本節簡單介紹了PostgreSQL的備份工具pg_basebackup原始碼中實際執行備份邏輯的函式BaseBackup.

### 一、資料結構
**option**
使用工具時儲存選項的資料結構
```

#ifndef HAVE_STRUCT_OPTION

//工具軟體選項
struct option
{
    const char *name;//名稱
    int         has_arg;//是否包含引數,no_argument/required_argument/optional_argument
    int        *flag;//標記
    int         val;//引數值
};

#define no_argument 0
#define required_argument 1
#define optional_argument 2
#endif

/*
 * On OpenBSD and some versions of Solaris, opterr and friends are defined in
 * core libc rather than in a separate getopt module.  Define these variables
 * only if configure found they aren't there by default; otherwise, this
 * module and its callers will just use libc's variables.  (We assume that
 * testing opterr is sufficient for all of these.)
 */
#ifndef HAVE_INT_OPTERR

int         opterr = 1,         /* if error message should be printed */
            optind = 1,         /* index into parent argv vector */
            optopt;             /* character checked for validity */
char       *optarg;             /* argument associated with option */

#endif

#define BADCH   (int)'?'
#define BADARG  (int)':'
#define EMSG    ""

```

**pg_result**
用於接收PQgetResult的返回結果.
```
 
struct pg_result
{
    //元組數量
    int         ntups;
    //屬性數量
    int         numAttributes;
    PGresAttDesc *attDescs;
    //PGresTuple陣列
    PGresAttValue **tuples;     /* each PGresTuple is an array of
                                 * PGresAttValue's */
    //元組陣列的大小
    int         tupArrSize;     /* allocated size of tuples array */
    //引數格式
    int         numParameters;
    //引數描述符
    PGresParamDesc *paramDescs;
    //執行狀態型別(列舉變數)
    ExecStatusType resultStatus;
    //從查詢返回的命令狀態
    char        cmdStatus[CMDSTATUS_LEN];   /* cmd status from the query */
    //1-二進位制的元組資料,否則為文字資料
    int         binary;         /* binary tuple values if binary == 1,
                                 * otherwise text */

/*
     * These fields are copied from the originating PGconn, so that operations
     * on the PGresult don't have to reference the PGconn.
     * 這些欄位從原始的PGconn中複製,以便不需要依賴PGconn
     */
    //鉤子函式
    PGNoticeHooks noticeHooks;
    PGEvent    *events;
    int         nEvents;
    int         client_encoding;    /* encoding id */

/*
     * Error information (all NULL if not an error result).  errMsg is the
     * "overall" error message returned by PQresultErrorMessage.  If we have
     * per-field info then it is stored in a linked list.
     * 錯誤資訊(如沒有錯誤,則全部為NULL)
     * errMsg是PQresultErrorMessage返回的"overall"錯誤資訊.
     * 如果存在per-field資訊,那麼會儲存在相互連結的連結串列中
     */
    //錯誤資訊
    char       *errMsg;         /* error message, or NULL if no error */
    //按欄位拆分的資訊
    PGMessageField *errFields;  /* message broken into fields */
    //如可用,觸發查詢的文字資訊
    char       *errQuery;       /* text of triggering query, if available */

/* All NULL attributes in the query result point to this null string */
    //查詢結果中的所有NULL屬性指向該null字串
    char        null_field[1];

/*
     * Space management information.  Note that attDescs and error stuff, if
     * not null, point into allocated blocks.  But tuples points to a
     * separately malloc'd block, so that we can realloc it.
     * 空間管理資訊.
     * 注意attDescs和error,如為not null,則指向已分配的blocks.
     * 但元組指向單獨的已分配的block,因此可以重新分配空間.
     */
    //最近已分配的block
    PGresult_data *curBlock;    /* most recently allocated block */
    //塊中空閒空間的開始偏移
    int         curOffset;      /* start offset of free space in block */
    //塊中剩餘的空閒位元組
    int         spaceLeft;      /* number of free bytes remaining in block */
    //該PGresult結構體總共的分配空間
    size_t      memorySize;     /* total space allocated for this PGresult */
};

/* Data about a single parameter of a prepared statement */
//prepared statement語句的單個引數的資料
typedef struct pgresParamDesc
{
    //型別ID
    Oid         typid;          /* type id */
} PGresParamDesc;

typedef enum
{
    //空查詢串
    PGRES_EMPTY_QUERY = 0,      /* empty query string was executed */
    //後臺程式正常執行了沒有結果返回的查詢命令
    PGRES_COMMAND_OK,           /* a query command that doesn't return
                                 * anything was executed properly by the
                                 * backend */
    //後臺程式正常執行了有元組返回的查詢命令
    //PGresult中有結果元組
    PGRES_TUPLES_OK,            /* a query command that returns tuples was
                                 * executed properly by the backend, PGresult
                                 * contains the result tuples */
    //複製資料OUT,傳輸中
    PGRES_COPY_OUT,             /* Copy Out data transfer in progress */
    //複製資料IN,傳輸中
    PGRES_COPY_IN,              /* Copy In data transfer in progress */
    //從後臺程式中收到非期望中的響應
    PGRES_BAD_RESPONSE,         /* an unexpected response was recv'd from the
                                 * backend */
    //提示或警告資訊
    PGRES_NONFATAL_ERROR,       /* notice or warning message */
    //查詢失敗
    PGRES_FATAL_ERROR,          /* query failed */
    //複製I/O,傳輸中
    PGRES_COPY_BOTH,            /* Copy In/Out data transfer in progress */
    //更大的結果集中的單個元組
    PGRES_SINGLE_TUPLE          /* single tuple from larger resultset */
} ExecStatusType;

typedef union pgresult_data PGresult_data;

union pgresult_data
{
    //連結到下一個block,或者為NULL
    PGresult_data *next;        /* link to next block, or NULL */
    //以位元組形式訪問塊
    char        space[1];       /* dummy for accessing block as bytes */
};

```

### 二、原始碼解讀
BaseBackup,實際執行備份的函式.
