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swift-experimental-subprocess's Introduction

Introducing Swift Subprocess

Revision History

  • v1: Initial draft
  • v2: Minor Updates:
    • Switched AsyncBytes to be backed by DispatchIO.
    • Introduced resolveExecutablePath(withEnvironment:) to enable explicit lookup of the executable path.
    • Added a new option, closeWhenDone, to automatically close the file descriptors passed in via .readFrom and friends.
    • Introduced a new parameter, shouldSendToProcessGroup, in the sendSignal function to control whether the signal should be sent to the process or the process group.
    • Introduced a section on "Future Directions."
  • v3: Minor updates:
    • Added a section describing Task Cancellation
    • Clarified for readFrom() and writeTo() Subprocess will close the passed in file descriptor right after spawning the process when closeWhenDone is set to true.
    • Adjusted argument orders in Arguments.
    • Added Subprocess.run(withConfiguration:...) in favor or Configuration.run().

Introduction

As Swift establishes itself as a general-purpose language for both compiled and scripting use cases, one persistent pain point for developers is process creation. The existing Foundation API for spawning a process, NSTask, originated in Objective-C. It was subsequently renamed to Process in Swift. As the language has continued to evolve, Process has not kept up. It lacks support for async/await, makes extensive use of completion handlers, and uses Objective-C exceptions to indicate developer error. This proposal introduces a new type called Subprocess, which addresses the ergonomic shortcomings of Process and enhances the experience of using Swift for scripting.

Motivation

Consider the following shell script that checks the list of changes in the current repository and announces the result:

#!/usr/bin/env bash

changedFiles=$(git diff --name-only)
if [[ -z "$changedFiles" ]]; then
    # No changed file
    say "No changed files"
else
    # Split changed files into comma-separated text
    changedFiles=$(echo "$changedFiles" | tr "\n" ", ")
    say "These files have changed: ${changedFiles}"
fi

If we were to rewrite this example in Swift script today with Process, it would look something like this:

#!/usr/bin/swift

import Foundation

let gitProcess = Process()
let gitProcessPipe = Pipe()                                         // <- 0
gitProcess.currentDirectoryURL = URL(fileURLWithPath: ".")
gitProcess.executableURL = URL(fileURLWithPath: "/usr/bin/git")     // <- 1
gitProcess.arguments = [
    "diff",
    "--name-only"
]
gitProcess.standardOutput = gitProcessPipe
try gitProcess.run()
let processOutput = gitProcessPipe
    .fileHandleForReading.readDataToEndOfFile()                     // <- 2
gitProcess.waitUntilExit()                                          // <- 3
var changedFiles = String(data: processOutput, encoding: .utf8)!
if changedFiles.isEmpty {
    changedFiles = "No changed files"
} else {
    changedFiles = changedFiles.split(separator: "\n").joined(separator: ", ")
    changedFiles = "These files have changed: \(changedFiles)"
}

let sayProcess = Process()
sayProcess.currentDirectoryURL = URL(fileURLWithPath: ".")
sayProcess.executableURL = URL(fileURLWithPath: "/usr/bin/say")
sayProcess.arguments = [
    changedFiles
]
try sayProcess.run()

While the Swift script above is functionally equivalent to the shell script, it is unnecessarily verbose and cumbersome to use. Specifically, we can observe the following issues (the relevant regions are marked in the example above):

  1. Process requires the user to explicitly set standard output and standard error before being able to access the process output. Furthermore, the standard IO properties, .standardInput, .standardOutput, .standardError, all have the type Any because they support both Pipe and FileHandle. This design can easily lead to confusion because one may attempt to directly access a Process' standardOutput without realizing the need to set these properties first.
  2. Process expects an explicit URL to point to its executable instead of trying to resolve the executable path using the $PATH variable. This design adds friction in Swift scripting because developers will have to explicitly look up the path to any executable they wish to run.
  3. Process expects developers to work with Pipe directly to read process output. It could be confusing to determine the correct fileHandle to read.
  4. Instead of async/await, Process uses blocking methods (.waitUntilExit()) and callbacks (.readabilityHandler) exclusively. This design leaves developers with the responsibility to manage asynchronicity and can easily introduce a "Pyramid of Doom."

