Make await for cancel the stream when breaking out of the loop

runtime/vm/parser.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/parser.h"
 
 #include "lib/invocation_mirror.h"
 #include "platform/utils.h"
 #include "vm/ast_transformer.h"
 #include "vm/bootstrap.h"
@@ skipping 6837 matching lines @@
   current_block_ = current_block_->parent;
   return statements;
 }
 
 
 SequenceNode* Parser::CloseAsyncFunction(const Function& closure,
                                          SequenceNode* closure_body) {
   TRACE_PARSER("CloseAsyncFunction");
   ASSERT(!closure.IsNull());
   ASSERT(closure_body != NULL);
+
   // The block for the async closure body has already been closed. Close the
   // corresponding function block.
   CloseBlock();
 
   closure_body->scope()->LookupVariable(Symbols::AwaitJumpVar(), false);
   closure_body->scope()->LookupVariable(Symbols::AwaitContextVar(), false);
   closure_body->scope()->CaptureVariable(Symbols::AsyncCompleter());
 
   // Create and return a new future that executes a closure with the current
   // body.
@@ skipping 1460 matching lines @@
   intptr_t loop_var_pos = TokenPos();
   const String* loop_var_name = ExpectIdentifier("variable name expected");
 
   // Parse stream expression.
   ExpectToken(Token::kIN);
   const intptr_t stream_pos = TokenPos();
   AstNode* stream_expr =
       ParseAwaitableExpr(kAllowConst, kConsumeCascades, NULL);
   ExpectToken(Token::kRPAREN);
 
+  // Open a block for the iterator variable and the try-finally
+  // statement that contains the loop.
   OpenBlock();
+  const Block* loop_block = current_block_;
 
   // Build creation of implicit StreamIterator.
   // var :for-in-iter = new StreamIterator(stream_expr).
   const Class& stream_iterator_cls =
       Class::ZoneHandle(Z, I->object_store()->stream_iterator_class());
   ASSERT(!stream_iterator_cls.IsNull());
   const Function& iterator_ctor =
       Function::ZoneHandle(Z, stream_iterator_cls.LookupFunction(
           Symbols::StreamIteratorConstructor()));
   ASSERT(!iterator_ctor.IsNull());
   ArgumentListNode* ctor_args = new (Z) ArgumentListNode(Scanner::kNoSourcePos);
   ctor_args->Add(stream_expr);
   ConstructorCallNode* ctor_call =
       new (Z) ConstructorCallNode(Scanner::kNoSourcePos,
                               TypeArguments::ZoneHandle(Z),
                               iterator_ctor,
                               ctor_args);
   const AbstractType& iterator_type = Type::ZoneHandle(Z, Type::DynamicType());
   LocalVariable* iterator_var = new(Z) LocalVariable(
       stream_pos, Symbols::ForInIter(), iterator_type);
   current_block_->scope->AddVariable(iterator_var);
   AstNode* iterator_init =
       new(Z) StoreLocalNode(stream_pos, iterator_var, ctor_call);
   current_block_->statements->Add(iterator_init);
 
+  // We need to ensure that the stream is cancelled after the loop.
+  // Thus, wrap the loop in a try-finally that calls :for-in-iter.close()
+  // in the finally clause. It is harmless to call close() if the stream
+  // is already cancelled (when moveNext() returns false).
+  // Note: even though this is async code, we do not need to set up
+  // the closurized saved_exception_var and saved_stack_trace_var because
+  // there can not be a suspend/resume event before the exception is
+  // rethrown in the catch clause. The catch block of the implicit
+  // try-finally is empty.
+  LocalVariable* context_var = NULL;
+  LocalVariable* exception_var = NULL;
+  LocalVariable* stack_trace_var = NULL;
+  LocalVariable* saved_exception_var = NULL;
+  LocalVariable* saved_stack_trace_var = NULL;
+  SetupExceptionVariables(current_block_->scope,
+                          false,  // Do not create the saved_ vars.
+                          &context_var,
+                          &exception_var,
+                          &stack_trace_var,
+                          &saved_exception_var,
+                          &saved_stack_trace_var);
+  OpenBlock();  // try block.
+  PushTry(current_block_);
+  SetupSavedTryContext(context_var);
+
+  // Build while loop condition.
+  // while (await :for-in-iter.moveNext())
   LocalScope* try_scope;
   int16_t try_index;
   LocalScope* outer_try_scope;
   int16_t outer_try_index;
   CheckAsyncOpInTryBlock(&try_scope, &try_index,
                          &outer_try_scope, &outer_try_index);
 
