PostgreSQL的学习心得和知识总结（一百三十八）|深入理解PostgreSQL数据库之Protocol message构造和解析逻辑

目录结构

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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》
2、参考书籍：《数据库事务处理的艺术：事务管理与并发控制》
3、PostgreSQL数据库仓库链接，点击前往
4、日本著名PostgreSQL数据库专家 铃木启修 网站主页，点击前往
5、参考书籍：《PostgreSQL中文手册》
6、参考书籍：《PostgreSQL指南：内幕探索》，点击前往
7、深度解析 PostgreSQL Protocol v3.0（一），点击前往
8、技术贴 | 深度解析 PostgreSQL Protocol v3.0（二）— 扩展查询，点击前往

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1、本文内容全部来源于开源社区 GitHub和以上博主的贡献，本文也免费开源（可能会存在问题，评论区等待大佬们的指正）
2、本文目的：开源共享 抛砖引玉 一起学习
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5、本文内容基于PostgreSQL master源码开发而成

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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。

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学习内容：（详见目录）

1、深入理解PostgreSQL数据库之Protocol message构造和解析逻辑

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学习时间：

2024年04月17日 22:14:16 星期三

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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

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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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功能使用背景说明

在之前的博客 PostgreSQL的学习心得和知识总结（一百三十七）|深入理解PostgreSQL数据库之Add support for Close on portals and statements，点击前往 的结尾的时候，简单介绍了一下close的message构造：

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这里解释一下close消息的构造，如下：

	/* construct the Close message */if (pqPutMsgStart(command, conn) < 0 ||pqPutc(type, conn) < 0 || // 1字节 SpqPuts(target, conn) < 0 || // strlen("p2") + 1 = 3pqPutMsgEnd(conn) < 0) // (conn->outMsgEnd - conn->outMsgStart)长度 4字节goto sendFailed;

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功能实现源码解析

辅助调试信息

上面是通过抓包工具得到的，接下来我们看一下PostgreSQL中提供的debug方式，如下：

#include <iostream>#include "libpq-fe.h"using namespace std;int main()
{const char *conninfo = "host=localhost port=5432 user=postgres dbname=postgres password=1";const char *dropTableCmd = "drop table if exists t1;";const char *createTableCmd = "create table t1(id int, name text);";const char *insertCmd = "insert into t1 values(1, 'Oracle'), (2, 'MySQL'), (3, 'SQL Server');";const char *prepareCmd0 = "table t1;";const char *prepareCmd1 = "select * from t1 where id < $1;";const char *prepareCmd2 = "select * from t1 where id = $1 and name = $2;";PGresult *result = NULL;ExecStatusType result_status;int n_rows = 0, ntups = 0;const char *values2[2] = {"2", "MySQL"};FILE *file = fopen("/home/postgres/test/bin/1.txt", "w+");PGconn *conn = PQconnectdb(conninfo);PQtrace(conn, file);if (PQstatus(conn) == CONNECTION_OK){cout << "连接PostgreSQL数据库 成功！" << endl;if (PQexec(conn, dropTableCmd) != NULL){cout << "删除表成功" << endl;}if (PQexec(conn, createTableCmd) != NULL){cout << "创建表成功" << endl;}if (PQexec(conn, insertCmd) != NULL){cout << "插入表成功" << endl<< endl;}/* ---------------------------------------------------------------- */// prepareif (PQsendPrepare(conn, "p23456789", prepareCmd2, 2, NULL) != NULL) // 异步{cout << "prepare p23456789 send成功" << endl;}while (NULL != (result = PQgetResult(conn))){result_status = PQresultStatus(result);if ((PGRES_EMPTY_QUERY != result_status) && (PGRES_COMMAND_OK != result_status) && (PGRES_TUPLES_OK != result_status) && (PGRES_NONFATAL_ERROR != result_status)){cout << "prepare p23456789 失败" << endl;}else{n_rows = atoi(PQcmdTuples(result));ntups = PQntuples(result);cout << "prepare p23456789 成功"<< " n_rows: " << n_rows << " ntups: " << ntups << endl<< endl;}PQclear(result);}/* ---------------------------------------------------------------- */// exec_preparedif (PQsendQueryPrepared(conn, "p23456789", 2, values2, NULL, NULL, 0) != NULL) // 异步{cout << "exec prepare p23456789 send成功" << endl;}while (NULL != (result = PQgetResult(conn))){result_status = PQresultStatus(result);if ((PGRES_EMPTY_QUERY != result_status) && (PGRES_COMMAND_OK != result_status) && (PGRES_TUPLES_OK != result_status) && (PGRES_NONFATAL_ERROR != result_status)){cout << "exec prepare p23456789 失败" << endl;}else{n_rows = atoi(PQcmdTuples(result));ntups = PQntuples(result);cout << "exec prepare p23456789 send成功"<< " n_rows: " << n_rows << " ntups: " << ntups << endl<< endl;}PQclear(result);}/* ---------------------------------------------------------------- */// deallocateresult = PQexec(conn, "select pg_sleep(60);"); // 同步PQclear(result);if (PQsendClosePrepared(conn, "p23456789") != NULL) // 异步{cout << "close prepare p23456789 send成功" << endl;}while (NULL != (result = PQgetResult(conn))){result_status = PQresultStatus(result);if ((PGRES_EMPTY_QUERY != result_status) && (PGRES_COMMAND_OK != result_status) && (PGRES_TUPLES_OK != result_status) && (PGRES_NONFATAL_ERROR != result_status)){cout << "close prepare p23456789 失败" << endl;}else{n_rows = atoi(PQcmdTuples(result));ntups = PQntuples(result);cout << "close prepare p23456789 成功"<< " n_rows: " << n_rows << " ntups: " << ntups << endl<< endl;}PQclear(result);}/* ---------------------------------------------------------------- */PQfinish(conn);PQuntrace(conn);cout << "与PostgreSQL数据库连接 关闭！" << endl;}else{cout << "连接失败！" << endl;}fclose(file);return 0;
}/** export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$PG_HOME/lib** g++ libpqtest2.cpp -lpq -L/home/postgres/test/lib -I/home/postgres/test/include -o main -w -g -O0*/

