fix: Read child-process output to EOF to prevent truncation and hangs

The previous implementation wrote the entire stdin payload synchronously
before any stdout was drained, then relied on NSFileHandleDataAvailable
notifications to collect output. For inputs larger than the 64 KB pipe
buffer the child blocked writing stdout, stopped reading stdin, and the
parent's blocking write deadlocked before waitUntilExit() was reached.

Drain stdout and stderr to EOF on background queues started before the
stdin write, join them after the process exits, then publish the buffers.
This collects output in full and removes the deadlock. Dropping the
notification observers also removes the unbalanced-observer teardown, so
no deinit is required.

Add a testable run(executableURL:) entry point and TaskTests covering a
>64 KB round-trip, stderr capture, and non-zero exit status.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: macmade

Shared/Task.swift

@@ -42,13 +42,18 @@ public class Task
             return nil
         }
 
-        guard FileManager.default.fileExists( atPath: executable.path )
+        return self.run( executableURL: executable, arguments: arguments, input: input )
+    }
+
+    public class func run( executableURL: URL, arguments: [ String ], input: Data? ) -> Task?
+    {
+        guard FileManager.default.fileExists( atPath: executableURL.path )
         else
         {
             return nil
         }
 
-        let task = Task( executable: executable, arguments: arguments )
+        let task = Task( executable: executableURL, arguments: arguments )
 
         task.run( input: input )
 
@@ -68,12 +73,6 @@ public class Task
 
         self.standardOutput = Data()
         self.standardError  = Data()
-
-        NotificationCenter.default.addObserver( self, selector: #selector( self.dataAvailableForStandardOutput( _: ) ), name: NSNotification.Name.NSFileHandleDataAvailable, object: self.pipeOut.fileHandleForReading )
-        NotificationCenter.default.addObserver( self, selector: #selector( self.dataAvailableForStandardError( _: )  ), name: NSNotification.Name.NSFileHandleDataAvailable, object: self.pipeErr.fileHandleForReading )
-
-        self.pipeOut.fileHandleForReading.waitForDataInBackgroundAndNotify()
-        self.pipeErr.fileHandleForReading.waitForDataInBackgroundAndNotify()
     }
 
     public func run( input: Data? )
@@ -85,6 +84,24 @@ public class Task
 
         self.task.launch()
 
+        // Drain both output pipes to EOF on background queues, started before we
+        // write stdin and before we block in waitUntilExit(). This guarantees the
+        // child can never deadlock by filling a pipe buffer we aren't reading, and
+        // that the full output is collected rather than truncated.
+        let group   = DispatchGroup()
+        var outData = Data()
+        var errData = Data()
+
+        DispatchQueue.global( qos: .userInitiated ).async( group: group )
+        {
+            outData = self.pipeOut.fileHandleForReading.readDataToEndOfFile()
+        }
+
+        DispatchQueue.global( qos: .userInitiated ).async( group: group )
+        {
+            errData = self.pipeErr.fileHandleForReading.readDataToEndOfFile()
+        }
+
         if let input = input, let pipe = self.task.standardInput as? Pipe
         {
             let handle = pipe.fileHandleForWriting
@@ -95,6 +112,12 @@ public class Task
 
         self.task.waitUntilExit()
 
+        // The child has exited and closed its pipe ends, so both reads have hit
+        // (or are about to hit) EOF. Join them before publishing the results.
+        group.wait()
+
+        self.standardOutput    = outData
+        self.standardError     = errData
         self.terminationStatus = self.task.terminationStatus
 
         #if DEBUG
@@ -104,32 +127,4 @@ public class Task
             }
         #endif
     }
-
-    @objc
-    private func dataAvailableForStandardOutput( _ notification: Notification )
-    {
-        guard let handle = notification.object as? FileHandle?,
-              let data   = handle?.availableData
-        else
-        {
-            return
-        }
-
-        self.standardOutput.append( data )
-        handle?.waitForDataInBackgroundAndNotify()
-    }
-
-    @objc
-    private func dataAvailableForStandardError( _ notification: Notification )
-    {
-        guard let handle = notification.object as? FileHandle?,
-              let data = handle?.availableData
-        else
-        {
-            return
-        }
-
-        self.standardError.append( data )
-        handle?.waitForDataInBackgroundAndNotify()
-    }
 }

