Nim/lib/pure/asyncmacro.nim

#
#
#            Nim's Runtime Library
#        (c) Copyright 2015 Dominik Picheta
#
#    See the file "copying.txt", included in this
#    distribution, for details about the copyright.
#

## AsyncMacro
## *************
## `asyncdispatch` module depends on the `asyncmacro` module to work properly.

import macros, strutils, asyncfutures

proc skipUntilStmtList(node: NimNode): NimNode {.compileTime.} =
  # Skips a nest of StmtList's.
  result = node
  if node[0].kind == nnkStmtList:
    result = skipUntilStmtList(node[0])

proc skipStmtList(node: NimNode): NimNode {.compileTime.} =
  result = node
  if node[0].kind == nnkStmtList:
    result = node[0]

template createCb(retFutureSym, iteratorNameSym,
                  strName, identName, futureVarCompletions: untyped) =
  bind finished

  var nameIterVar = iteratorNameSym
  #{.push stackTrace: off.}
  proc identName {.closure.} =
    try:
      if not nameIterVar.finished:
        var next = nameIterVar()
        # Continue while the yielded future is already finished.
        while (not next.isNil) and next.finished:
          next = nameIterVar()
          if nameIterVar.finished:
            break

        if next == nil:
          if not retFutureSym.finished:
            let msg = "Async procedure ($1) yielded `nil`, are you await'ing a " &
                    "`nil` Future?"
            raise newException(AssertionError, msg % strName)
        else:
          {.gcsafe.}:
            {.push hint[ConvFromXtoItselfNotNeeded]: off.}
            next.callback = (proc() {.closure, gcsafe.})(identName)
            {.pop.}
    except:
      futureVarCompletions

      if retFutureSym.finished:
        # Take a look at tasyncexceptions for the bug which this fixes.
        # That test explains it better than I can here.
        raise
      else:
        retFutureSym.fail(getCurrentException())

  identName()
  #{.pop.}

template useVar(result: var NimNode, futureVarNode: NimNode, valueReceiver,
                rootReceiver: untyped, fromNode: NimNode) =
  ## Params:
  ##    futureVarNode: The NimNode which is a symbol identifying the Future[T]
  ##                   variable to yield.
  ##    fromNode: Used for better debug information (to give context).
  ##    valueReceiver: The node which defines an expression that retrieves the
  ##                   future's value.
  ##
  ##    rootReceiver: ??? TODO
  # -> yield future<x>
  result.add newNimNode(nnkYieldStmt, fromNode).add(futureVarNode)
  # -> future<x>.read
  valueReceiver = newDotExpr(futureVarNode, newIdentNode("read"))
  result.add rootReceiver

template createVar(result: var NimNode, futSymName: string,
                   asyncProc: NimNode,
                   valueReceiver, rootReceiver: untyped,
                   fromNode: NimNode) =
  result = newNimNode(nnkStmtList, fromNode)
  var futSym = genSym(nskVar, "future")
  result.add newVarStmt(futSym, asyncProc) # -> var future<x> = y
  useVar(result, futSym, valueReceiver, rootReceiver, fromNode)

proc createFutureVarCompletions(futureVarIdents: seq[NimNode],
    fromNode: NimNode): NimNode {.compileTime.} =
  result = newNimNode(nnkStmtList, fromNode)
  # Add calls to complete each FutureVar parameter.
  for ident in futureVarIdents:
    # Only complete them if they have not been completed already by the user.
    # TODO: Once https://github.com/nim-lang/Nim/issues/5617 is fixed.
    # TODO: Add line info to the complete() call!
    # In the meantime, this was really useful for debugging :)
    #result.add(newCall(newIdentNode("echo"), newStrLitNode(fromNode.lineinfo)))
    result.add newIfStmt(
      (
        newCall(newIdentNode("not"),
                newDotExpr(ident, newIdentNode("finished"))),
        newCall(newIdentNode("complete"), ident)
      )
    )

proc processBody(node, retFutureSym: NimNode,
                 subTypeIsVoid: bool,
                 futureVarIdents: seq[NimNode]): NimNode {.compileTime.} =
  #echo(node.treeRepr)
  result = node
  case node.kind
  of nnkReturnStmt:
    result = newNimNode(nnkStmtList, node)