执行结果以及打印信息，如下：

[postgres@localhost:~/test/bin]$ ./main 
连接PostgreSQL数据库 成功！
删除表成功
创建表成功
插入表成功prepare p23456789 send成功
prepare p23456789 成功 n_rows: 0 ntups: 0exec prepare p23456789 send成功
exec prepare p23456789 send成功 n_rows: 1 ntups: 1close prepare p23456789 send成功
close prepare p23456789 成功 n_rows: 0 ntups: 0与PostgreSQL数据库连接 关闭！
[postgres@localhost:~/test/bin]$ cat 1.txt 
2024-04-16 20:22:43.371152      F       29      Query    "drop table if exists t1;"
2024-04-16 20:22:43.381514      B       15      CommandComplete  "DROP TABLE"
2024-04-16 20:22:43.381531      B       5       ReadyForQuery    I
2024-04-16 20:22:43.381561      F       40      Query    "create table t1(id int, name text);"
2024-04-16 20:22:43.388197      B       17      CommandComplete  "CREATE TABLE"
2024-04-16 20:22:43.388215      B       5       ReadyForQuery    I
2024-04-16 20:22:43.388239      F       73      Query    "insert into t1 values(1, 'Oracle'), (2, 'MySQL'), (3, 'SQL Server');"
2024-04-16 20:22:43.388729      B       15      CommandComplete  "INSERT 0 3"
2024-04-16 20:22:43.388736      B       5       ReadyForQuery    I
2024-04-16 20:22:43.388758      F       62      Parse    "p23456789" "select * from t1 where id = $1 and name = $2;" 0
2024-04-16 20:22:43.388762      F       4       Sync
2024-04-16 20:22:43.390728      B       4       ParseComplete
2024-04-16 20:22:43.390767      B       5       ReadyForQuery    I
2024-04-16 20:22:43.390772      F       37      Bind     "" "p23456789" 0 2 1 '2' 5 'MySQL' 1 0
2024-04-16 20:22:43.390775      F       6       Describe         P ""
2024-04-16 20:22:43.390777      F       9       Execute  "" 0
2024-04-16 20:22:43.390779      F       4       Sync
2024-04-16 20:22:43.392433      B       4       BindComplete
2024-04-16 20:22:43.392483      B       50      RowDescription   2 "id" 16403 1 23 4 -1 0 "name" 16403 2 25 65535 -1 0
2024-04-16 20:22:43.392494      B       20      DataRow  2 1 '2' 5 'MySQL'
2024-04-16 20:22:43.392500      B       13      CommandComplete  "SELECT 1"
2024-04-16 20:22:43.392644      B       5       ReadyForQuery    I
2024-04-16 20:22:43.392725      F       25      Query    "select pg_sleep(60);"
2024-04-16 20:23:43.406609      B       33      RowDescription   1 "pg_sleep" 0 0 2278 4 -1 0
2024-04-16 20:23:43.406637      B       10      DataRow  1 0 ''
2024-04-16 20:23:43.406642      B       13      CommandComplete  "SELECT 1"
2024-04-16 20:23:43.406645      B       5       ReadyForQuery    I
2024-04-16 20:23:43.406654      F       15      Close    S "p23456789"
2024-04-16 20:23:43.406657      F       4       Sync
2024-04-16 20:23:43.406885      B       4       CloseComplete
2024-04-16 20:23:43.406928      B       5       ReadyForQuery    I
2024-04-16 20:23:43.406938      F       4       Terminate
[postgres@localhost:~/test/bin]$