Proposed solution

We propose a new type, struct Subprocess, that will eventually replace Process as the canonical way to launch a process in Swift.

Here's the above script rewritten using Subprocess:

#!/usr/bin/swift

import FoundationEssentials

let gitResult = try await Subprocess.run(   // <- 0
    executing: .named("git"),               // <- 1
    arguments: ["diff", "--name-only"]
)

var changedFiles = String(
    data: gitResult.standardOutput!,
    encoding: .utf8)!
if changedFiles.isEmpty {
    changedFiles = "No changed files"
}
_ = try await Subprocess.run(
    executing: .named("say"),
    arguments: [changedFiles]
)

Let's break down the example above:

  1. Subprocess is constructed entirely on the async/await paradigm. The run() method utilizes await to allow the child process to finish, asynchronously returning an ExecutionResult. Additionally, there is an closure based overload of run() that offers more granulated control, which will be discussed later.
  2. There are two primary ways to configure the executable being run:
    • The default approach, recommended for most developers, is .at(FilePath). This allows developers to specify a full path to an executable.
    • Alternatively, developers can use .named(String) to instruct Subprocess to look up the executable path based on heuristics such as the $PATH environment variable.

Detailed Design

The New Subprocess Type

We propose a new struct Subprocess. Developers primarily interact with Subprocess via the static run methods which asynchronously executes a subprocess.

@available(FoundationPreview 0.4, *)
@available(iOS, unavailable)
@available(tvOS, unavailable)
@available(watchOS, unavailable)
extension Subprocess {
    public static func run(
        executing executable: Executable,
        arguments: Arguments = [],
        environment: Environment = .inherit,
        workingDirectory: FilePath? = nil,
        platformOptions: PlatformOptions = .default,
        input: InputMethod = .noInput,
        output: CollectedOutputMethod = .collect,
        error: CollectedOutputMethod = .collect
    ) async throws -> CollectedResult

    public static func run(
        executing executable: Executable,
        arguments: Arguments = [],
        environment: Environment = .inherit,
        workingDirectory: FilePath? = nil,
        platformOptions: PlatformOptions = .default,
        input: some Sequence<UInt8>,
        output: CollectedOutputMethod = .collect,
        error: CollectedOutputMethod = .collect
    ) async throws -> CollectedResult

    public static func run<S: AsyncSequence>(
        executing executable: Executable,
        arguments: Arguments = [],
        environment: Environment = .inherit,
        workingDirectory: FilePath? = nil,
        platformOptions: PlatformOptions = .default,
        input: S,
        output: CollectedOutputMethod = .collect,
        error: CollectedOutputMethod = .collect
    ) async throws -> CollectedResult where S.Element == UInt8
}

// MARK: - Custom Execution Body
extension Subprocess {
    public static func run<R>(
        executing executable: Executable,
        arguments: Arguments = [],
        environment: Environment = .inherit,
        workingDirectory: FilePath? = nil,
        platformOptions: PlatformOptions = .default,
        input: InputMethod = .noInput,
        output: RedirectedOutputMethod = .redirect,
        error: RedirectedOutputMethod = .discard,
        _ body: (@Sendable @escaping (Subprocess) async throws -> R)
    ) async throws -> Result<R>

    public static func run<R>(
        executing executable: Executable,
        arguments: Arguments = [],
        environment: Environment = .inherit,
        platformOptions: PlatformOptions = .default,
        input: some Sequence<UInt8>,
        output: RedirectedOutputMethod = .redirect,
        error: RedirectedOutputMethod = .discard,
        _ body: (@Sendable @escaping (Subprocess) async throws -> R)
    ) async throws -> Result<R>

