In the perhaps-unlikely scenario where your session default settings
specify read-only, deferrable, serializable transactions, the implicit
transaction created by ProcessIncomingNotify() honored those settings
and would wait for a safe snapshot to be creatable; thereby getting
blocked by any long-running concurrent transaction. This is a bit
surprising, especially if you think you are committing a transaction
that doesn't have those properties.
There doesn't seem to be any good semantic reason for NOTIFY reporting
to wait for a safe snapshot, so create a code pathway that lets it
just get a plain snapshot, independently of transaction mode.
Given the lack of field complaints about such issues, I don't feel a
need to back-patch this.
Author: shihao zhong <zhong950419@gmail.com>
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAGRkXqQ1bmVwBw2CXiyZeWWP=XkHPXLPcQ7BSg2Q5JKSvDAnTA@mail.gmail.com
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