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如上是通过直接调用libpq中的函数PQtrace和PQuntrace，如下：

// src/interfaces/libpq/fe-trace.c/* Enable tracing */
void
PQtrace(PGconn *conn, FILE *debug_port)
{if (conn == NULL)return;PQuntrace(conn);if (debug_port == NULL)return;conn->Pfdebug = debug_port;conn->traceFlags = 0;
}/* Disable tracing */
void
PQuntrace(PGconn *conn)
{if (conn == NULL)return;if (conn->Pfdebug){fflush(conn->Pfdebug);conn->Pfdebug = NULL;}conn->traceFlags = 0;
}

当然如上两函数是trace的开关函数，真正实现打印的逻辑 如下：

// src/interfaces/libpq/fe-trace.c/** Print the given message to the trace output stream.* 将给定消息打印到跟踪输出流*/
void
pqTraceOutputMessage(PGconn *conn, const char *message, bool toServer)
{char		id;int			length;char	   *prefix = toServer ? "F" : "B"; // 这里指的是 Front / Backendint			logCursor = 0;bool		regress;if ((conn->traceFlags & PQTRACE_SUPPRESS_TIMESTAMPS) == 0){char		timestr[128];pqTraceFormatTimestamp(timestr, sizeof(timestr));fprintf(conn->Pfdebug, "%s\t", timestr);}regress = (conn->traceFlags & PQTRACE_REGRESS_MODE) != 0;id = message[logCursor++];memcpy(&length, message + logCursor, 4);length = (int) pg_ntoh32(length);logCursor += 4;/** In regress mode, suppress the length of ErrorResponse and* NoticeResponse.  The F (file name), L (line number) and R (routine* name) fields can change as server code is modified, and if their* lengths differ from the originals, that would break tests.*  * 在回归模式下，抑制ErrorResponse和NoticeResponse的长度* F（文件名）、L（行号）和 R（例程名称）字段可能会随着服务器代码的修改而更改* 如果它们的长度与原始长度不同，则会破坏测试*/if (regress && !toServer && (id == 'E' || id == 'N'))fprintf(conn->Pfdebug, "%s\tNN\t", prefix);elsefprintf(conn->Pfdebug, "%s\t%d\t", prefix, length);switch (id){case PqMsg_ParseComplete:fprintf(conn->Pfdebug, "ParseComplete");/* No message content */break;case PqMsg_BindComplete:fprintf(conn->Pfdebug, "BindComplete");/* No message content */break;case PqMsg_CloseComplete:fprintf(conn->Pfdebug, "CloseComplete");/* No message content */break;case PqMsg_NotificationResponse:pqTraceOutputA(conn->Pfdebug, message, &logCursor, regress);break;case PqMsg_Bind:pqTraceOutputB(conn->Pfdebug, message, &logCursor);break;case PqMsg_CopyDone:fprintf(conn->Pfdebug, "CopyDone");/* No message content */break;case PqMsg_CommandComplete:/* Close(F) and CommandComplete(B) use the same identifier. */Assert(PqMsg_Close == PqMsg_CommandComplete);pqTraceOutputC(conn->Pfdebug, toServer, message, &logCursor);break;case PqMsg_CopyData:/* Drop COPY data to reduce the overhead of logging. */break;case PqMsg_Describe:/* Describe(F) and DataRow(B) use the same identifier. */Assert(PqMsg_Describe == PqMsg_DataRow);pqTraceOutputD(conn->Pfdebug, toServer, message, &logCursor);break;case PqMsg_Execute:/* Execute(F) and ErrorResponse(B) use the same identifier. */Assert(PqMsg_Execute == PqMsg_ErrorResponse);pqTraceOutputE(conn->Pfdebug, toServer, message, &logCursor,regress);break;case PqMsg_CopyFail:pqTraceOutputf(conn->Pfdebug, message, &logCursor);break;case PqMsg_FunctionCall:pqTraceOutputF(conn->Pfdebug, message, &logCursor, regress);break;case PqMsg_CopyInResponse:pqTraceOutputG(conn->Pfdebug, message, &logCursor);break;case PqMsg_Flush:/* Flush(F) and CopyOutResponse(B) use the same