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walsender.c
walsender.c 22.42 KiB
/*-------------------------------------------------------------------------
*
* walsender.c
*
* The WAL sender process (walsender) is new as of Postgres 8.5. It takes
* charge of XLOG streaming sender in the primary server. At first, it is
* started by the postmaster when the walreceiver in the standby server
* connects to the primary server and requests XLOG streaming replication,
* i.e., unlike any auxiliary process, it is not an always-running process.
* It attempts to keep reading XLOG records from the disk and sending them
* to the standby server, as long as the connection is alive (i.e., like
* any backend, there is an one to one relationship between a connection
* and a walsender process).
*
* Normal termination is by SIGTERM, which instructs the walsender to
* close the connection and exit(0) at next convenient moment. Emergency
* termination is by SIGQUIT; like any backend, the walsender will simply
* abort and exit on SIGQUIT. A close of the connection and a FATAL error
* are treated as not a crash but approximately normal termination;
* the walsender will exit quickly without sending any more XLOG records.
*
* If the server is shut down, postmaster sends us SIGUSR2 after all
* regular backends have exited and the shutdown checkpoint has been written.
* This instruct walsender to send any outstanding WAL, including the
* shutdown checkpoint record, and then exit.
*
* Note that there can be more than one walsender process concurrently.
*
* Portions Copyright (c) 2010-2010, PostgreSQL Global Development Group
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/replication/walsender.c,v 1.2 2010/01/15 11:47:15 heikki Exp $
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include <unistd.h>
#include "access/xlog_internal.h"
#include "catalog/pg_type.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "replication/walsender.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/lock.h"
#include "storage/pmsignal.h"
#include "tcop/tcopprot.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
/* Array of WalSnds in shared memory */
WalSndCtlData *WalSndCtl = NULL;
/* My slot in the shared memory array */
static WalSnd *MyWalSnd = NULL;
/* Global state */
bool am_walsender = false; /* Am I a walsender process ? */
/* User-settable parameters for walsender */
int MaxWalSenders = 0; /* the maximum number of concurrent walsenders */
int WalSndDelay = 200; /* max sleep time between some actions */
#define NAPTIME_PER_CYCLE 100 /* max sleep time between cycles (100ms) */
/*
* These variables are used similarly to openLogFile/Id/Seg/Off,
* but for walsender to read the XLOG.
*/
static int sendFile = -1;
static uint32 sendId = 0;
static uint32 sendSeg = 0;
static uint32 sendOff = 0;
/*
* How far have we sent WAL already? This is also advertised in
* MyWalSnd->sentPtr.
*/
static XLogRecPtr sentPtr = {0, 0};
/* Flags set by signal handlers for later service in main loop */
static volatile sig_atomic_t got_SIGHUP = false;
static volatile sig_atomic_t shutdown_requested = false;
static volatile sig_atomic_t ready_to_stop = false;
/* Signal handlers */
static void WalSndSigHupHandler(SIGNAL_ARGS);
static void WalSndShutdownHandler(SIGNAL_ARGS);
static void WalSndQuickDieHandler(SIGNAL_ARGS);
/* Prototypes for private functions */
static int WalSndLoop(void);
static void InitWalSnd(void);
static void WalSndHandshake(void);
static void WalSndKill(int code, Datum arg);
static void XLogRead(char *buf, XLogRecPtr recptr, Size nbytes);
static bool XLogSend(StringInfo outMsg);
static void CheckClosedConnection(void);
/*
* How much WAL to send in one message? Must be >= XLOG_BLCKSZ.
*/
#define MAX_SEND_SIZE (XLOG_SEG_SIZE / 2)
/* Main entry point for walsender process */
int
WalSenderMain(void)
{
MemoryContext walsnd_context;
if (!superuser())
ereport(FATAL,
(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
errmsg("must be superuser to start walsender")));
/* Create a per-walsender data structure in shared memory */
InitWalSnd();
/*
* Create a memory context that we will do all our work in. We do this so
* that we can reset the context during error recovery and thereby avoid
* possible memory leaks. Formerly this code just ran in
* TopMemoryContext, but resetting that would be a really bad idea.
