Re: libpq async duplicate error results - Mailing list pgsql-hackers

I wrote:
>>> An idea that I don't have time to pursue right now is to track
>>> how much of conn->errorMessage has been read out by PQgetResult,
>>> and only report newly-added text in later PQgetResult calls of
>>> a series, while PQerrorMessage would continue to report the
>>> whole thing.  Buffer resets would occur where they do now.

> I spent a bit more time poking at this issue.  I no longer like the
> idea of reporting only the tail part of conn->errorMessage; I'm
> afraid that would convert "edge cases sometimes return redundant text"
> into "edge cases sometimes miss returning text at all", which is not
> an improvement.

I figured out what seems like an adequately bulletproof solution to
this problem.  The thing that was scaring me before is that there are
code paths in which we'll discard a pending conn->result PGresult
(which could be an error) and make a new one; it wasn't entirely
clear that we could track which result represents what text.
The solution to that has two components:

1. Never advance the how-much-text-has-been-reported counter until
we are just about to return an error PGresult to the application.
This reduces the risk of missing or duplicated text to what seems
an acceptable level.  I found that pqPrepareAsyncResult can be used
to handle this logic -- it's only called at outer levels, and it
already has some state-updating behavior, so we'd already have bugs
if it were called improperly.

2. Don't make error PGresults at all until we reach pqPrepareAsyncResult.
This saves some cycles in the paths where we previously replaced one
error with another.  Importantly, it also means that we can now have
a bulletproof solution to the problem of how to return an error when
we get OOM on any attempt to make a PGresult.  The attached patch
includes a statically-declared PGresult that we can return in that
case.

It's not very practical to do #2 exactly, because in the case of
an error received from the server, we need to build a PGresult that
includes the broken-down error fields.  But all internally-generated
errors can be handled in this style.

This seems workable, and you'll notice it fixes the duplicated text
in the test case Andres was worried about.

Note that this patch is on top of the event-proc changes I posted
in [1].  Without that change, it's not very clear how to cope
with PGEVT_RESULTCREATE failure after we've already updated the
error status.

            regards, tom lane

[1] https://www.postgresql.org/message-id/3390587.1645035110%40sss.pgh.pa.us

diff --git a/contrib/test_decoding/expected/slot_creation_error.out
b/contrib/test_decoding/expected/slot_creation_error.out
index 321648c339..043bdae0a2 100644
--- a/contrib/test_decoding/expected/slot_creation_error.out
+++ b/contrib/test_decoding/expected/slot_creation_error.out
@@ -98,7 +98,6 @@ t

 step s2_init: <... completed>
 FATAL:  terminating connection due to administrator command
-FATAL:  terminating connection due to administrator command
 server closed the connection unexpectedly
     This probably means the server terminated abnormally
     before or while processing the request.
diff --git a/src/interfaces/libpq/fe-auth.c b/src/interfaces/libpq/fe-auth.c
index f8f4111fef..6fceff561b 100644
--- a/src/interfaces/libpq/fe-auth.c
+++ b/src/interfaces/libpq/fe-auth.c
@@ -1237,7 +1237,7 @@ PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user,
     if (!conn)
         return NULL;

-    resetPQExpBuffer(&conn->errorMessage);
+    pqClearConnErrorState(conn);

     /* If no algorithm was given, ask the server. */
     if (algorithm == NULL)
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 9c9416e8ff..2a3d68b4d1 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -3685,7 +3685,7 @@ keep_going:                        /* We will come back to here until there is
                  * (and it seems some clients expect it to be empty after a
                  * successful connection).
                  */
-                resetPQExpBuffer(&conn->errorMessage);
+                pqClearConnErrorState(conn);

                 /* We are open for business! */
                 conn->status = CONNECTION_OK;
@@ -4231,7 +4231,7 @@ closePGconn(PGconn *conn)