identifier */Assert(PqMsg_CopyOutResponse == PqMsg_Flush);if (!toServer)pqTraceOutputH(conn->Pfdebug, message, &logCursor);elsefprintf(conn->Pfdebug, "Flush");	/* no message content */break;case PqMsg_EmptyQueryResponse:fprintf(conn->Pfdebug, "EmptyQueryResponse");/* No message content */break;case PqMsg_BackendKeyData:pqTraceOutputK(conn->Pfdebug, message, &logCursor, regress);break;case PqMsg_NoData:fprintf(conn->Pfdebug, "NoData");/* No message content */break;case PqMsg_NoticeResponse:pqTraceOutputNR(conn->Pfdebug, "NoticeResponse", message,&logCursor, regress);break;case PqMsg_Parse:pqTraceOutputP(conn->Pfdebug, message, &logCursor, regress);break;case PqMsg_Query:pqTraceOutputQ(conn->Pfdebug, message, &logCursor);break;case PqMsg_AuthenticationRequest:pqTraceOutputR(conn->Pfdebug, message, &logCursor);break;case PqMsg_PortalSuspended:fprintf(conn->Pfdebug, "PortalSuspended");/* No message content */break;case PqMsg_Sync:/* Parameter Status(B) and Sync(F) use the same identifier */Assert(PqMsg_ParameterStatus == PqMsg_Sync);if (!toServer)pqTraceOutputS(conn->Pfdebug, message, &logCursor);elsefprintf(conn->Pfdebug, "Sync"); /* no message content */break;case PqMsg_ParameterDescription:pqTraceOutputt(conn->Pfdebug, message, &logCursor, regress);break;case PqMsg_RowDescription:pqTraceOutputT(conn->Pfdebug, message, &logCursor, regress);break;case PqMsg_NegotiateProtocolVersion:pqTraceOutputv(conn->Pfdebug, message, &logCursor);break;case PqMsg_FunctionCallResponse:pqTraceOutputV(conn->Pfdebug, message, &logCursor);break;case PqMsg_CopyBothResponse:pqTraceOutputW(conn->Pfdebug, message, &logCursor, length);break;case PqMsg_Terminate:fprintf(conn->Pfdebug, "Terminate");/* No message content */break;case PqMsg_ReadyForQuery:pqTraceOutputZ(conn->Pfdebug, message, &logCursor);break;default:fprintf(conn->Pfdebug, "Unknown message: %02x", id);break;}fputc('\n', conn->Pfdebug);/** Verify the printing routine did it right.  Note that the one-byte* message identifier is not included in the length, but our cursor does* include it.*  * 验证打印例程是否正确* 请注意，一字节消息标识符不包括在长度中，但我们的光标确实包括它*/if (logCursor - 1 != length)fprintf(conn->Pfdebug,"mismatched message length: consumed %d, expected %d\n",logCursor - 1, length);
}

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接下来我们重点看一下close消息的打印，如下：

pqTraceOutputMessage|pqTraceOutputC/* Close(F) or CommandComplete(B) */
static void
pqTraceOutputC(FILE *f, bool toServer, const char *message, int *cursor)
{if (toServer){fprintf(f, "Close\t");pqTraceOutputByte1(f, message, cursor);pqTraceOutputString(f, message, cursor, false);}else{fprintf(f, "CommandComplete\t");pqTraceOutputString(f, message, cursor, false);}
}	

/**   pqTraceOutputByte1: output a 1-char message to the log*	 pqTraceOutputByte1：将 1 个字符的消息输出到日志*/
static void
pqTraceOutputByte1(FILE *pfdebug, const char *data, int *cursor)
{const char *v = data + *cursor;/** Show non-printable data in hex format, including the terminating \0* that completes ErrorResponse and NoticeResponse messages.*  * 以十六进制格式显示不可打印的数据* 包括完成ErrorResponse 和NoticeResponse 消息的终止\0*/if (!isprint((unsigned char) *v))fprintf(pfdebug, " \\x%02x", *v);elsefprintf(pfdebug, " %c", *v);*cursor += 1;
}