*
* XXX: we don't actually attempt error recovery in walsender, we just
* close the connection and exit.
*/
walsnd_context = AllocSetContextCreate(TopMemoryContext,
"Wal Sender",
ALLOCSET_DEFAULT_MINSIZE,
ALLOCSET_DEFAULT_INITSIZE,
ALLOCSET_DEFAULT_MAXSIZE);
MemoryContextSwitchTo(walsnd_context);
/* Unblock signals (they were blocked when the postmaster forked us) */
PG_SETMASK(&UnBlockSig);
/* Tell the standby that walsender is ready for receiving commands */
ReadyForQuery(DestRemote);
/* Handle handshake messages before streaming */
WalSndHandshake();
/* Main loop of walsender */
return WalSndLoop();
}
static void
WalSndHandshake(void)
{
StringInfoData input_message;
bool replication_started = false;
initStringInfo(&input_message);
while (!replication_started)
{
int firstchar;
/* Wait for a command to arrive */
firstchar = pq_getbyte();
/*
* Check for any other interesting events that happened while we
* slept.
*/
if (got_SIGHUP)
{
got_SIGHUP = false;
ProcessConfigFile(PGC_SIGHUP);
}
if (firstchar == EOF)
{
/* standby disconnected */
ereport(COMMERROR,
(errcode(ERRCODE_PROTOCOL_VIOLATION),
errmsg("unexpected EOF on standby connection")));
}
else
{
/*
* Read the message contents. This is expected to be done without
* blocking because we've been able to get message type code.
*/
if (pq_getmessage(&input_message, 0))
firstchar = EOF; /* suitable message already logged */
}
/* Handle the very limited subset of commands expected in this phase */
switch (firstchar)
{
case 'Q': /* Query message */
{
const char *query_string;
XLogRecPtr recptr;
query_string = pq_getmsgstring(&input_message);
pq_getmsgend(&input_message);
if (strcmp(query_string, "IDENTIFY_SYSTEM") == 0)
{ StringInfoData buf;
char sysid[32];
char tli[11];
/*
* Reply with a result set with one row, two columns.
* First col is system ID, and second if timeline ID
*/
snprintf(sysid, sizeof(sysid), UINT64_FORMAT,
GetSystemIdentifier());
snprintf(tli, sizeof(tli), "%u", ThisTimeLineID);
/* Send a RowDescription message */
pq_beginmessage(&buf, 'T');
pq_sendint(&buf, 2, 2); /* 2 fields */
/* first field */
pq_sendstring(&buf, "systemid"); /* col name */
pq_sendint(&buf, 0, 4); /* table oid */
pq_sendint(&buf, 0, 2); /* attnum */
pq_sendint(&buf, TEXTOID, 4); /* type oid */
pq_sendint(&buf, -1, 2); /* typlen */
pq_sendint(&buf, 0, 4); /* typmod */
pq_sendint(&buf, 0, 2); /* format code */
/* second field */
pq_sendstring(&buf, "timeline"); /* col name */
pq_sendint(&buf, 0, 4); /* table oid */
pq_sendint(&buf, 0, 2); /* attnum */
pq_sendint(&buf, INT4OID, 4); /* type oid */
pq_sendint(&buf, 4, 2); /* typlen */
pq_sendint(&buf, 0, 4); /* typmod */
pq_sendint(&buf, 0, 2); /* format code */
pq_endmessage(&buf);
/* Send a DataRow message */
pq_beginmessage(&buf, 'D');
pq_sendint(&buf, 2, 2); /* # of columns */
pq_sendint(&buf, strlen(sysid), 4); /* col1 len */
pq_sendbytes(&buf, (char *) &sysid, strlen(sysid));
pq_sendint(&buf, strlen(tli), 4); /* col2 len */
pq_sendbytes(&buf, (char *) tli, strlen(tli));
pq_endmessage(&buf);
/* Send CommandComplete and ReadyForQuery messages */
EndCommand("SELECT", DestRemote);
ReadyForQuery(DestRemote);
}
else if (sscanf(query_string, "START_REPLICATION %X/%X",
&recptr.xlogid, &recptr.xrecoff) == 2)
{
StringInfoData buf;
/* Send a CopyOutResponse message, and start streaming */
pq_beginmessage(&buf, 'H');
pq_sendbyte(&buf, 0);
pq_sendint(&buf, 0, 2);
pq_endmessage(&buf);
/*
* Initialize position to the received one, then
* the xlog records begin to be shipped from that position
*/
sentPtr = recptr;
/* break out of the loop */
replication_started = true;
}
else {
ereport(FATAL,
(errcode(ERRCODE_PROTOCOL_VIOLATION),
errmsg("invalid standby query string: %s", query_string)));
}
break;
}
/* 'X' means that the standby is closing the connection */
case 'X':
proc_exit(0);
case EOF:
ereport(ERROR,
(errcode(ERRCODE_PROTOCOL_VIOLATION),
errmsg("unexpected EOF on standby connection")));
default:
ereport(FATAL,
(errcode(ERRCODE_PROTOCOL_VIOLATION),
errmsg("invalid standby handshake message type %d", firstchar)));
}
}
}
/*
* Check if the remote end has closed the connection.