-  // Build while loop condition.
-  // while (await :for-in-iter.moveNext())
   ArgumentListNode* no_args = new(Z) ArgumentListNode(stream_pos);
   AstNode* iterator_moveNext = new(Z) InstanceCallNode(
       stream_pos,
       new(Z) LoadLocalNode(stream_pos, iterator_var),
                            Symbols::MoveNext(),
                            no_args);
-  AstNode* await_moveNext = new (Z) AwaitNode(stream_pos,
-                                              iterator_moveNext,
-                                              try_scope,
-                                              try_index,
-                                              outer_try_scope,
-                                              outer_try_index);
+  AstNode* await_moveNext =
+      new(Z) AwaitNode(stream_pos,
+                       iterator_moveNext,
+                       try_scope,
+                       try_index,
+                       outer_try_scope,
+                       outer_try_index);
   OpenBlock();
   AwaitTransformer at(current_block_->statements, async_temp_scope_);
-  AstNode* transformed_await = at.Transform(await_moveNext);
+  await_moveNext = at.Transform(await_moveNext);
   SequenceNode* await_preamble = CloseBlock();
 
   // Parse the for loop body. Ideally, we would use ParseNestedStatement()
   // here, but that does not work well because we have to insert an implicit
   // variable assignment and potentially a variable declaration in the
   // loop body.
   OpenLoopBlock();
+
   SourceLabel* label =
       SourceLabel::New(await_for_pos, label_name, SourceLabel::kFor);
   current_block_->scope->AddLabel(label);
   const intptr_t loop_var_assignment_pos = TokenPos();
 
   AstNode* iterator_current = new(Z) InstanceGetterNode(
       loop_var_assignment_pos,
       new(Z) LoadLocalNode(loop_var_assignment_pos, iterator_var),
       Symbols::Current());
 
@@ skipping 17 matching lines @@
   } else {
     AstNode* loop_var_primary =
         ResolveIdent(loop_var_pos, *loop_var_name, false);
     ASSERT(!loop_var_primary->IsPrimaryNode());
     loop_var_assignment = CreateAssignmentNode(loop_var_primary,
                                                iterator_current,
                                                loop_var_name,
                                                loop_var_assignment_pos);
     ASSERT(loop_var_assignment != NULL);
   }
+
   current_block_->statements->Add(loop_var_assignment);
 
   // Now parse the for-in loop statement or block.
   if (CurrentToken() == Token::kLBRACE) {
     ConsumeToken();
     ParseStatementSequence();
     ExpectToken(Token::kRBRACE);
   } else {
     AstNode* statement = ParseStatement();
     if (statement != NULL) {
       current_block_->statements->Add(statement);
     }
   }
-  SequenceNode* for_loop_statement = CloseBlock();
+  SequenceNode* for_loop_block = CloseBlock();
 
   WhileNode* while_node = new (Z) WhileNode(await_for_pos,
                                             label,
-                                            transformed_await,
+                                            await_moveNext,
                                             await_preamble,
-                                            for_loop_statement);
+                                            for_loop_block);
+  // Add the while loop to the try block.
   current_block_->statements->Add(while_node);
 
+  SequenceNode* try_block = CloseBlock();
+
+

[regis, 2015/03/24 23:46:27]

remove extra blank line

[hausner, 2015/03/24 23:57:29]

Done.

+  // Create an empty "catch all" block that rethrows the current
+  // exception and stacktrace.
+  try_stack_->enter_catch();
+  SequenceNode* catch_block = new(Z) SequenceNode(await_for_pos, NULL);
+
+  if (outer_try_scope != NULL) {
+    // TODO(hausner): Do we need to restore the saved try context here? The
+    // code does not touch any captured variables, it just rethrows the
+    // current exception.