/**   pqTraceOutputString: output a string message to the log*   pqTraceOutputString：输出字符串消息到日志*/
static void
pqTraceOutputString(FILE *pfdebug, const char *data, int *cursor, bool suppress)
{int			len;if (suppress){fprintf(pfdebug, " \"SSSS\"");*cursor += strlen(data + *cursor) + 1;}else{len = fprintf(pfdebug, " \"%s\"", data + *cursor);/** This is a null-terminated string. So add 1 after subtracting 3* which is the double quotes and space length from len.* * 这是一个以 null 结尾的字符串* 因此，从 len 中减去双引号和空格长度 3 后加 1*/*cursor += (len - 3 + 1);}
}

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接下来我们继续以close消息为例，详细看一下message的构造和解析过程！

## len = 152024-04-16 20:23:43.406654	F	15	Close	 S "p23456789"

前端信息构造

此时的函数堆栈，如下：

libpq.so.5!PQsendTypedCommand(PGconn * conn, char command, char type, const char * target) (\home\postgres\postgres\src\interfaces\libpq\fe-exec.c:2605)
libpq.so.5!PQsendClosePrepared(PGconn * conn, const char * stmt) (\home\postgres\postgres\src\interfaces\libpq\fe-exec.c:2558)
main() (\home\postgres\test\bin\libpqtest2.cpp:110)

接下来就详细看一下这里的处理逻辑，如下：

第一步：

// src/interfaces/libpq/fe-exec.c/** pqPutMsgStart: begin construction of a message to the server* pqPutMsgStart：开始向服务器构造消息** msg_type is the message type byte, or 0 for a message without type byte* (only startup messages have no type byte)* msg_type 是消息类型字节，如果没有类型字节，则为 0（只有启动消息没有类型字节）** Returns 0 on success, EOF on error* 成功时返回 0，错误时返回 EOF** The idea here is that we construct the message in conn->outBuffer,* beginning just past any data already in outBuffer (ie, at* outBuffer+outCount).  We enlarge the buffer as needed to hold the message.* When the message is complete, we fill in the length word (if needed) and* then advance outCount past the message, making it eligible to send.* 这里的想法是，我们在 conn->outBuffer 中构造消息，从 outBuffer 中已有的任何数据开始（即，在 outBuffer+outCount 处）* 我们根据需要扩大缓冲区来保存消息* 消息完成后，我们填写长度字（如果需要），然后将 outCount 提前到消息后面，使其符合发送条件** The state variable conn->outMsgStart points to the incomplete message's* length word: it is either outCount or outCount+1 depending on whether* there is a type byte.  The state variable conn->outMsgEnd is the end of* the data collected so far.* 状态变量 conn->outMsgStart 指向不完整消息的长度字：* 它是 outCount 或 outCount+1，具体取决于是否存在类型字节* 状态变量 conn->outMsgEnd 是迄今为止收集的数据的结尾*/
int
pqPutMsgStart(char msg_type, PGconn *conn)
{int			lenPos;int			endPos;/* allow room for message type byte */if (msg_type)endPos = conn->outCount + 1;elseendPos = conn->outCount;/* do we want a length word? */lenPos = endPos;/* allow room for message length */endPos += 4;/* make sure there is room for message header */if (pqCheckOutBufferSpace(endPos, conn))return EOF;/* okay, save the message type byte if any */if (msg_type)conn->outBuffer[conn->outCount] = msg_type;/* set up the message pointers */conn->outMsgStart = lenPos;conn->outMsgEnd = endPos;/* length word, if needed, will be filled in by pqPutMsgEnd */return 0;
}

解释一下：

• msg_type此时是 C
• endPos += 4; 就代表了 已经把消息长度的空间分配好了
• conn->outCount = 0
• conn->outBuffer[conn->outCount] = msg_type; // 放的就是 C
• conn->outMsgStart = 1
• conn->outMsgEnd = 5

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第二步：

libpq.so.5!pqPutMsgBytes(const void * buf, size_t len, PGconn * conn) (\home\postgres\postgres\src\interfaces\libpq\fe-misc.c:497)
libpq.so.5!pqPutc(char c, PGconn * conn) (\home\postgres\postgres\src\interfaces\libpq\fe-misc.c:94)
libpq.so.5!PQsendTypedCommand(PGconn * conn, char command, char type, const char * target) (\home\postgres\postgres\src\interfaces\libpq\fe-exec.c:2606)
libpq.so.5!PQsendClosePrepared(PGconn * conn, const char * stmt) (\home\postgres\postgres\src\interfaces\libpq\fe-exec.c:2558)
main() (\home\postgres\test\bin\libpqtest2.cpp:110)

