BUG #17948: libpq seems to misbehave in a pipelining corner case
От | PG Bug reporting form |
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Тема | BUG #17948: libpq seems to misbehave in a pipelining corner case |
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Msg-id | 17948-fcace7557e449957@postgresql.org обсуждение исходный текст |
Список | pgsql-bugs |
The following bug has been logged on the website: Bug reference: 17948 Logged by: Ivan Trofimov Email address: i.trofimow@yandex.ru PostgreSQL version: 15.3 Operating system: Ubuntu 20.04 Description: 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 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, so PQgetResult returns NULL right away). I am able to reproduce this behavior reliably via https://pastebin.com/raw/4j3v3QzC, linking against libpq5=15.3 and running the program against PostgreSQL 15.2. Who is at fault here, is it libpq or me misunderstanding/misusing libpq?
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Сообщение: BUG #17949: Adding an index introduces serialisation anomalies.