[regis, 2015/03/24 23:46:27]

The code may not access captured variables, but if you set a breakpoint in the
debugger to inspect these variables, you will need a valid current context.

[hausner, 2015/03/24 23:57:29]

True, thank you. Removed comment.

+    catch_block->Add(AwaitTransformer::RestoreSavedTryContext(
+        Z, outer_try_scope, outer_try_index));
+  }
+
+  // We don't need to copy the current execption and stack trace variables

[regis, 2015/03/24 23:46:27]

typo

[hausner, 2015/03/24 23:57:29]

Done.

+  // into :saved_exception_var and :saved_stack_trace_var here because there
+  // is no code in the catch clause that could suspend the function.
+
+  // Rethrow the exception.
+  catch_block->Add(new(Z) ThrowNode(
+      await_for_pos,
+      new(Z) LoadLocalNode(await_for_pos, exception_var),
+      new(Z) LoadLocalNode(await_for_pos, stack_trace_var)));
+
+  TryStack* try_statement = PopTry();
+  ASSERT(try_index == try_statement->try_index());
+
+  // The finally block contains a call to cancel the stream.
+  // :for-in-iter.cancel()
+
+  // Inline the finally block to the exit points in the try block.
+  intptr_t node_index = 0;
+  SequenceNode* finally_clause = NULL;
+  do {
+    OpenBlock();
+    ArgumentListNode* no_args =
+        new(Z) ArgumentListNode(Scanner::kNoSourcePos);
+    current_block_->statements->Add(
+        new(Z) InstanceCallNode(Scanner::kNoSourcePos,
+            new(Z) LoadLocalNode(Scanner::kNoSourcePos, iterator_var),
+            Symbols::Cancel(),
+            no_args));
+    finally_clause = CloseBlock();
+    AstNode* node_to_inline = try_statement->GetNodeToInlineFinally(node_index);
+    if (node_to_inline != NULL) {
+      InlinedFinallyNode* node =
+          new(Z) InlinedFinallyNode(Scanner::kNoSourcePos,
+                                    finally_clause,
+                                    context_var,
+                                    outer_try_index);
+      finally_clause = NULL;
+      AddFinallyBlockToNode(true, node_to_inline, node);
+      node_index++;
+    }
+  } while (finally_clause == NULL);
+
+  // Create the try-statement and add to the current sequence, which is
+  // the block around the loop statement.
+
+  const Type& dynamic_type = Type::ZoneHandle(Z, Type::DynamicType());
+  const Array& handler_types = Array::ZoneHandle(Z, Array::New(1, Heap::kOld));
+  handler_types.SetAt(0, dynamic_type);  // Catch block handles all exceptions.
+
+  CatchClauseNode* catch_clause = new(Z) CatchClauseNode(await_for_pos,
+      catch_block,
+      handler_types,
+      context_var,
+      exception_var,
+      stack_trace_var,
+      exception_var,
+      stack_trace_var,
+      AllocateTryIndex(),
+      true);  // Needs stack trace.
+
+  AstNode* try_catch_node =
+      new(Z) TryCatchNode(await_for_pos,
+                         try_block,
+                         context_var,
+                         catch_clause,
+                         finally_clause,
+                         try_index);
+
+  ASSERT(current_block_ == loop_block);
+  loop_block->statements->Add(try_catch_node);
+
   return CloseBlock();  // Implicit block around while loop.
 }
 
 
 AstNode* Parser::ParseForInStatement(intptr_t forin_pos,
                                      SourceLabel* label) {
   TRACE_PARSER("ParseForInStatement");
   bool loop_var_is_final = (CurrentToken() == Token::kFINAL);
   if (CurrentToken() == Token::kCONST) {
     ReportError("Loop variable cannot be 'const'");
@@ skipping 4817 matching lines @@
 void Parser::SkipQualIdent() {
   ASSERT(IsIdentifier());
   ConsumeToken();
   if (CurrentToken() == Token::kPERIOD) {
     ConsumeToken();  // Consume the kPERIOD token.
     ExpectIdentifier("identifier expected after '.'");
   }
 }
 
 }  // namespace dart