Add advisory file locking API

Sources/System/FileControl.swift

@@ -0,0 +1,24 @@
+/*
+ This source file is part of the Swift System open source project
+
+ Copyright (c) 2021 Apple Inc. and the Swift System project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+ */
+
+// Strongly typed, Swifty interfaces to the most common and useful `fcntl`
+// commands.
+
+extension FileDescriptor {
+  internal func _fcntl(
+    _ cmd: Command, _ lock: inout FileDescriptor.FileLock,
+    retryOnInterrupt: Bool
+  ) -> Result<(), Errno> {
+    nothingOrErrno(retryOnInterrupt: retryOnInterrupt) {
+      withUnsafeMutablePointer(to: &lock) {
+        system_fcntl(self.rawValue, cmd.rawValue, $0)
+      }
+    }
+  }
+}

Sources/System/FileControlRaw.swift

@@ -0,0 +1,63 @@
+/*
+ This source file is part of the Swift System open source project
+
+ Copyright (c) 2021 Apple Inc. and the Swift System project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+ */
+
+#if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
+import Darwin
+#elseif os(Linux) || os(FreeBSD) || os(Android)
+import Glibc
+#elseif os(Windows)
+// Nothing
+#else
+#error("Unsupported Platform")
+#endif
+
+
+#if !os(Windows)
+
+// - MARK: Commands
+
+// TODO: Make below API as part of broader `fcntl` support.
+extension FileDescriptor {
+  /// Commands (and various constants) to pass to `fcntl`.
+  internal struct Command: RawRepresentable, Hashable {
+    internal let rawValue: CInt
+
+    internal init(rawValue: CInt) { self.rawValue = rawValue }
+
+    private init(_ raw: CInt) { self.init(rawValue: raw) }
+
+    /// Get open file description record locking information.
+    ///
+    /// The corresponding C constant is `F_GETLK`.
+    internal static var getOFDLock: Command { Command(_F_OFD_GETLK) }
+
+    /// Set open file description record locking information.
+    ///
+    /// The corresponding C constant is `F_SETLK`.
+    internal static var setOFDLock: Command { Command(_F_OFD_SETLK) }
+
+    /// Set open file description record locking information and wait until
+    /// the request can be completed.
+    ///
+    /// The corresponding C constant is `F_SETLKW`.
+    internal static var setOFDLockWait: Command { Command(_F_OFD_SETLKW) }
+
+#if !os(Linux)
+    /// Set open file description record locking information and wait until
+    /// the request can be completed, returning on timeout.
+    ///
+    /// The corresponding C constant is `F_SETLKWTIMEOUT`.
+    internal static var setOFDLockWaitTimout: Command {
+      Command(_F_OFD_SETLKWTIMEOUT)
+    }
+#endif
+
+  }
+}
+#endif