fields are set by assign hooks associated with entries in * "vars".They should not be set directly except by those hook * functions. */boolautocommit;boolon_error_stop;boolquiet;boolsingleline;boolsinglestep;intfetch_count;PSQL_ECHOecho;PSQL_ECHO_HIDDEN echo_hidden;PSQL_ERROR_ROLLBACK on_error_rollback;HistControl histcontrol;const char *prompt1;const char *prompt2;const char *prompt3;PGVerbosity verbosity;/* current error verbosity level */} PsqlSettings;extern PsqlSettings pset;

scan_result = psql_scan(scan_state, query_buf, &prompt_tmp)  返回状态包括如下：

/* Termination states for psql_scan() */
typedef enum
{
  PSCAN_SEMICOLON, // 以分号结尾
  PSCAN_BACKSLASH, //反斜杠命令
  PSCAN_INCOMPLETE,//SQL命令遇到行尾但是未完成命令
   PSCAN_EOL //EOF
} PsqlScanResult;