Fix referenced watcher hanging event loop if worker is not joined

Fixes amphp/file#75.

src/Context/Internal/AbstractContext.php

@@ -3,6 +3,7 @@
 namespace Amp\Parallel\Context\Internal;
 
 use Amp\Cancellation;
+use Amp\CancelledException;
 use Amp\ForbidCloning;
 use Amp\ForbidSerialization;
 use Amp\Future;
@@ -24,33 +25,13 @@ abstract class AbstractContext implements Context
     use ForbidCloning;
     use ForbidSerialization;
 
-    /** @var Future<ExitResult> */
-    private readonly Future $result;
+    /** @var Future<ExitResult<TResult>>|null */
+    private ?Future $result = null;
 
     protected function __construct(
         private readonly Channel $ipcChannel,
-        Channel $resultChannel,
+        private readonly Channel $resultChannel,
     ) {
-        $this->result = async(static function () use ($resultChannel): ExitResult {
-            try {
-                $data = $resultChannel->receive();
-            } catch (\Throwable $exception) {
-                throw new ContextException("Failed to receive result from context", previous: $exception);
-            } finally {
-                $resultChannel->close();
-            }
-
-            if (!$data instanceof ExitResult) {
-                throw new ContextException(\sprintf(
-                    "The context sent data instead of exiting: %s",
-                    flattenArgument($data),
-                ));
-            }
-
-            return $data;
-        });
-
-        $this->result->ignore();
     }
 
     public function receive(?Cancellation $cancellation = null): mixed
@@ -116,6 +97,41 @@ public function onClose(\Closure $onClose): void
 
     protected function receiveExitResult(?Cancellation $cancellation = null): ExitResult
     {
-        return $this->result->await($cancellation);
+        while ($this->result) {
+            try {
+                $this->result->await($cancellation);
+            } catch (CancelledException) {
+                // Ignore cancellation from a prior join request, throw only if this request was cancelled.
+                $cancellation?->throwIfRequested();
+            }
+        }
+
+        $this->result = async(function () use ($cancellation): ExitResult {
+            try {
+                $data = $this->resultChannel->receive($cancellation);
+                $this->resultChannel->close();
+            } catch (CancelledException $exception) {
+                throw $exception;
+            } catch (\Throwable $exception) {
+                $this->resultChannel->close();
+                throw new ContextException("Failed to receive result from context", previous: $exception);
+            }
+
+            if (!$data instanceof ExitResult) {
+                throw new ContextException(\sprintf(
+                    "The context sent data instead of exiting: %s",
+                    flattenArgument($data),
+                ));
+            }
+
+            return $data;
+        });
+
+        try {
+            return $this->result->await();
+        } catch (CancelledException $exception) {
+            $this->result = null;
+            throw $exception;
+        }
     }
 }