    public static func run<R, S: AsyncSequence>(
        executing executable: Executable,
        arguments: Arguments = [],
        environment: Environment = .inherit,
        workingDirectory: FilePath? = nil,
        platformOptions: PlatformOptions = .default,
        input: S,
        output: RedirectedOutputMethod = .redirect,
        error: RedirectedOutputMethod = .discard,
        _ body: (@Sendable @escaping (Subprocess) async throws -> R)
    ) async throws -> Result<R> where S.Element == UInt8

    public static func run<R>(
        executing executable: Executable,
        arguments: Arguments = [],
        environment: Environment = .inherit,
        workingDirectory: FilePath? = nil,
        platformOptions: PlatformOptions = .default,
        output: RedirectedOutputMethod = .redirect,
        error: RedirectedOutputMethod = .discard,
        _ body: (@Sendable @escaping (Subprocess, StandardInputWriter) async throws -> R)
    ) async throws -> Result<R>

    public static func run<R>(
        withConfiguration configuration: Configuration,
        output: RedirectedOutputMethod = .redirect,
        error: RedirectedOutputMethod = .redirect,
        _ body: (@Sendable @escaping (Subprocess, StandardInputWriter) async throws -> R)
    ) async throws -> Result<R>
}

The run methods can generally be divided into two categories, each addressing distinctive use cases of Subprocess:

  • The first category returns a simple CollectedResult object, encapsulating information such as ProcessIdentifier, TerminationStatus, as well as collected standard output and standard error if requested. These methods are designed for straightforward use cases of Subprocess, where developers are primarily interested in the output or termination status of a process. Here are some examples:
// Simple ls with no standard input
let ls = try await Subprocess.run(
    executing: .named("ls"),
    output: .collect)
print("Items in current directory: \(String(data: ls.standardOutput!, encoding: .utf8)!)")

// Launch VSCode with arguments
let code = try await Subprocess.run(
    executing: .named("code"),
    arguments: ["/some/directory"])
print("Code launched successfully: \(result.terminationStatus.isSuccess)")

// Launch `cat` with sequence written to standardInput
let inputData = "Hello SwiftFoundation".utf8CString.map { UInt8($0) }
let cat = try await Subprocess.run(
    executing: .named("cat"),
    input: inputData,
    output: .collect
)
print("Cat result: \(String(data: cat.standardOutput!, encoding: .utf8)!)")
  • Alternatively, developers can leverage the closure-based approach. These methods spawn the child process and invoke the provided body closure with a Subprocess object. Developers can send signals to the running subprocess or transform standardOutput or standardError to the desired result type within the closure. One additional variation of the closure-based methods provides the body closure with an additional Subprocess.StandardInputWriter object, allowing developers to write to the standard input of the subprocess directly. These methods asynchronously wait for the child process to exit before returning the result.
// Use curl to call REST API
struct MyType: Codable { ... }

let result = try await Subprocess.run(
    executing: .named("curl"),
    arguments: ["/some/rest/api"],
    output: .redirect) {
        let output = try await Array($0.standardOutput!)
        return try JSONDecoder().decode(MyType.self, from: Data(output))
}
// Result will have type `MyType`
print("Result: \(result)")

// Perform custom write and write the standard output
let result = try await Subprocess.run(
    executing: .at("/some/executable"),
    output: .redirect) { subprocess, writer in
    try await writer.write("Hello World".utf8CString)
    try await writer.finish()
    return try await Array(subprocess.standardOutput!.lines)
}

Both styles of the run methods provide convenient overloads that allow developers to pass a Sequence<UInt8> or AsyncSequence<UInt8> to the standard input of the subprocess.

The Subprocess object itself is designed to represent an executed process. This execution could be either in progress or completed. Direct construction of Subprocess instances is not supported; instead, a Subprocess object is passed to the body closure of run(). This object is only valid within the scope of the closure, and developers may use it to send signals to the child process or retrieve the child's standard I/Os via AsyncSequences.