     /*
      * Close the connection, reset all transient state, flush I/O buffers.
-     * Note that this includes clearing conn->errorMessage; we're no longer
+     * Note that this includes clearing conn's error state; we're no longer
      * interested in any failures associated with the old connection, and we
      * want a clean slate for any new connection attempt.
      */
@@ -4241,7 +4241,7 @@ closePGconn(PGconn *conn)
     conn->xactStatus = PQTRANS_IDLE;
     conn->pipelineStatus = PQ_PIPELINE_OFF;
     pqClearAsyncResult(conn);    /* deallocate result */
-    resetPQExpBuffer(&conn->errorMessage);
+    pqClearConnErrorState(conn);
     release_conn_addrinfo(conn);

     /* Reset all state obtained from server, too */
@@ -5236,7 +5236,7 @@ ldapServiceLookup(const char *purl, PQconninfoOption *options,
  * Returns 0 on success, nonzero on failure.  On failure, if errorMessage
  * isn't null, also store an error message there.  (Note: the only reason
  * this function and related ones don't dump core on errorMessage == NULL
- * is the undocumented fact that printfPQExpBuffer does nothing when passed
+ * is the undocumented fact that appendPQExpBuffer does nothing when passed
  * a null PQExpBuffer pointer.)
  */
 static int
diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index c7c48d07dc..ceeb0a4fd9 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -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;
         }

@@ -657,9 +669,12 @@ 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 +689,7 @@ 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 +708,14 @@ 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 (res == unconstify(PGresult *, &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 +768,39 @@ 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 +827,79 @@ 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
@@ -819,6 +909,7 @@ pqPrepareAsyncResult(PGconn *conn)
      */
     conn->result = conn->next_result;
     conn->next_result = NULL;
+    conn->error_result = false; /* next_result is never an error */

     return res;
 }
@@ -1278,7 +1369,7 @@ pqAppendCmdQueueEntry(PGconn *conn, PGcmdQueueEntry *entry)
              */
             if (conn->asyncStatus == PGASYNC_IDLE)
             {
-                resetPQExpBuffer(&conn->errorMessage);
+                pqClearConnErrorState(conn);
                 pqPipelineProcessQueue(conn);
             }
             break;
@@ -1626,10 +1717,10 @@ 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 +1778,8 @@ PQsendQueryStart(PGconn *conn, bool newQuery)

         /* reset single-row processing mode */
         conn->singleRowMode = false;
-
     }
+
     /* ready to send command message */
     return true;
 }
@@ -1884,7 +1975,7 @@ 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 +2106,7 @@ 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 +2141,7 @@ 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 +2205,9 @@ 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 +2358,9 @@ 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 +2915,9 @@ 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 +2927,7 @@ 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 +3797,9 @@ 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 +3991,7 @@ PQescapeStringConn(PGconn *conn,
         return 0;
     }

-    resetPQExpBuffer(&conn->errorMessage);
+    pqClearConnErrorState(conn);

     return PQescapeStringInternal(conn, to, from, length, error,
                                   conn->client_encoding,
@@ -3939,7 +4029,7 @@ 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 +4294,7 @@ PQescapeByteaConn(PGconn *conn,
     if (!conn)
         return NULL;

-    resetPQExpBuffer(&conn->errorMessage);
+    pqClearConnErrorState(conn);

     return PQescapeByteaInternal(conn, from, from_length, to_length,
                                  conn->std_strings,
diff --git a/src/interfaces/libpq/fe-lobj.c b/src/interfaces/libpq/fe-lobj.c
index 48399a90cb..075a5ed85b 100644
--- a/src/interfaces/libpq/fe-lobj.c
+++ b/src/interfaces/libpq/fe-lobj.c
@@ -665,8 +665,8 @@ lo_import_internal(PGconn *conn, const char *filename, Oid oid)
     if (conn == NULL)
         return InvalidOid;

-    /* Since this is the beginning of a query cycle, reset the error buffer */
-    resetPQExpBuffer(&conn->errorMessage);
+    /* Since this is the beginning of a query cycle, reset the error state */
+    pqClearConnErrorState(conn);

