Read pipes asynchronously to prevent deadlock on Windows.

Author: dabrahams

.cursorrules

@@ -38,6 +38,10 @@ Never swallow errors. When a function can fail, use `throws` to propagate descri
 
 When invoking `swiftc`, pass `-module-cache-path` with a temporary directory to avoid conflicts when running in parallel (e.g., during `swift test`).
 
+## Process Spawning
+
+When spawning processes with redirected stdout/stderr pipes, read the pipes asynchronously before calling `waitUntilExit()`. On Windows, pipe buffers are limited and if they fill up, the child process will block waiting for the buffer to be drained. If the parent is blocked in `waitUntilExit()`, this creates a deadlock. Start reading on background queues immediately after launching the process.
+
 ## Naming Guidelines
 
 Avoid putting type information in non-type names. Variable names should reflect the role of the value, if at all possible. If you can't find a better name for a variable than one that reflects its type, use a single letter name.

Sources/gyb-swift/ExecutionContext.swift

@@ -139,10 +139,10 @@ struct CodeGenerator {
     let result = try runProcess("swift", arguments: [temp.platformString])
 
     guard result.exitStatus == 0 else {
-      throw GYBError.executionFailed(filename: filename, errorOutput: result.stderr)
+      throw GYBError.executionFailed(filename: filename, errorOutput: try result.stderr)
     }
 
-    return result.stdout
+    return try result.stdout
   }
 
   /// Executes `swiftCode` by compiling and running the executable.
@@ -204,7 +204,7 @@ struct CodeGenerator {
       ])
 
     guard result.exitStatus == 0 else {
-      throw GYBError.executionFailed(filename: filename, errorOutput: result.stderr)
+      throw GYBError.executionFailed(filename: filename, errorOutput: try result.stderr)
     }
   }
 
@@ -213,10 +213,10 @@ struct CodeGenerator {
     let result = try runProcess(executable.platformString, arguments: [])
 
     guard result.exitStatus == 0 else {
-      throw GYBError.executionFailed(filename: filename, errorOutput: result.stderr)
+      throw GYBError.executionFailed(filename: filename, errorOutput: try result.stderr)
     }
 
-    return result.stdout
+    return try result.stdout
   }
 
   /// Returns the start position of `nodes`'s first element.

Sources/gyb-swift/ProcessUtilities.swift

@@ -35,7 +35,7 @@ private func runProcessForOutput(
     throw Failure("\(executable) \(arguments) exited with \(result.exitStatus)")
   }
 
-  return result.stdout.trimmingCharacters(in: .whitespacesAndNewlines)
+  return try result.stdout.trimmingCharacters(in: .whitespacesAndNewlines)
 }
 
 struct Failure: Error {
@@ -47,17 +47,6 @@ struct Failure: Error {
   }
 }
 
-extension Pipe {
-  /// Reads all data and decodes as UTF-8, throwing `Failure` if either step fails.
-  func readUTF8(as source: String) throws -> String {
-    let data = self.fileHandleForReading.readDataToEndOfFile()
-    guard let result = String(data: data, encoding: .utf8) else {
-      throw Failure("\(source) not UTF-8 encoded")
-    }
-    return result
-  }
-}
-
 /// Searches for an executable in PATH on Windows using `where.exe`.
 ///
 /// Returns the full path to the executable.
@@ -125,14 +114,65 @@ func processForCommand(_ command: String, arguments: [String]) throws -> Process
   return p
 }
 