主要邏輯是透過libpq介面向伺服器端發起備份請求(BASE_BACKUP命令)
```
static void
BaseBackup(void)
{
    PGresult   *res;
    char       *sysidentifier;
    TimeLineID  latesttli;
    TimeLineID  starttli;
    char       *basebkp;
    char        escaped_label[MAXPGPATH];
    char       *maxrate_clause = NULL;
    int         i;
    char        xlogstart[64];
    char        xlogend[64];
    int         minServerMajor,
                maxServerMajor;
    int         serverVersion,
                serverMajor;
    //資料庫連線
    Assert(conn != NULL);

/*
     * Check server version. BASE_BACKUP command was introduced in 9.1, so we
     * can't work with servers older than 9.1.
     * 檢查伺服器版本.BASE_BACKUP在9.1+才出現,資料庫版本不能低於9.1.
     */
    minServerMajor = 901;
    maxServerMajor = PG_VERSION_NUM / 100;
    serverVersion = PQserverVersion(conn);
    serverMajor = serverVersion / 100;
    if (serverMajor < minServerMajor || serverMajor > maxServerMajor)
    {
        const char *serverver = PQparameterStatus(conn, "server_version");

fprintf(stderr, _("%s: incompatible server version %s\n"),
                progname, serverver ? serverver : "'unknown'");
        exit(1);
    }

/*
     * If WAL streaming was requested, also check that the server is new
     * enough for that.
     * 要求WAL streaming,檢查資料庫是否支援
     */
    if (includewal == STREAM_WAL && !CheckServerVersionForStreaming(conn))
    {
        /*
         * Error message already written in CheckServerVersionForStreaming(),
         * but add a hint about using -X none.
         * 錯誤資訊已在CheckServerVersionForStreaming()中體現,這裡新增-X提示.
         */
        fprintf(stderr, _("HINT: use -X none or -X fetch to disable log streaming\n"));
        exit(1);
    }

/*
     * Build contents of configuration file if requested
     * 如需要建立recovery.conf檔案
     */
    if (writerecoveryconf)
        GenerateRecoveryConf(conn);

/*
     * Run IDENTIFY_SYSTEM so we can get the timeline
     * 執行RunIdentifySystem,獲取時間線
     */
    if (!RunIdentifySystem(conn, &sysidentifier, &latesttli, NULL, NULL))
        exit(1);

/*
     * Start the actual backup
     * 開始實際的backup
     */
    PQescapeStringConn(conn, escaped_label, label, sizeof(escaped_label), &i);

if (maxrate > 0)
        maxrate_clause = psprintf("MAX_RATE %u", maxrate);

if (verbose)
        //提示資訊
        fprintf(stderr,
                _("%s: initiating base backup, waiting for checkpoint to complete\n"),
                progname);

if (showprogress && !verbose)
    {
        //進度資訊
        fprintf(stderr, "waiting for checkpoint");
        if (isatty(fileno(stderr)))
            fprintf(stderr, "\r");
        else
            fprintf(stderr, "\n");
    }
    //base backup命令
    basebkp =
        psprintf("BASE_BACKUP LABEL '%s' %s %s %s %s %s %s %s",
                 escaped_label,
                 showprogress ? "PROGRESS" : "",
                 includewal == FETCH_WAL ? "WAL" : "",
                 fastcheckpoint ? "FAST" : "",
                 includewal == NO_WAL ? "" : "NOWAIT",
                 maxrate_clause ? maxrate_clause : "",
                 format == 't' ? "TABLESPACE_MAP" : "",
                 verify_checksums ? "" : "NOVERIFY_CHECKSUMS");
    //呼叫API
    if (PQsendQuery(conn, basebkp) == 0)
    {
        fprintf(stderr, _("%s: could not send replication command \"%s\": %s"),
                progname, "BASE_BACKUP", PQerrorMessage(conn));
        exit(1);
    }

/*
     * Get the starting WAL location
     * 獲取WAL起始位置
     */
    //獲取PQ執行結果
    res = PQgetResult(conn);
    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, _("%s: could not initiate base backup: %s"),
                progname, PQerrorMessage(conn));
        exit(1);
    }
    //判斷ntuples
    if (PQntuples(res) != 1)
    {
        fprintf(stderr,
                _("%s: server returned unexpected response to BASE_BACKUP command; got %d rows and %d fields, expected %d rows and %d fields\n"),
                progname, PQntuples(res), PQnfields(res), 1, 2);
        exit(1);
    }
    //獲取WAL start位置
    strlcpy(xlogstart, PQgetvalue(res, 0, 0), sizeof(xlogstart));

if (verbose)
        fprintf(stderr, _("%s: checkpoint completed\n"), progname);

/*
     * 9.3 and later sends the TLI of the starting point. With older servers,
     * assume it's the same as the latest timeline reported by
     * IDENTIFY_SYSTEM.
     * 9.3+在起始點就傳送了TLI.
