[nullsafe][refactor] Restructure code around node typechecking

Summary:
`TypeCheck.typecheck_node` is one of central parts in nullsafe
typechecking.

Lets make the code clearer:
1. Clear contract of the method - return a named record instead of a
tuple of lists.
2. Comments.
3. Making code functional, use List.fold and aggregate all needed data
there instead of storing information in references
4. Helper functions.

Reviewed By: skcho

Differential Revision: D20246273

fbshipit-source-id: 75f56497e

infer/src/nullsafe/eradicate.ml

@@ -95,11 +95,14 @@ module MkCallback (Extension : ExtensionT) : CallBackT = struct
         NodePrinter.with_session ~pp_name node ~f:(fun () ->
             State.set_node node ;
             if Config.write_html then L.d_printfln "before:@\n%a@\n" TypeState.pp typestate ;
-            let typestates_succ, typestates_exn =
+            let TypeCheck.{normal_flow_typestate; exception_flow_typestates} =
               TypeCheck.typecheck_node tenv calls_this checks idenv curr_pname curr_pdesc
                 find_canonical_duplicate annotated_signature typestate node linereader
             in
-            (typestates_succ, typestates_exn) )
+            let normal_flow_typestates =
+              Option.value_map normal_flow_typestate ~f:(fun a -> [a]) ~default:[]
+            in
+            (normal_flow_typestates, exception_flow_typestates) )
 
 
       let proc_throws _ = DontKnow

infer/src/nullsafe/typeCheck.ml

@@ -10,7 +10,8 @@ module F = Format
 module L = Logging
 module DExp = DecompiledExp
 
-(** Module for type checking. *)
+type typecheck_result =
+  {normal_flow_typestate: TypeState.t option; exception_flow_typestates: TypeState.t list}
 
 (** Module to treat selected complex expressions as constants. *)
 module ComplexExpressions = struct
@@ -1176,61 +1177,79 @@ let typecheck_instr tenv calls_this checks (node : Procdesc.Node.t) idenv curr_p
         ~nullsafe_mode ~node:node' ~original_node:node normalized_cond
 