*/
static void
CheckClosedConnection(void)
{
unsigned char firstchar;
int r;
r = pq_getbyte_if_available(&firstchar);
if (r < 0)
{
/* no data available */
if (errno == EAGAIN || errno == EWOULDBLOCK)
return;
/*
* Ok if interrupted, though it shouldn't really happen with
* a non-blocking operation.
*/
if (errno == EINTR)
return;
ereport(COMMERROR,
(errcode_for_socket_access(),
errmsg("could not receive data from client: %m")));
}
if (r == 0)
{
/* standby disconnected unexpectedly */
ereport(ERROR,
(errcode(ERRCODE_PROTOCOL_VIOLATION),
errmsg("unexpected EOF on standby connection")));
}
/* Handle the very limited subset of commands expected in this phase */
switch (firstchar)
{
/*
* 'X' means that the standby is closing down the socket. EOF means
* unexpected loss of standby connection. Either way, perform normal
* shutdown.
*/
case 'X':
proc_exit(0);
default:
ereport(FATAL,
(errcode(ERRCODE_PROTOCOL_VIOLATION),
errmsg("invalid standby closing message type %d",
firstchar)));
}
}
/* Main loop of walsender process */
static int
WalSndLoop(void)
{
StringInfoData output_message;
initStringInfo(&output_message);
/* Loop forever */
for (;;)
{
int remain; /* remaining time (ms) */
/*
* Emergency bailout if postmaster has died. This is to avoid the
* necessity for manual cleanup of all postmaster children.
*/
if (!PostmasterIsAlive(true))
exit(1);
/* Process any requests or signals received recently */
if (got_SIGHUP)
{
got_SIGHUP = false;
ProcessConfigFile(PGC_SIGHUP);
}
/*
* When SIGUSR2 arrives, we send all outstanding logs up to the
* shutdown checkpoint record (i.e., the latest record) and exit.
*/
if (ready_to_stop)
{
XLogSend(&output_message);
shutdown_requested = true;
}
/* Normal exit from the walsender is here */
if (shutdown_requested)
{
/* Inform the standby that XLOG streaming was done */
pq_puttextmessage('C', "COPY 0");
pq_flush();
proc_exit(0);
}
/*
* Nap for the configured time or until a message arrives.
*
* On some platforms, signals won't interrupt the sleep. To ensure we
* respond reasonably promptly when someone signals us, break down the
* sleep into NAPTIME_PER_CYCLE (ms) increments, and check for
* interrupts after each nap.
*/
remain = WalSndDelay;
while (remain > 0)
{
if (got_SIGHUP || shutdown_requested || ready_to_stop)
break;
/* * Check to see whether a message from the standby or an interrupt
* from other processes has arrived.
*/
pg_usleep(remain > NAPTIME_PER_CYCLE ? NAPTIME_PER_CYCLE : remain);
CheckClosedConnection();
remain -= NAPTIME_PER_CYCLE;
}
/* Attempt to send the log once every loop */
if (!XLogSend(&output_message))
goto eof;
}
/* can't get here because the above loop never exits */
return 1;
eof:
/*
* Reset whereToSendOutput to prevent ereport from attempting
* to send any more messages to the standby.
