Rename extern function is_foreign_expr() to mysql_is_foreign_expr().

postgres_fdw has the same-named extern function.  So if we have
both the extensions installed, then we have observed some weird
behavior i.e. sometimes for MySQL server query, postgres_fdw's
is_foreign_expr() is getting called, leading to a server crash.
So rename it by appending "mysql_" to the function name.

Suraj Kharage, reviewed by Vaibhav Dalvi.

Author: jeevanchalke

deparse.c

@@ -1712,7 +1712,7 @@ foreign_expr_walker(Node *node, foreign_glob_cxt *glob_cxt,
  * Returns true if given expr is safe to evaluate on the foreign server.
  */
 bool
-is_foreign_expr(PlannerInfo *root, RelOptInfo *baserel, Expr *expr)
+mysql_is_foreign_expr(PlannerInfo *root, RelOptInfo *baserel, Expr *expr)
 {
 	foreign_glob_cxt glob_cxt;
 	foreign_loc_cxt loc_cxt;

mysql_fdw.c

@@ -776,7 +776,7 @@ mysqlGetForeignRelSize(PlannerInfo *root, RelOptInfo *baserel,
 	{
 		RestrictInfo *ri = (RestrictInfo *) lfirst(lc);
 
-		if (is_foreign_expr(root, baserel, ri->clause))
+		if (mysql_is_foreign_expr(root, baserel, ri->clause))
 			fpinfo->remote_conds = lappend(fpinfo->remote_conds, ri);
 		else
 			fpinfo->local_conds = lappend(fpinfo->local_conds, ri);
@@ -1045,7 +1045,7 @@ mysqlGetForeignPlan(PlannerInfo *root, RelOptInfo *foreignrel,
 		}
 		else if (list_member_ptr(fpinfo->local_conds, rinfo))
 			local_exprs = lappend(local_exprs, rinfo->clause);
-		else if (is_foreign_expr(root, foreignrel, rinfo->clause))
+		else if (mysql_is_foreign_expr(root, foreignrel, rinfo->clause))
 		{
 			remote_conds = lappend(remote_conds, rinfo);
 			remote_exprs = lappend(remote_exprs, rinfo->clause);

mysql_fdw.h

@@ -161,8 +161,6 @@ typedef struct MySQLColumn
 	int			atttype;		/* Attribute type */
 } MySQLColumn;
 
-extern bool is_foreign_expr(PlannerInfo *root, RelOptInfo *baserel,
-							Expr *expr);
 
 extern int ((mysql_options) (MYSQL *mysql, enum mysql_option option,
 							 const void *arg));
@@ -229,6 +227,8 @@ extern void mysql_append_where_clause(StringInfo buf, PlannerInfo *root,
 									  RelOptInfo *baserel, List *exprs,
 									  bool is_first, List **params);
 extern void mysql_deparse_analyze(StringInfo buf, char *dbname, char *relname);
+extern bool mysql_is_foreign_expr(PlannerInfo *root, RelOptInfo *baserel,
+								  Expr *expr);
 
 
 /* connection.c headers */