From 91d1a0b70e36d2557229af2655d5a041a02324a8 Mon Sep 17 00:00:00 2001
From: Max Desiatov <m_desiatov@apple.com>
Date: Tue, 12 Dec 2023 18:53:27 +0000
Subject: [PATCH] Use a concurrent queue to wait for process completion (#450)

Calling blocking `waitpid` on Swift concurrency threads can cause freezes for `async` code when spawning too many processes with the existing `Process` API.
---
 Sources/TSCBasic/CMakeLists.txt               |  6 ++--
 Sources/TSCBasic/{ => Process}/Process.swift  | 30 ++++++++-----------
 .../TSCBasic/{ => Process}/ProcessEnv.swift   |  2 +-
 .../TSCBasic/{ => Process}/ProcessSet.swift   |  0
 4 files changed, 16 insertions(+), 22 deletions(-)
 rename Sources/TSCBasic/{ => Process}/Process.swift (98%)
 rename Sources/TSCBasic/{ => Process}/ProcessEnv.swift (97%)
 rename Sources/TSCBasic/{ => Process}/ProcessSet.swift (100%)

diff --git a/Sources/TSCBasic/CMakeLists.txt b/Sources/TSCBasic/CMakeLists.txt
index 0f30dfff..a1f5bcc2 100644
--- a/Sources/TSCBasic/CMakeLists.txt
+++ b/Sources/TSCBasic/CMakeLists.txt
@@ -37,9 +37,9 @@ add_library(TSCBasic
   WritableByteStream.swift
   Path.swift
   PathShims.swift
-  Process.swift
-  ProcessEnv.swift
-  ProcessSet.swift
+  Process/Process.swift
+  Process/ProcessEnv.swift
+  Process/ProcessSet.swift
   RegEx.swift
   Result.swift
   SortedArray.swift
diff --git a/Sources/TSCBasic/Process.swift b/Sources/TSCBasic/Process/Process.swift
similarity index 98%
rename from Sources/TSCBasic/Process.swift
rename to Sources/TSCBasic/Process/Process.swift
index 5a396823..12a144c0 100644
--- a/Sources/TSCBasic/Process.swift
+++ b/Sources/TSCBasic/Process/Process.swift
@@ -130,11 +130,15 @@ public struct ProcessResult: CustomStringConvertible, Sendable {
     }
 }
 
-#if swift(<5.6)
-extension Process: UnsafeSendable {}
-#else
 extension Process: @unchecked Sendable {}
-#endif
+
+extension DispatchQueue {
+    // a shared concurrent queue for running concurrent asynchronous operations
+    static let processConcurrent = DispatchQueue(
+        label: "swift.org.swift-tsc.process.concurrent",
+        attributes: .concurrent
+    )
+}
 
 /// Process allows spawning new subprocesses and working with them.
 ///
@@ -829,25 +833,15 @@ public final class Process {
     @available(macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
     @discardableResult
     public func waitUntilExit() async throws -> ProcessResult {
-        #if compiler(>=5.6)
-        return try await withCheckedThrowingContinuation { continuation in
-            waitUntilExit(continuation.resume(with:))
-        }
-        #else
-        if #available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *) {
-            return try await withCheckedThrowingContinuation { continuation in
-                waitUntilExit(continuation.resume(with:))
+        try await withCheckedThrowingContinuation { continuation in
+            DispatchQueue.processConcurrent.async {
+                self.waitUntilExit(continuation.resume(with:))
             }
-        } else {
-            preconditionFailure("Unsupported with Swift 5.5 on this OS version")
         }
-        #endif
     }
 
     /// Blocks the calling process until the subprocess finishes execution.
-//    #if compiler(>=5.8)
-//    @available(*, noasync)
-//    #endif
+    @available(*, noasync)
     @discardableResult
     public func waitUntilExit() throws -> ProcessResult {
         let group = DispatchGroup()
diff --git a/Sources/TSCBasic/ProcessEnv.swift b/Sources/TSCBasic/Process/ProcessEnv.swift
similarity index 97%
rename from Sources/TSCBasic/ProcessEnv.swift
rename to Sources/TSCBasic/Process/ProcessEnv.swift
index bf7e3c62..226406aa 100644
--- a/Sources/TSCBasic/ProcessEnv.swift
+++ b/Sources/TSCBasic/Process/ProcessEnv.swift
@@ -11,7 +11,7 @@
 import Foundation
 import TSCLibc
 
-/// Provides functionality related a process's enviorment.
+/// Provides functionality related a process's environment.
 public enum ProcessEnv {
 
     /// Returns a dictionary containing the current environment.
diff --git a/Sources/TSCBasic/ProcessSet.swift b/Sources/TSCBasic/Process/ProcessSet.swift
similarity index 100%
rename from Sources/TSCBasic/ProcessSet.swift
rename to Sources/TSCBasic/Process/ProcessSet.swift