*/
if (whereToSendOutput == DestRemote)
whereToSendOutput = DestNone;
proc_exit(0);
return 1; /* keep the compiler quiet */
}
/* Initialize a per-walsender data structure for this walsender process */
static void
InitWalSnd(void)
{
/* use volatile pointer to prevent code rearrangement */
int i;
/*
* WalSndCtl should be set up already (we inherit this by fork() or
* EXEC_BACKEND mechanism from the postmaster).
*/
Assert(WalSndCtl != NULL);
Assert(MyWalSnd == NULL);
/*
* Find a free walsender slot and reserve it. If this fails, we must be
* out of WalSnd structures.
*/
for (i = 0; i < MaxWalSenders; i++)
{
volatile WalSnd *walsnd = &WalSndCtl->walsnds[i];
SpinLockAcquire(&walsnd->mutex);
if (walsnd->pid != 0)
{
SpinLockRelease(&walsnd->mutex);
continue;
}
else
{
/* found */
MyWalSnd = (WalSnd *) walsnd;
walsnd->pid = MyProcPid;
MemSet(&MyWalSnd->sentPtr, 0, sizeof(XLogRecPtr));
SpinLockRelease(&walsnd->mutex);
break;
}
}
if (MyWalSnd == NULL)
ereport(FATAL, (errcode(ERRCODE_TOO_MANY_CONNECTIONS),
errmsg("sorry, too many standbys already")));
/* Arrange to clean up at walsender exit */
on_shmem_exit(WalSndKill, 0);
}
/* Destroy the per-walsender data structure for this walsender process */
static void
WalSndKill(int code, Datum arg)
{
Assert(MyWalSnd != NULL);
/*
* Mark WalSnd struct no longer in use. Assume that no lock is required
* for this.
*/
MyWalSnd->pid = 0;
/* WalSnd struct isn't mine anymore */
MyWalSnd = NULL;
}
/*
* Read 'nbytes' bytes from WAL into 'buf', starting at location 'recptr'
*/
void
XLogRead(char *buf, XLogRecPtr recptr, Size nbytes)
{
char path[MAXPGPATH];
uint32 startoff;
while (nbytes > 0)
{
int segbytes;
int readbytes;
startoff = recptr.xrecoff % XLogSegSize;
if (sendFile < 0 || !XLByteInSeg(recptr, sendId, sendSeg))
{
/* Switch to another logfile segment */
if (sendFile >= 0)
close(sendFile);
XLByteToSeg(recptr, sendId, sendSeg);
XLogFilePath(path, ThisTimeLineID, sendId, sendSeg);
sendFile = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
if (sendFile < 0)
ereport(FATAL, /* XXX: Why FATAL? */
(errcode_for_file_access(),
errmsg("could not open file \"%s\" (log file %u, segment %u): %m",
path, sendId, sendSeg)));
sendOff = 0;
}
/* Need to seek in the file? */
if (sendOff != startoff)
{
if (lseek(sendFile, (off_t) startoff, SEEK_SET) < 0)
ereport(FATAL,
(errcode_for_file_access(),
errmsg("could not seek in log file %u, segment %u to offset %u: %m",
sendId, sendSeg, startoff)));
sendOff = startoff;
}
/* How many bytes are within this segment? */
if (nbytes > (XLogSegSize - startoff)) segbytes = XLogSegSize - startoff;
else
segbytes = nbytes;
readbytes = read(sendFile, buf, segbytes);
if (readbytes <= 0)
ereport(FATAL,
(errcode_for_file_access(),
errmsg("could not read from log file %u, segment %u, offset %u, "
"length %lu: %m",
sendId, sendSeg, sendOff, (unsigned long) segbytes)));
/* Update state for read */
XLByteAdvance(recptr, readbytes);
sendOff += readbytes;
nbytes -= readbytes;
buf += readbytes;
}
}
/*
* Read all WAL that's been written (and flushed) since last cycle, and send
* it to client.
