🐛 Fix issue with concurrent output in RealShell.attach

phpmon/Common/Shell/RealShell.swift

@@ -219,6 +219,7 @@ class RealShell: ShellProtocol {
         process.standardError = errorPipe
 
         let output = ShellOutput.empty()
+        let serialQueue = DispatchQueue(label: "com.nicoverbruggen.phpmon.shell_output")
 
         return try await withCheckedThrowingContinuation({ continuation in
             let timeoutTask = Task {
@@ -235,19 +236,23 @@ class RealShell: ShellProtocol {
             outputPipe.fileHandleForReading.readabilityHandler = { fileHandle in
                 let data = fileHandle.availableData
                 if !data.isEmpty, let string = String(data: data, encoding: .utf8) {
+                    serialQueue.async {
                         output.out += string
                         didReceiveOutput(string, .stdOut)
                     }
                 }
+            }
 
             // Set up background reading for stderr
             errorPipe.fileHandleForReading.readabilityHandler = { fileHandle in
                 let data = fileHandle.availableData
                 if !data.isEmpty, let string = String(data: data, encoding: .utf8) {
+                    serialQueue.async {
                         output.err += string
                         didReceiveOutput(string, .stdErr)
                     }
                 }
+            }
 
             process.terminationHandler = { process in
                 timeoutTask.cancel()
@@ -260,6 +265,7 @@ class RealShell: ShellProtocol {
                 let remainingOut = outputPipe.fileHandleForReading.readDataToEndOfFile()
                 let remainingErr = errorPipe.fileHandleForReading.readDataToEndOfFile()
 
+                serialQueue.async {
                     if !remainingOut.isEmpty, let string = String(data: remainingOut, encoding: .utf8) {
                         output.out += string
                         didReceiveOutput(string, .stdOut)
@@ -276,6 +282,7 @@ class RealShell: ShellProtocol {
                         continuation.resume(returning: (process, .out(output.out)))
                     }
                 }
+            }
 
             process.launch()
         })

phpmon/Common/Shell/ShellProtocol.swift

@@ -67,7 +67,7 @@ enum ShellStream: Codable {
     case stdOut, stdErr, stdIn
 }
 
-class ShellOutput {
+class ShellOutput: @unchecked Sendable {
     var out: String
     var err: String
 

tests/unit/Testables/Shell/RealShellTest.swift

@@ -77,4 +77,43 @@ struct RealShellTest {
         let duration = start.duration(to: .now)
         #expect(duration < .milliseconds(2000)) // Should complete in ~700ms if parallel
     }
+
+    @Test func attach_handles_concurrent_stdout_stderr_writes_safely() async throws {
+        // This test verifies that concurrent writes to output.out and output.err
+        // from multiple readability handlers don't cause data races or crashes.
+        // Without the serial queue, rapid interleaved output causes undefined behavior.
+
+        let script = """
+        for i in {1..200}; do
+            echo "stdout-$i" >&1
+            echo "stderr-$i" >&2
+        done
+        """
+
+        var receivedChunks = 0
+        let (_, shellOutput) = try await container.shell.attach(
+            script,
+            didReceiveOutput: { _, _ in
+                receivedChunks += 1
+            },
+            withTimeout: 5.0
+        )
+
+        // Verify all output was captured without corruption
+        let stdoutLines = shellOutput.out
+            .components(separatedBy: "\n")
+            .filter { !$0.isEmpty }
+        let stderrLines = shellOutput.err
+            .components(separatedBy: "\n")
+            .filter { !$0.isEmpty }
+
+        #expect(stdoutLines.count == 200)
+        #expect(stderrLines.count == 200)
+
+        // Verify content integrity - each line should match the pattern
+        for i in 1...200 {
+            #expect(stdoutLines.contains("stdout-\(i)"))
+            #expect(stderrLines.contains("stderr-\(i)"))
+        }
+    }
 }