@available(FoundationPreview 0.4, *)
@available(iOS, unavailable)
@available(tvOS, unavailable)
@available(watchOS, unavailable)
public struct Subprocess: Sendable {
    public let processIdentifier: ProcessIdentifier
    // The standard output of the child process, expressed as AsyncSequence<UInt8>
    // This property is `nil` if the standard output is discarded or written to disk
    public var standardOutput: AsyncBytes? { get }
    // The standard error of the child process, expressed as AsyncSequence<UInt8>
    // This property is `nil` if the standard error is discarded or written to disk
    public var standardError: AsyncBytes? { get }
    // If `shouldSendToProcessGroup` is `true`, the signal will be send to the entire process
    // group instead of the current process.
    public func sendSignal(_ signal: Signal, toProcessGroup shouldSendToProcessGroup: Bool) throws
}

extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct ProcessIdentifier: Sendable, Hashable {
        let value: pid_t

        public init(value: pid_t) {
            self.value = value
        }
    }
}

Signals

Subprocess uses struct Subprocess.Signal to represent the signal that could be sent via sendSignal(). Developers could either initialize Signal directly using the raw signal value or use one of the common values defined as static property.

extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct Signal : Hashable, Sendable {
        public let rawValue: Int32

        public static var interrupt: Self { get }
        public static var terminate: Self { get }
        public static var suspend: Self { get }
        public static var resume: Self { get }
        public static var kill: Self { get }
        public static var terminalClosed: Self { get }
        public static var quit: Self { get }
        public static var userDefinedOne: Self { get }
        public static var userDefinedTwo: Self { get }
        public static var alarm: Self { get }
        public static var windowSizeChange: Self { get }

        public init(rawValue: Int32)
    }
}

Subprocess.StandardInputWriter

StandardInputWriter provides developers with direct control over writing to the child process's standard input. Similar to the Subprocess object itself, developers should use the StandardInputWriter object passed to the body closure, and this object is only valid within the body of the closure.

Note: Developers must call finish() when they have completed writing to signal that the standard input file descriptor should be closed.

extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct StandardInputWriter: Sendable {
        public func write<S>(_ sequence: S) async throws where S : Sequence, S.Element == UInt8
        public func write<S>(_ sequence: S) async throws where S : Sequence, S.Element == CChar

        public func write<S: AsyncSequence>(_ asyncSequence: S) async throws where S.Element == CChar
        public func write<S: AsyncSequence>(_ asyncSequence: S) async throws where S.Element == UInt8

        public func finish() async throws
    }
}

Subprocess.Configuration

In contrast to the monolithic Process, Subprocess utilizes various types to model the lifetime of a process. Subprocess.Configuration represents the collection of information needed to spawn a process. This type is designed to be very similar to the existing Process, enabling you to configure your process in a manner akin to NSTask:

public extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct Configuration : Sendable {
        // Configurable properties
        public var executable: Executable
        public var arguments: Arguments
        public var environment: Environment
        public var workingDirectory: FilePath
        public var platformOptions: PlatformOptions

        public init(
            executing executable: Executable,
            arguments: Arguments = [],
            environment: Environment = .inherit,
            workingDirectory: FilePath? = nil,
            platformOptions: PlatformOptions = .default
        )
    }
}

Note:

  • The .workingDirectory property defaults to the current working directory of the calling process.

The static methods on Subprocess are simply syntactic sugar for calling Configuration().run(). Beyond the configurable parameters exposed by these static run methods, Subprocess.Configuration also provides platform-specific launch options via PlatformOptions.

