Fixes the order in which FutureVar and return completions are made.

This caused a pretty bad and subtle bug in the asynchttpserver.
As far as I can understand, the fact that the returned future was
being completed first meant that the underlying async procedure
could continue running and thus clean() the FutureVar
and request new data. The control then went back and the
FutureVar was completed again causing an error.

lib/pure/asyncmacro.nim

@@ -40,6 +40,8 @@ template createCb(retFutureSym, iteratorNameSym,
         else:
           next.callback = cb
     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.
@@ -47,7 +49,6 @@ template createCb(retFutureSym, iteratorNameSym,
       else:
         retFutureSym.fail(getCurrentException())
 
-      futureVarCompletions
   cb()
   #{.pop.}
 proc generateExceptionCheck(futSym,
@@ -123,12 +124,16 @@ template createVar(result: var NimNode, futSymName: string,
   result.add newVarStmt(futSym, asyncProc) # -> var future<x> = y
   useVar(result, futSym, valueReceiver, rootReceiver, fromNode)
 
-proc createFutureVarCompletions(futureVarIdents: seq[NimNode]): NimNode
+proc createFutureVarCompletions(futureVarIdents: seq[NimNode],
-                                {.compileTime.} =
+    fromNode: NimNode): NimNode {.compileTime.} =
-  result = newStmtList()
+  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"),
@@ -145,6 +150,10 @@ proc processBody(node, retFutureSym: NimNode,
   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,
@@ -158,8 +167,6 @@ proc processBody(node, retFutureSym: NimNode,
       else:
         result.add newCall(newIdentNode("complete"), retFutureSym, x)
 
-    result.add createFutureVarCompletions(futureVarIdents)
-
     result.add newNimNode(nnkReturnStmt, node).add(newNilLit())
     return # Don't process the children of this return stmt
   of nnkCommand, nnkCall:
@@ -347,6 +354,8 @@ proc asyncSingleProc(prc: NimNode): NimNode {.compileTime.} =
                                     futureVarIdents, nil)
   # 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(
@@ -366,8 +375,6 @@ proc asyncSingleProc(prc: NimNode): NimNode {.compileTime.} =
       # -> complete(retFuture)
       procBody.add(newCall(newIdentNode("complete"), retFutureSym))
 
-    procBody.add(createFutureVarCompletions(futureVarIdents))
-
     var closureIterator = newProc(iteratorNameSym, [newIdentNode("FutureBase")],
                                   procBody, nnkIteratorDef)
     closureIterator[4] = newNimNode(nnkPragma, prc[6]).add(newIdentNode("closure"))
@@ -377,7 +384,7 @@ proc asyncSingleProc(prc: NimNode): NimNode {.compileTime.} =
     #var cbName = newIdentNode("cb")
     var procCb = getAst createCb(retFutureSym, iteratorNameSym,
                          newStrLitNode(prc[0].getName),
-                         createFutureVarCompletions(futureVarIdents))
+                         createFutureVarCompletions(futureVarIdents, nil))
     outerProcBody.add procCb
 
     # -> return retFuture
@@ -398,7 +405,7 @@ proc asyncSingleProc(prc: NimNode): NimNode {.compileTime.} =
   if procBody.kind != nnkEmpty:
     result[6] = outerProcBody
   #echo(treeRepr(result))
-  #if prc[0].getName == "beta":
+  #if prc[0].getName == "recvLineInto":
   #  echo(toStrLit(result))
 
 macro async*(prc: untyped): untyped =