Fix contrib/dblink to handle inconsistent DateStyle/IntervalStyle safely.

If the remote database's settings of these GUCs are different from ours,
ambiguous datetime values may be read incorrectly.  To fix, temporarily
adopt the remote server's settings while we ingest a query result.

This is not a complete fix, since it doesn't do anything about ambiguous
values in commands sent to the remote server; but there seems little we
can do about that end of it given dblink's entirely textual API for
transmitted commands.

Back-patch to 9.2.  The hazard exists in all versions, but this patch
would need more work to apply before 9.2.  Given the lack of field
complaints about this issue, it doesn't seem worth the effort at present.

Daniel Farina and Tom Lane

1 files changed, 100 insertions, 5 deletions

diff --git a/contrib/dblink/dblink.c b/contrib/dblink/dblink.c
index bba0d2adc3c..e8ad94ba841 100644
--- a/contrib/dblink/dblink.c
+++ b/contrib/dblink/dblink.c
@@ -53,6 +53,7 @@
 #include "utils/acl.h"
 #include "utils/builtins.h"
 #include "utils/fmgroids.h"
+#include "utils/guc.h"
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
@@ -86,7 +87,8 @@ typedef struct storeInfo
  */
 static Datum dblink_record_internal(FunctionCallInfo fcinfo, bool is_async);
 static void prepTuplestoreResult(FunctionCallInfo fcinfo);
-static void materializeResult(FunctionCallInfo fcinfo, PGresult *res);
+static void materializeResult(FunctionCallInfo fcinfo, PGconn *conn,
+				  PGresult *res);
 static void materializeQueryResult(FunctionCallInfo fcinfo,
 					   PGconn *conn,
 					   const char *conname,
@@ -118,6 +120,8 @@ static void validate_pkattnums(Relation rel,
 				   int **pkattnums, int *pknumatts);
 static bool is_valid_dblink_option(const PQconninfoOption *options,
 					   const char *option, Oid context);
+static int	applyRemoteGucs(PGconn *conn);
+static void restoreLocalGucs(int nestlevel);
 
 /* Global */
 static remoteConn *pconn = NULL;
@@ -605,7 +609,7 @@ dblink_fetch(PG_FUNCTION_ARGS)
 				 errmsg("cursor \"%s\" does not exist", curname)));
 	}
 
-	materializeResult(fcinfo, res);
+	materializeResult(fcinfo, conn, res);
 	return (Datum) 0;
 }
 
@@ -750,7 +754,7 @@ dblink_record_internal(FunctionCallInfo fcinfo, bool is_async)
 				}
 				else
 				{
-					materializeResult(fcinfo, res);
+					materializeResult(fcinfo, conn, res);
 				}
 			}
 		}
@@ -806,7 +810,7 @@ prepTuplestoreResult(FunctionCallInfo fcinfo)
  * The PGresult will be released in this function.
  */
 static void
-materializeResult(FunctionCallInfo fcinfo, PGresult *res)
+materializeResult(FunctionCallInfo fcinfo, PGconn *conn, PGresult *res)
 {
 	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
 
@@ -816,7 +820,7 @@ materializeResult(FunctionCallInfo fcinfo, PGresult *res)
 	PG_TRY();
 	{
 		TupleDesc	tupdesc;
-		bool		is_sql_cmd = false;
+		bool		is_sql_cmd;
 		int			ntuples;
 		int			nfields;
 
@@ -877,6 +881,7 @@ materializeResult(FunctionCallInfo fcinfo, PGresult *res)
 		if (ntuples > 0)
 		{
 			AttInMetadata *attinmeta;
+			int			nestlevel = -1;
 			Tuplestorestate *tupstore;
 			MemoryContext oldcontext;
 			int			row;
@@ -884,6 +889,10 @@ materializeResult(FunctionCallInfo fcinfo, PGresult *res)
 
 			attinmeta = TupleDescGetAttInMetadata(tupdesc);
 
+			/* Set GUCs to ensure we read GUC-sensitive data types correctly */
+			if (!is_sql_cmd)
+				nestlevel = applyRemoteGucs(conn);
+
 			oldcontext = MemoryContextSwitchTo(
 									rsinfo->econtext->ecxt_per_query_memory);
 			tupstore = tuplestore_begin_heap(true, false, work_mem);
@@ -920,6 +929,9 @@ materializeResult(FunctionCallInfo fcinfo, PGresult *res)
 				tuplestore_puttuple(tupstore, tuple);
 			}
 