Subprocess.PlatformOptions

While Subprocess.Configuration provides configuration options to the essential launch parameters such as arguments and environment, PlatformOptions provides additional options to configure platform-specific behavior.

extension Subprocess {
    /// The collection of platform-specific configurations
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct PlatformOptions: Sendable {
        public var qualityOfService: QualityOfService
        // Set user ID for the subprocess
        public var userID: Int?
        // Set group ID for the subprocess
        public var groupID: Int?
        // Set list of supplementary group IDs for the subprocess
        public var supplementaryGroups: [Int]?
        // Creates a session and sets the process group ID
        // i.e. Detach from the terminal.
        public var createSession: Bool
        // Create a new process group
        public var createProcessGroup: Bool
        public var launchRequirementData: Data?
        public var additionalSpawnAttributeConfigurator: (@Sendable (inout posix_spawnattr_t?) throws -> Void)?
        public var additionalFileAttributeConfigurator: (@Sendable (inout posix_spawn_file_actions_t?) throws -> Void)?

        public init(
            qualityOfService: QualityOfService,
            userID: Int? = nil,
            groupID: Int? = nil,
            supplementaryGroups: [Int]?,
            createSession: Bool,
            createProcessGroup: Bool,
            launchRequirementData: Data?
        )

        public static var `default`: Self
    }
}

PlatformOptions also supports "escape hatches" that enable developers to configure the underlying platform-specific objects directly if Subprocess lacks corresponding high-level APIs.

For Darwin, we are proposing two such APIs:

  • .additionalSpawnAttributeConfigurator: (@Sendable (inout posix_spawnattr_t?) throws -> Void)? gives developers an opportunity to configure the posix_spawnattr_t object just before it's passed to posix_spawn(). For instance, developers can set additional spawn flags:
let config = Subprocess.Configuration(executing: .at("/my/executable"))
config.additionalSpawnAttributeConfigurator = { spawnAttr in
    let flags: Int32 = POSIX_SPAWN_CLOEXEC_DEFAULT |
        POSIX_SPAWN_SETSIGMASK |
        POSIX_SPAWN_SETSIGDEF |
        POSIX_SPAWN_START_SUSPENDED
    posix_spawnattr_setflags(&spawnAttr, Int16(flags))
}
  • Similarly, .additionalFileAttributeConfigurator: (@Sendable (inout posix_spawn_file_actions_t?) throws -> Void) allows developers to customize posix_spawn_file_actions_t. For instance, a developer might want to bind child file descriptors, other than standard input (fd 0), standard output (fd 1), and standard error (fd 2), to parent file descriptors:
let config = Subprocess.Configuration(executing: .at("/my/executable"))
// Bind child fd 4 to a parent fd
config.additionalFileAttributeConfigurator = { fileAttr in
    let parentFd: FileDescriptor = ...
    posix_spawn_file_actions_adddup2(&fileAttr, parentFd.rawValue, 4)
}

(We welcome community input on which Linux and Windows "escape hatches" we should add)

Subprocess.InputMethod

In addition to supporting the direct passing of Sequence<UInt8> and AsyncSequence<UInt8> as the standard input to the child process, Subprocess also provides a Subprocess.InputMethod type that includes two additional input options:

  • .noInput: Specifies that the subprocess does not require any standard input. This is the default value.
  • .readFrom: Specifies that the subprocess should read its standard input from a file descriptor provided by the developer. Subprocess will automatically close the file descriptor after the process is spawned if closeWhenDone is set to true.
extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct InputMethod: Sendable, Hashable {
        public static var noInput: Self
        public static func readFrom(_ fd: FileDescriptor, closeWhenDone: Bool) -> Self
    }
}

Here are some examples:

// By default `InputMethod` is set to `.noInput`
let ls = try await Subprocess.run(executing: .named("ls"))

// Alteratively, developers could pass in a file descriptor
let fd: FileDescriptor = ...
let cat = try await Subprocess.run(executing: .named("cat"), input: .readFrom(fd, closeWhenDone: true))

// Pass in a async sequence directly
let sequence: AsyncSequence = ...
let exe = try await Subprocess.run(executing: .at("/some/executable"), input: sequence)

Subprocess Output Methods

Subprocess uses two types to describe where the standard output and standard error of the child process should be redirected. These two types, Subprocess.collectOutputMethod and Subprocess.redirectOutputMethod, correspond to the two general categories of run methods mentioned above. Similar to InputMethod, both OutputMethods add two general output destinations:

  • .discard: Specifies that the child process's output should be discarded, effectively written to /dev/null.
  • .writeTo: Specifies that the child process should write its output to a file descriptor provided by the developer. Subprocess will automatically close the file descriptor after the process is spawned if closeWhenDone is set to true.