    # As I've painfully found out, the order here really DOES matter.
    result.add createFutureVarCompletions(futureVarIdents, node)

    if node[0].kind == nnkEmpty:
      if not subTypeIsVoid:
        result.add newCall(newIdentNode("complete"), retFutureSym,
            newIdentNode("result"))
      else:
        result.add newCall(newIdentNode("complete"), retFutureSym)
    else:
      let x = node[0].processBody(retFutureSym, subTypeIsVoid,
                                  futureVarIdents)
      if x.kind == nnkYieldStmt: result.add x
      else:
        result.add newCall(newIdentNode("complete"), retFutureSym, x)

    result.add newNimNode(nnkReturnStmt, node).add(newNilLit())
    return # Don't process the children of this return stmt
  of nnkCommand, nnkCall:
    if node[0].kind == nnkIdent and node[0].eqIdent("await"):
      case node[1].kind
      of nnkIdent, nnkInfix, nnkDotExpr, nnkCall, nnkCommand:
        # await x
        # await x or y
        # await foo(p, x)
        # await foo p, x
        var futureValue: NimNode
        result.createVar("future" & $node[1][0].toStrLit, node[1], futureValue,
                  futureValue, node)
      else:
        error("Invalid node kind in 'await', got: " & $node[1].kind)
    elif node.len > 1 and node[1].kind == nnkCommand and
         node[1][0].kind == nnkIdent and node[1][0].eqIdent("await"):
      # foo await x
      var newCommand = node
      result.createVar("future" & $node[0].toStrLit, node[1][1], newCommand[1],
                newCommand, node)

  of nnkVarSection, nnkLetSection:
    case node[0][2].kind
    of nnkCommand:
      if node[0][2][0].kind == nnkIdent and node[0][2][0].eqIdent("await"):
        # var x = await y
        var newVarSection = node # TODO: Should this use copyNimNode?
        result.createVar("future" & node[0][0].strVal, node[0][2][1],
          newVarSection[0][2], newVarSection, node)
    else: discard
  of nnkAsgn:
    case node[1].kind
    of nnkCommand:
      if node[1][0].eqIdent("await"):
        # x = await y
        var newAsgn = node
        result.createVar("future" & $node[0].toStrLit, node[1][1], newAsgn[1], newAsgn, node)
    else: discard
  of nnkDiscardStmt:
    # discard await x
    if node[0].kind == nnkCommand and node[0][0].kind == nnkIdent and
          node[0][0].eqIdent("await"):
      var newDiscard = node
      result.createVar("futureDiscard_" & $toStrLit(node[0][1]), node[0][1],
                newDiscard[0], newDiscard, node)
  of RoutineNodes-{nnkTemplateDef}:
    # skip all the nested procedure definitions
    return
  else: discard

  for i in 0 ..< result.len:
    result[i] = processBody(result[i], retFutureSym, subTypeIsVoid,
                            futureVarIdents)

proc getName(node: NimNode): string {.compileTime.} =
  case node.kind
  of nnkPostfix:
    return node[1].strVal
  of nnkIdent, nnkSym:
    return node.strVal
  of nnkEmpty:
    return "anonymous"
  else:
    error("Unknown name.")

proc getFutureVarIdents(params: NimNode): seq[NimNode] {.compileTime.} =
  result = @[]
  for i in 1 ..< len(params):
    expectKind(params[i], nnkIdentDefs)
    if params[i][1].kind == nnkBracketExpr and
       params[i][1][0].eqIdent("futurevar"):
      result.add(params[i][0])

proc isInvalidReturnType(typeName: string): bool =
  return typeName notin ["Future"] #, "FutureStream"]

proc verifyReturnType(typeName: string) {.compileTime.} =
  if typeName.isInvalidReturnType:
    error("Expected return type of 'Future' got '$1'" %
          typeName)

proc asyncSingleProc(prc: NimNode): NimNode {.compileTime.} =
  ## This macro transforms a single procedure into a closure iterator.
  ## The ``async`` macro supports a stmtList holding multiple async procedures.
  if prc.kind notin {nnkProcDef, nnkLambda, nnkMethodDef, nnkDo}:
      error("Cannot transform this node kind into an async proc." &
            " proc/method definition or lambda node expected.")

