Avoid unbounded stack consumption for synchronous control flow

Previously, as sequence of state transitions that did not pass through
an asynchrous boundary incurred stack frames. The trivial loop in
the enclosed test case would then overflow the stack.

This commit merges the `resume` and `apply(tr: Try[Any])` methods into
a `apply`. It changes the body of this method to be an infinite loop
with returns at the terminal points in the state machine (or at a
terminal failure.)

To allow merging of these previously separate matches, states that
contain an await are now allocated two state ids: one for the setup
code that calls `onComplete`, and one for the code in the continuation
that records the result and advances the state machine.

Fixes #93

1 files changed, 7 insertions, 17 deletions

diff --git a/src/main/scala/scala/async/internal/AsyncTransform.scala b/src/main/scala/scala/async/internal/AsyncTransform.scala
index 7d56043..c7a0c65 100644
--- a/src/main/scala/scala/async/internal/AsyncTransform.scala
+++ b/src/main/scala/scala/async/internal/AsyncTransform.scala
@@ -24,31 +24,29 @@ trait AsyncTransform {
 
     val anfTree = futureSystemOps.postAnfTransform(anfTree0)
 
-    val resumeFunTreeDummyBody = DefDef(Modifiers(), name.resume, Nil, List(Nil), Ident(definitions.UnitClass), Literal(Constant(())))
-
     val applyDefDefDummyBody: DefDef = {
       val applyVParamss = List(List(ValDef(Modifiers(Flag.PARAM), name.tr, TypeTree(futureSystemOps.tryType[Any]), EmptyTree)))
-      DefDef(NoMods, name.apply, Nil, applyVParamss, TypeTree(definitions.UnitTpe), Literal(Constant(())))
+      DefDef(NoMods, name.apply, Nil, applyVParamss, TypeTree(definitions.UnitTpe), literalUnit)
     }
 
     // Create `ClassDef` of state machine with empty method bodies for `resume` and `apply`.
     val stateMachine: ClassDef = {
       val body: List[Tree] = {
-        val stateVar = ValDef(Modifiers(Flag.MUTABLE | Flag.PRIVATE | Flag.LOCAL), name.state, TypeTree(definitions.IntTpe), Literal(Constant(0)))
+        val stateVar = ValDef(Modifiers(Flag.MUTABLE | Flag.PRIVATE | Flag.LOCAL), name.state, TypeTree(definitions.IntTpe), Literal(Constant(StateAssigner.Initial)))
         val result = ValDef(NoMods, name.result, TypeTree(futureSystemOps.promType[T](uncheckedBoundsResultTag)), futureSystemOps.createProm[T](uncheckedBoundsResultTag).tree)
         val execContextValDef = ValDef(NoMods, name.execContext, TypeTree(), execContext)
 
         val apply0DefDef: DefDef = {
           // We extend () => Unit so we can pass this class as the by-name argument to `Future.apply`.
           // See SI-1247 for the the optimization that avoids creatio
-          DefDef(NoMods, name.apply, Nil, Nil, TypeTree(definitions.UnitTpe), Apply(Ident(name.resume), Nil))
+          DefDef(NoMods, name.apply, Nil, Nil, TypeTree(definitions.UnitTpe), Apply(Ident(name.apply), literalNull :: Nil))
         }
         val extraValDef: ValDef = {
           // We extend () => Unit so we can pass this class as the by-name argument to `Future.apply`.
           // See SI-1247 for the the optimization that avoids creatio
-          ValDef(NoMods, newTermName("extra"), TypeTree(definitions.UnitTpe), Literal(Constant(())))
+          ValDef(NoMods, newTermName("extra"), TypeTree(definitions.UnitTpe), literalUnit)
         }
-        List(emptyConstructor, stateVar, result, execContextValDef) ++ List(resumeFunTreeDummyBody, applyDefDefDummyBody, apply0DefDef, extraValDef)
+        List(emptyConstructor, stateVar, result, execContextValDef) ++ List(applyDefDefDummyBody, apply0DefDef, extraValDef)
       }
 
       val tryToUnit = appliedType(definitions.FunctionClass(1), futureSystemOps.tryType[Any], typeOf[Unit])
@@ -90,8 +88,7 @@ trait AsyncTransform {
       val stateMachineSpliced: Tree = spliceMethodBodies(
         liftedFields,
         stateMachine,
-        atMacroPos(asyncBlock.onCompleteHandler[T]),
-        atMacroPos(asyncBlock.resumeFunTree[T].rhs)
+        atMacroPos(asyncBlock.onCompleteHandler[T])
       )
 
       def selectStateMachine(selection: TermName) = Select(Ident(name.stateMachine), selection)
@@ -131,10 +128,9 @@ trait AsyncTransform {
    *  @param  liftables  trees of definitions that are lifted to fields of the state machine class
    *  @param  tree       `ClassDef` tree of the state machine class
    *  @param  applyBody  tree of onComplete handler (`apply` method)
-   *  @param  resumeBody RHS of definition tree of `resume` method
    *  @return            transformed `ClassDef` tree of the state machine class
    */
-  def spliceMethodBodies(liftables: List[Tree], tree: ClassDef, applyBody: Tree, resumeBody: Tree): Tree = {
+  def spliceMethodBodies(liftables: List[Tree], tree: ClassDef, applyBody: Tree): Tree = {
     val liftedSyms = liftables.map(_.symbol).toSet
     val stateMachineClass = tree.symbol
     liftedSyms.foreach {
@@ -211,12 +207,6 @@ trait AsyncTransform {
         (ctx: analyzer.Context) =>
           val typedTree = fixup(dd, changeOwner(applyBody, callSiteTyper.context.owner, dd.symbol), ctx)
           typedTree
-
-      case dd@DefDef(_, name.resume, _, _, _, _) if dd.symbol.owner == stateMachineClass =>
-        (ctx: analyzer.Context) =>
-          val changed = changeOwner(resumeBody, callSiteTyper.context.owner, dd.symbol)
-          val res = fixup(dd, changed, ctx)
-          res
     }
     result
   }