CollectedOutMethod adds one more option to non-closure-based run methods that return a CollectedResult: .collect and its variation .collect(limit:). This option specifies that Subprocess should collect the output as Data. Since the output of a child process could be arbitrarily large, Subprocess imposes a limit on how many bytes it will collect. By default, this limit is 16kb (when specifying .collect). Developers can override this limit by specifying .collect(limit: newLimit):

extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct CollectedOutputMethod: Sendable, Hashable {
        // Discard the output (write to /dev/null)
        public static var discard: Self
        // Collect the output as Data with the default 16kb limit
        public static var collect: Self
        // Write the output directly to a FileDescriptor
        public static func writeTo(_ fd: FileDescriptor, closeWhenDone: Bool) -> Self
        // Collect the output as Data with modified limit (in bytes).
        public static func collect(limit limit: Int) -> Self
    }
}

On the other hand, RedirectedOutputMethod adds one more option, .redirect, to the closure-based run methods to signify that output should be redirected to the .standardOutput or .standardError property of Subprocess passed to the closure as AsyncSequence. Since AsyncSequence is not push-based, there is no byte limit for this option:

extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct RedirectedOutputMethod: Sendable, Hashable {
        // Discard the output (write to /dev/null)
        public static var discard: Self
        // Redirect the output as AsyncSequence
        public static var redirect: Self
        // Write the output directly to a FileDescriptor
        public static func writeTo(_ fd: FileDescriptor, closeWhenDone: Bool) -> Self
    }
}

Here are some examples of using both output methods:

let ls = try await Subprocess.run(executing: .named("ls"), output: .collect)
// The output has been collected as `Data`, up to 16kb limit
print("ls output: \(String(data: ls.standardOutput!, encoding: .utf8)!)")

// Increase the default buffer limit to 256kb
let curl = try await Subprocess.run(
    executing: .named("curl"),
    output: .collect(limit: 256 * 1024)
)
print("curl output: \(String(data: curl.standardOutput!, encoding: .utf8)!)")

// Write to a specific file descriptor
let fd: FileDescriptor = try .open(...)
let result = try await Subprocess.run(
    executing: .at("/some/script"), output: .writeTo(fd, closeWhenDone: true))

// Redirect the output as AsyncSequence
let result2 = try await Subprocess.run(executing: .named("/some/script"), output: .redirect) { subprocess in
    // Output can be access via `subprocess.standardOutput` here
    for try await item in subprocess.standardOutput! {
        print(item)
    }
    return "Done"
}

Result Types

Subprocess provides two "Result" types corresponding to the two categories of run methods: Subprocess.CollectedResult and Subprocess.Result<T>.

Subprocess.collectResult is essentially a collection of properties that represent the result of an execution after the child process has exited. It is used by the non-closure-based run methods. In many ways, CollectedResult can be seen as the "synchronous" version of Subprocess—instead of the asynchronous AsyncSequence<UInt8>, the standard IOs can be retrieved via synchronous Data.

extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct CollectedResult: Sendable, Hashable {
        public let processIdentifier: ProcessIdentifier
        public let terminationStatus: TerminationStatus
        public let standardOutput: Data?
        public let standardError: Data?
    }
}

Subprocess.Result is a simple wrapper around the generic result returned by the run closures with the corresponding TerminationStatus of the child process:

extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct Result<T: Sendable>: Sendable {
        public let terminationStatus: TerminationStatus
        public let value: T
    }
}

