Introudce CombinedErrorOutput

CombinedErrorOutput is a concrete error output type for subprocesses that combines the standard error
output with the standard output stream.

When `CombinedErrorOutput` is used as the error output for a subprocess, both
standard output and standard error from the child process are merged into a
single output stream. This is equivalent to using shell redirection like `2>&1`.

Author: iCharlesHu

Sources/Subprocess/API.swift

@@ -32,7 +32,7 @@
 public func run<
     Input: InputProtocol,
     Output: OutputProtocol,
-    Error: OutputProtocol
+    Error: ErrorOutputProtocol
 >(
     _ executable: Executable,
     arguments: Arguments = [],
@@ -74,7 +74,7 @@ public func run<
 public func run<
     InputElement: BitwiseCopyable,
     Output: OutputProtocol,
-    Error: OutputProtocol
+    Error: ErrorOutputProtocol
 >(
     _ executable: Executable,
     arguments: Arguments = [],
@@ -117,7 +117,7 @@ public func run<
 ///   - isolation: the isolation context to run the body closure.
 ///   - body: The custom execution body to manually control the running process
 /// - Returns: an `ExecutableResult` type containing the return value of the closure.
-public func run<Result, Input: InputProtocol, Output: OutputProtocol, Error: OutputProtocol>(
+public func run<Result, Input: InputProtocol, Output: OutputProtocol, Error: ErrorOutputProtocol>(
     _ executable: Executable,
     arguments: Arguments = [],
     environment: Environment = .inherit,
@@ -164,7 +164,7 @@ public func run<Result, Input: InputProtocol, Output: OutputProtocol, Error: Out
 ///   - isolation: the isolation context to run the body closure.
 ///   - body: The custom execution body to manually control the running process.
 /// - Returns: an `ExecutableResult` type containing the return value of the closure.
-public func run<Result, Input: InputProtocol, Error: OutputProtocol>(
+public func run<Result, Input: InputProtocol, Error: ErrorOutputProtocol>(
     _ executable: Executable,
     arguments: Arguments = [],
     environment: Environment = .inherit,
@@ -257,7 +257,7 @@ public func run<Result, Input: InputProtocol, Output: OutputProtocol>(
 ///   - isolation: the isolation context to run the body closure.
 ///   - body: The custom execution body to manually control the running process
 /// - Returns: An `ExecutableResult` type containing the return value of the closure.
-public func run<Result, Error: OutputProtocol>(
+public func run<Result, Error: ErrorOutputProtocol>(
     _ executable: Executable,
     arguments: Arguments = [],
     environment: Environment = .inherit,
@@ -391,7 +391,7 @@ public func run<Result>(
 public func run<
     InputElement: BitwiseCopyable,
     Output: OutputProtocol,
-    Error: OutputProtocol
+    Error: ErrorOutputProtocol
 >(
     _ configuration: Configuration,
     input: borrowing Span<InputElement>,
@@ -451,7 +451,7 @@ public func run<
 public func run<
     Input: InputProtocol,
     Output: OutputProtocol,
-    Error: OutputProtocol
+    Error: ErrorOutputProtocol
 >(
     _ configuration: Configuration,
     input: Input = .none,
@@ -463,10 +463,13 @@ public func run<
         standardOutput: Output.OutputType,
         standardError: Error.OutputType
     )
+    let inputPipe = try input.createPipe()
+    let outputPipe = try output.createPipe()
+    let errorPipe = try error.createPipe(from: outputPipe)
     let result = try await configuration.run(
-        input: try input.createPipe(),
-        output: try output.createPipe(),
-        error: try error.createPipe()
+        input: inputPipe,
+        output: outputPipe,
+        error: errorPipe
     ) { (execution, inputIO, outputIO, errorIO) -> RunResult in
         // Write input, capture output and error in parallel
         var inputIOBox: IOChannel? = consume inputIO
@@ -540,18 +543,21 @@ public func run<
 ///   - body: The custom execution body to manually control the running process
 /// - Returns an executableResult type containing the return value
 ///     of the closure.
-public func run<Result, Input: InputProtocol, Output: OutputProtocol, Error: OutputProtocol>(
+public func run<Result, Input: InputProtocol, Output: OutputProtocol, Error: ErrorOutputProtocol>(
     _ configuration: Configuration,
     input: Input = .none,
     output: Output = .discarded,
     error: Error = .discarded,
     isolation: isolated (any Actor)? = #isolation,
     body: ((Execution) async throws -> Result)
 ) async throws -> ExecutionResult<Result> where Error.OutputType == Void {
+    let inputPipe = try input.createPipe()
+    let outputPipe = try output.createPipe()
+    let errorPipe = try error.createPipe(from: outputPipe)
     return try await configuration.run(
-        input: try input.createPipe(),
-        output: try output.createPipe(),
-        error: try error.createPipe()
+        input: inputPipe,
+        output: outputPipe,
+        error: errorPipe
     ) { execution, inputIO, outputIO, errorIO in
         var inputIOBox: IOChannel? = consume inputIO
         return try await withThrowingTaskGroup(
@@ -590,7 +596,7 @@ public func run<Result, Input: InputProtocol, Output: OutputProtocol, Error: Out
 ///   - body: The custom execution body to manually control the running process
 /// - Returns an executableResult type containing the return value
 ///     of the closure.
-public func run<Result, Input: InputProtocol, Error: OutputProtocol>(
+public func run<Result, Input: InputProtocol, Error: ErrorOutputProtocol>(
     _ configuration: Configuration,
     input: Input = .none,
     error: Error = .discarded,
@@ -599,10 +605,13 @@ public func run<Result, Input: InputProtocol, Error: OutputProtocol>(
     body: ((Execution, AsyncBufferSequence) async throws -> Result)
 ) async throws -> ExecutionResult<Result> where Error.OutputType == Void {
     let output = SequenceOutput()
+    let inputPipe = try input.createPipe()
+    let outputPipe = try output.createPipe()
+    let errorPipe = try error.createPipe(from: outputPipe)
     return try await configuration.run(
-        input: try input.createPipe(),
-        output: try output.createPipe(),
-        error: try error.createPipe()
+        input: inputPipe,
+        output: outputPipe,
+        error: errorPipe
     ) { execution, inputIO, outputIO, errorIO in
         var inputIOBox: IOChannel? = consume inputIO
         var outputIOBox: IOChannel? = consume outputIO
@@ -702,7 +711,7 @@ public func run<Result, Input: InputProtocol, Output: OutputProtocol>(
 ///   - body: The custom execution body to manually control the running process
 /// - Returns an executableResult type containing the return value
 ///     of the closure.
-public func run<Result, Error: OutputProtocol>(
+public func run<Result, Error: ErrorOutputProtocol>(
     _ configuration: Configuration,
     error: Error = .discarded,
     preferredBufferSize: Int? = nil,
@@ -711,10 +720,13 @@ public func run<Result, Error: OutputProtocol>(
 ) async throws -> ExecutionResult<Result> where Error.OutputType == Void {
     let input = CustomWriteInput()
     let output = SequenceOutput()
+    let inputPipe = try input.createPipe()
+    let outputPipe = try output.createPipe()
+    let errorPipe = try error.createPipe(from: outputPipe)
     return try await configuration.run(
-        input: try input.createPipe(),
-        output: try output.createPipe(),
-        error: try error.createPipe()
+        input: inputPipe,
+        output: outputPipe,
+        error: errorPipe
     ) { execution, inputIO, outputIO, errorIO in
         let writer = StandardInputWriter(diskIO: inputIO!)
         let outputSequence = AsyncBufferSequence(

