As far as i understand, there is an invariant in libpq, that if PQpipelineSync call finished successfully,
PQresultStatus will eventually return PGRES_PIPELINE_SYNC, or the connection will be in CONNECTION_BAD state.
This is highlighted in several places in the docs:
1. "PGRES_PIPELINE_SYNC is reported exactly once for each PQpipelineSync at the corresponding point in the pipeline."
2. "From the client's perspective, after PQresultStatus returns PGRES_FATAL_ERROR, the pipeline is flagged as aborted.
PQresultStatus will report a PGRES_PIPELINE_ABORTED result for each remaining queued operation in an aborted pipeline.
The result for PQpipelineSync is reported as PGRES_PIPELINE_SYNC to signal the end of the aborted pipeline and
resumption of normal result processing. The client must process results with PQgetResult during error recovery."
So i expect a code like this
if (!PQpipelineSync(conn)) exit(1);
while (PQstatus(conn) != CONNECTION_BAD) {
res = PQgetResult(conn);
if (PQresultStatus(res) == PGRES_PIPELINE_SYNC) {
break;
}
}
to eventually exit the loop.
However, if instead of expected "ReadyToQuery" response server sends an error and closes the connection (
say, backend terminated by administrator or with a proxy in place and upstream PG server being dead) this loop
gets stuck in busy-loop.
My understanding is that transitions in pipelining state-machine go like this:
1. Initial PQgetResult call gets here https://github.com/postgres/postgres/blob/a817edbf6f302c376f5c0012d19a0474b6bdea88/src/interfaces/libpq/fe-exec.c#L2081
2. parseInput gets an error the server send https://github.com/postgres/postgres/blob/a817edbf6f302c376f5c0012d19a0474b6bdea88/src/interfaces/libpq/fe-protocol3.c#L216
and switches the state into PGASYNC_READY
3. PQgetResult continues and here https://github.com/postgres/postgres/blob/a817edbf6f302c376f5c0012d19a0474b6bdea88/src/interfaces/libpq/fe-exec.c#L2126
advances the queue, so the Sync entry is gone, and switches into PGASYNC_PIPELINE_IDLE later.
4. Next PQgetResult call gets here https://github.com/postgres/postgres/blob/a817edbf6f302c376f5c0012d19a0474b6bdea88/src/interfaces/libpq/fe-exec.c#LL2110C4-L2110C26,
and pqPipelineProcessQueue switches into PGASYNC_IDLE here https://github.com/postgres/postgres/blob/a817edbf6f302c376f5c0012d19a0474b6bdea88/src/interfaces/libpq/fe-exec.c#L3084
Now we are stuck in the position where libpq considers the connection being in CONNECTION_OK state
(because no reads/writes over half-closed socket have been issues),
asyncStatus being PGASYNC_IDLE, pipeline being in PQ_PIPELINE_ABORTED state,
and the expected PGRES_PIPELINE_SYNC never came and never will (because PGASYNC_IDLE).
Who is at fault here, is it postgres, libpq or me misunderstanding/misusing libpq?