Flush chttpd_db monitor refs on demonitor #4906
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nickva
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rnewson
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This ensures that the coordinator process flushes on demonitoring of the attachment refs in chttpd_db. The problem here is that it's possible to receive a 'DOWN' message for the monitor ref that is not receive'ed, causing it to stick around in the coordinator message queue while the next http request is handled. The pending message will not become apparent until the next fabric call is invoked, as fabric expects to have full access to all messages in the calling process, an expectation which is violated by the pending message and causes a case clause crash in the fabric receive message callbacks. I noticed this during eunit runs with stubbed attachment handles that generate an immediate noproc message on the monitor call. Normal operations should not result in an immediate noproc result on monitoring the attachment process, however, any failure that causes the attachment process to fail between acquisition of the refs and the demonitor calls will induce this bug, causing the next http request handled by the particular chttpd coordinator pool processs to fail on whatever next fabric call is made.
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chewbranca
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We need to potentially extract the usage delta from the incoming RPC
message. Rather than pattern match on all possible message formats that
could potentially include usage deltas, we instead utilize
rexi_util:extract_delta which matches against tuples ending in
`{delta, Delta}`, and separates that out from the underlying message.
The subtlety here is that receiving the message to extract the delta
changes the behavior as this was previously doing a selective receive
keyed off of the Ref, and then ignoring any other messages that arrived.
I don't know if the selective receive was intended, but I don't think
it's appropriate to leave unexpected messages floating around,
especially given things like issue #4909.
Instead of utilizing a selective receive, this switches to extracting
the message and delta like we need to do, and then in the event it finds
unexpected messages they're logged and skipped.
This selective receive was masking the lack of unlink on the linked
rexi_mon pid in fix #4906. I've also noticed some rpc responses arriving
late as well, but I haven't tracked that down, so let's log when it does
happen.
chewbranca
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Feb 12, 2024
We need to potentially extract the usage delta from the incoming RPC
message. Rather than pattern match on all possible message formats that
could potentially include usage deltas, we instead utilize
rexi_util:extract_delta which matches against tuples ending in
`{delta, Delta}`, and separates that out from the underlying message.
The subtlety here is that receiving the message to extract the delta
changes the behavior as this was previously doing a selective receive
keyed off of the Ref, and then ignoring any other messages that arrived.
I don't know if the selective receive was intended, but I don't think
it's appropriate to leave unexpected messages floating around,
especially given things like issue #4909.
Instead of utilizing a selective receive, this switches to extracting
the message and delta like we need to do, and then in the event it finds
unexpected messages they're logged and skipped.
This selective receive was masking the lack of unlink on the linked
rexi_mon pid in fix #4906. I've also noticed some rpc responses arriving
late as well, but I haven't tracked that down, so let's log when it does
happen.
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This ensures that the coordinator process flushes on demonitoring of the attachment refs in chttpd_db. The problem here is that it's possible to receive a 'DOWN' message for the monitor ref that is not receive'ed, causing it to stick around in the coordinator message queue while the next http request is handled. The pending message will not become apparent until the next fabric call is invoked, as fabric expects to have full access to all messages in the calling process, an expectation which is violated by the pending message, causing a case clause crash in the fabric receive message callbacks.
I noticed this during eunit runs with stubbed attachment handles that generate an immediate noproc message on the monitor call. Normal operations should not result in an immediate noproc result on monitoring the attachment process, however, any failure that causes the attachment process to fail between acquisition of the refs and the demonitor calls will induce this bug, causing the next http request handled by the particular chttpd coordinator pool processs to fail on whatever next fabric call is made.