     */
    if (PQnfields(res) >= 2)
        starttli = atoi(PQgetvalue(res, 0, 1));
    else
        starttli = latesttli;
    PQclear(res);
    MemSet(xlogend, 0, sizeof(xlogend));

if (verbose && includewal != NO_WAL)
        fprintf(stderr, _("%s: write-ahead log start point: %s on timeline %u\n"),
                progname, xlogstart, starttli);

/*
     * Get the header
     * 獲取頭部資訊
     */
    res = PQgetResult(conn);
    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, _("%s: could not get backup header: %s"),
                progname, PQerrorMessage(conn));
        exit(1);
    }
    if (PQntuples(res) < 1)
    {
        fprintf(stderr, _("%s: no data returned from server\n"), progname);
        exit(1);
    }

/*
     * Sum up the total size, for progress reporting
     * 統計總大小,用於進度報告
     */
    totalsize = totaldone = 0;
    tablespacecount = PQntuples(res);
    for (i = 0; i < PQntuples(res); i++)
    {
        totalsize += atol(PQgetvalue(res, i, 2));

/*
         * Verify tablespace directories are empty. Don't bother with the
         * first once since it can be relocated, and it will be checked before
         * we do anything anyway.
         * 驗證表空間目錄是否為空.
         * 首次驗證不需要報警,因為可以重新定位並且在作其他事情前會檢查.
         */
        if (format == 'p' && !PQgetisnull(res, i, 1))
        {
            char       *path = unconstify(char *, get_tablespace_mapping(PQgetvalue(res, i, 1)));

verify_dir_is_empty_or_create(path, &made_tablespace_dirs, &found_tablespace_dirs);
        }
    }

/*
     * When writing to stdout, require a single tablespace
     * 在寫入stdout時,要求一個獨立的表空間.
     */
    if (format == 't' && strcmp(basedir, "-") == 0 && PQntuples(res) > 1)
    {
        fprintf(stderr,
                _("%s: can only write single tablespace to stdout, database has %d\n"),
                progname, PQntuples(res));
        exit(1);
    }

/*
     * If we're streaming WAL, start the streaming session before we start
     * receiving the actual data chunks.
     * 如果正在streaming WAL,開始接收實際的資料chunks前,開始streaming session.
     */
    if (includewal == STREAM_WAL)
    {
        if (verbose)
            fprintf(stderr, _("%s: starting background WAL receiver\n"),
                    progname);
        StartLogStreamer(xlogstart, starttli, sysidentifier);
    }

/*
     * Start receiving chunks
     * 開始接收chunks
     */
    for (i = 0; i < PQntuples(res); i++)//所有的表空間
    {
        if (format == 't')
            //tar包
            ReceiveTarFile(conn, res, i);
        else
            //普通檔案
            ReceiveAndUnpackTarFile(conn, res, i);
    }                           /* Loop over all tablespaces */

if (showprogress)
    {
        progress_report(PQntuples(res), NULL, true);
        if (isatty(fileno(stderr)))
            fprintf(stderr, "\n");  /* Need to move to next line */
    }

PQclear(res);

/*
     * Get the stop position
     */
    res = PQgetResult(conn);
    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr,
                _("%s: could not get write-ahead log end position from server: %s"),
                progname, PQerrorMessage(conn));
        exit(1);
    }
    if (PQntuples(res) != 1)
    {
        fprintf(stderr,
                _("%s: no write-ahead log end position returned from server\n"),
                progname);
        exit(1);
    }
    strlcpy(xlogend, PQgetvalue(res, 0, 0), sizeof(xlogend));
    if (verbose && includewal != NO_WAL)
        fprintf(stderr, _("%s: write-ahead log end point: %s\n"), progname, xlogend);
    PQclear(res);
    //
    res = PQgetResult(conn);
    if (PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        const char *sqlstate = PQresultErrorField(res, PG_DIAG_SQLSTATE);

if (sqlstate &&
            strcmp(sqlstate, ERRCODE_DATA_CORRUPTED) == 0)
        {
            fprintf(stderr, _("%s: checksum error occurred\n"),
                    progname);
            checksum_failure = true;
        }
        else
        {
            fprintf(stderr, _("%s: final receive failed: %s"),
                    progname, PQerrorMessage(conn));
        }
        exit(1);
    }

if (bgchild > 0)
    {
#ifndef WIN32
        int         status;
        pid_t       r;
#else
        DWORD       status;

/*
         * get a pointer sized version of bgchild to avoid warnings about
         * casting to a different size on WIN64.
         */
        intptr_t    bgchild_handle = bgchild;
        uint32      hi,
                    lo;
#endif

if (verbose)
            fprintf(stderr,
                    _("%s: waiting for background process to finish streaming ...\n"), progname);

#ifndef WIN32//WIN32
        if (write(bgpipe[1], xlogend, strlen(xlogend)) != strlen(xlogend))
        {
            fprintf(stderr,
                    _("%s: could not send command to background pipe: %s\n"),
                    progname, strerror(errno));
            exit(1);
        }

/* Just wait for the background process to exit */
        r = waitpid(bgchild, &status, 0);
        if (r == (pid_t) -1)
        {
            fprintf(stderr, _("%s: could not wait for child process: %s\n"),
                    progname, strerror(errno));
            exit(1);
        }
        if (r != bgchild)
        {
            fprintf(stderr, _("%s: child %d died, expected %d\n"),
                    progname, (int) r, (int) bgchild);
            exit(1);
        }
        if (status != 0)
        {
            fprintf(stderr, "%s: %s\n",
                    progname, wait_result_to_str(status));
            exit(1);
        }
        /* Exited normally, we're happy! */
#else                           /* WIN32 */

/*
         * On Windows, since we are in the same process, we can just store the
         * value directly in the variable, and then set the flag that says
         * it's there.