/** pqPutc: write 1 char to the current message*/
int
pqPutc(char c, PGconn *conn)
{if (pqPutMsgBytes(&c, 1, conn))return EOF;return 0;
}.../** pqPutMsgBytes: add bytes to a partially-constructed message* pqPutMsgBytes：向部分构造的消息添加字节** Returns 0 on success, EOF on error*/
static int
pqPutMsgBytes(const void *buf, size_t len, PGconn *conn)
{/* make sure there is room for it */if (pqCheckOutBufferSpace(conn->outMsgEnd + len, conn))return EOF;/* okay, save the data */memcpy(conn->outBuffer + conn->outMsgEnd, buf, len);conn->outMsgEnd += len;/* no Pfdebug call here, caller should do it */return 0;
}

解释一下：

• 因为 type 是’S’，因此这里写的就是它
• 因为内存copy是从 conn->outBuffer + conn->outMsgEnd 开始的，也就放到了conn->outBuffer[5]
• conn->outMsgEnd += 1

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第三步：

libpq.so.5!pqPutMsgBytes(const void * buf, size_t len, PGconn * conn) (\home\postgres\postgres\src\interfaces\libpq\fe-misc.c:500)
libpq.so.5!pqPuts(const char * s, PGconn * conn) (\home\postgres\postgres\src\interfaces\libpq\fe-misc.c:154)
libpq.so.5!PQsendTypedCommand(PGconn * conn, char command, char type, const char * target) (\home\postgres\postgres\src\interfaces\libpq\fe-exec.c:2607)
libpq.so.5!PQsendClosePrepared(PGconn * conn, const char * stmt) (\home\postgres\postgres\src\interfaces\libpq\fe-exec.c:2558)
main() (\home\postgres\test\bin\libpqtest2.cpp:110)

/** pqPuts: write a null-terminated string to the current message* pqPuts：将空终止字符串写入当前消息*/
int
pqPuts(const char *s, PGconn *conn)
{if (pqPutMsgBytes(s, strlen(s) + 1, conn))return EOF;return 0;
}

解释一下：

• len = strlen + 1; strlen(“p23456789”) + 1 = 10
• 内存copy是从 conn->outMsgEnd = 6 开始的，len = 10。自然结果如上
• conn->outMsgEnd += 10

---

第四步：

libpq.so.5!pqPutMsgEnd(PGconn * conn) (\home\postgres\postgres\src\interfaces\libpq\fe-misc.c:520)
libpq.so.5!PQsendTypedCommand(PGconn * conn, char command, char type, const char * target) (\home\postgres\postgres\src\interfaces\libpq\fe-exec.c:2608)
libpq.so.5!PQsendClosePrepared(PGconn * conn, const char * stmt) (\home\postgres\postgres\src\interfaces\libpq\fe-exec.c:2558)
main() (\home\postgres\test\bin\libpqtest2.cpp:110)

/** pqPutMsgEnd: finish constructing a message and possibly send it* pqPutMsgEnd：完成构建消息并可能发送它** Returns 0 on success, EOF on error** We don't actually send anything here unless we've accumulated at least* 8K worth of data (the typical size of a pipe buffer on Unix systems).* This avoids sending small partial packets.  The caller must use pqFlush* when it's important to flush all the data out to the server.*  * 除非我们已经积累了至少 8K 的数据（Unix 系统上管道缓冲区的典型大小），否则我们实际上不会在这里发送任何内容* 这避免了发送小部分数据包* 当需要将所有数据刷新到服务器时，调用者必须使用 pqFlush*/
int
pqPutMsgEnd(PGconn *conn)
{/* Fill in length word if needed */if (conn->outMsgStart >= 0){uint32		msgLen = conn->outMsgEnd - conn->outMsgStart;msgLen = pg_hton32(msgLen);memcpy(conn->outBuffer + conn->outMsgStart, &msgLen, 4);}/* trace client-to-server message */if (conn->Pfdebug){if (conn->outCount < conn->outMsgStart)pqTraceOutputMessage(conn, conn->outBuffer + conn->outCount, true);elsepqTraceOutputNoTypeByteMessage(conn,conn->outBuffer + conn->outMsgStart);}/* Make message eligible to send */conn->outCount = conn->outMsgEnd;if (conn->outCount >= 8192){int			toSend = conn->outCount - (conn->outCount % 8192);if (pqSendSome(conn, toSend) < 0)return EOF;/* in nonblock mode, don't complain if unable to send it all */}return 0;
}

