Strict Future value / error access

Strict future access is now always enabled, making it a panic to access
value/error when the Future is still pending - `read` and `readError`
raise catchable exceptions instead which may be preferable to use.

* remove `chronosStrictFutureAccess` enabling its functionality by
default
* disallow setting a cancellation callback on a non-pending future

chronos/config.nim

@@ -4,10 +4,6 @@
 ## in transition periods leading up to a breaking release that starts enforcing
 ## them and removes the option.
 ##
-## `chronosPreviewV4` is a preview flag to enable v4 semantics - in particular,
-## it enables strict exception checking and disables parts of the deprecated
-## API and other changes being prepared for the upcoming release
-##
 ## `chronosDebug` can be defined to enable several debugging helpers that come
 ## with a runtime cost - it is recommeneded to not enable these in production
 ## code.
@@ -26,8 +22,6 @@ const
     ##
     ## `Exception` handling may be removed in future chronos versions.
 
-  chronosStrictFutureAccess* {.booldefine.}: bool = defined(chronosPreviewV4)
-
   chronosStackTrace* {.booldefine.}: bool = defined(chronosDebug)
     ## Include stack traces in futures for creation and completion points
 

chronos/futures.nim

@@ -209,31 +209,27 @@ func value*[T: not void](future: Future[T]): lent T =
   ##
   ## See `read` for a version that raises a catchable error when future
   ## has not completed.
-  when chronosStrictFutureAccess:
-    if not future.completed():
-      raiseFutureDefect("Future not completed while accessing value", future)
+  if not future.completed():
+    raiseFutureDefect("Future not completed while accessing value", future)
 
   future.internalValue
 
 func value*(future: Future[void]) =
-  ## Return the value in a completed future - raises Defect when
-  ## `fut.completed()` is `false`.
+  ## Raises Defect when `fut.completed()` is `false`.
   ##
   ## See `read` for a version that raises a catchable error when future
   ## has not completed.
-  when chronosStrictFutureAccess:
-    if not future.completed():
-      raiseFutureDefect("Future not completed while accessing value", future)
+  if not future.completed():
+    raiseFutureDefect("Future not completed while accessing value", future)
 
 func error*(future: FutureBase): ref CatchableError =
   ## Return the error of `future`, or `nil` if future did not fail.
   ##
   ## See `readError` for a version that raises a catchable error when the
   ## future has not failed.
-  when chronosStrictFutureAccess:
-    if not future.failed() and not future.cancelled():
-      raiseFutureDefect(
-        "Future not failed/cancelled while accessing error", future)
+  if not future.failed() and not future.cancelled():
+    raiseFutureDefect(
+      "Future not failed/cancelled while accessing error", future)
 
   future.internalError
 

chronos/internal/asyncfutures.nim

@@ -283,10 +283,9 @@ proc tryCancel(future: FutureBase, loc: ptr SrcLoc): bool =
   if not(isNil(future.internalChild)):
     # If you hit this assertion, you should have used the `CancelledError`
     # mechanism and/or use a regular `addCallback`
-    when chronosStrictFutureAccess:
-      doAssert future.internalCancelcb.isNil,
-        "futures returned from `{.async.}` functions must not use " &
-        "`cancelCallback`"
+    doAssert future.internalCancelcb.isNil,
+      "futures returned from `{.async.}` functions must not use " &
+      "`cancelCallback`"
     tryCancel(future.internalChild, loc)
   else:
     if not(isNil(future.internalCancelcb)):
@@ -353,9 +352,9 @@ proc `cancelCallback=`*(future: FutureBase, cb: CallbackFunc) =
   ## This callback will be called immediately as ``future.cancel()`` invoked and
   ## must be set before future is finished.
 
-  when chronosStrictFutureAccess:
-    doAssert not future.finished(),
-      "cancellation callback must be set before finishing the future"
+  doAssert not future.finished(),
+    "cancellation callback must be set before finishing the future"
+
   future.internalCancelcb = cb
 
 {.push stackTrace: off.}