         * 在Windows平臺,因為在同一個程式中,只需要直接儲存值到遍歷中,然後設定標記即可.
         */
        if (sscanf(xlogend, "%X/%X", &hi, &lo) != 2)
        {
            fprintf(stderr,
                    _("%s: could not parse write-ahead log location \"%s\"\n"),
                    progname, xlogend);
            exit(1);
        }
        xlogendptr = ((uint64) hi) << 32 | lo;
        InterlockedIncrement(&has_xlogendptr);

/* First wait for the thread to exit */
        if (WaitForSingleObjectEx((HANDLE) bgchild_handle, INFINITE, FALSE) !=
            WAIT_OBJECT_0)
        {
            _dosmaperr(GetLastError());
            fprintf(stderr, _("%s: could not wait for child thread: %s\n"),
                    progname, strerror(errno));
            exit(1);
        }
        if (GetExitCodeThread((HANDLE) bgchild_handle, &status) == 0)
        {
            _dosmaperr(GetLastError());
            fprintf(stderr, _("%s: could not get child thread exit status: %s\n"),
                    progname, strerror(errno));
            exit(1);
        }
        if (status != 0)
        {
            fprintf(stderr, _("%s: child thread exited with error %u\n"),
                    progname, (unsigned int) status);
            exit(1);
        }
        /* Exited normally, we're happy */
#endif
    }

/* Free the configuration file contents */
    //釋放配置檔案記憶體
    destroyPQExpBuffer(recoveryconfcontents);

/*
     * End of copy data. Final result is already checked inside the loop.
     * 複製資料完成.最終結果已在迴圈中檢查.
     */
    PQclear(res);
    PQfinish(conn);
    conn = NULL;

/*
     * Make data persistent on disk once backup is completed. For tar format
     * once syncing the parent directory is fine, each tar file created per
     * tablespace has been already synced. In plain format, all the data of
     * the base directory is synced, taking into account all the tablespaces.
     * Errors are not considered fatal.
     * 在備份結束後,持久化資料在磁碟上.
     * 對於tar格式只需要同步父目錄即可,每一個表空間建立一個tar檔案,這些檔案已同步.
     * 對於普通格式,基礎目錄中的所有資料已同步,已兼顧了所有的表空間.
     * 錯誤不會認為是致命的異常.
     */
    if (do_sync)
    {
        if (verbose)
            fprintf(stderr,
                    _("%s: syncing data to disk ...\n"), progname);
        if (format == 't')
        {
            if (strcmp(basedir, "-") != 0)
                (void) fsync_fname(basedir, true, progname);
        }
        else
        {
            (void) fsync_pgdata(basedir, progname, serverVersion);
        }
    }

if (verbose)
        fprintf(stderr, _("%s: base backup completed\n"), progname);
}

/*
 * PQgetvalue:
 *  return the value of field 'field_num' of row 'tup_num'
 *  返回tuples陣列第field_num欄位的第tup_num行.
 */
char *
PQgetvalue(const PGresult *res, int tup_num, int field_num)
{
    if (!check_tuple_field_number(res, tup_num, field_num))
        return NULL;
    return res->tuples[tup_num][field_num].value;
}

```

### 三、跟蹤分析
備份命令
```
pg_basebackup -h localhost -U xdb -p 5432 -D /data/backup -P -Xs -R

```
啟動gdb跟蹤
```
[xdb@localhost ~]$ gdb pg_basebackup
GNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-110.el7
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http:>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
For bug reporting instructions, please see:
<http:></http:>...
Reading symbols from /appdb/atlasdb/pg11.2/bin/pg_basebackup...done.
(gdb) b BaseBackup
(gdb) set args -h localhost -U xdb -p 5432 -D /data/backup -P -Xs -R 
(gdb) r
Starting program: /appdb/atlasdb/pg11.2/bin/pg_basebackup -h localhost -U xdb -p 5432 -D /data/backup -P -Xs -R 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Breakpoint 1, BaseBackup () at pg_basebackup.c:1740
1740        char       *maxrate_clause = NULL;
(gdb) 
```
連線pg_conn結構體
```
(gdb) n
1749        Assert(conn != NULL);
(gdb) p *conn
$1 = {pghost = 0x6282d0 "localhost", pghostaddr = 0x0, pgport = 0x6282f0 "5432", pgtty = 0x628310 "", 
  connect_timeout = 0x0, client_encoding_initial = 0x0, pgoptions = 0x628330 "", appname = 0x0, 
  fbappname = 0x628350 "pg_basebackup", dbName = 0x6282b0 "replication", replication = 0x6283d0 "true", 