@available(FoundationPreview 0.4, *)
@available(iOS, unavailable)
@available(tvOS, unavailable)
@available(watchOS, unavailable)
extension Subprocess.Result: Equatable where T : Equatable {}

@available(FoundationPreview 0.4, *)
@available(iOS, unavailable)
@available(tvOS, unavailable)
@available(watchOS, unavailable)
extension Subprocess.Result : Hashable where T : Hashable {}

Subprocess.Executable

Subprocess utilizes Executable to configure how the executable is resolved. Developers can create an Executable using two static methods: .named(), indicating that an executable name is provided, and Subprocess should try to automatically resolve the executable path, and .at(), signaling that an executable path is provided, and Subprocess should use it unmodified.

extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct Executable: Sendable, Hashable {
        /// Create an `Executable` with an executable name such as `ls`
        public static func named(_ executableName: String) -> Self
        /// Create an `Executable` with an executable path
        /// such as `/bin/ls`
        public static func at(_ filePath: FilePath) -> Self
        // Resolves the executable path with the given `Environment` value
        public func resolveExecutablePath(in environment: Environment) -> FilePath?
    }
}

Subprocess.Environment

struct Environment is used to configure how should the process being launched receive its environment values:

extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct Environment: Sendable {
        /// A copy of the current environment value of the launching process
        public static var inherit: Self { get }
        /// Update or insert the environment values of self with
        /// the supplied values
        public func updating(
            _ newValue: [String : String]) -> Self
        public func updating(
            _ newValue: [Data : Data]) -> Self
        /// Use the supplied values directly
        public static func custom(
            _ newValue: [String : String]) -> Self
        public static func custom(
            _ newValue: [Data : Data]) -> Self
    }
}

Developers have the option to:

  • Inherit the same environment variables as the launching process by using .inherit. This is the default option.
  • Inherit the environment variables from the launching process with overrides via .inherit.updating().
  • Specify custom values for environment variables using .custom().
// Override the `PATH` environment value from launching process
let result = try await Subprocess.run(
    executing: .at("/some/executable"),
    environment: .inherit.updating(
        ["PATH" : "/some/new/path"]
    )
)

// Use custom values
let result2 = try await Subprocess.run(
    executing: .at("/at"),
    environment: .custom([
        "PATH" : "/some/path"
        "HOME" : "/Users/Charles"
    ])
)

Environment is designed to support both String and raw bytes for the use case where the environment values might not be valid UTF8 strings.

Subprocess.Arguments

Subprocess.Arguments is used to configure the spawn arguments sent to the child process. It conforms to ExpressibleByArrayLiteral. In most cases, developers can simply pass in an array [String] with the desired arguments. However, there might be scenarios where a developer wishes to override the first argument (i.e., the executable path). This is particularly useful because some processes might behave differently based on the first argument provided. The ability to override the executable path can be achieved by specifying the pathOverride parameter:

extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct Arguments: Sendable, ExpressibleByArrayLiteral {
        public typealias ArrayLiteralElement = String
        /// Creates an Arguments object using the given literal values
        public init(arrayLiteral elements: ArrayLiteralElement...)
        /// Overrides the first arguments (aka the executable path)
        /// with the given value. If `executablePathOverride` is nil,
        /// `Arguments` will automatically use the executable path
        /// as the first argument.
        public init(executablePathOverride: String?, remainingValues: [String])
        /// Overrides the first arguments (aka the executable path)
        /// with the given value. If `executablePathOverride` is nil,
        /// `Arguments` will automatically use the executable path
        /// as the first argument.
        public init(executablePathOverride: Data?, remainingValues: [Data])
    }
}

Similar to Environment, Arguments also supports raw bytes in addition to String.

