6fea65508a
ComputeXidHorizons (nee GetOldestXmin) thought that it could identify walsenders by checking for proc->databaseId == 0. Perhaps that was safe when the code was written, but it's been wrong at least since autovacuum was invented. Background processes that aren't connected to any particular database, such as the autovacuum launcher and logical replication launcher, look like that too. This imprecision is harmful because when such a process advertises an xmin, the result is to hold back dead-tuple cleanup in all databases, though it'd be sufficient to hold it back in shared catalogs (which are the only relations such a process can access). Aside from being generally inefficient, this has recently been seen to cause regression test failures in the buildfarm, as a consequence of the logical replication launcher's startup transaction preventing VACUUM from marking pages of a user table as all-visible. We only want that global hold-back effect for the case where a walsender is advertising a hot standby feedback xmin. Therefore, invent a new PGPROC flag that says that a process' xmin should be considered globally, and check that instead of using the incorrect databaseId == 0 test. Currently only a walsender sets that flag, and only if it is not connected to any particular database. (This is for bug-compatibility with the undocumented behavior of the existing code, namely that feedback sent by a client who has connected to a particular database would not be applied globally. I'm not sure this is a great definition; however, such a client is capable of issuing plain SQL commands, and I don't think we want xmins advertised for such commands to be applied globally. Perhaps this could do with refinement later.) While at it, I rewrote the comment in ComputeXidHorizons, and re-ordered the commented-upon if-tests, to make them match up for intelligibility's sake. This is arguably a back-patchable bug fix, but given the lack of complaints I think it prudent to let it age awhile in HEAD first. Discussion: https://postgr.es/m/1346227.1649887693@sss.pgh.pa.us
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