  pguser = 0x628290 "xdb", pgpass = 0x0, pgpassfile = 0x628d30 "/home/xdb/.pgpass", keepalives = 0x0, 
  keepalives_idle = 0x0, keepalives_interval = 0x0, keepalives_count = 0x0, sslmode = 0x628370 "prefer", 
  sslcompression = 0x628390 "0", sslkey = 0x0, sslcert = 0x0, sslrootcert = 0x0, sslcrl = 0x0, requirepeer = 0x0, 
  krbsrvname = 0x6283b0 "postgres", target_session_attrs = 0x6283f0 "any", Pfdebug = 0x0, noticeHooks = {
    noticeRec = 0x7ffff7b9eab4 <defaultnoticereceiver>, noticeRecArg = 0x0, 
    noticeProc = 0x7ffff7b9eb09 <defaultnoticeprocessor>, noticeProcArg = 0x0}, events = 0x0, nEvents = 0, 
  eventArraySize = 0, status = CONNECTION_OK, asyncStatus = PGASYNC_IDLE, xactStatus = PQTRANS_IDLE, 
  queryclass = PGQUERY_SIMPLE, last_query = 0x61f1c0 "SHOW wal_segment_size", last_sqlstate = "\000\000\000\000\000", 
  options_valid = true, nonblocking = false, singleRowMode = false, copy_is_binary = 0 '\000', copy_already_done = 0, 
  notifyHead = 0x0, notifyTail = 0x0, nconnhost = 1, whichhost = 0, connhost = 0x627a50, sock = 7, laddr = {addr = {
      ss_family = 10, __ss_padding = "\307\326", '\000' <repeats times="">, "\001", '\000' <repeats times="">, 
      __ss_align = 0}, salen = 28}, raddr = {addr = {ss_family = 10, 
      __ss_padding = "\025\070", '\000' <repeats times="">, "\001", '\000' <repeats times="">, __ss_align = 0}, 
    salen = 28}, pversion = 196608, sversion = 110002, auth_req_received = true, password_needed = false, 
  sigpipe_so = false, sigpipe_flag = true, try_next_addr = false, try_next_host = false, addr_cur = 0x0, 
  setenv_state = SETENV_STATE_IDLE, next_eo = 0x0, send_appname = true, be_pid = 1435, be_key = -828773845, 
  pstatus = 0x629570, client_encoding = 0, std_strings = true, verbosity = PQERRORS_DEFAULT, 
  show_context = PQSHOW_CONTEXT_ERRORS, lobjfuncs = 0x0, inBuffer = 0x61f600 "T", inBufSize = 16384, inStart = 75, 
  inCursor = 75, inEnd = 75, outBuffer = 0x623610 "Q", outBufSize = 16384, outCount = 0, outMsgStart = 1, outMsgEnd = 27, 
  rowBuf = 0x627620, rowBufLen = 32, result = 0x0, next_result = 0x0, sasl_state = 0x0, ssl_in_use = false, 
  allow_ssl_try = false, wait_ssl_try = false, ssl = 0x0, peer = 0x0, engine = 0x0, gctx = 0x0, gtarg_nam = 0x0, 
  errorMessage = {data = 0x627830 "", len = 0, maxlen = 256}, workBuffer = {data = 0x627940 "SELECT", len = 6, 
    maxlen = 256}, addrlist = 0x0, addrlist_family = 0}
(gdb) 
```
判斷版本,是否支援BaseBackup
```
(gdb) n
1755        minServerMajor = 901;
(gdb) 
1756        maxServerMajor = PG_VERSION_NUM / 100;
(gdb) p PG_VERSION_NUM
$2 = 110002
(gdb) n
1757        serverVersion = PQserverVersion(conn);
(gdb) 
1758        serverMajor = serverVersion / 100;
(gdb) 
1759        if (serverMajor < minServerMajor || serverMajor > maxServerMajor)
(gdb) p serverVersion
$3 = 110002
(gdb) n
```
判斷伺服器是否支援WAL streaming
```
(gdb) n
1772        if (includewal == STREAM_WAL && !CheckServerVersionForStreaming(conn))
(gdb) 
```
如需要,生成recovery.conf檔案
```
1785        if (writerecoveryconf)
(gdb) p includewal
$4 = STREAM_WAL
(gdb) p writerecoveryconf
$5 = true
(gdb) n
1786            GenerateRecoveryConf(conn);
(gdb) 
```
獲取系統識別符號
```
1791        if (!RunIdentifySystem(conn, &sysidentifier, &latesttli, NULL, NULL))
(gdb) 
1797        PQescapeStringConn(conn, escaped_label, label, sizeof(escaped_label), &i);
(gdb) p sysidentifier
$6 = 0x6292d0 "6662151435832250464"
(gdb) p *sysidentifier
$7 = 54 '6'
(gdb) p latesttli
$8 = 1
(gdb) 
```
開始實際的備份工作
```
(gdb) p escaped_label
$9 = "pg_basebackup base backup\000\000\000\001", '\000' <repeats times="">, "\"`-\360\377\177\000\000\000\000\000\000\377\177\000\000` u\367\377\177\000\000\000\001\000\000\000\000\000\000\001\000\000\000\002\000\000\000]VA\000\000\000\000\000@\335\377\377\377\177\000\000b\343\377\377\377\177", '\000' <repeats times="">, "\343\377\377\377\177\000\000B\335\377\377\377\177\000\000\370Ǹ\367\377\177\000\000\220\325\227\367\377\177\000\000\000\341\377\377\377\177\000\000\000\000\000\000\000\000\000\000\371D"...