// In most cases, simply pass in an array
let result = try await Subprocess.run(
    executing: .at("/some/executable"),
    arguments: ["arg1", "arg2"]
)

// Override the executable path
let result2 = try await Subprocess.run(
    executing: .at("/some/executable"),
    arguments: .init(["arg1", "arg2"], overrideExecutablePathWith: "/new/executable/path")
)

Subprocess.TerminationStatus

Subprocess.TerminationStatus is used to communicate the exit statuses of a process: exited, signalled, or stillActive on Windows.

extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public enum TerminationStatus: Sendable, Hashable {
    #if canImport(WinSDK)
        public typealias Code = DWORD
    #else
        public typealias Code = CInt
    #endif

    #if canImport(WinSDK)
        case stillActive
    #endif
        case exit(Code)
        case unhandledException(Code)
        // A process is terminated successfully when it exited 0
        public var isSuccess: Bool
        public var wasUnhandledException: Bool
    }
}

Subprocess.AsyncBytes

Subprocess vends AsyncBytes as the concrete implementation of AsyncSequence<UInt8>, used by Subprocess as the standardOutput and standardError properties.

extension Subprocess {
    @available(FoundationPreview 0.4, *)
    @available(iOS, unavailable)
    @available(tvOS, unavailable)
    @available(watchOS, unavailable)
    public struct AsyncBytes: AsyncSequence, Sendable {
        public typealias Element = UInt8
        public typealias AsyncIterator = Iterator

        public func makeAsyncIterator() -> Iterator

        @_nonSendable
        public struct Iterator: AsyncIteratorProtocol {
            public typealias Element = UInt8

            public mutating func next() async throws -> UInt8?
        }
    }
}

Task Cancellation

If the task running Subprocess.run is cancelled while the child process is running, Subprocess will attempt to release all the resources it acquired (i.e. file descriptors) and then terminate the child process via SIGKILL.

Impact on Existing Code

No impact on existing code is anticipated. All introduced changes are additive.

Future Directions

Automatic Splitting of Arguments

Ideally, the Arguments feature should automatically split a string, such as "-a -n 1024 -v 'abc'", into an array of arguments. This enhancement would enable Arguments to conform to ExpressibleByStringLiteral, allowing developers to conveniently pass either a String or [String] as Arguments.

I decided to defer this feature because it turned out to be a "hard problem" -- different platforms handle arguments differently, requiring careful consideration to ensure correctness.

For reference, Python uses shlex.split, which could serve as a valuable starting point for implementation.

Combined stdout and stderr

In Python's Subprocess, developers can merge standard output and standard error into a single stream. This is particularly useful when an executable improperly utilizes standard error as standard output (or vice versa). We should explore the most effective way to achieve this enhancement without introducing confusion to existing parameters—perhaps by introducing a new property.

Alternatives Considered

Improving Process vs Creating New Type

We explored improving Process itself instead of creating a new type (Subprocess). However, it was found challenging to add all the desired features while preserving binary compatibility with the existing Process type.

Other Naming Schemes

We considered naming this new type Command (which is what Rust uses), but ultimately decided to go with the more "familiar" name "Subprocess". "Subprocess" also communicates to the developers that the new process being launch is a child, or "sub" process of the parent, and the parent will await the child to finish.

Considerations Between URL and FilePath:

While Process historically used URL to represent both the executable path and the working directory, Subprocess has opted for FilePath. This choice is made because, in the context of Subprocess, the executable is on disk, and FilePath aligns more closely reflects this concept.

Opaque Environment and Arguments vs Array and Dictionary

We chose to use opaque Environment and Arguments to represent the environment and argument values passed to subprocess instead of using plain [String] and [String : String] for two reasons:

  • Opaque types allows raw byte support. There are cases where the argument passed to child process isn't a valid UTF8 string and both Environment and Arguments support this use case
  • Opaque types gives us room to support more features in the future.

Acknowledgments

Special thanks to @AndrewHoos, @FranzBusch, @MaxDesiatov, and @weissi for their prior work on Process, which significantly influenced the development of Subprocess.

swift-experimental-subprocess's People

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