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Up until now, logical replication actions have been performed as the subscription owner, who will generally be a superuser. Commit cec57b1a0fbcd3833086ba686897c5883e0a2afc documented hazards associated with that situation, namely, that any user who owns a table on the subscriber side could assume the privileges of the subscription owner by attaching a trigger, expression index, or some other kind of executable code to it. As a remedy, it suggested not creating configurations where users who are not fully trusted own tables on the subscriber. Although that will work, it basically precludes using logical replication in the way that people typically want to use it, namely, to replicate a database from one node to another without necessarily having any restrictions on which database users can own tables. So, instead, change logical replication to execute INSERT, UPDATE, DELETE, and TRUNCATE operations as the table owner when they are replicated. Since this involves switching the active user frequently within a session that is authenticated as the subscription user, also impose SECURITY_RESTRICTED_OPERATION restrictions on logical replication code. As an exception, if the table owner can SET ROLE to the subscription owner, these restrictions have no security value, so don't impose them in that case. Subscription owners are now required to have the ability to SET ROLE to every role that owns a table that the subscription is replicating. If they don't, replication will fail. Superusers, who normally own subscriptions, satisfy this property by default. Non-superusers users who own subscriptions will need to be granted the roles that own relevant tables. Patch by me, reviewed (but not necessarily in its entirety) by Jelte Fennema, Jeff Davis, and Noah Misch. Discussion: http://postgr.es/m/CA+TgmoaSCkg9ww9oppPqqs+9RVqCexYCE6Aq=UsYPfnOoDeFkw@mail.gmail.com
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 a set of functions with details about each one of them provided in src/include/replication/walreceiver.h. 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 primary server, and writes and flushes it to disk (in pg_wal), it updates WalRcvData->flushedUpto and signals the startup process to know how far WAL replay can advance. Walreceiver sends information about replication progress to the primary 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 primary 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.