(gdb) p label
$10 = 0x412610 "pg_basebackup base backup"
(gdb) p i
$11 = 0
(gdb) 
```
構造backup命令
```
(gdb) n
1802        if (verbose)
(gdb) 
1807        if (showprogress && !verbose)
(gdb) 
1809            fprintf(stderr, "waiting for checkpoint");
(gdb) 
waiting for checkpoint1810          if (isatty(fileno(stderr)))
(gdb) 
1811                fprintf(stderr, "\r");
(gdb) 
1817            psprintf("BASE_BACKUP LABEL '%s' %s %s %s %s %s %s %s",
(gdb) 
1824                     format == 't' ? "TABLESPACE_MAP" : "",
(gdb) 
1817            psprintf("BASE_BACKUP LABEL '%s' %s %s %s %s %s %s %s",
(gdb) 
1822                     includewal == NO_WAL ? "" : "NOWAIT",
(gdb) 
1817            psprintf("BASE_BACKUP LABEL '%s' %s %s %s %s %s %s %s",
(gdb) 
1820                     includewal == FETCH_WAL ? "WAL" : "",
(gdb) 
1817            psprintf("BASE_BACKUP LABEL '%s' %s %s %s %s %s %s %s",
(gdb) 
1816        basebkp =
(gdb) 
1827        if (PQsendQuery(conn, basebkp) == 0)
(gdb) 
(gdb) p basebkp
$12 = 0x6291f0 "BASE_BACKUP LABEL 'pg_basebackup base backup' PROGRESS   NOWAIT   "
(gdb) 
```
傳送命令到伺服器端,獲取執行結果
```
(gdb) n
1837        res = PQgetResult(conn);
(gdb) 
1838        if (PQresultStatus(res) != PGRES_TUPLES_OK)
(gdb) 
```
返回結果
```
(gdb) p *res
$13 = {ntups = 1, numAttributes = 2, attDescs = 0x629688, tuples = 0x629e90, tupArrSize = 128, numParameters = 0, 
  paramDescs = 0x0, resultStatus = PGRES_TUPLES_OK, 
  cmdStatus = "SELECT\000\000\000\000\000\000\000\000\000\000\027\000\000\000\004\000\000\000\377\377\377\377:\226b", '\000' <repeats times="">, "\031\000\000\000\377\377\377\377\377\377\377\377B\226b", binary = 0, noticeHooks = {
    noticeRec = 0x7ffff7b9eab4 <defaultnoticereceiver>, noticeRecArg = 0x0, 
    noticeProc = 0x7ffff7b9eb09 <defaultnoticeprocessor>, noticeProcArg = 0x0}, events = 0x0, nEvents = 0, 
  client_encoding = 0, errMsg = 0x0, errFields = 0x0, errQuery = 0x0, null_field = "", curBlock = 0x629680, 
  curOffset = 133, spaceLeft = 1915}
(gdb)
(gdb) p *res->attDescs
$14 = {name = 0x6296c8 "recptr", tableid = 0, columnid = 0, format = 0, typid = 25, typlen = -1, atttypmod = 0}
(gdb) p *res->tuples
$15 = (PGresAttValue *) 0x6296d8
(gdb) p **res->tuples
$16 = {len = 10, value = 0x6296f8 "1/57000028"}
(gdb) p *res->tuples[2]
Cannot access memory at address 0x0
(gdb) p *res->tuples[0]
$17 = {len = 10, value = 0x6296f8 "1/57000028"}
(gdb) p *res->tuples[1]
Cannot access memory at address 0x15171
(gdb) 
```
判斷ntuples,獲取WAL start位置
```
(gdb) n
1844        if (PQntuples(res) != 1)
(gdb) 
1852        strlcpy(xlogstart, PQgetvalue(res, 0, 0), sizeof(xlogstart));
(gdb) p PQgetvalue(res, 0, 0)
$18 = 0x6296f8 "1/57000028"
(gdb) p xlogstart
$19 = " `-\360\377\177\000\000\353\340\377\377\377\177\000\000\360\340a", '\000' <repeats times="">, "\360\337\377\377\377\177", '\000' <repeats times="">
(gdb) n
1854        if (verbose)
(gdb) p xlogstart
$20 = "1/57000028\000\377\377\177\000\000\360\340a", '\000' <repeats times="">, "\360\337\377\377\377\177", '\000' <repeats times="">
(gdb) 
```
獲取時間線timeline
```
(gdb) n
1862        if (PQnfields(res) >= 2)
(gdb) p PQnfields(res)
$21 = 2
(gdb) n
1863            starttli = atoi(PQgetvalue(res, 0, 1));
(gdb) 
1866        PQclear(res);
(gdb) p atoi(PQgetvalue(res, 0, 1))
$22 = 1
(gdb)  p res->tuples[1]
$23 = (PGresAttValue *) 0x15171
(gdb)  p res->tuples[0]
$24 = (PGresAttValue *) 0x6296d8
(gdb) 
(gdb) n
1867        MemSet(xlogend, 0, sizeof(xlogend));
(gdb) 
1869        if (verbose && includewal != NO_WAL)
(gdb) p xlogend
$25 = '\000' <repeats times="">
```
Get the header
```
(gdb) n
1876        res = PQgetResult(conn);
(gdb) n
1877        if (PQresultStatus(res) != PGRES_TUPLES_OK)
(gdb) p *res
$26 = {ntups = 1, numAttributes = 3, attDescs = 0x629688, tuples = 0x629e90, tupArrSize = 128, numParameters = 0, 
  paramDescs = 0x0, resultStatus = PGRES_TUPLES_OK, 
  cmdStatus = "SELECT\000\000\000\000\000\000\000\000\000\000\027\000\000\000\004\000\000\000\377\377\377\377:\226b", '\000' <repeats times="">, "\031\000\000\000\377\377\377\377\377\377\377\377B\226b", binary = 0, noticeHooks = {
    noticeRec = 0x7ffff7b9eab4 <defaultnoticereceiver>, noticeRecArg = 0x0, 
    noticeProc = 0x7ffff7b9eb09 <defaultnoticeprocessor>, noticeProcArg = 0x0}, events = 0x0, nEvents = 0, 
  client_encoding = 0, errMsg = 0x0, errFields = 0x0, errQuery = 0x0, null_field = "", curBlock = 0x629680, 
  curOffset = 183, spaceLeft = 1865}
```
統計總大小,用於進度報告
```
(gdb) 
1892        totalsize = totaldone = 0;
(gdb) n
1893        tablespacecount = PQntuples(res);
(gdb) 
1894        for (i = 0; i < PQntuples(res); i++)
(gdb) p tablespacecount
$29 = 1
(gdb) n
1896            totalsize += atol(PQgetvalue(res, i, 2));
(gdb) p PQgetvalue(res, i, 2)
$30 = 0x629730 "445480"
(gdb) p atol(PQgetvalue(res, i, 2))
$31 = 445480
(gdb) n
1903            if (format == 'p' && !PQgetisnull(res, i, 1))
(gdb) 
1894        for (i = 0; i < PQntuples(res); i++)
(gdb) p res->tuples[0][0]
$33 = {len = -1, value = 0x629658 ""}
(gdb) p res->tuples[0][1]
$34 = {len = -1, value = 0x629658 ""}
(gdb) p res->tuples[0][2]
$35 = {len = 6, value = 0x629730 "445480"}
(gdb) 
```
開始接收實際的資料chunks前,開始streaming session.