  let prcName = prc.name.getName

  let returnType = prc.params[0]
  var baseType: NimNode
  # Verify that the return type is a Future[T]
  if returnType.kind == nnkBracketExpr:
    let fut = repr(returnType[0])
    verifyReturnType(fut)
    baseType = returnType[1]
  elif returnType.kind in nnkCallKinds and returnType[0].eqIdent("[]"):
    let fut = repr(returnType[1])
    verifyReturnType(fut)
    baseType = returnType[2]
  elif returnType.kind == nnkEmpty:
    baseType = returnType
  else:
    verifyReturnType(repr(returnType))

  let subtypeIsVoid = returnType.kind == nnkEmpty or
        (baseType.kind == nnkIdent and returnType[1].eqIdent("void"))

  let futureVarIdents = getFutureVarIdents(prc.params)

  var outerProcBody = newNimNode(nnkStmtList, prc.body)

  # -> var retFuture = newFuture[T]()
  var retFutureSym = genSym(nskVar, "retFuture")
  var subRetType =
    if returnType.kind == nnkEmpty: newIdentNode("void")
    else: baseType
  outerProcBody.add(
    newVarStmt(retFutureSym,
      newCall(
        newNimNode(nnkBracketExpr, prc.body).add(
          newIdentNode("newFuture"),
          subRetType),
      newLit(prcName)))) # Get type from return type of this proc

  # -> iterator nameIter(): FutureBase {.closure.} =
  # ->   {.push warning[resultshadowed]: off.}
  # ->   var result: T
  # ->   {.pop.}
  # ->   <proc_body>
  # ->   complete(retFuture, result)
  var iteratorNameSym = genSym(nskIterator, $prcName & "Iter")
  var procBody = prc.body.processBody(retFutureSym, subtypeIsVoid,
                                    futureVarIdents)
  # don't do anything with forward bodies (empty)
  if procBody.kind != nnkEmpty:
    procBody.add(createFutureVarCompletions(futureVarIdents, nil))

    if not subtypeIsVoid:
      procBody.insert(0, newNimNode(nnkPragma).add(newIdentNode("push"),
        newNimNode(nnkExprColonExpr).add(newNimNode(nnkBracketExpr).add(
          newIdentNode("warning"), newIdentNode("resultshadowed")),
        newIdentNode("off")))) # -> {.push warning[resultshadowed]: off.}

      procBody.insert(1, newNimNode(nnkVarSection, prc.body).add(
        newIdentDefs(newIdentNode("result"), baseType))) # -> var result: T

      procBody.insert(2, newNimNode(nnkPragma).add(
        newIdentNode("pop"))) # -> {.pop.})

      procBody.add(
        newCall(newIdentNode("complete"),
          retFutureSym, newIdentNode("result"))) # -> complete(retFuture, result)
    else:
      # -> complete(retFuture)
      procBody.add(newCall(newIdentNode("complete"), retFutureSym))

    var closureIterator = newProc(iteratorNameSym, [newIdentNode("FutureBase")],
                                  procBody, nnkIteratorDef)
    closureIterator.pragma = newNimNode(nnkPragma, lineInfoFrom=prc.body)
    closureIterator.addPragma(newIdentNode("closure"))

    # If proc has an explicit gcsafe pragma, we add it to iterator as well.
    if prc.pragma.findChild(it.kind in {nnkSym, nnkIdent} and $it == "gcsafe") != nil:
      closureIterator.addPragma(newIdentNode("gcsafe"))
    outerProcBody.add(closureIterator)

    # -> createCb(retFuture)
    # NOTE: The "_continue" suffix is checked for in asyncfutures.nim to produce
    # friendlier stack traces:
    var cbName = genSym(nskProc, prcName & "_continue")
    var procCb = getAst createCb(retFutureSym, iteratorNameSym,
                         newStrLitNode(prcName),
                         cbName,
                         createFutureVarCompletions(futureVarIdents, nil))
    outerProcBody.add procCb

    # -> return retFuture
    outerProcBody.add newNimNode(nnkReturnStmt, prc.body[^1]).add(retFutureSym)

  result = prc

  if subtypeIsVoid:
    # Add discardable pragma.
    if returnType.kind == nnkEmpty:
      # Add Future[void]
      result.params[0] = parseExpr("Future[void]")
  if procBody.kind != nnkEmpty:
    result.body = outerProcBody
  #echo(treeRepr(result))
  #if prcName == "recvLineInto":
  #  echo(toStrLit(result))

macro async*(prc: untyped): untyped =
  ## Macro which processes async procedures into the appropriate
  ## iterators and yield statements.
  if prc.kind == nnkStmtList:
    result = newStmtList()
    for oneProc in prc:
      result.add asyncSingleProc(oneProc)
  else:
    result = asyncSingleProc(prc)
  when defined(nimDumpAsync):
    echo repr result


