Re: pg_basebackup behavior on non-existent slot

On Tue, Sep 28, 2021 at 12:03 PM Dimitri Fontaine
<Dimitri(dot)Fontaine(at)microsoft(dot)com> wrote:
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> Hi,
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> When developing pg_auto_failover we found a bug where if the target replication slot given to pg_basebackup does not exist, then a full copy of the source PGDATA is completed before erroring out. You could easily end-up copying 100GB of data over the network just to see pg_basebackup remove them all at the end, and then when using the –progress option, you have to scroll up to the very start of the output to see the error message.
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> Please find attached a patch that shows a way to fix the issue. The patch is missing windows compatibility, I don’t know how to cast the WNOHANG spell on this platform. Please use the patch as you see fit, either inspiration, or maybe something you would like to commit to fix the bug.
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> Here what it looks like without the patch:
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> $ ./src/bin/pg_basebackup/pg_basebackup -p 5501 -D /tmp/bb -X stream -S SlotDoesNotExists -P
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> pg_basebackup: error: could not send replication command "START_REPLICATION": ERROR: replication slot "SlotDoesNotExists" does not exist
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> 32971/32971 kB (100%), 1/1 tablespace
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> pg_basebackup: error: child process exited with exit code 1
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> pg_basebackup: removing data directory "/tmp/bb"
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> Here’s what it looks like with the patch applied locally:
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> $ ./src/bin/pg_basebackup/pg_basebackup -p 5501 -D /tmp/bb -X stream -S SlotDoesNotExists -P
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> pg_basebackup: error: could not send replication command "START_REPLICATION": ERROR: replication slot "SlotDoesNotExists" does not exist
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> pg_basebackup: error: child process exited with exit code 1
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> pg_basebackup: removing data directory "/tmp/bb"

Isn't this solving basically the same thing as
https://commitfest.postgresql.org/34/3302/ (which does have Windows
support)? Does this implementations have some advantages over the one
Daniel posted, or should we just focus on that one since it does have
Windows support in it?

--
Magnus Hagander
Me: https://www.hagander.net/
Work: https://www.redpill-linpro.com/