     /*
      * open the file to be read in
@@ -730,7 +730,8 @@ lo_import_internal(PGconn *conn, const char *filename, Oid oid)
         (void) lo_close(conn, lobj);
         (void) close(fd);
         /* deliberately overwrite any error from lo_close */
-        printfPQExpBuffer(&conn->errorMessage,
+        pqClearConnErrorState(conn);
+        appendPQExpBuffer(&conn->errorMessage,
                           libpq_gettext("could not read from file \"%s\": %s\n"),
                           filename,
                           strerror_r(save_errno, sebuf, sizeof(sebuf)));
@@ -785,7 +786,8 @@ lo_export(PGconn *conn, Oid lobjId, const char *filename)

         (void) lo_close(conn, lobj);
         /* deliberately overwrite any error from lo_close */
-        printfPQExpBuffer(&conn->errorMessage,
+        pqClearConnErrorState(conn);
+        appendPQExpBuffer(&conn->errorMessage,
                           libpq_gettext("could not open file \"%s\": %s\n"),
                           filename,
                           strerror_r(save_errno, sebuf, sizeof(sebuf)));
@@ -806,7 +808,8 @@ lo_export(PGconn *conn, Oid lobjId, const char *filename)
             (void) lo_close(conn, lobj);
             (void) close(fd);
             /* deliberately overwrite any error from lo_close */
-            printfPQExpBuffer(&conn->errorMessage,
+            pqClearConnErrorState(conn);
+            appendPQExpBuffer(&conn->errorMessage,
                               libpq_gettext("could not write to file \"%s\": %s\n"),
                               filename,
                               strerror_r(save_errno, sebuf, sizeof(sebuf)));
@@ -863,8 +866,8 @@ lo_initialize(PGconn *conn)
     if (conn == NULL)
         return -1;

-    /* Since this is the beginning of a query cycle, reset the error buffer */
-    resetPQExpBuffer(&conn->errorMessage);
+    /* Since this is the beginning of a query cycle, reset the error state */
+    pqClearConnErrorState(conn);

     /* Nothing else to do if we already collected info */
     if (conn->lobjfuncs != NULL)
diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index 26dbeaed97..dc12a8b1f5 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -316,8 +316,9 @@ pqParseInput3(PGconn *conn)
                         return;
                     break;
                 case 'T':        /* Row Description */
-                    if (conn->result != NULL &&
-                        conn->result->resultStatus == PGRES_FATAL_ERROR)
+                    if (conn->error_result ||
+                        (conn->result != NULL &&
+                         conn->result->resultStatus == PGRES_FATAL_ERROR))
                     {
                         /*
                          * We've already choked for some reason.  Just discard
@@ -387,8 +388,9 @@ pqParseInput3(PGconn *conn)
                         if (getAnotherTuple(conn, msgLength))
                             return;
                     }
-                    else if (conn->result != NULL &&
-                             conn->result->resultStatus == PGRES_FATAL_ERROR)
+                    else if (conn->error_result ||
+                             (conn->result != NULL &&
+                              conn->result->resultStatus == PGRES_FATAL_ERROR))
                     {
                         /*
                          * We've already choked for some reason.  Just discard
@@ -966,10 +968,18 @@ pqGetErrorNotice3(PGconn *conn, bool isError)
      */
     if (isError)
     {
-        if (res)
-            pqSetResultError(res, &workBuf);
         pqClearAsyncResult(conn);    /* redundant, but be safe */
-        conn->result = res;
+        if (res)
+        {
+            pqSetResultError(res, &workBuf, 0);
+            conn->result = res;
+        }
+        else
+        {
+            /* Fall back to using the internal-error processing paths */
+            conn->error_result = true;
+        }
+
         if (PQExpBufferDataBroken(workBuf))
             appendPQExpBufferStr(&conn->errorMessage,
                                  libpq_gettext("out of memory\n"));
@@ -2116,10 +2126,33 @@ pqFunctionCall3(PGconn *conn, Oid fnid,
                     continue;
                 /* consume the message and exit */
                 conn->inStart += 5 + msgLength;
-                /* if we saved a result object (probably an error), use it */
-                if (conn->result)
-                    return pqPrepareAsyncResult(conn);
-                return PQmakeEmptyPGresult(conn, status);
+
+                /*
+                 * If we already have a result object (probably an error), use
+                 * that.  Otherwise, if we saw a function result message,
+                 * report COMMAND_OK.  Otherwise, the backend violated the
+                 * protocol, so complain.
+                 */
+                if (!(conn->result || conn->error_result))
+                {
+                    if (status == PGRES_COMMAND_OK)
+                    {
+                        conn->result = PQmakeEmptyPGresult(conn, status);
+                        if (!conn->result)
+                        {
+                            appendPQExpBufferStr(&conn->errorMessage,
+                                                 libpq_gettext("out of memory\n"));
+                            pqSaveErrorResult(conn);
+                        }
+                    }
+                    else
+                    {
+                        appendPQExpBuffer(&conn->errorMessage,
+                                          libpq_gettext("protocol error: no function result\n"));
+                        pqSaveErrorResult(conn);
+                    }
+                }
+                return pqPrepareAsyncResult(conn);
             case 'S':            /* parameter status */
                 if (getParameterStatus(conn))
                     continue;
diff --git a/src/interfaces/libpq/libpq-int.h b/src/interfaces/libpq/libpq-int.h
index 4290553482..e0cee4b142 100644
--- a/src/interfaces/libpq/libpq-int.h
+++ b/src/interfaces/libpq/libpq-int.h
@@ -496,8 +496,17 @@ struct pg_conn
     PGdataValue *rowBuf;        /* array for passing values to rowProcessor */
     int            rowBufLen;        /* number of entries allocated in rowBuf */