+			/* clean up GUC settings, if we changed any */
+			restoreLocalGucs(nestlevel);
+
 			/* clean up and return the tuplestore */
 			tuplestore_donestoring(tupstore);
 		}
@@ -1053,6 +1065,7 @@ static PGresult *
 storeQueryResult(storeInfo *sinfo, PGconn *conn, const char *sql)
 {
 	bool		first = true;
+	int			nestlevel = -1;
 	PGresult   *res;
 
 	if (!PQsendQuery(conn, sql))
@@ -1072,6 +1085,15 @@ storeQueryResult(storeInfo *sinfo, PGconn *conn, const char *sql)
 		if (PQresultStatus(sinfo->cur_res) == PGRES_SINGLE_TUPLE)
 		{
 			/* got one row from possibly-bigger resultset */
+
+			/*
+			 * Set GUCs to ensure we read GUC-sensitive data types correctly.
+			 * We shouldn't do this until we have a row in hand, to ensure
+			 * libpq has seen any earlier ParameterStatus protocol messages.
+			 */
+			if (first && nestlevel < 0)
+				nestlevel = applyRemoteGucs(conn);
+
 			storeRow(sinfo, sinfo->cur_res, first);
 
 			PQclear(sinfo->cur_res);
@@ -1092,6 +1114,9 @@ storeQueryResult(storeInfo *sinfo, PGconn *conn, const char *sql)
 		}
 	}
 
+	/* clean up GUC settings, if we changed any */
+	restoreLocalGucs(nestlevel);
+
 	/* return last_res */
 	res = sinfo->last_res;
 	sinfo->last_res = NULL;
@@ -2898,3 +2923,73 @@ is_valid_dblink_option(const PQconninfoOption *options, const char *option,
 
 	return true;
 }
+
+/*
+ * Copy the remote session's values of GUCs that affect datatype I/O
+ * and apply them locally in a new GUC nesting level.  Returns the new
+ * nestlevel (which is needed by restoreLocalGucs to undo the settings),
+ * or -1 if no new nestlevel was needed.
+ *
+ * We use the equivalent of a function SET option to allow the settings to
+ * persist only until the caller calls restoreLocalGucs.  If an error is
+ * thrown in between, guc.c will take care of undoing the settings.
+ */
+static int
+applyRemoteGucs(PGconn *conn)
+{
+	static const char *const GUCsAffectingIO[] = {
+		"DateStyle",
+		"IntervalStyle"
+	};
+
+	int			nestlevel = -1;
+	int			i;
+
+	for (i = 0; i < lengthof(GUCsAffectingIO); i++)
+	{
+		const char *gucName = GUCsAffectingIO[i];
+		const char *remoteVal = PQparameterStatus(conn, gucName);
+		const char *localVal;
+
+		/*
+		 * If the remote server is pre-8.4, it won't have IntervalStyle, but
+		 * that's okay because its output format won't be ambiguous.  So just
+		 * skip the GUC if we don't get a value for it.  (We might eventually
+		 * need more complicated logic with remote-version checks here.)
+		 */
+		if (remoteVal == NULL)
+			continue;
+
+		/*
+		 * Avoid GUC-setting overhead if the remote and local GUCs already
+		 * have the same value.
+		 */
+		localVal = GetConfigOption(gucName, false, false);
+		Assert(localVal != NULL);
+
+		if (strcmp(remoteVal, localVal) == 0)
+			continue;
+
+		/* Create new GUC nest level if we didn't already */
+		if (nestlevel < 0)
+			nestlevel = NewGUCNestLevel();
+
+		/* Apply the option (this will throw error on failure) */
+		(void) set_config_option(gucName, remoteVal,
+								 PGC_USERSET, PGC_S_SESSION,
+								 GUC_ACTION_SAVE, true, 0);
+	}
+
+	return nestlevel;
+}
+
+/*
+ * Restore local GUCs after they have been overlaid with remote settings.
+ */
+static void
+restoreLocalGucs(int nestlevel)
+{
+	/* Do nothing if no new nestlevel was created */
+	if (nestlevel > 0)
+		AtEOXact_GUC(true, nestlevel);
+}