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

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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
This commit is contained in:
Tom Lane
2022-02-18 15:35:15 -05:00
parent 6c417bbcc8
commit 618c16707a
7 changed files with 224 additions and 65 deletions
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180
src/interfaces/libpq/fe-exec.c
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@ -44,6 +44,13 @@ char *const pgresStatus[] = {
"PGRES_PIPELINE_ABORTED"
};
/* We return this if we're unable to make a PGresult at all */
static const PGresult OOM_result = {
.resultStatus = PGRES_FATAL_ERROR,
.client_encoding = PG_SQL_ASCII,
.errMsg = "out of memory\n",
};
/*
* static state needed by PQescapeString and PQescapeBytea; initialize to
* values that result in backward-compatible behavior
@ -141,6 +148,10 @@ static int pqPipelineFlush(PGconn *conn);
* returns a newly allocated, initialized PGresult with given status.
* If conn is not NULL and status indicates an error, the conn's
* errorMessage is copied. Also, any PGEvents are copied from the conn.
*
* Note: the logic to copy the conn's errorMessage is now vestigial;
* no internal caller uses it. However, that behavior is documented for
* outside callers, so we'd better keep it.
*/
PGresult *
PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status)
@ -191,7 +202,8 @@ PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status)
/* non-error cases */
break;
default:
pqSetResultError(result, &conn->errorMessage);
/* we intentionally do not use or modify errorReported here */
pqSetResultError(result, &conn->errorMessage, 0);
break;
}
@ -235,8 +247,12 @@ PQsetResultAttrs(PGresult *res, int numAttributes, PGresAttDesc *attDescs)
{
int i;
/* Fail if argument is NULL or OOM_result */
if (!res || (const PGresult *) res == &OOM_result)
return false;
/* If attrs already exist, they cannot be overwritten. */
if (!res || res->numAttributes > 0)
if (res->numAttributes > 0)
return false;
/* ignore no-op request */
@ -435,7 +451,11 @@ PQsetvalue(PGresult *res, int tup_num, int field_num, char *value, int len)
PGresAttValue *attval;
const char *errmsg = NULL;
/* Note that this check also protects us against null "res" */
/* Fail if argument is NULL or OOM_result */
if (!res || (const PGresult *) res == &OOM_result)
return false;
/* Invalid field_num? */
if (!check_field_number(res, field_num))
return false;
@ -519,6 +539,10 @@ fail:
void *
PQresultAlloc(PGresult *res, size_t nBytes)
{
/* Fail if argument is NULL or OOM_result */
if (!res || (const PGresult *) res == &OOM_result)
return NULL;
return pqResultAlloc(res, nBytes, true);
}
@ -657,9 +681,12 @@ pqResultStrdup(PGresult *res, const char *str)
/*
* pqSetResultError -
* assign a new error message to a PGresult
*
* Copy text from errorMessage buffer beginning at given offset
* (it's caller's responsibility that offset is valid)
*/
void
pqSetResultError(PGresult *res, PQExpBuffer errorMessage)
pqSetResultError(PGresult *res, PQExpBuffer errorMessage, int offset)
{
char *msg;
@ -674,7 +701,7 @@ pqSetResultError(PGresult *res, PQExpBuffer errorMessage)
* at a constant "out of memory" string.
*/
if (!PQExpBufferBroken(errorMessage))
msg = pqResultStrdup(res, errorMessage->data);
msg = pqResultStrdup(res, errorMessage->data + offset);
else
msg = NULL;
if (msg)
@ -693,9 +720,14 @@ PQclear(PGresult *res)
PGresult_data *block;
int i;
/* As a convenience, do nothing for a NULL pointer */
if (!res)
return;
/* Also, do nothing if the argument is OOM_result */
if ((const PGresult *) res == &OOM_result)
return;
/* Close down any events we may have */
for (i = 0; i < res->nEvents; i++)
{
/* only send DESTROY to successfully-initialized event procs */
@ -748,24 +780,39 @@ pqClearAsyncResult(PGconn *conn)
if (conn->result)
PQclear(conn->result);
conn->result = NULL;
conn->error_result = false;
if (conn->next_result)
PQclear(conn->next_result);
conn->next_result = NULL;
}
/*
* This subroutine deletes any existing async result, sets conn->result
* to a PGresult with status PGRES_FATAL_ERROR, and stores the current
* contents of conn->errorMessage into that result.
* pqSaveErrorResult -
* remember that we have an error condition
*
* In much of libpq, reporting an error just requires appending text to
* conn->errorMessage and returning a failure code to one's caller.