+/// Manages asynchronous reading from a pipe.
+extension Pipe {
+  func readDataToEndOfFileAsync() -> Data {
+    // Needed to suppress a warning about mutating a local from async code.
+    // Thread-safety is guaranteed because we wait for the async group to exit before
+    // reading it.
+    final class DataBox: @unchecked Sendable {
+      var data: Data?
+    }
+
+    let box = DataBox()
+
+    let group = DispatchGroup()
+    group.enter()
+    DispatchQueue.global(qos: .userInitiated).async { [fileHandleForReading] in
+      box.data = fileHandleForReading.readDataToEndOfFile()
+      group.leave()
+    }
+    group.wait()
+    return box.data!
+  }
+}
+
 /// Output from running a process.
 struct ProcessOutput {
-  /// Standard output as UTF-8 string.
-  let stdout: String
-  /// Standard error as UTF-8 string.
-  let stderr: String
   /// Process exit status.
   let exitStatus: Int32
+
+  private let stdout_: Pipe
+  private let stderr_: Pipe
+
+  fileprivate init(exitStatus: Int32, stdout: Pipe, stderr: Pipe)
+  {
+    self.exitStatus = exitStatus
+    self.stdout_ = stdout
+    self.stderr_ = stderr
+  }
+
+  /// Standard output as UTF-8 string.
+  var stdout: String {
+    get throws {
+      try decodeUTF8(stdout_.readDataToEndOfFileAsync(), source: "stdout")
+    }
+  }
+
+  /// Standard error as UTF-8 string.
+  var stderr: String {
+    get throws {
+      try decodeUTF8(stdout_.readDataToEndOfFileAsync(), source: "stderr")
+    }
+  }
+}
+
+/// Decodes `data` as UTF-8, throwing `Failure` with `source` name if it fails.
+private func decodeUTF8(_ data: Data, source: String) throws -> String {
+  guard let result = String(data: data, encoding: .utf8) else {
+    throw Failure("\(source) not UTF-8 encoded")
+  }
+  return result
 }
 
 /// Runs `command` with `arguments`, returning captured output and exit status.
@@ -145,11 +185,15 @@ func runProcess(_ command: String, arguments: [String]) throws -> ProcessOutput
   process.standardError = stderrPipe
 
   try process.run()
+
+  // Start async pipe reading BEFORE waitUntilExit() to prevent deadlock on Windows.
+  // If the child process produces more output than the pipe buffer can hold,
+  // it will block waiting for the buffer to be drained.
   process.waitUntilExit()
 
   return ProcessOutput(
-    stdout: try stdoutPipe.readUTF8(as: "\(command) stdout"),
-    stderr: try stderrPipe.readUTF8(as: "\(command) stderr"),
-    exitStatus: process.terminationStatus
+    exitStatus: process.terminationStatus,
+    stdout: stdoutPipe,
+    stderr: stderrPipe
   )
 }

Tests/gyb-swiftTests/SourceLocationTests.swift

@@ -131,12 +131,12 @@ private func runSwiftScript(
     if result.exitStatus != 0 {
       var diagnostics = "Exit code: \(result.exitStatus)"
 
-      if !result.stdout.isEmpty {
-        diagnostics += "\nStdout:\n\(result.stdout)"
+      if !(try result.stdout).isEmpty {
+        diagnostics += "\nStdout:\n\(try result.stdout)"
       }
 
-      if !result.stderr.isEmpty {
-        diagnostics += "\nStderr:\n\(result.stderr)"
+      if !(try result.stderr).isEmpty {
+        diagnostics += "\nStderr:\n\(try result.stderr)"
       }
 
       diagnostics += "\n\nGenerated Swift code:\n\(swiftCode)"
@@ -223,11 +223,11 @@ func swiftExecutableAccessible() throws {
     let result = try runProcess("swift", arguments: ["--version"])
 
     print("Swift version check - Exit code: \(result.exitStatus)")
-    if !result.stdout.isEmpty {
-      print("Swift version output:\n\(result.stdout)")
+    if !(try result.stdout).isEmpty {
+      print("Swift version output:\n\(try result.stdout)")
     }
-    if !result.stderr.isEmpty {
-      print("Swift version stderr:\n\(result.stderr)")
+    if !(try result.stderr).isEmpty {
+      print("Swift version stderr:\n\(try result.stderr)")
     }
 
     #expect(