fix: Sort private attributes for improved stable encoding (#401)

To help increase cache hits, we want to always use a stable encoding for
the context. This was done in a previous commit when we added the
`encoder.outputFormatting = [.sortedKeys]` modification.

A context's private attributes are sorted as a set, which makes no
guarantee about encoding order. To address this, we are sorting the
private attributes prior to the JSON encoding phase. As a result, we
would expect to see an increase in cache hits for customers.

LaunchDarkly/LaunchDarkly/Models/Context/LDContext.swift

@@ -97,7 +97,8 @@ public struct LDContext: Encodable, Equatable {
             let includePrivateAttributes = encoder.userInfo[UserInfoKeys.includePrivateAttributes] as? Bool ?? false
 
             if let privateAttributes = privateAttributes, !privateAttributes.isEmpty, includePrivateAttributes {
-                try container.encodeIfPresent(privateAttributes, forKey: .privateAttributes)
+                let sorted = privateAttributes.sorted { $0.raw() < $1.raw() }
+                try container.encodeIfPresent(sorted, forKey: .privateAttributes)
             }
 
             if let redactedAttributes = redactedAttributes, !redactedAttributes.isEmpty {

LaunchDarkly/LaunchDarklyTests/Models/Context/LDContextSpec.swift

@@ -386,11 +386,63 @@ final class LDContextSpec: XCTestCase {
         }
     }
 
+    func testAttributeOrderDefinitionDoesNotAffectJsonEncoding() {
+        var builder1 = LDContextBuilder(key: "context-key")
+        builder1.kind("org")
+        builder1.trySetValue("example-attribute", LDValue(stringLiteral: "Hi there"))
+        builder1.name("Example name")
+        builder1.addPrivateAttribute(Reference.init(literal: "first"))
+        builder1.addPrivateAttribute(Reference.init(literal: "second"))
+        builder1.addPrivateAttribute(Reference.init(literal: "third"))
+
+        guard case .success(let context1) = builder1.build()
+        else {
+            XCTFail("builder1.build should not have failed")
+            return
+        }
+
+        var builder2 = LDContextBuilder(key: "context-key")
+        builder2.trySetValue("example-attribute", LDValue(stringLiteral: "Hi there"))
+        builder2.name("Example name")
+        builder2.kind("org")
+        builder2.addPrivateAttribute(Reference.init(literal: "second"))
+        builder2.addPrivateAttribute(Reference.init(literal: "third"))
+        builder2.addPrivateAttribute(Reference.init(literal: "first"))
+
+        guard case .success(let context2) = builder2.build()
+        else {
+            XCTFail("builder1.build should not have failed")
+            return
+        }
+
+        let encoder = JSONEncoder()
+        encoder.userInfo[LDContext.UserInfoKeys.includePrivateAttributes] = true
+        encoder.userInfo[LDContext.UserInfoKeys.redactAttributes] = false
+        encoder.outputFormatting = [.sortedKeys]
+
+        guard let context1JsonData = try? encoder.encode(context1)
+        else {
+            XCTFail("failed to encode context1")
+            return
+        }
+
+        guard let context2JsonData = try? encoder.encode(context2)
+        else {
+            XCTFail("failed to encode context2")
+            return
+        }
+
+        XCTAssertEqual(context1JsonData, context2JsonData)
+    }
+
     func testAttributeOrderDefinitionDoesNotAffectContextHash() {
         var builder1 = LDContextBuilder(key: "context-key")
         builder1.kind("org")
         builder1.trySetValue("example-attribute", LDValue(stringLiteral: "Hi there"))
         builder1.name("Example name")
+        builder1.addPrivateAttribute(Reference.init(literal: "first"))
+        builder1.addPrivateAttribute(Reference.init(literal: "second"))
+        builder1.addPrivateAttribute(Reference.init(literal: "third"))
 
         guard case .success(let context1) = builder1.build()
         else {
@@ -402,6 +454,9 @@ final class LDContextSpec: XCTestCase {
         builder2.trySetValue("example-attribute", LDValue(stringLiteral: "Hi there"))
         builder2.name("Example name")
         builder2.kind("org")
+        builder2.addPrivateAttribute(Reference.init(literal: "second"))
+        builder2.addPrivateAttribute(Reference.init(literal: "third"))
+        builder2.addPrivateAttribute(Reference.init(literal: "first"))
 
         guard case .success(let context2) = builder2.build()
         else {