[HACKERS] Getting server crash on Windows when using ICU collation

Hi All,

The backtrace observed in the core dump is,

> postgres.exe!varstr_cmp(char * arg1, int len1, char * arg2, int len2, unsigned int collid)  Line 1494 C
  postgres.exe!text_cmp(varlena * arg1, varlena * arg2, unsigned int collid)  Line 1627 C
  postgres.exe!text_gt(FunctionCallInfoData * fcinfo)  Line 1738 + 0x12 bytes C
  postgres.exe!ExecInterpExpr(ExprState * state, ExprContext * econtext, char * isnull)  Line 650 + 0xa bytes C
  postgres.exe!evaluate_expr(Expr * expr, unsigned int result_type, int result_typmod, unsigned int result_collation)  Line 4719 C
  postgres.exe!evaluate_function(unsigned int funcid, unsigned int result_type, int result_typmod, unsigned int result_collid, unsigned int input_collid, List * args, char funcvariadic, HeapTupleData * func_tuple, eval_const_expressions_context * context)  Line 4272 + 0x50 bytes C
  postgres.exe!simplify_function(unsigned int funcid, unsigned int result_type, int result_typmod, unsigned int result_collid, unsigned int input_collid, List * * args_p, char funcvariadic, char process_args, char allow_non_const, eval_const_expressions_context * context)  Line 3914 + 0x44 bytes C
  postgres.exe!eval_const_expressions_mutator(Node * node, eval_const_expressions_context * context)  Line 2627 C
  postgres.exe!expression_tree_mutator(Node * node, Node * (void)* mutator, void * context)  Line 2735 + 0x37 bytes C
  postgres.exe!expression_tree_mutator(Node * node, Node * (void)* mutator, void * context)  Line 2932 + 0x8 bytes C
  postgres.exe!eval_const_expressions(PlannerInfo * root, Node * node)  Line 2413 C
  postgres.exe!subquery_planner(PlannerGlobal * glob, Query * parse, PlannerInfo * parent_root, char hasRecursion, double tuple_fraction)  Line 623 + 0x2d bytes C

RCA:
====
As seen in the backtrace, the crash is basically happening in varstr_cmp() function. AFAICU, this crash is only happening on Windows because in varstr_cmp(), if the encoding type is UTF8, we don't even think of calling ICU functions to compare the string. Infact, we directly attempt to call the OS function wsccoll*().

The point is, if collation provider is ICU, then we should tryto call ICU functions for collation support instead of calling OS functions. This thing is being taken care inside varstr_cmp() for Linux but surprisingly it's not handled for Windows. Please have a look at below code snippet in varstr_cmp() to know on how it is being taken care for linux,

#endif   /* WIN32 */