Limit the maximum number of hh_client processes to run in parallel

src/Linters/HHClientLinter.hack

@@ -62,13 +62,14 @@ final class HHClientLinter implements Linter {
 
   public async function getLintErrorsAsync(
   ): Awaitable<vec<HHClientLintError>> {
-    $lines = await __Private\execute_async(
-      'hh_client',
-      '--lint',
-      $this->getFile()->getPath(),
-      '--json',
-      '--from',
-      'hhast',
+    $lines = await __Private\ProcessExecutionQueues::HH_CLIENT->waitForAsync(
+      vec[
+        '--lint',
+        $this->getFile()->getPath(),
+        '--json',
+        '--from',
+        'hhast',
+      ],
     );
     $hh_client_lint_result = TypeAssert\matches<this::TJSONResult>(
       \json_decode(

src/__Private/ProcessExecutionQueue.hack

@@ -0,0 +1,36 @@
+/*
+ *  Copyright (c) 2017-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the MIT license found in the
+ *  LICENSE file in the root directory of this source tree.
+ *
+ */
+
+namespace Facebook\HHAST\__Private;
+
+use namespace HH\Lib\Async;
+
+final class ProcessExecutionQueue {
+
+  <<__Memoize>>
+  private function impl(): Async\Semaphore<vec<string>, vec<string>> {
+    return new Async\Semaphore(
+      $this->limit,
+      async (vec<string> $args) ==>
+        await execute_async($this->process_name, ...$args),
+    );
+  }
+
+  public function __construct(
+    private int $limit,
+    private string $process_name,
+  )[] {
+  }
+
+  public async function waitForAsync(
+    vec<string> $args,
+  ): Awaitable<vec<string>> {
+    return await $this->impl()->waitForAsync($args);
+  }
+}

src/__Private/ProcessExecutionQueues.hack

@@ -0,0 +1,21 @@
+/*
+ *  Copyright (c) 2017-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the MIT license found in the
+ *  LICENSE file in the root directory of this source tree.
+ *
+ */
+
+namespace Facebook\HHAST\__Private;
+
+enum class ProcessExecutionQueues: ProcessExecutionQueue {
+
+    // Random number; it might seem high, but it's likely that `hh_parse` will
+    // execute quick enough that most of the processes are waiting to be cleaned
+    // up
+    ProcessExecutionQueue HH_CLIENT =
+        new ProcessExecutionQueue(32, 'hh_client');
+    ProcessExecutionQueue HH_PARSE = new ProcessExecutionQueue(32, 'hh_parse');
+
+}

src/entrypoints.hack

@@ -30,7 +30,8 @@ async function from_file_async(
   );
 
   try {
-    $results = await __Private\ParserQueue::get()->waitForAsync($args);
+    $results = await __Private\ProcessExecutionQueues::HH_PARSE
+      ->waitForAsync($args);
   } catch (__Private\SubprocessException $e) {
     throw new HHParseError(
       $path,