-    /* Status for asynchronous result construction */
+    /*
+     * Status for asynchronous result construction.  If result isn't NULL, it
+     * is a result being constructed or ready to return.  If result is NULL
+     * and error_result is true, then we need to return a PGRES_FATAL_ERROR
+     * result, but haven't yet constructed it; text for the error has been
+     * appended to conn->errorMessage.  (Delaying construction simplifies
+     * dealing with out-of-memory cases.)  If next_result isn't NULL, it is a
+     * PGresult that will replace "result" after we return that one.
+     */
     PGresult   *result;            /* result being constructed */
+    bool        error_result;    /* do we need to make an ERROR result? */
     PGresult   *next_result;    /* next result (used in single-row mode) */

     /* Assorted state for SASL, SSL, GSS, etc */
@@ -567,8 +576,14 @@ struct pg_conn
      * Buffer for current error message.  This is cleared at the start of any
      * connection attempt or query cycle; after that, all code should append
      * messages to it, never overwrite.
+     *
+     * In some situations we might report an error more than once in a query
+     * cycle.  If so, errorMessage accumulates text from all the errors, and
+     * errorReported tracks how much we've already reported, so that the
+     * individual error PGresult objects don't contain duplicative text.
      */
     PQExpBufferData errorMessage;    /* expansible string */
+    int            errorReported;    /* # bytes of string already reported */

     /* Buffer for receiving various parts of messages */
     PQExpBufferData workBuffer; /* expansible string */
@@ -644,7 +659,7 @@ extern pgthreadlock_t pg_g_threadlock;

 /* === in fe-exec.c === */

-extern void pqSetResultError(PGresult *res, PQExpBuffer errorMessage);
+extern void pqSetResultError(PGresult *res, PQExpBuffer errorMessage, int offset);
 extern void *pqResultAlloc(PGresult *res, size_t nBytes, bool isBinary);
 extern char *pqResultStrdup(PGresult *res, const char *str);
 extern void pqClearAsyncResult(PGconn *conn);
@@ -830,6 +845,13 @@ extern void pqTraceOutputNoTypeByteMessage(PGconn *conn, const char *message);

 /* === miscellaneous macros === */

+/*
+ * Reset the conn's error-reporting state.
+ */
+#define pqClearConnErrorState(conn) \
+    (resetPQExpBuffer(&(conn)->errorMessage), \
+     (conn)->errorReported = 0)
+
 /*
  * this is so that we can check if a connection is non-blocking internally
  * without the overhead of a function call