*
* Returns true if OK, false if trouble.
*/
static bool
XLogSend(StringInfo outMsg)
{
XLogRecPtr SendRqstPtr;
char activitymsg[50];
/* use volatile pointer to prevent code rearrangement */
volatile WalSnd *walsnd = MyWalSnd;
/*
* Invalid position means that we have not yet received the initial
* CopyData message from the slave that indicates where to start the
* streaming.
*/
if (sentPtr.xlogid == 0 &&
sentPtr.xrecoff == 0)
return true;
/* Attempt to send all the records which were written to the disk */
SendRqstPtr = GetWriteRecPtr();
/* Quick exit if nothing to do */
if (!XLByteLT(sentPtr, SendRqstPtr))
return true;
/*
* Since successive pages in a segment are consecutively written,
* we can gather multiple records together by issuing just one
* read() call, and send them as one CopyData message at one time;
* nmsgs is the number of CopyData messages sent in this XLogSend;
* npages is the number of pages we have determined can be read and
* sent together; startpos is the starting position of reading and
* sending in the first page, startoff is the file offset at which
* it should go and endpos is the end position of reading and
* sending in the last page. We must initialize all of them to
* keep the compiler quiet.
*/
while (XLByteLT(sentPtr, SendRqstPtr))
{
XLogRecPtr startptr;
XLogRecPtr endptr;
Size nbytes;
/*
* Figure out how much to send in one message. If there's less than
* MAX_SEND_SIZE bytes to send, send everything. Otherwise send
* MAX_SEND_SIZE bytes, but round to page boundary for efficiency.
*/
startptr = sentPtr;
endptr = startptr;
XLByteAdvance(endptr, MAX_SEND_SIZE);
/*
* Round down to page boundary. This is not only for performance
* reasons, walreceiver relies on the fact that we never split a WAL
* record across two messages. Since a long WAL record is split at
* page boundary into continuation records, page boundary is always
* safe cut-off point. We also assume that SendRqstPtr never points
* in the middle of a WAL record.
*/
endptr.xrecoff -= (endptr.xrecoff % XLOG_BLCKSZ);
if (XLByteLT(SendRqstPtr, endptr))
endptr = SendRqstPtr;
/*
* OK to read and send the log.
*
* We don't need to convert the xlogid/xrecoff from host byte order
* to network byte order because the both server can be expected to
* have the same byte order. If they have the different order, we
* don't reach here.
*/
pq_sendbytes(outMsg, (char *) &startptr, sizeof(startptr));
if (endptr.xlogid != startptr.xlogid)
{
Assert(endptr.xlogid == startptr.xlogid + 1);
nbytes = endptr.xrecoff + XLogFileSize - startptr.xrecoff;
}
else
nbytes = endptr.xrecoff - startptr.xrecoff;
sentPtr = endptr;
/*
* Read the log into the output buffer directly to prevent
* extra memcpy calls.
*/
enlargeStringInfo(outMsg, nbytes);
XLogRead(&outMsg->data[outMsg->len], startptr, nbytes);
outMsg->len += nbytes;
outMsg->data[outMsg->len] = '\0';
pq_putmessage('d', outMsg->data, outMsg->len);
resetStringInfo(outMsg);
}
/* Update shared memory status */
SpinLockAcquire(&walsnd->mutex);
walsnd->sentPtr = sentPtr;
SpinLockRelease(&walsnd->mutex);
/* Flush pending output */
if (pq_flush())
return false;
/* Report progress of XLOG streaming in PS display */
snprintf(activitymsg, sizeof(activitymsg), "streaming %X/%X",
sentPtr.xlogid, sentPtr.xrecoff);
set_ps_display(activitymsg, false);
return true;
}
/* SIGHUP: set flag to re-read config file at next convenient time */
static void
WalSndSigHupHandler(SIGNAL_ARGS)
{
got_SIGHUP = true;
}
/* SIGTERM: set flag to shut down */
static void
WalSndShutdownHandler(SIGNAL_ARGS)
{
shutdown_requested = true;
}
/*
* WalSndQuickDieHandler() occurs when signalled SIGQUIT by the postmaster.
