Implement access control for baggage items via key properties

Sources/CoreBaggage/Baggage.swift

@@ -185,6 +185,7 @@ extension Baggage {
         return self._storage.count
     }
 
+    /// Returns true if the baggage has no items stored.
     public var isEmpty: Bool {
         return self._storage.isEmpty
     }
@@ -193,12 +194,26 @@ extension Baggage {
     ///
     /// Order of those invocations is NOT guaranteed and should not be relied on.
     ///
+    /// ### Access Control
+    ///
+    /// By default accesses ALL `public` and `publicExceptLogging` values.
+    /// Baggage items whose keys are marked with `private` access, are never accessed by a forEach.
+    ///
     /// - Parameter body: A closure invoked with the type erased key and value stored for the key in this baggage.
-    public func forEach(_ body: (AnyBaggageKey, Any) throws -> Void) rethrows {
+    public func forEach(access: ForEachAccessType = .allPublic, _ body: (AnyBaggageKey, Any) throws -> Void) rethrows {
         try self._storage.forEach { key, value in
-            try body(key, value)
+            if key.access.rawValue <= access.rawValue {
+                try body(key, value)
+            }
         }
     }
+
+    public enum ForEachAccessType: Int, Hashable {
+        /// Access all accessible values (i.e. `public` and `publicExceptLogging` values; `private` values are skipped)
+        case allPublic = 1
+        /// Access all accessible values for purposes of logging (i.e. ONLY `public`, skipping `publicExceptLogging` and `private` values)
+        case logging = 0
+    }
 }
 
 extension Baggage: CustomStringConvertible {

Sources/CoreBaggage/BaggageAccessPolicy.swift

@@ -0,0 +1,56 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Distributed Tracing Baggage open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift Distributed Tracing Baggage project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+/// Configures the policy related to values stored using this key.
+///
+/// This can be used to ensure that a value should never be logged automatically by a logger associated to a context.
+///
+/// Summary:
+/// - `public` - items are accessible by other modules via `baggage.forEach` and direct key lookup,
+///    and will be logged by the `DefaultContext` `logger.
+/// - `publicExceptLogging` - items are accessible by other modules via `baggage.forEach` and direct key lookup,
+///    however will NOT be logged by the `DefaultContext` `logger.
+/// - `private` - items are NOT accessible by other modules via `baggage.forEach` nor are they logged by default.
+///    The only way to gain access to a private baggage item is through it's key or accessor, which means that
+///    access is controlled using Swift's native access control mechanism, i.e. a `private`/`internal` `Key` and `set` accessor,
+///    will result in a baggage item that may only be set by the owning module, but read by anyone via the (`public`) accessor.
+public enum BaggageAccessPolicy: Int, Hashable {
+    /// Access to this baggage item is NOT restricted.
+    /// This baggage item will be listed when `baggage.forEach` is invoked, and thus modules other than the defining
+    /// module may gain access to it and potentially log or pass it to other parts of the system.
+    ///
+    /// Note that this can happen regardless of the key being declared private or internal.
+    ///
+    /// ### Example
+    /// When module `A` defines `AKey` and keeps it `private`, any other module still may call `baggage.forEach`
+    case `public` = 0
+
+    /// Access to this baggage item is NOT restricted, however the `DefaultContext` (and any other well-behaved context)
+    /// MUST NOT log this baggage item.
+    ///
+    /// This policy can be useful if some user sensitive value must be carried in baggage context, however it should never
+    /// appear in log statements. While usually such items should not be put into baggage, we offer this mode as a way of
+    /// threading through a system values which should not be logged nor pollute log statements.
+    case publicExceptLogging = 1
+
+    /// Access to this baggage item is RESTRICTED and can only be performed by a direct subscript lookup into the baggage.
+    ///
+    /// This effectively restricts the access to the baggage item, to any party which has access to the associated
+    /// `BaggageKey`. E.g. if the baggage key is defined internal or private, and the `set` accessor is also internal or
+    /// private, no other module would be able to modify this baggage once it was set on a baggage context.
+    case `private` = 2
+}
+
+extension Baggage {
+    public typealias AccessPolicy = BaggageAccessPolicy
+}

Sources/CoreBaggage/BaggageKey.swift

@@ -53,17 +53,37 @@ public protocol BaggageKey {
     ///
     /// Defaults to `nil`.
     static var nameOverride: String? { get }
+
+    /// Configures the policy related to values stored using this key.
+    ///
+    /// This can be used to ensure that a value should never be logged automatically by a logger associated to a context.
+    ///
+    /// Summary:
+    /// - `public` - items are accessible by other modules via `baggage.forEach` and direct key lookup,
+    ///    and will be logged by the `DefaultContext` `logger.
+    /// - `publicExceptLogging` - items are accessible by other modules via `baggage.forEach` and direct key lookup,
+    ///    however will NOT be logged by the `DefaultContext` `logger.
+    /// - `private` - items are NOT accessible by other modules via `baggage.forEach` nor are they logged by default.
+    ///    The only way to gain access to a private baggage item is through it's key or accessor, which means that
+    ///    access is controlled using Swift's native access control mechanism, i.e. a `private`/`internal` `Key` and `set` accessor,
+    ///    will result in a baggage item that may only be set by the owning module, but read by anyone via the (`public`) accessor.
+    ///
+    /// Defaults to `.public`.
+    static var access: Baggage.AccessPolicy { get }
 }
 
 extension BaggageKey {
     public static var nameOverride: String? { return nil }
+    public static var access: Baggage.AccessPolicy { return .public }
 }
 
 /// A type-erased `BaggageKey` used when iterating through the `Baggage` using its `forEach` method.
 public struct AnyBaggageKey {
     /// The key's type represented erased to an `Any.Type`.
     public let keyType: Any.Type
 
+    public let access: Baggage.AccessPolicy
+
     private let _nameOverride: String?
 