Sources/Subprocess/Configuration.swift

@@ -811,6 +811,19 @@ internal struct IODescriptor: ~Copyable {
         self.closeWhenDone = closeWhenDone
     }
 
+    internal init?(duplicating ioDescriptor: borrowing IODescriptor?) throws {
+        let descriptor = try ioDescriptor?.duplicate()
+        if let descriptor {
+            self = descriptor
+        } else {
+            return nil
+        }
+    }
+
+    func duplicate() throws -> IODescriptor {
+        return try IODescriptor(self.descriptor.duplicate(), closeWhenDone: self.closeWhenDone)
+    }
+
     consuming func createIOChannel() -> IOChannel {
         let shouldClose = self.closeWhenDone
         self.closeWhenDone = false
@@ -962,6 +975,13 @@ internal struct CreatedPipe: ~Copyable {
         return self._writeFileDescriptor.take()
     }
 
+    internal init(duplicating createdPipe: borrowing CreatedPipe) throws {
+        self.init(
+            readFileDescriptor: try IODescriptor(duplicating: createdPipe._readFileDescriptor),
+            writeFileDescriptor: try IODescriptor(duplicating: createdPipe._writeFileDescriptor)
+        )
+    }
+
     internal init(closeWhenDone: Bool, purpose: Purpose) throws {
         #if canImport(WinSDK)
         /// On Windows, we need to create a named pipe.
@@ -1213,3 +1233,32 @@ extension Set {
         return self.remove(element) != nil
     }
 }
+
+#if canImport(WinSDK)
+extension HANDLE {
+    func duplicate() throws -> HANDLE {
+        var handle: HANDLE? = nil
+        guard
+            DuplicateHandle(
+                GetCurrentProcess(),
+                self,
+                GetCurrentProcess(),
+                &handle,
+                0, true, DWORD(DUPLICATE_SAME_ACCESS)
+            )
+        else {
+            throw SubprocessError(
+                code: .init(.failedToCreatePipe),
+                underlyingError: .init(rawValue: GetLastError())
+            )
+        }
+        guard let handle else {
+            throw SubprocessError(
+                code: .init(.failedToCreatePipe),
+                underlyingError: .init(rawValue: GetLastError())
+            )
+        }
+        return handle
+    }
+}
+#endif

