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

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.

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)

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

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;
 		}
 
@@ -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
+ * pqSaveErrorResult -
- * to a PGresult with status PGRES_FATAL_ERROR, and stores the current
+ *	  remember that we have an error condition
- * contents of conn->errorMessage into that result.
+ *
+ * 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)
+	if (res)
-		res = PQmakeEmptyPGresult(conn, PGRES_FATAL_ERROR);
+	{
+		/*
+		 * 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)
 		{
-			/*
+			/* Report the error saved by pqWait or pqReadData */
-			 * conn->errorMessage has been set by pqWait or pqReadData. We
-			 * want to append it to any already-received error message.
-			 */
 			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,

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 */
+	/* Since this is the beginning of a query cycle, reset the error state */
-	resetPQExpBuffer(&conn->errorMessage);
+	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 */
+	/* Since this is the beginning of a query cycle, reset the error state */
-	resetPQExpBuffer(&conn->errorMessage);
+	pqClearConnErrorState(conn);
 
 	/* Nothing else to do if we already collected info */
 	if (conn->lobjfuncs != NULL)

src/interfaces/libpq/fe-protocol3.c

@@ -316,8 +316,9 @@ pqParseInput3(PGconn *conn)
 						return;
 					break;
 				case 'T':		/* Row Description */
-					if (conn->result != NULL &&
+					if (conn->error_result ||
-						conn->result->resultStatus == PGRES_FATAL_ERROR)
+						(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 &&
+					else if (conn->error_result ||
-							 conn->result->resultStatus == PGRES_FATAL_ERROR)
+							 (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);
+				 * If we already have a result object (probably an error), use
-				return PQmakeEmptyPGresult(conn, status);
+				 * 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
+					{
+						appendPQExpBufferStr(&conn->errorMessage,
+											 libpq_gettext("protocol error: no function result\n"));
+						pqSaveErrorResult(conn);
+					}
+				}
+				return pqPrepareAsyncResult(conn);
 			case 'S':			/* parameter status */
 				if (getParameterStatus(conn))
 					continue;

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