```
(gdb) n
1914        if (format == 't' && strcmp(basedir, "-") == 0 && PQntuples(res) > 1)
(gdb) 
1926        if (includewal == STREAM_WAL)
(gdb) 
1928            if (verbose)
(gdb) 
1931            StartLogStreamer(xlogstart, starttli, sysidentifier);
(gdb) n
Detaching after fork from child process 1511.
1937        for (i = 0; i < PQntuples(res); i++)
(gdb) 
```
檢視作業系統中的日誌目錄
```
[xdb@localhost backup]$ ll
total 0
drwx------. 3 xdb xdb 60 Mar 15 15:46 pg_wal
[xdb@localhost backup]$ ll ./pg_wal/
total 16384
-rw-------. 1 xdb xdb 16777216 Mar 15 15:46 000000010000000100000057
drwx------. 2 xdb xdb        6 Mar 15 15:46 archive_status
[xdb@localhost backup]$ 
```
Start receiving chunks,開始接收chunks
```
(gdb) n
Detaching after fork from child process 1511.
1937        for (i = 0; i < PQntuples(res); i++)
(gdb) n
1939            if (format == 't')
(gdb) 
1942                ReceiveAndUnpackTarFile(conn, res, i);
(gdb) 
193789/445489 kB
(gdb) for (i = 0; i < PQntuples(res); i++)
```
檢視作業系統中的備份目錄
```
[xdb@localhost backup]$ ll
total 56
-rw-------. 1 xdb xdb   226 Mar 15 15:47 backup_label
drwx------. 6 xdb xdb    58 Mar 15 15:48 base
drwx------. 2 xdb xdb  4096 Mar 15 15:48 global
drwx------. 2 xdb xdb     6 Mar 15 15:47 pg_commit_ts
drwx------. 2 xdb xdb     6 Mar 15 15:47 pg_dynshmem
-rw-------. 1 xdb xdb  4513 Mar 15 15:48 pg_hba.conf
-rw-------. 1 xdb xdb  1636 Mar 15 15:48 pg_ident.conf
drwx------. 4 xdb xdb    68 Mar 15 15:48 pg_logical
drwx------. 4 xdb xdb    36 Mar 15 15:47 pg_multixact
drwx------. 2 xdb xdb     6 Mar 15 15:47 pg_notify
drwx------. 2 xdb xdb     6 Mar 15 15:48 pg_replslot
drwx------. 2 xdb xdb     6 Mar 15 15:47 pg_serial
drwx------. 2 xdb xdb     6 Mar 15 15:47 pg_snapshots
drwx------. 2 xdb xdb     6 Mar 15 15:48 pg_stat
drwx------. 2 xdb xdb     6 Mar 15 15:48 pg_stat_tmp
drwx------. 2 xdb xdb     6 Mar 15 15:47 pg_subtrans
drwx------. 2 xdb xdb     6 Mar 15 15:48 pg_tblspc
drwx------. 2 xdb xdb     6 Mar 15 15:47 pg_twophase
-rw-------. 1 xdb xdb     3 Mar 15 15:48 PG_VERSION
drwx------. 3 xdb xdb    92 Mar 15 15:48 pg_wal
drwx------. 2 xdb xdb    18 Mar 15 15:48 pg_xact
-rw-------. 1 xdb xdb    88 Mar 15 15:48 postgresql.auto.conf
-rw-------. 1 xdb xdb 23812 Mar 15 15:48 postgresql.conf
-rw-------. 1 xdb xdb   183 Mar 15 15:48 recovery.conf
[xdb@localhost backup]$ 
```
顯示進度
```
(gdb) n
1945        if (showprogress)
(gdb) 
1947            progress_report(PQntuples(res), NULL, true);
(gdb) 
194889/445489 kB (100%),if (isatty(fileno(stderr)))
(gdb) 
(gdb) n
1949                fprintf(stderr, "\n");  /* Need to move to next line */
(gdb)

1952        PQclear(res);
(gdb) 
```
Get the stop position
```
(gdb) 
1957        res = PQgetResult(conn);
(gdb) n
1958        if (PQresultStatus(res) != PGRES_TUPLES_OK)
(gdb) p *res
$36 = {ntups = 1, numAttributes = 2, attDescs = 0x6295a8, tuples = 0x629db0, tupArrSize = 128, numParameters = 0, 
  paramDescs = 0x0, resultStatus = PGRES_TUPLES_OK, 
  cmdStatus = "SELECT", '\000' <repeats times="">, "\300!", <incomplete sequence="">, binary = 0, noticeHooks = {
    noticeRec = 0x7ffff7b9eab4 <defaultnoticereceiver>, noticeRecArg = 0x0, 
    noticeProc = 0x7ffff7b9eb09 <defaultnoticeprocessor>, noticeProcArg = 0x0}, events = 0x0, nEvents = 0, 
  client_encoding = 0, errMsg = 0x0, errFields = 0x0, errQuery = 0x0, null_field = "", curBlock = 0x6295a0, 
  curOffset = 133, spaceLeft = 1915}
(gdb) 
(gdb) n
1965        if (PQntuples(res) != 1)
(gdb) 
1972        strlcpy(xlogend, PQgetvalue(res, 0, 0), sizeof(xlogend));
(gdb) p xlogend
$37 = '\000' <repeats times="">
(gdb) p *xlogend
$38 = 0 '\000'
(gdb) n
1973        if (verbose && includewal != NO_WAL)
(gdb) 
1975        PQclear(res);
(gdb) 
```
COMMAND is OK
```
(gdb) 
1977        res = PQgetResult(conn);
(gdb) 
1978        if (PQresultStatus(res) != PGRES_COMMAND_OK)
(gdb) p *res