 
-(** Typecheck the instructions in a cfg node. *)
+let can_instrunction_throw tenv node instr =
-let typecheck_node tenv calls_this checks idenv curr_pname curr_pdesc find_canonical_duplicate
-    annotated_signature typestate node linereader =
-  let instrs = Procdesc.Node.get_instrs node in
-  let instr_ref_gen = TypeErr.InstrRef.create_generator node in
-  let typestates_exn = ref [] in
-  let noreturn = ref false in
-  let handle_exceptions typestate instr =
   match instr with
-    | Sil.Call (_, Exp.Const (Const.Cfun callee_pname), _, _, _)
+  | Sil.Call (_, Exp.Const (Const.Cfun callee_pname), _, _, _) -> (
-      when Models.is_noreturn callee_pname ->
-        noreturn := true
-    | Sil.Call (_, Exp.Const (Const.Cfun callee_pname), _, _, _) ->
       let callee_attributes_opt = PatternMatch.lookup_attributes tenv callee_pname in
-        (* check if the call might throw an exception *)
+      (* We assume if the function is not annotated with throws(), it can not throw an exception.
-        let has_exceptions =
+         This is unsound.
+         TODO(T63305137) nullsafe should assume all methods can throw.
+      *)
       match callee_attributes_opt with
       | Some callee_attributes ->
           not (List.is_empty callee_attributes.ProcAttributes.exceptions)
       | None ->
-              false
+          false )
-        in
-        if has_exceptions then typestates_exn := typestate :: !typestates_exn
   | Sil.Store {e1= Exp.Lvar pv}
     when Pvar.is_return pv
          && Procdesc.Node.equal_nodekind (Procdesc.Node.get_kind node) Procdesc.Node.throw_kind ->
-        (* throw instruction *)
+      (* Explicit throw instruction *)
-        typestates_exn := typestate :: !typestates_exn
+      true
   | _ ->
-        ()
+      false
-  in
+
+
+(* true if after this instruction the program interrupts *)
+let is_noreturn_instruction = function
+  | Sil.Call (_, Exp.Const (Const.Cfun callee_pname), _, _, _) when Models.is_noreturn callee_pname
+    ->
+      true
+  | _ ->
+      false
+
+
+(** Typecheck the instructions in a cfg node. *)
+let typecheck_node tenv calls_this checks idenv curr_pname curr_pdesc find_canonical_duplicate
+    annotated_signature typestate node linereader =
+  if Procdesc.Node.equal_nodekind (Procdesc.Node.get_kind node) Procdesc.Node.exn_sink_kind then
+    {normal_flow_typestate= None; exception_flow_typestates= []}
+  else
+    let instrs = Procdesc.Node.get_instrs node in
+    let instr_ref_gen = TypeErr.InstrRef.create_generator node in
     let canonical_node = find_canonical_duplicate node in
-  let do_instruction typestate instr =
+    (* typecheck the instruction and accumulate result *)
+    let fold_instruction
+        ( { normal_flow_typestate= normal_typestate_prev_opt
+          ; exception_flow_typestates= exception_flow_typestates_prev } as prev_result ) instr =
+      match normal_typestate_prev_opt with
+      | None ->
+          (* no input typestate - abort typechecking and propagate the current result *)
+          prev_result
+      | Some normal_flow_typestate_prev ->
           let instr_ref =
             (* keep unique instruction reference per-node *)
             TypeErr.InstrRef.gen instr_ref_gen
           in
-    let post =
+          let normal_flow_typestate =
             typecheck_instr tenv calls_this checks node idenv curr_pname curr_pdesc
-        find_canonical_duplicate annotated_signature instr_ref linereader typestate instr
+              find_canonical_duplicate annotated_signature instr_ref linereader
+              normal_flow_typestate_prev instr
+          in
+          if is_noreturn_instruction instr then {prev_result with normal_flow_typestate= None}
+          else
+            let exception_flow_typestates =
+              if can_instrunction_throw tenv node instr then
+                (* add the typestate after this instruction to the list of exception typestates *)
+                normal_flow_typestate :: exception_flow_typestates_prev
+              else exception_flow_typestates_prev
             in
             if Config.write_html then (
               L.d_printfln "instr: %a@\n" (Sil.pp_instr ~print_types:true Pp.text) instr ;
-      L.d_printfln "new state:@\n%a@\n" TypeState.pp post ) ;
+              L.d_printfln "new state:@\n%a@\n" TypeState.pp normal_flow_typestate ) ;
-    handle_exceptions typestate instr ;
+            {normal_flow_typestate= Some normal_flow_typestate; exception_flow_typestates}
-    post
     in
     (* Reset 'always' field for forall errors to false. *)
     (* This is used to track if it is set to true for all visit to the node. *)
     TypeErr.node_reset_forall canonical_node ;
-  let typestate_succ = Instrs.fold ~f:do_instruction ~init:typestate instrs in
+    Instrs.fold instrs ~f:fold_instruction
-  let dont_propagate =
+      ~init:{normal_flow_typestate= Some typestate; exception_flow_typestates= []}
-    Procdesc.Node.equal_nodekind (Procdesc.Node.get_kind node) Procdesc.Node.exn_sink_kind
-    (* don't propagate exceptions *)
-    || !noreturn
-  in
-  if dont_propagate then ([], []) (* don't propagate to exit node *)
-  else ([typestate_succ], !typestates_exn)

infer/src/nullsafe/typeCheck.mli

@@ -15,6 +15,16 @@ type find_canonical_duplicate = Procdesc.Node.t -> Procdesc.Node.t
 
 type checks = {eradicate: bool; check_ret_type: check_return_type list}
 
+type typecheck_result =
+  { normal_flow_typestate: TypeState.t option
+        (** Typestate at the exit of the node. [None] if node is determined dead end (e.g. noreturn
+            function). Will be passed to all output nodes of the current node. *)
+  ; exception_flow_typestates: TypeState.t list
+        (** If an exception might be thrown after this node, this list should contain all possible
+            states at which the exception can be thrown. (Can be several states because different
+            instructions in the single node can potentially throw). These typestates (joined
+            together) will be passed to all "exception output" nodes of the current node. *) }
+
 val typecheck_node :
      Tenv.t
   -> bool ref
@@ -27,4 +37,5 @@ val typecheck_node :
   -> TypeState.t
   -> Procdesc.Node.t
   -> Printer.LineReader.t
-  -> TypeState.t list * TypeState.t list
+  -> typecheck_result
+(** Main entry point. Typecheck the CFG node given input typestate, and report issues, if found *)