Fix failing Process::join()

If the process is killed while joining, an exception was thrown from Process::kill().

lib/Context/Process.php

@@ -267,8 +267,10 @@ final class Process implements Context
                     throw new SynchronizationError("Did not receive an exit result from process");
                 }
             } catch (\Throwable $exception) {
-                $this->kill();
-                throw $exception;
+                if ($this->isRunning()) {
+                    $this->kill();
+                }
+                throw new ContextException("Failed to receive result from process", 0, $exception);
             }
 
             $this->channel->close();

test/Context/ProcessTest.php

@@ -35,12 +35,12 @@ class ProcessTest extends TestCase
 
     /**
      * @expectedException \Amp\Parallel\Sync\PanicError
-     * @expectedExceptionMessage No script found at 'test-process.php'
+     * @expectedExceptionMessage No script found at '../test-process.php'
      */
     public function testInvalidScriptPath()
     {
         Loop::run(function () {
-            $process = new Process("test-process.php");
+            $process = new Process("../test-process.php");
             yield $process->start();
             yield $process->join();
         });
@@ -84,4 +84,23 @@ class ProcessTest extends TestCase
             \var_dump(yield $process->join());
         });
     }
+
+    /**
+     * @expectedException \Amp\Parallel\Context\ContextException
+     * @expectedExceptionMessage Failed to receive result from process
+     */
+    public function testKillWhenJoining()
+    {
+        Loop::run(function () {
+            $process = new Process([
+                __DIR__ . "/sleep-process.php",
+                5,
+            ]);
+            yield $process->start();
+            $promise = $process->join();
+            $process->kill();
+            $this->assertFalse($process->isRunning());
+            yield $promise;
+        });
+    }
 }

test/Context/sleep-process.php

@@ -0,0 +1,7 @@
+<?php
+
+use Amp\Parallel\Sync\Channel;
+
+return function (Channel $channel) use ($argv) {
+    \sleep((int) ($argv[1] ?? 1));
+};