* Where returning a failure code is impractical, instead call this
* function to remember that an error needs to be reported.
*
* (It might seem that appending text to conn->errorMessage should be
* sufficient, but we can't rely on that working under out-of-memory
* conditions. The OOM hazard is also why we don't try to make a new
* PGresult right here.)
*/
void
pqSaveErrorResult(PGconn *conn)
{
/* Drop any pending result ... */
pqClearAsyncResult(conn);
conn->result = PQmakeEmptyPGresult(conn, PGRES_FATAL_ERROR);
/* ... and set flag to remember to make an error result later */
conn->error_result = true;
}
/*
* pqSaveWriteError -
* report a write failure
*
* As above, after appending conn->write_err_msg to whatever other error we
* have. This is used when we've detected a write failure and have exhausted
* our chances of reporting something else instead.
@ -792,24 +839,79 @@ pqSaveWriteError(PGconn *conn)
}
/*
* This subroutine prepares an async result object for return to the caller.
* pqPrepareAsyncResult -
* prepare the current async result object for return to the caller
*
* If there is not already an async result object, build an error object
* using whatever is in conn->errorMessage. In any case, clear the async
* result storage.
* result storage, and update our notion of how much error text has been
* returned to the application.
*/
PGresult *
pqPrepareAsyncResult(PGconn *conn)
{
PGresult *res;
/*
* conn->result is the PGresult to return. If it is NULL (which probably
* shouldn't happen) we assume there is an appropriate error message in
* conn->errorMessage.
*/
res = conn->result;
if (!res)
res = PQmakeEmptyPGresult(conn, PGRES_FATAL_ERROR);
if (res)
{
/*
* If the pre-existing result is an ERROR (presumably something
* received from the server), assume that it represents whatever is in
* conn->errorMessage, and advance errorReported.
*/
if (res->resultStatus == PGRES_FATAL_ERROR)
conn->errorReported = conn->errorMessage.len;
}
else
{
/*
* We get here after internal-to-libpq errors. We should probably
* always have error_result = true, but if we don't, gin up some error
* text.
*/
if (!conn->error_result)
appendPQExpBufferStr(&conn->errorMessage,
libpq_gettext("no error text available\n"));
/* Paranoia: be sure errorReported offset is sane */
if (conn->errorReported < 0 ||
conn->errorReported >= conn->errorMessage.len)
conn->errorReported = 0;
/*
* Make a PGresult struct for the error. We temporarily lie about the
* result status, so that PQmakeEmptyPGresult doesn't uselessly copy
* all of conn->errorMessage.
*/
res = PQmakeEmptyPGresult(conn, PGRES_EMPTY_QUERY);
if (res)
{
/*
* Report whatever new error text we have, and advance
* errorReported.
*/
res->resultStatus = PGRES_FATAL_ERROR;
pqSetResultError(res, &conn->errorMessage, conn->errorReported);
conn->errorReported = conn->errorMessage.len;
}
else
{
/*
* Ouch, not enough memory for a PGresult. Fortunately, we have a
* card up our sleeve: we can use the static OOM_result. Casting
* away const here is a bit ugly, but it seems best to declare
* OOM_result as const, in hopes it will be allocated in read-only
* storage.
*/
res = unconstify(PGresult *, &OOM_result);
/*
* Don't advance errorReported. Perhaps we'll be able to report
* the text later.
*/
}
}
/*
* Replace conn->result with next_result, if any. In the normal case
@ -818,6 +920,7 @@ pqPrepareAsyncResult(PGconn *conn)
* it was before we created the current single-row result.
*/
conn->result = conn->next_result;
conn->error_result = false; /* next_result is never an error */
conn->next_result = NULL;
return res;
@ -1278,7 +1381,7 @@ pqAppendCmdQueueEntry(PGconn *conn, PGcmdQueueEntry *entry)
*/
if (conn->asyncStatus == PGASYNC_IDLE)
{
resetPQExpBuffer(&conn->errorMessage);
pqClearConnErrorState(conn);
pqPipelineProcessQueue(conn);
}
break;
@ -1626,10 +1729,10 @@ PQsendQueryStart(PGconn *conn, bool newQuery)
return false;
/*
* If this is the beginning of a query cycle, reset the error buffer.
* If this is the beginning of a query cycle, reset the error state.
