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125f551c8b
For reasons lost in the mists of time, most postmaster child processes
reset SIGTTIN/SIGTTOU signal handling to SIG_DFL, with the major exception
that backend sessions do not. It seems like a pretty bad idea for any
postmaster children to do that: if stderr is connected to the terminal,
and the user has put the postmaster in background, any log output would
result in the child process freezing up. Hence, switch them all to
doing what backends do, ie, nothing. This allows them to inherit the
postmaster's SIG_IGN setting. On the other hand, manually-launched
processes such as standalone backends will have default processing,
which seems fine.
In passing, also remove useless resets of SIGCONT and SIGWINCH signal
processing. Perhaps the postmaster once changed those to something
besides SIG_DFL, but it doesn't now, so these are just wasted (and
confusing) syscalls.
Basically, this propagates the changes made in commit 8e2998d8a from
backends to other postmaster children. Probably the only reason these
calls now exist elsewhere is that I missed changing pgstat.c along with
postgres.c at the time.
Given the lack of field complaints that can be traced to this, I don't
presently feel a need to back-patch.
Discussion: https://postgr.es/m/5627.1542477392@sss.pgh.pa.us
src/backend/replication/README Walreceiver - libpqwalreceiver API ---------------------------------- The transport-specific part of walreceiver, responsible for connecting to the primary server, receiving WAL files and sending messages, is loaded dynamically to avoid having to link the main server binary with libpq. The dynamically loaded module is in libpqwalreceiver subdirectory. The dynamically loaded module implements four functions: bool walrcv_connect(char *conninfo, XLogRecPtr startpoint) Establish connection to the primary, and starts streaming from 'startpoint'. Returns true on success. int walrcv_receive(char **buffer, pgsocket *wait_fd) Retrieve any message available without blocking through the connection. If a message was successfully read, returns its length. If the connection is closed, returns -1. Otherwise returns 0 to indicate that no data is available, and sets *wait_fd to a socket descriptor which can be waited on before trying again. On success, a pointer to the message payload is stored in *buffer. The returned buffer is valid until the next call to walrcv_* functions, and the caller should not attempt to free it. void walrcv_send(const char *buffer, int nbytes) Send a message to XLOG stream. void walrcv_disconnect(void); Disconnect. This API should be considered internal at the moment, but we could open it up for 3rd party replacements of libpqwalreceiver in the future, allowing pluggable methods for receiving WAL. Walreceiver IPC --------------- When the WAL replay in startup process has reached the end of archived WAL, restorable using restore_command, it starts up the walreceiver process to fetch more WAL (if streaming replication is configured). Walreceiver is a postmaster subprocess, so the startup process can't fork it directly. Instead, it sends a signal to postmaster, asking postmaster to launch it. Before that, however, startup process fills in WalRcvData->conninfo and WalRcvData->slotname, and initializes the starting point in WalRcvData->receiveStart. As walreceiver receives WAL from the master server, and writes and flushes it to disk (in pg_wal), it updates WalRcvData->receivedUpto and signals the startup process to know how far WAL replay can advance. Walreceiver sends information about replication progress to the master server whenever it either writes or flushes new WAL, or the specified interval elapses. This is used for reporting purpose. Walsender IPC ------------- At shutdown, postmaster handles walsender processes differently from regular backends. It waits for regular backends to die before writing the shutdown checkpoint and terminating pgarch and other auxiliary processes, but that's not desirable for walsenders, because we want the standby servers to receive all the WAL, including the shutdown checkpoint, before the master is shut down. Therefore postmaster treats walsenders like the pgarch process, and instructs them to terminate at PM_SHUTDOWN_2 phase, after all regular backends have died and checkpointer has issued the shutdown checkpoint. When postmaster accepts a connection, it immediately forks a new process to handle the handshake and authentication, and the process initializes to become a backend. Postmaster doesn't know if the process becomes a regular backend or a walsender process at that time - that's indicated in the connection handshake - so we need some extra signaling to let postmaster identify walsender processes. When walsender process starts up, it marks itself as a walsender process in the PMSignal array. That way postmaster can tell it apart from regular backends. Note that no big harm is done if postmaster thinks that a walsender is a regular backend; it will just terminate the walsender earlier in the shutdown phase. A walsender will look like a regular backend until it's done with the initialization and has marked itself in PMSignal array, and at process termination, after unmarking the PMSignal slot. Each walsender allocates an entry from the WalSndCtl array, and tracks information about replication progress. User can monitor them via statistics views. Walsender - walreceiver protocol -------------------------------- See manual.