解释一下：

• 如上 message_len = 16 - 1 = 15，然后大端存储
• 内存copy是从 conn->outMsgStart = 1开始，长度是4 自然就是上面的四字节
• conn->outCount = conn->outMsgEnd = 16

至此，该15字节的close消息构建完毕！

---

后端消息解析

我们这里调试服务进程，如下：

---

第一步：消息读取，如下：

pq_recvbuf() (\home\postgres\postgres\src\backend\libpq\pqcomm.c:926)
pq_getbyte() (\home\postgres\postgres\src\backend\libpq\pqcomm.c:969)
SocketBackend(StringInfo inBuf) (\home\postgres\postgres\src\backend\tcop\postgres.c:370)
ReadCommand(StringInfo inBuf) (\home\postgres\postgres\src\backend\tcop\postgres.c:493)
PostgresMain(const char * dbname, const char * username) (\home\postgres\postgres\src\backend\tcop\postgres.c:4612)
BackendMain(char * startup_data, size_t startup_data_len) (\home\postgres\postgres\src\backend\tcop\backend_startup.c:105)
postmaster_child_launch(BackendType child_type, char * startup_data, size_t startup_data_len, ClientSocket * client_sock) (\home\postgres\postgres\src\backend\postmaster\launch_backend.c:265)
BackendStartup(ClientSocket * client_sock) (\home\postgres\postgres\src\backend\postmaster\postmaster.c:3593)
ServerLoop() (\home\postgres\postgres\src\backend\postmaster\postmaster.c:1674)
PostmasterMain(int argc, char ** argv) (\home\postgres\postgres\src\backend\postmaster\postmaster.c:1372)
main(int argc, char ** argv) (\home\postgres\postgres\src\backend\main\main.c:197)

// src/backend/libpq/pqcomm.c/* --------------------------------*		pq_recvbuf - load some bytes into the input buffer*		pq_recvbuf - 将一些字节加载到输入缓冲区中**		returns 0 if OK, EOF if trouble* --------------------------------*/
static int
pq_recvbuf(void)
{if (PqRecvPointer > 0){if (PqRecvLength > PqRecvPointer){/* still some unread data, left-justify it in the buffer */// 还有一些未读数据，将其在缓冲区中左对齐memmove(PqRecvBuffer, PqRecvBuffer + PqRecvPointer,PqRecvLength - PqRecvPointer);PqRecvLength -= PqRecvPointer;PqRecvPointer = 0;}elsePqRecvLength = PqRecvPointer = 0;}/* Ensure that we're in blocking mode */// 确保我们处于阻塞模式socket_set_nonblocking(false);/* Can fill buffer from PqRecvLength and upwards */// 可以从 PqRecvLength 及以上填充缓冲区for (;;){int			r;errno = 0;r = secure_read(MyProcPort, PqRecvBuffer + PqRecvLength,PQ_RECV_BUFFER_SIZE - PqRecvLength);if (r < 0){if (errno == EINTR)continue;		/* Ok if interrupted *//** Careful: an ereport() that tries to write to the client would* cause recursion to here, leading to stack overflow and core* dump!  This message must go *only* to the postmaster log.* 小心：尝试写入客户端的 ereport() 会导致递归到此处，从而导致堆栈溢出和核心转储* 此消息必须 *仅* 发送到邮件管理员日志** If errno is zero, assume it's EOF and let the caller complain.* 如果 errno 为零，则假设它是 EOF 并让调用者抱怨*/if (errno != 0)ereport(COMMERROR,(errcode_for_socket_access(),errmsg("could not receive data from client: %m")));return EOF;}if (r == 0){/** EOF detected.  We used to write a log message here, but it's* better to expect the ultimate caller to do that.* 检测到 EOF* 我们曾经在这里编写日志消息，但最好期望最终调用者这样做*/return EOF;}/* r contains number of bytes read, so just incr length */// r 包含读取的字节数，因此只需增加长度PqRecvLength += r;return 0;}
}

