BUG #14516: misleading error from libpq on out-of-memory

The following bug has been logged on the website:

Bug reference: 14516
Logged by: Andrew Gierth
Email address: andrew(at)tao11(dot)riddles(dot)org(dot)uk
PostgreSQL version: 9.4.8
Operating system: any
Description:

This came up on IRC:

Running psql with a restricted memory ulimit to reproduce, one gets this
error message:

postgres=# \copy (select repeat('a',120000000)) to '/dev/null'
lost synchronization with server: got message type "d", length 120000001

Googling this finds some old bugs, but in the reported case, none of those
applied and the only problem was an actual lack of memory; the database
contained a large (60MB) bytea value, and pg_dump would fail with the "lost
synchronization" error.

So the failure is expected, but the fact that the error message doesn't even
hint at "out of memory" being the cause is a problem.