Rearrange pgstat_bestart() to avoid failures within its critical section.
We long ago decided to design the shared PgBackendStatus data structure to minimize the cost of writing status updates, which means that writers just have to increment the st_changecount field twice. That isn't hooked into any sort of resource management mechanism, which means that if something were to throw error between the two increments, the st_changecount field would be left odd indefinitely. That would cause readers to lock up. Now, since it's also a bad idea to leave the field odd for longer than absolutely necessary (because readers will spin while we have it set), the expectation was that we'd treat these segments like spinlock critical sections, with only short, more or less straight-line, code in them. That was fine as originally designed, but commit 9029f4b3 broke it by inserting a significant amount of non-straight-line code into pgstat_bestart(), code that is very capable of throwing errors, not to mention taking a significant amount of time during which readers will spin. We have a report from Neeraj Kumar of readers actually locking up, which I suspect was due to an encoding conversion error in X509_NAME_to_cstring, though conceivably it was just a garden-variety OOM failure. Subsequent commits have loaded even more dubious code into pgstat_bestart's critical section (and commit fc70a4b0 deserves some kind of booby prize for managing to miss the critical section entirely, although the negative consequences seem minimal given that the PgBackendStatus entry should be seen by readers as inactive at that point). The right way to fix this mess seems to be to compute all these values into a local copy of the process' PgBackendStatus struct, and then just copy the data back within the critical section proper. This plan can't be implemented completely cleanly because of the struct's heavy reliance on out-of-line strings, which we must initialize separately within the critical section. But still, the critical section is far smaller and safer than it was before. In hopes of forestalling future errors of the same ilk, rename the macros for st_changecount management to make it more apparent that the writer-side macros create a critical section. And to prevent the worst consequences if we nonetheless manage to mess it up anyway, adjust those macros so that they really are a critical section, ie they now bump CritSectionCount. That doesn't add much overhead, and it guarantees that if we do somehow throw an error while the counter is odd, it will lead to PANIC and a database restart to reset shared memory. Back-patch to 9.5 where the problem was introduced. In HEAD, also fix an oversight in commit b0b39f72: it failed to teach pgstat_read_current_status to copy st_gssstatus data from shared memory to local memory. Hence, subsequent use of that data within the transaction would potentially see changing data that it shouldn't see. Discussion: https://postgr.es/m/CAPR3Wj5Z17=+eeyrn_ZDG3NQGYgMEOY6JV6Y-WRRhGgwc16U3Q@mail.gmail.com
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