解释一下：

• PqRecvLength = 21 这个其实是两条消息 如下：

	/* construct the Close message */ // 0 - 15if (pqPutMsgStart(command, conn) < 0 ||pqPutc(type, conn) < 0 ||pqPuts(target, conn) < 0 ||pqPutMsgEnd(conn) < 0)goto sendFailed;/* construct the Sync message */ // 16 - 20if (conn->pipelineStatus == PQ_PIPELINE_OFF){if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||pqPutMsgEnd(conn) < 0)goto sendFailed;}

---

第二步：

/* --------------------------------*		pq_getbyte	- get a single byte from connection, or return EOF* --------------------------------*/
int
pq_getbyte(void)
{Assert(PqCommReadingMsg);while (PqRecvPointer >= PqRecvLength){if (pq_recvbuf())		/* If nothing in buffer, then recv some */return EOF;			/* Failed to recv data */}return (unsigned char) PqRecvBuffer[PqRecvPointer++];
}

解释一下：

• 此时 PqRecvPointer = 0，自然返回的是 'C'

---

第三步：

.../** In protocol version 3, all frontend messages have a length word next* after the type code; we can read the message contents independently of* the type.* * 在协议版本 3 中，所有前端消息在类型代码之后都有一个长度字* 我们可以独立于类型来读取消息内容*/if (pq_getmessage(inBuf, maxmsglen))return EOF;				/* suitable message already logged */RESUME_CANCEL_INTERRUPTS();
...

/* --------------------------------*		pq_getmessage	- get a message with length word from connection*		pq_getmessage - 从连接获取长度字的消息**		The return value is placed in an expansible StringInfo, which has*		already been initialized by the caller.*		Only the message body is placed in the StringInfo; the length word*		is removed.  Also, s->cursor is initialized to zero for convenience*		in scanning the message contents.*		返回值放置在可扩展的 StringInfo 中，该 StringInfo 已由调用者初始化*		StringInfo中只放置消息体； 长度词被删除*		另外，为了方便扫描消息内容，s->cursor 被初始化为零**		maxlen is the upper limit on the length of the*		message we are willing to accept.  We abort the connection (by*		returning EOF) if client tries to send more than that.*		maxlen 是我们愿意接受的消息长度的上限*		如果客户端尝试发送更多内容，我们将中止连接（通过返回 EOF）**		returns 0 if OK, EOF if trouble* --------------------------------*/
int
pq_getmessage(StringInfo s, int maxlen)
{int32		len;Assert(PqCommReadingMsg);resetStringInfo(s);/* Read message length word */if (pq_getbytes((char *) &len, 4) == EOF){ereport(COMMERROR,(errcode(ERRCODE_PROTOCOL_VIOLATION),errmsg("unexpected EOF within message length word")));return EOF;}len = pg_ntoh32(len);if (len < 4 || len > maxlen){ereport(COMMERROR,(errcode(ERRCODE_PROTOCOL_VIOLATION),errmsg("invalid message length")));return EOF;}len -= 4;					/* discount length itself */if (len > 0){/** Allocate space for message.  If we run out of room (ridiculously* large message), we will elog(ERROR), but we want to discard the* message body so as not to lose communication sync.*/PG_TRY();{enlargeStringInfo(s, len);}PG_CATCH();{if (pq_discardbytes(len) == EOF)ereport(COMMERROR,(errcode(ERRCODE_PROTOCOL_VIOLATION),errmsg("incomplete message from client")));/* we discarded the rest of the message so we're back in sync. */PqCommReadingMsg = false;PG_RE_THROW();}PG_END_TRY();/* And grab the message */if (pq_getbytes(s->data, len) == EOF){ereport(COMMERROR,(errcode(ERRCODE_PROTOCOL_VIOLATION),errmsg("incomplete message from client")));return EOF;}s->len = len;/* Place a trailing null per StringInfo convention */s->data[len] = '\0';}/* finished reading the message. */PqCommReadingMsg = false;return 0;
}

解释一下：

• 读取长度 4字节：memcpy(s, PqRecvBuffer + PqRecvPointer, amount);此时 PqRecvPointer = 1
• pg_ntoh32 网络序–>主机序 len = 15
• len -= 4; /* discount length itself */
• 读取长度 11 字节：memcpy(s, PqRecvBuffer + PqRecvPointer, amount);此时 PqRecvPointer = 5
• 此时读取的内容：0x1e54c88 "Sp23456789"，也就是 input_message

接下来ReadCommand的结果，如下：

firstchar = ReadCommand(&input_message); // C

---

最后一步，如下：

OK，至此 服务端解析完毕！