Add spawnedOrFailed (#47)

Author: JordanMartinez

CHANGELOG.md

@@ -61,6 +61,9 @@ New features:
   - spawnFile
 - Added unsafe, uncurried API of all ChildProcess-creating functions (#46 by @JordanMartinez)
 - Added safe variant of `spawnSync`/`spawnSync'` (#46 by @JordanMartinez)
+- Added `Aff`-based `waitSpawned` to safely get `Pid` (#47 by @JordanMartinez)
+
+  Blocks until child process either successfully spawns or fails to spawn.
 
 Bugfixes:
 

src/Node/ChildProcess/Aff.purs

@@ -0,0 +1,41 @@
+module Node.ChildProcess.Aff where
+
+import Prelude
+
+import Control.Parallel (parOneOf)
+import Data.Either (Either(..))
+import Data.Maybe (fromJust)
+import Data.Posix (Pid)
+import Effect.Aff (Aff, effectCanceler, makeAff)
+import Effect.Ref as Ref
+import Node.ChildProcess (ChildProcess, pid)
+import Node.ChildProcess as CP
+import Node.Errors.SystemError (SystemError)
+import Node.EventEmitter (once)
+import Partial.Unsafe (unsafePartial)
+
+-- | Blocks until either a `spawn` or `error` event is fired.
+-- | If a `spawn` event fired, child process was successfully started
+-- | and the `pid` of the process can be obtained.
+-- | If an `error` event fires, child process was not started successfully.
+waitSpawned :: ChildProcess -> Aff (Either SystemError Pid)
+waitSpawned cp = parOneOf [ pidOnSpawn, errored ]
+  where
+  pidOnSpawn = makeAff \done -> do
+    ref <- Ref.new mempty
+    removeListener <- cp # once CP.spawnH do
+      join $ Ref.read ref
+      pid' <- pid cp
+      done $ Right $ Right $ unsafePartial $ fromJust pid'
+    Ref.write removeListener ref
+    pure $ effectCanceler do
+      removeListener
+
+  errored = makeAff \done -> do
+    ref <- Ref.new mempty
+    removeListener <- cp # once CP.errorH \sysErr -> do
+      join $ Ref.read ref
+      done $ Right $ Left sysErr
+    Ref.write removeListener ref
+    pure $ effectCanceler do
+      removeListener