*/
if (newQuery)
resetPQExpBuffer(&conn->errorMessage);
pqClearConnErrorState(conn);
/* Don't try to send if we know there's no live connection. */
if (conn->status != CONNECTION_OK)
@ -1687,8 +1790,8 @@ PQsendQueryStart(PGconn *conn, bool newQuery)
/* reset single-row processing mode */
conn->singleRowMode = false;
}
/* ready to send command message */
return true;
}
@ -1884,7 +1987,7 @@ PQsetSingleRowMode(PGconn *conn)
(conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE &&
conn->cmd_queue_head->queryclass != PGQUERY_EXTENDED))
return 0;
if (conn->result)
if (conn->result || conn->error_result)
return 0;
/* OK, set flag */
@ -2015,10 +2118,7 @@ PQgetResult(PGconn *conn)
pqWait(true, false, conn) ||
pqReadData(conn) < 0)
{
/*
* conn->errorMessage has been set by pqWait or pqReadData. We
* want to append it to any already-received error message.
*/
/* Report the error saved by pqWait or pqReadData */
pqSaveErrorResult(conn);
conn->asyncStatus = PGASYNC_IDLE;
return pqPrepareAsyncResult(conn);
@ -2053,7 +2153,7 @@ PQgetResult(PGconn *conn)
* is the start of the results of the next query, clear any
* prior error message.
*/
resetPQExpBuffer(&conn->errorMessage);
pqClearConnErrorState(conn);
pqPipelineProcessQueue(conn);
}
break;
@ -2117,7 +2217,9 @@ PQgetResult(PGconn *conn)
appendPQExpBuffer(&conn->errorMessage,
libpq_gettext("unexpected asyncStatus: %d\n"),
(int) conn->asyncStatus);
res = PQmakeEmptyPGresult(conn, PGRES_FATAL_ERROR);
pqSaveErrorResult(conn);
conn->asyncStatus = PGASYNC_IDLE; /* try to restore valid state */
res = pqPrepareAsyncResult(conn);
break;
}
@ -2268,9 +2370,9 @@ PQexecStart(PGconn *conn)
}
/*
* Since this is the beginning of a query cycle, reset the error buffer.
* Since this is the beginning of a query cycle, reset the error state.
*/
resetPQExpBuffer(&conn->errorMessage);
pqClearConnErrorState(conn);
/*
* Silently discard any prior query result that application didn't eat.
@ -2825,9 +2927,9 @@ PQfn(PGconn *conn,
return NULL;
/*
* Since this is the beginning of a query cycle, reset the error buffer.
* Since this is the beginning of a query cycle, reset the error state.
*/
resetPQExpBuffer(&conn->errorMessage);
pqClearConnErrorState(conn);
if (conn->pipelineStatus != PQ_PIPELINE_OFF)
{
@ -2837,7 +2939,7 @@ PQfn(PGconn *conn,
}
if (conn->sock == PGINVALID_SOCKET || conn->asyncStatus != PGASYNC_IDLE ||
conn->result != NULL)
conn->result || conn->error_result)
{
appendPQExpBufferStr(&conn->errorMessage,
libpq_gettext("connection in wrong state\n"));
@ -3707,9 +3809,9 @@ PQsetnonblocking(PGconn *conn, int arg)
* behavior. this is ok because either they are making a transition _from_
* or _to_ blocking mode, either way we can block them.
*
* Clear errorMessage in case pqFlush adds to it.
* Clear error state in case pqFlush adds to it.
*/
resetPQExpBuffer(&conn->errorMessage);
pqClearConnErrorState(conn);
/* if we are going from blocking to non-blocking flush here */
if (pqFlush(conn))
@ -3901,7 +4003,7 @@ PQescapeStringConn(PGconn *conn,
return 0;
}
resetPQExpBuffer(&conn->errorMessage);
pqClearConnErrorState(conn);
return PQescapeStringInternal(conn, to, from, length, error,
conn->client_encoding,
@ -3939,7 +4041,7 @@ PQescapeInternal(PGconn *conn, const char *str, size_t len, bool as_ident)
if (!conn)
return NULL;
resetPQExpBuffer(&conn->errorMessage);
pqClearConnErrorState(conn);
/* Scan the string for characters that must be escaped. */
for (s = str; (s - str) < len && *s != '\0'; ++s)
@ -4204,7 +4306,7 @@ PQescapeByteaConn(PGconn *conn,
if (!conn)
return NULL;
resetPQExpBuffer(&conn->errorMessage);
pqClearConnErrorState(conn);
return PQescapeByteaInternal(conn, from, from_length, to_length,
conn->std_strings,