$39 = {ntups = 0, numAttributes = 0, attDescs = 0x0, tuples = 0x0, tupArrSize = 0, numParameters = 0, paramDescs = 0x0, 
  resultStatus = PGRES_COMMAND_OK, cmdStatus = "SELECT", '\000' <repeats times="">, "\300!", <incomplete sequence="">, 
  binary = 0, noticeHooks = {noticeRec = 0x7ffff7b9eab4 <defaultnoticereceiver>, noticeRecArg = 0x0, 
    noticeProc = 0x7ffff7b9eb09 <defaultnoticeprocessor>, noticeProcArg = 0x0}, events = 0x0, nEvents = 0, 
  client_encoding = 0, errMsg = 0x0, errFields = 0x0, errQuery = 0x0, null_field = "", curBlock = 0x0, curOffset = 0, 
  spaceLeft = 0}
(gdb) 
```
善後工作,如在備份結束後,持久化資料在磁碟上等
```
(gdb) n
1997        if (bgchild > 0)
(gdb) 
2014            if (verbose)
(gdb) 
2019            if (write(bgpipe[1], xlogend, strlen(xlogend)) != strlen(xlogend))
(gdb) 
2028            r = waitpid(bgchild, &status, 0);
(gdb) p bgchild
$40 = 1511
(gdb) n
2029            if (r == -1)
(gdb) 
2035            if (r != bgchild)
(gdb) p r
$41 = 1511
(gdb) n
2041            if (!WIFEXITED(status))
(gdb) 
2047            if (WEXITSTATUS(status) != 0)
(gdb) 
2098        destroyPQExpBuffer(recoveryconfcontents);
(gdb) 
2103        PQclear(res);
(gdb) 
2104        PQfinish(conn);
(gdb) 
2113        if (do_sync)
(gdb) 
2115            if (format == 't')
(gdb) 
2122                (void) fsync_pgdata(basedir, progname, serverVersion);
(gdb) 
2126        if (verbose)
(gdb) 
2128    }
(gdb) 
main (argc=12, argv=0x7fffffffe4b8) at pg_basebackup.c:2534
2534        success = true;
(gdb) 
```

16:01:56 (xdb@[local]:5432)testdb=# 
16:01:56 (xdb@[local]:5432)testdb=
```

複製資料
```
(gdb) 
1942                ReceiveAndUnpackTarFile(conn, res, i);
(gdb) 
193789/445489 kBfor (i = 0; i < PQntuples(res); i++)
(gdb) 
1945        if (showprogress)
(gdb) 
```
資料庫活動
```
...
-[ RECORD 3 ]----+------------------------------
datid            | 
datname          | 
pid              | 1352
usesysid         | 
usename          | 
application_name | 
client_addr      | 
client_hostname  | 
client_port      | 
backend_start    | 2019-03-15 15:03:01.923092+08
xact_start       | 
query_start      | 
state_change     | 
wait_event_type  | Activity
wait_event       | WalWriterMain
state            | 
backend_xid      | 
backend_xmin     | 
query            | 
backend_type     | walwriter
```
執行善後工作
```
2113        if (do_sync)
(gdb) 
2115            if (format == 't')
(gdb) 
2122                (void) fsync_pgdata(basedir, progname, serverVersion);
(gdb) 
2126        if (verbose)
(gdb) 
2128    }
(gdb) 
```
資料庫活動
```
16:05:01 (xdb@[local]:5432)testdb=# select * from pg_stat_activity
where backend_type not in ('checkpointer', 'background writer', 'logical replication launcher', 'autovacuum launcher') and query not like '%pg_stat_activity%';
-[ RECORD 1 ]----+------------------------------
datid            | 
datname          | 
pid              | 1352
usesysid         | 
usename          | 
application_name | 
client_addr      | 
client_hostname  | 
client_port      | 
backend_start    | 2019-03-15 15:03:01.923092+08
xact_start       | 
query_start      | 
state_change     | 
wait_event_type  | Activity
wait_event       | WalWriterMain
state            | 
backend_xid      | 
backend_xmin     | 
query            | 
backend_type     | walwriter

```
DONE!

### 四、參考資料
[PG Source Code](https://doxygen.postgresql.org)</defaultnoticeprocessor></defaultnoticereceiver></incomplete></repeats></repeats></defaultnoticeprocessor></defaultnoticereceiver></incomplete></repeats></defaultnoticeprocessor></defaultnoticereceiver></repeats></repeats></repeats></repeats></repeats></repeats></defaultnoticeprocessor></defaultnoticereceiver></repeats></repeats></repeats></repeats></repeats></repeats></repeats></defaultnoticeprocessor></defaultnoticereceiver></http:>

PostgreSQL 原始碼解讀（150）- PG Tools#2（BaseBackup函式）

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