Introduce Swift Subprocess

Package.swift

@@ -33,7 +33,10 @@ let package = Package(
             exact: "0.0.5"),
         .package(
             url: "https://github.com/apple/swift-syntax.git",
-            from: "509.0.2")
+            from: "509.0.2"),
+        .package(
+            url: "https://github.com/apple/swift-system",
+            from: "1.0.0")
     ],
     targets: [
         // Foundation (umbrella)
@@ -63,6 +66,7 @@ let package = Package(
             "_CShims",
             "FoundationMacros",
             .product(name: "_RopeModule", package: "swift-collections"),
+            .product(name: "SystemPackage", package: "swift-system"),
           ],
           cSettings: [
             .define("_GNU_SOURCE", .when(platforms: [.linux]))

Sources/FoundationEssentials/AsyncLineSequence.swift

@@ -0,0 +1,168 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2019 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+public struct AsyncLineSequence<Base: AsyncSequence>: AsyncSequence where Base.Element == UInt8 {
+    public typealias Element = String
+
+    var base: Base
+
+    public struct AsyncIterator: AsyncIteratorProtocol {
+        public typealias Element = String
+
+        var byteSource: Base.AsyncIterator
+        var buffer: Array<UInt8> = []
+        var leftover: UInt8? = nil
+
+        internal init(underlyingIterator: Base.AsyncIterator) {
+            byteSource = underlyingIterator
+        }
+
+        // We'd like to reserve flexibility to improve the implementation of
+        // next() in the future, so aren't marking it @inlinable. Manually
+        // specializing for the common source types helps us get back some of
+        // the performance we're leaving on the table.
+        @_specialize(where Base == Subprocess.AsyncBytes)
+        public mutating func next() async rethrows -> String? {
+            /*
+             0D 0A: CR-LF
+             0A | 0B | 0C | 0D: LF, VT, FF, CR
+             E2 80 A8:  U+2028 (LINE SEPARATOR)
+             E2 80 A9:  U+2029 (PARAGRAPH SEPARATOR)
+             */
+            let _CR: UInt8 = 0x0D
+            let _LF: UInt8 = 0x0A
+            let _NEL_PREFIX: UInt8 = 0xC2
+            let _NEL_SUFFIX: UInt8 = 0x85
+            let _SEPARATOR_PREFIX: UInt8 = 0xE2
+            let _SEPARATOR_CONTINUATION: UInt8 = 0x80
+            let _SEPARATOR_SUFFIX_LINE: UInt8 = 0xA8
+            let _SEPARATOR_SUFFIX_PARAGRAPH: UInt8 = 0xA9
+
+            func yield() -> String? {
+                defer {
+                    buffer.removeAll(keepingCapacity: true)
+                }
+                if buffer.isEmpty {
+                    return nil
+                }
+                return String(decoding: buffer, as: UTF8.self)
+            }
+
+            func nextByte() async throws -> UInt8? {
+                defer { leftover = nil }
+                if let leftover = leftover {
+                    return leftover
+                }
+                return try await byteSource.next()
+            }
+
+            while let first = try await nextByte() {
+                switch first {
+                case _CR:
+                    let result = yield()
+                    // Swallow up any subsequent LF
+                    guard let next = try await byteSource.next() else {
+                        return result //if we ran out of bytes, the last byte was a CR
+                    }
+                    if next != _LF {
+                        leftover = next
+                    }
+                    if let result = result {
+                        return result
+                    }
+                    continue
+                case _LF..<_CR:
+                    guard let result = yield() else {
+                        continue
+                    }
+                    return result
+                case _NEL_PREFIX: // this may be used to compose other UTF8 characters
+                    guard let next = try await byteSource.next() else {
+                        // technically invalid UTF8 but it should be repaired to "\u{FFFD}"
+                        buffer.append(first)
+                        return yield()
+                    }
+                    if next != _NEL_SUFFIX {
+                        buffer.append(first)
+                        buffer.append(next)
+                    } else {
+                        guard let result = yield() else {
+                            continue
+                        }
+                        return result
+                    }
+                case _SEPARATOR_PREFIX:
+                    // Try to read: 80 [A8 | A9].
+                    // If we can't, then we put the byte in the buffer for error correction
+                    guard let next = try await byteSource.next() else {
+                        buffer.append(first)
+                        return yield()
+                    }
+                    guard next == _SEPARATOR_CONTINUATION else {
+                        buffer.append(first)
+                        buffer.append(next)
+                        continue
+                    }
+                    guard let fin = try await byteSource.next() else {
+                        buffer.append(first)
+                        buffer.append(next)
+                        return yield()
+
+                    }
+                    guard fin == _SEPARATOR_SUFFIX_LINE || fin == _SEPARATOR_SUFFIX_PARAGRAPH else {
+                        buffer.append(first)
+                        buffer.append(next)
+                        buffer.append(fin)
+                        continue
+                    }
+                    if let result = yield() {
+                        return result
+                    }
+                    continue
+                default:
+                    buffer.append(first)
+                }
+            }
+            // Don't emit an empty newline when there is no more content (e.g. end of file)
+            if !buffer.isEmpty {
+                return yield()
+            }
+            return nil
+        }
+
+    }
+
+    public func makeAsyncIterator() -> AsyncIterator {
+        return AsyncIterator(underlyingIterator: base.makeAsyncIterator())
+    }
+
+    internal init(underlyingSequence: Base) {
+        base = underlyingSequence
+    }
+}
+
+@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+extension AsyncLineSequence : Sendable where Base : Sendable {}
+@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+extension AsyncLineSequence.AsyncIterator : Sendable where Base.AsyncIterator : Sendable {}
+
+@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+public extension AsyncSequence where Self.Element == UInt8 {
+    /**
+     A non-blocking sequence of newline-separated `Strings` created by decoding the elements of `self` as UTF8.
+     */
+    @available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+    var lines: AsyncLineSequence<Self> {
+        AsyncLineSequence(underlyingSequence: self)
+    }
+}