Sources/Subprocess/IO/Output.swift

@@ -56,7 +56,7 @@ extension OutputProtocol {
 /// On Unix-like systems, `DiscardedOutput` redirects the
 /// standard output of the subprocess to `/dev/null`, while on Windows,
 /// redirects the output to `NUL`.
-public struct DiscardedOutput: OutputProtocol {
+public struct DiscardedOutput: OutputProtocol, ErrorOutputProtocol {
     /// The type for the output.
     public typealias OutputType = Void
 
@@ -82,7 +82,7 @@ public struct DiscardedOutput: OutputProtocol {
 ///
 /// Developers have the option to instruct the `Subprocess` to automatically
 /// close the related `FileDescriptor` after the subprocess is spawned.
-public struct FileDescriptorOutput: OutputProtocol {
+public struct FileDescriptorOutput: OutputProtocol, ErrorOutputProtocol {
     /// The type for this output.
     public typealias OutputType = Void
 
@@ -115,7 +115,7 @@ public struct FileDescriptorOutput: OutputProtocol {
 
 /// A concrete `Output` type for subprocesses that collects output
 /// from the subprocess as `String` with the given encoding.
-public struct StringOutput<Encoding: Unicode.Encoding>: OutputProtocol {
+public struct StringOutput<Encoding: Unicode.Encoding>: OutputProtocol, ErrorOutputProtocol {
     /// The type for this output.
     public typealias OutputType = String?
     /// The max number of bytes to collect.
@@ -147,7 +147,7 @@ public struct StringOutput<Encoding: Unicode.Encoding>: OutputProtocol {
 
 /// A concrete `Output` type for subprocesses that collects output from
 /// the subprocess as `[UInt8]`.
-public struct BytesOutput: OutputProtocol {
+public struct BytesOutput: OutputProtocol, ErrorOutputProtocol {
     /// The output type for this output option
     public typealias OutputType = [UInt8]
     /// The max number of bytes to collect
@@ -286,6 +286,57 @@ extension OutputProtocol where Self == BytesOutput {
     }
 }
 
+// MARK: - ErrorOutputProtocol
+
+/// Error output protocol specifies the set of methods that a type must implement to
+/// serve as the error output target for a subprocess.
+///
+/// Instead of developing custom implementations of `ErrorOutputProtocol`, use the
+/// default implementations provided by the `Subprocess` library to specify the
+/// output handling requirements.
+public protocol ErrorOutputProtocol: OutputProtocol {}
+
+/// A concrete error output type for subprocesses that combines the standard error
+/// output with the standard output stream.
+///
+/// When `CombinedErrorOutput` is used as the error output for a subprocess, both
+/// standard output and standard error from the child process are merged into a
+/// single output stream. This is equivalent to using shell redirection like `2>&1`.
+///
+/// This output type is useful when you want to capture or redirect both output
+/// streams together, making it easier to process all subprocess output as a unified
+/// stream rather than handling standard output and standard error separately.
+public struct CombinedErrorOutput: ErrorOutputProtocol {
+    public typealias OutputType = Void
+}
+
+extension ErrorOutputProtocol {
+    internal func createPipe(from outputPipe: borrowing CreatedPipe) throws -> CreatedPipe {
+        if self is CombinedErrorOutput {
+            return try CreatedPipe(duplicating: outputPipe)
+        }
+        return try createPipe()
+    }
+}
+
+extension ErrorOutputProtocol where Self == CombinedErrorOutput {
+    /// Creates an error output that combines standard error with standard output.
+    ///
+    /// When using `combineWithOutput`, both standard output and standard error from
+    /// the child process are merged into a single output stream. This is equivalent
+    /// to using shell redirection like `2>&1`.
+    ///
+    /// This is useful when you want to capture or redirect both output streams
+    /// together, making it easier to process all subprocess output as a unified
+    /// stream rather than handling standard output and standard error separately
+    ///
+    /// - Returns: A `CombinedErrorOutput` instance that merges standard error
+    ///   with standard output.
+    public static var combineWithOutput: Self {
+        return CombinedErrorOutput()
+    }
+}
+
 // MARK: - Span Default Implementations
 #if SubprocessSpan
 extension OutputProtocol {

Sources/Subprocess/SubprocessFoundation/Output+Foundation.swift

@@ -21,7 +21,7 @@ import FoundationEssentials
 
 /// A concrete `Output` type for subprocesses that collects output
 /// from the subprocess as data.
-public struct DataOutput: OutputProtocol {
+public struct DataOutput: OutputProtocol, ErrorOutputProtocol {
     /// The output type for this output option
     public typealias OutputType = Data
     /// The maximum number of bytes to collect.