     /// A human-readable String representation of the underlying key.
@@ -75,6 +95,13 @@ public struct AnyBaggageKey {
     init<Key>(_ keyType: Key.Type) where Key: BaggageKey {
         self.keyType = keyType
         self._nameOverride = keyType.nameOverride
+        self.access = keyType.access
+    }
+}
+
+extension AnyBaggageKey: CustomStringConvertible {
+    public var description: String {
+        return "AnyBaggageKey(\(self.name), access: \(self.access))"
     }
 }
 

Tests/CoreBaggageTests/BaggageTests+XCTest.swift

@@ -30,6 +30,8 @@ extension BaggageTests {
                 ("testEmptyBaggageDescription", testEmptyBaggageDescription),
                 ("testSingleKeyBaggageDescription", testSingleKeyBaggageDescription),
                 ("testMultiKeysBaggageDescription", testMultiKeysBaggageDescription),
+                ("test_forEach_forLogging_respects_BaggageAccessPolicy", test_forEach_forLogging_respects_BaggageAccessPolicy),
+                ("test_forEach_respects_BaggageAccessPolicy", test_forEach_respects_BaggageAccessPolicy),
                 ("test_todo_context", test_todo_context),
                 ("test_topLevel", test_topLevel),
            ]

Tests/CoreBaggageTests/BaggageTests.swift

@@ -55,7 +55,7 @@ final class BaggageTests: XCTestCase {
     func testMultiKeysBaggageDescription() {
         var baggage = Baggage.topLevel
         baggage.testID = 42
-        baggage[SecondTestIDKey.self] = "test"
+        baggage[Baggage.SecondTestIDKey.self] = "test"
 
         let description = String(describing: baggage)
         XCTAssert(description.starts(with: "Baggage(keys: ["), "Was: \(description)")
@@ -64,6 +64,51 @@ final class BaggageTests: XCTestCase {
         XCTAssert(description.contains("ExplicitKeyName"), "Was: \(description)")
     }
 
+    // ==== ----------------------------------------------------------------------------------------------------------------
+    // MARK: Access Policy
+
+    func test_forEach_forLogging_respects_BaggageAccessPolicy() {
+        var baggage = Baggage.topLevel
+
+        baggage.testID = 42
+        baggage.publicExceptLogging = "dont-log-me"
+        baggage.private = "dont-log-me-either-not-even-foreach"
+
+        XCTAssertEqual(baggage.testID, 42)
+        XCTAssertEqual(baggage.publicExceptLogging, "dont-log-me")
+        XCTAssertEqual(baggage.private, "dont-log-me-either-not-even-foreach")
+
+        var count = 0
+        baggage.forEach(access: .logging) { key, value in
+            print("  access: \(key) = \(value)")
+            XCTAssertNotEqual(key.name, "\(Baggage.PublicExceptLoggingKey.self)")
+            XCTAssertNotEqual(key.name, "\(Baggage.PrivateKey.self)")
+            count += 1
+        }
+        XCTAssertEqual(count, 1)
+    }
+
+    func test_forEach_respects_BaggageAccessPolicy() {
+        var baggage = Baggage.topLevel
+
+        baggage.testID = 42
+        baggage.publicExceptLogging = "dont-log-me"
+        baggage.private = "dont-log-me-either-not-even-foreach"
+
+        XCTAssertEqual(baggage.testID, 42)
+        XCTAssertEqual(baggage.publicExceptLogging, "dont-log-me")
+        XCTAssertEqual(baggage.private, "dont-log-me-either-not-even-foreach")
+
+        var count = 0
+        baggage.forEach { key, value in
+            print("  access: \(key) = \(value)")
+            // the "dont log" may still appear here; but our `DefaultLoggingContext` in can handle it properly (other package)
+            XCTAssertNotEqual(key.name, "\(Baggage.PrivateKey.self)")
+            count += 1
+        }
+        XCTAssertEqual(count, 2)
+    }
+
     // ==== ------------------------------------------------------------------------------------------------------------
     // MARK: Factories
 
@@ -83,7 +128,7 @@ private enum TestIDKey: Baggage.Key {
     typealias Value = Int
 }
 
-private extension Baggage {
+extension Baggage {
     var testID: Int? {
         get {
             return self[TestIDKey.self]
@@ -92,10 +137,42 @@ private extension Baggage {
             self[TestIDKey.self] = newValue
         }
     }
+
+    enum SecondTestIDKey: Baggage.Key {
+        typealias Value = String
+
+        static let nameOverride: String? = "ExplicitKeyName"
+    }
 }
 
-private enum SecondTestIDKey: Baggage.Key {
-    typealias Value = String
+extension Baggage {
+    var publicExceptLogging: String? {
+        get {
+            return self[PublicExceptLoggingKey.self]
+        }
+        set {
+            self[PublicExceptLoggingKey.self] = newValue
+        }
+    }
 
-    static let nameOverride: String? = "ExplicitKeyName"
+    var `private`: String? {
+        get {
+            return self[PrivateKey.self]
+        }
+        set {
+            self[PrivateKey.self] = newValue
+        }
+    }
+
+    enum PublicExceptLoggingKey: Baggage.Key {
+        typealias Value = String
+        static let nameOverride: String? = "public-except-logging"
+        static let access: BaggageAccessPolicy = .publicExceptLogging
+    }
+
+    enum PrivateKey: Baggage.Key {
+        typealias Value = String
+        static let nameOverride: String? = "private-DONT_SHOW_IN_LOGS_PLEASE"
+        static let access: BaggageAccessPolicy = .private
+    }
 }