SharedTests/TaskTests.swift

@@ -0,0 +1,69 @@
+/*******************************************************************************
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2022, Jean-David Gadina - www.xs-labs.com
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the Software), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED AS IS, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ ******************************************************************************/
+
+import Foundation
+import Testing
+
+@Suite( "Task — child-process I/O" )
+struct TaskTests
+{
+    // A payload several times larger than the 64 KB kernel pipe buffer. If the
+    // output is not drained while the child writes, the child blocks on write
+    // and the parent deadlocks in waitUntilExit(); if reads are torn down too
+    // early, the tail is truncated. Either way this round-trip fails.
+    private static let largePayload = Data( ( 0 ..< ( 256 * 1024 ) ).map { UInt8( $0 & 0xFF ) } )
+
+    @Test( "Collects complete stdout for input larger than the pipe buffer", .timeLimit( .minutes( 1 ) ) )
+    func largeInputRoundTrip() throws
+    {
+        let task = try #require( Task.run( executableURL: URL( fileURLWithPath: "/bin/cat" ), arguments: [], input: Self.largePayload ) )
+
+        #expect( task.terminationStatus == 0 )
+        #expect( task.standardOutput.count == Self.largePayload.count )
+        #expect( task.standardOutput == Self.largePayload )
+    }
+
+    @Test( "Captures standard error", .timeLimit( .minutes( 1 ) ) )
+    func capturesStandardError() throws
+    {
+        let task = try #require( Task.run( executableURL: URL( fileURLWithPath: "/bin/sh" ), arguments: [ "-c", "printf 'boom' 1>&2" ], input: nil ) )
+
+        #expect( task.terminationStatus == 0 )
+        #expect( String( data: task.standardError, encoding: .utf8 ) == "boom" )
+    }
+
+    @Test( "Reports a non-zero termination status", .timeLimit( .minutes( 1 ) ) )
+    func nonZeroTerminationStatus() throws
+    {
+        let task = try #require( Task.run( executableURL: URL( fileURLWithPath: "/bin/sh" ), arguments: [ "-c", "exit 3" ], input: nil ) )
+
+        #expect( task.terminationStatus == 3 )
+    }
+
+    @Test( "Returns nil for a non-existent executable" )
+    func missingExecutable()
+    {
+        #expect( Task.run( executableURL: URL( fileURLWithPath: "/nonexistent/executable" ), arguments: [], input: nil ) == nil )
+    }
+}

XcodeFormat.xcodeproj/project.pbxproj

@@ -63,6 +63,7 @@
 		415B69E897EF098F1D497C6B /* String.swift in Sources */ = {isa = PBXBuildFile; fileRef = 05744ED528EC2EDB00A88503 /* String.swift */; };
 		422C428BA7CBC2B94FD892A0 /* Task.swift in Sources */ = {isa = PBXBuildFile; fileRef = 05BED21928EC4A2A008039F6 /* Task.swift */; };
 		52C07AA4ACA79DEA083917ED /* URL.swift in Sources */ = {isa = PBXBuildFile; fileRef = 05744ECD28EC2E5000A88503 /* URL.swift */; };
+		54AF1A10772285FD73793431 /* TaskTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 913F32B63DB056684754D787 /* TaskTests.swift */; };
 		6AF1DB0943F9AA065EA80860 /* URLTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = B907F7DC445EA71845A03EE9 /* URLTests.swift */; };
 		7CA215582B396CDC75D0E7A5 /* Data.swift in Sources */ = {isa = PBXBuildFile; fileRef = 05744ED228EC2ED400A88503 /* Data.swift */; };
 		EF9CF118DCDD7951D6C90043 /* Configuration.swift in Sources */ = {isa = PBXBuildFile; fileRef = 050E0F6128EB66390080C562 /* Configuration.swift */; };
@@ -241,6 +242,7 @@
 		2B7221495008B0E46F3003F1 /* SharedTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = SharedTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
 		754426165B1A6F7B9A8AA57D /* Cocoa.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = Cocoa.framework; path = Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.0.sdk/System/Library/Frameworks/Cocoa.framework; sourceTree = DEVELOPER_DIR; };
 		78DA3F1F404A081C0DE85CA4 /* ConfigurationTests.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = ConfigurationTests.swift; sourceTree = "<group>"; };
+		913F32B63DB056684754D787 /* TaskTests.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = TaskTests.swift; sourceTree = "<group>"; };
 		B907F7DC445EA71845A03EE9 /* URLTests.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = URLTests.swift; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
@@ -571,6 +573,7 @@
 			children = (
 				B907F7DC445EA71845A03EE9 /* URLTests.swift */,
 				78DA3F1F404A081C0DE85CA4 /* ConfigurationTests.swift */,
+				913F32B63DB056684754D787 /* TaskTests.swift */,
 			);
 			name = SharedTests;
 			path = SharedTests;
@@ -812,6 +815,7 @@
 				415B69E897EF098F1D497C6B /* String.swift in Sources */,
 				52C07AA4ACA79DEA083917ED /* URL.swift in Sources */,
 				422C428BA7CBC2B94FD892A0 /* Task.swift in Sources */,
+				54AF1A10772285FD73793431 /* TaskTests.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};