pgsql: Rearrange libpq's error reporting to avoid duplicated error text

Rearrange libpq's error reporting to avoid duplicated error text.

Since commit ffa2e4670, libpq accumulates text in conn->errorMessage
across a whole query cycle. In some situations, we may report more
than one error event within a cycle: the easiest case to reach is
where we report a FATAL error message from the server, and then a
bit later we detect loss of connection. Since, historically, each
error PGresult bears the entire content of conn->errorMessage,
this results in duplication of the FATAL message in any output that
concatenates the contents of the PGresults.

Accumulation in errorMessage still seems like a good idea, especially
in view of the number of places that did ad-hoc error concatenation
before ffa2e4670. So to fix this, let's track how much of
conn->errorMessage has been read out into error PGresults, and only
include new text in later PGresults. The tricky part of that is
to be sure that we never discard an error PGresult once made (else
we'd risk dropping some text, a problem much worse than duplication).
While libpq formerly did that in some code paths, a little bit of
rearrangement lets us postpone making an error PGresult at all until
we are about to return it.

A side benefit of that postponement is that it now becomes practical
to return a dummy static PGresult in cases where we hit out-of-memory
while trying to manufacture an error PGresult. This eliminates the
admittedly-very-rare case where we'd return NULL from PQgetResult,
indicating successful query completion, even though what actually
happened was an OOM failure.

Discussion: https://postgr.es/m/ab4288f8-be5c-57fb-2400-e3e857f53e46@enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/618c16707a6d6e8f5c83ede2092975e4670201ad

Modified Files
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.../test_decoding/expected/slot_creation_error.out | 1 -
src/interfaces/libpq/fe-auth.c | 2 +-
src/interfaces/libpq/fe-connect.c | 8 +-
src/interfaces/libpq/fe-exec.c | 180 ++++++++++++++++-----
src/interfaces/libpq/fe-lobj.c | 17 +-
src/interfaces/libpq/fe-protocol3.c | 55 +++++--
src/interfaces/libpq/libpq-int.h | 26 ++-
7 files changed, 224 insertions(+), 65 deletions(-)