# Multisync
proc emptyNoop[T](x: T): T =
  # The ``await``s are replaced by a call to this for simplicity.
  when T isnot void:
    return x

proc stripAwait(node: NimNode): NimNode =
  ## Strips out all ``await`` commands from a procedure body, replaces them
  ## with ``emptyNoop`` for simplicity.
  result = node

  let emptyNoopSym = bindSym("emptyNoop")

  case node.kind
  of nnkCommand, nnkCall:
    if node[0].kind == nnkIdent and node[0].eqIdent("await"):
      node[0] = emptyNoopSym
    elif node.len > 1 and node[1].kind == nnkCommand and
         node[1][0].kind == nnkIdent and node[1][0].eqIdent("await"):
      # foo await x
      node[1][0] = emptyNoopSym
  of nnkVarSection, nnkLetSection:
    case node[0][2].kind
    of nnkCommand:
      if node[0][2][0].kind == nnkIdent and node[0][2][0].eqIdent("await"):
        # var x = await y
        node[0][2][0] = emptyNoopSym
    else: discard
  of nnkAsgn:
    case node[1].kind
    of nnkCommand:
      if node[1][0].eqIdent("await"):
        # x = await y
        node[1][0] = emptyNoopSym
    else: discard
  of nnkDiscardStmt:
    # discard await x
    if node[0].kind == nnkCommand and node[0][0].kind == nnkIdent and
          node[0][0].eqIdent("await"):
      node[0][0] = emptyNoopSym
  else: discard

  for i in 0 ..< result.len:
    result[i] = stripAwait(result[i])

proc splitParamType(paramType: NimNode, async: bool): NimNode =
  result = paramType
  if paramType.kind == nnkInfix and paramType[0].strVal in ["|", "or"]:
    let firstAsync = "async" in paramType[1].strVal.normalize
    let secondAsync = "async" in paramType[2].strVal.normalize

    if firstAsync:
      result = paramType[if async: 1 else: 2]
    elif secondAsync:
      result = paramType[if async: 2 else: 1]

proc stripReturnType(returnType: NimNode): NimNode =
  # Strip out the 'Future' from 'Future[T]'.
  result = returnType
  if returnType.kind == nnkBracketExpr:
    let fut = repr(returnType[0])
    verifyReturnType(fut)
    result = returnType[1]

proc splitProc(prc: NimNode): (NimNode, NimNode) =
  ## Takes a procedure definition which takes a generic union of arguments,
  ## for example: proc (socket: Socket | AsyncSocket).
  ## It transforms them so that ``proc (socket: Socket)`` and
  ## ``proc (socket: AsyncSocket)`` are returned.

  result[0] = prc.copyNimTree()
  # Retrieve the `T` inside `Future[T]`.
  let returnType = stripReturnType(result[0][3][0])
  result[0][3][0] = splitParamType(returnType, async=false)
  for i in 1 ..< result[0][3].len:
    # Sync proc (0) -> FormalParams (3) -> IdentDefs, the parameter (i) ->
    # parameter type (1).
    result[0][3][i][1] = splitParamType(result[0][3][i][1], async=false)
  result[0][6] = stripAwait(result[0][6])

  result[1] = prc.copyNimTree()
  if result[1][3][0].kind == nnkBracketExpr:
    result[1][3][0][1] = splitParamType(result[1][3][0][1], async=true)
  for i in 1 ..< result[1][3].len:
    # Async proc (1) -> FormalParams (3) -> IdentDefs, the parameter (i) ->
    # parameter type (1).
    result[1][3][i][1] = splitParamType(result[1][3][i][1], async=true)

macro multisync*(prc: untyped): untyped =
  ## Macro which processes async procedures into both asynchronous and
  ## synchronous procedures.
  ##
  ## The generated async procedures use the ``async`` macro, whereas the
  ## generated synchronous procedures simply strip off the ``await`` calls.
  let (sync, asyncPrc) = splitProc(prc)
  result = newStmtList()
  result.add(asyncSingleProc(asyncPrc))
  result.add(sync)