Create an improved FDW option validator function for contrib/dblink.

dblink now has its own validator function dblink_fdw_validator(), which is
better than the core function postgresql_fdw_validator() because it gets
the list of legal options from libpq instead of having a hard-wired list.

Make the dblink extension module provide a standard foreign data wrapper
dblink_fdw that encapsulates use of this validator, and recommend use of
that wrapper instead of making up wrappers on the fly.

Unfortunately, because ad-hoc wrappers *were* recommended practice
previously, it's not clear when we can get rid of postgresql_fdw_validator
without causing upgrade problems.  But this is a step in the right
direction.

Shigeru Hanada, reviewed by KaiGai Kohei

contrib/dblink/dblink.c

@@ -37,9 +37,12 @@
 #include "libpq-fe.h"
 
 #include "access/htup_details.h"
+#include "access/reloptions.h"
 #include "catalog/indexing.h"
 #include "catalog/namespace.h"
+#include "catalog/pg_foreign_server.h"
 #include "catalog/pg_type.h"
+#include "catalog/pg_user_mapping.h"
 #include "executor/spi.h"
 #include "foreign/foreign.h"
 #include "funcapi.h"
@@ -113,6 +116,8 @@ static char *escape_param_str(const char *from);
 static void validate_pkattnums(Relation rel,
 				   int2vector *pkattnums_arg, int32 pknumatts_arg,
 				   int **pkattnums, int *pknumatts);
+static bool is_valid_dblink_option(const PQconninfoOption *options,
+					   const char *option, Oid context);
 
 /* Global */
 static remoteConn *pconn = NULL;
@@ -1912,6 +1917,75 @@ dblink_get_notify(PG_FUNCTION_ARGS)
 	return (Datum) 0;
 }
 
+/*
+ * Validate the options given to a dblink foreign server or user mapping.
+ * Raise an error if any option is invalid.
+ *
+ * We just check the names of options here, so semantic errors in options,
+ * such as invalid numeric format, will be detected at the attempt to connect.
+ */
+PG_FUNCTION_INFO_V1(dblink_fdw_validator);
+Datum
+dblink_fdw_validator(PG_FUNCTION_ARGS)
+{
+	List	   *options_list = untransformRelOptions(PG_GETARG_DATUM(0));
+	Oid			context = PG_GETARG_OID(1);
+	ListCell   *cell;
+
+	static const PQconninfoOption *options = NULL;
+
+	/*
+	 * Get list of valid libpq options.
+	 *
+	 * To avoid unnecessary work, we get the list once and use it throughout
+	 * the lifetime of this backend process.  We don't need to care about
+	 * memory context issues, because PQconndefaults allocates with malloc.
+	 */
+	if (!options)
+	{
+		options = PQconndefaults();
+		if (!options)			/* assume reason for failure is OOM */
+			ereport(ERROR,
+					(errcode(ERRCODE_FDW_OUT_OF_MEMORY),
+					 errmsg("out of memory"),
+					 errdetail("could not get libpq's default connection options")));
+	}
+
+	/* Validate each supplied option. */
+	foreach(cell, options_list)
+	{
+		DefElem    *def = (DefElem *) lfirst(cell);
+
+		if (!is_valid_dblink_option(options, def->defname, context))
+		{
+			/*
+			 * Unknown option, or invalid option for the context specified,
+			 * so complain about it.  Provide a hint with list of valid
+			 * options for the context.
+			 */
+			StringInfoData buf;
+			const PQconninfoOption *opt;
+
+			initStringInfo(&buf);
+			for (opt = options; opt->keyword; opt++)
+			{
+				if (is_valid_dblink_option(options, opt->keyword, context))
+					appendStringInfo(&buf, "%s%s",
+									 (buf.len > 0) ? ", " : "",
+									 opt->keyword);
+			}
+			ereport(ERROR,
+					(errcode(ERRCODE_FDW_OPTION_NAME_NOT_FOUND),
+					 errmsg("invalid option \"%s\"", def->defname),
+					 errhint("Valid options in this context are: %s",
+							 buf.data)));
+		}
+	}
+
+	PG_RETURN_VOID();
+}
+
+
 /*************************************************************
  * internal functions
  */
@@ -2768,3 +2842,59 @@ validate_pkattnums(Relation rel,
 					 errmsg("invalid attribute number %d", pkattnum)));
 	}
 }
+
+/*
+ * Check if the specified connection option is valid.
+ *
+ * We basically allow whatever libpq thinks is an option, with these
+ * restrictions:
+ *		debug options: disallowed
+ *		"client_encoding": disallowed
+ *		"user": valid only in USER MAPPING options
+ *		secure options (eg password): valid only in USER MAPPING options
+ *		others: valid only in FOREIGN SERVER options
+ *
+ * We disallow client_encoding because it would be overridden anyway via
+ * PQclientEncoding; allowing it to be specified would merely promote
+ * confusion.
+ */
+static bool
+is_valid_dblink_option(const PQconninfoOption *options, const char *option,
+					   Oid context)
+{
+	const PQconninfoOption *opt;
+
+	/* Look up the option in libpq result */
+	for (opt = options; opt->keyword; opt++)
+	{
+		if (strcmp(opt->keyword, option) == 0)
+			break;
+	}
+	if (opt->keyword == NULL)
+		return false;
+
+	/* Disallow debug options (particularly "replication") */
+	if (strchr(opt->dispchar, 'D'))
+		return false;
+
+	/* Disallow "client_encoding" */
+	if (strcmp(opt->keyword, "client_encoding") == 0)
+		return false;
+
+	/*
+	 * If the option is "user" or marked secure, it should be specified only
+	 * in USER MAPPING.  Others should be specified only in SERVER.
+	 */
+	if (strcmp(opt->keyword, "user") == 0 || strchr(opt->dispchar, '*'))
+	{
+		if (context != UserMappingRelationId)
+			return false;
+	}
+	else
+	{
+		if (context != ForeignServerRelationId)
+			return false;
+	}
+
+	return true;
+}