*
* Some backend has bought the farm,
* so we need to stop what we're doing and exit.
*/
static void
WalSndQuickDieHandler(SIGNAL_ARGS)
{
PG_SETMASK(&BlockSig);
/*
* We DO NOT want to run proc_exit() callbacks -- we're here because
* shared memory may be corrupted, so we don't want to try to clean up our
* transaction. Just nail the windows shut and get out of town. Now that
* there's an atexit callback to prevent third-party code from breaking
* things by calling exit() directly, we have to reset the callbacks
* explicitly to make this work as intended.
*/
on_exit_reset();
/*
* Note we do exit(2) not exit(0). This is to force the postmaster into a
* system reset cycle if some idiot DBA sends a manual SIGQUIT to a random
* backend. This is necessary precisely because we don't clean up our
* shared memory state. (The "dead man switch" mechanism in pmsignal.c
* should ensure the postmaster sees this as a crash, too, but no harm
* in being doubly sure.)
*/
exit(2);
}
/* SIGUSR2: set flag to do a last cycle and shut down afterwards */
static void
WalSndLastCycleHandler(SIGNAL_ARGS)
{
ready_to_stop = true;
}
/* Set up signal handlers */
void
WalSndSignals(void)
{
/* Set up signal handlers */
pqsignal(SIGHUP, WalSndSigHupHandler); /* set flag to read config file */
pqsignal(SIGINT, SIG_IGN); /* not used */
pqsignal(SIGTERM, WalSndShutdownHandler); /* request shutdown */
pqsignal(SIGQUIT, WalSndQuickDieHandler); /* hard crash time */
pqsignal(SIGALRM, SIG_IGN);
pqsignal(SIGPIPE, SIG_IGN);
pqsignal(SIGUSR1, SIG_IGN); /* not used */
pqsignal(SIGUSR2, WalSndLastCycleHandler); /* request a last cycle and shutdown */
/* Reset some signals that are accepted by postmaster but not here */
pqsignal(SIGCHLD, SIG_DFL);
pqsignal(SIGTTIN, SIG_DFL);
pqsignal(SIGTTOU, SIG_DFL);
pqsignal(SIGCONT, SIG_DFL);
pqsignal(SIGWINCH, SIG_DFL);
}
/* Report shared-memory space needed by WalSndShmemInit */
Size
WalSndShmemSize(void)
{
Size size = 0;
size = offsetof(WalSndCtlData, walsnds);
size = add_size(size, mul_size(MaxWalSenders, sizeof(WalSnd)));
return size;
}
/* Allocate and initialize walsender-related shared memory */
void
WalSndShmemInit(void)
{
bool found;
int i;
WalSndCtl = (WalSndCtlData *)
ShmemInitStruct("Wal Sender Ctl", WalSndShmemSize(), &found);
if (WalSndCtl == NULL)
ereport(FATAL,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("not enough shared memory for walsender")));
if (found)
return; /* already initialized */
/* Initialize the data structures */
MemSet(WalSndCtl, 0, WalSndShmemSize());
for (i = 0; i < MaxWalSenders; i++)
{
WalSnd *walsnd = &WalSndCtl->walsnds[i];
SpinLockInit(&walsnd->mutex);
}
}
/*
* Returns the oldest Send position among walsenders. Or InvalidXLogRecPtr
* if none.
*/
XLogRecPtr
GetOldestWALSendPointer(void)
{
XLogRecPtr oldest = {0, 0};
int i;
bool found = false;
for (i = 0; i < MaxWalSenders; i++)
{
/* use volatile pointer to prevent code rearrangement */
volatile WalSnd *walsnd = &WalSndCtl->walsnds[i];
XLogRecPtr recptr;
if (walsnd->pid == 0)
continue;
SpinLockAcquire(&walsnd->mutex);
recptr = walsnd->sentPtr;
SpinLockRelease(&walsnd->mutex);
if (recptr.xlogid == 0 && recptr.xrecoff == 0)
continue;
if (!found || XLByteLT(recptr, oldest))
oldest = recptr;
found = true;
}
return oldest;
}