Fix encoding and decoding of HTTPHeaders (#3116)

* Fix encoding and decoding of HTTPHeaders with multiple of the same header name via Codable. Add test.
* Add back the ability to decode the previous encoding, in case anyone was using it

Sources/Vapor/HTTP/Headers/HTTPHeaders.swift

@@ -35,20 +35,37 @@ extension HTTPHeaders {
 }
 
 extension HTTPHeaders: Codable {
-    public init(from decoder: Decoder) throws {
-        let dictionary = try decoder.singleValueContainer().decode([String: String].self)
+    private enum CodingKeys: String, CodingKey { case name, value }
+
+    public init(from decoder: any Decoder) throws {
         self.init()
-        for (name, value) in dictionary {
-            self.add(name: name, value: value)
+        do {
+            var container = try decoder.unkeyedContainer()
+
+            while !container.isAtEnd {
+                let nested = try container.nestedContainer(keyedBy: Self.CodingKeys.self)
+                let name = try nested.decode(String.self, forKey: .name)
+                let value = try nested.decode(String.self, forKey: .value)
+
+                self.add(name: name, value: value)
+            }
+        } catch DecodingError.typeMismatch(let type, _) where "\(type)".starts(with: "Array<") {
+            // Try the old format
+            let container = try decoder.singleValueContainer()
+            let dict = try container.decode([String: String].self)
+
+            self.add(contentsOf: dict.map { ($0.key, $0.value) })
         }
     }
 
-    public func encode(to encoder: Encoder) throws {
-        var dictionary: [String: String] = [:]
+    public func encode(to encoder: any Encoder) throws {
+        var container = encoder.unkeyedContainer()
+
         for (name, value) in self {
-            dictionary[name] = value
+            var nested = container.nestedContainer(keyedBy: Self.CodingKeys.self)
+
+            try nested.encode(name, forKey: .name)
+            try nested.encode(value, forKey: .value)
         }
-        var container = encoder.singleValueContainer()
-        try container.encode(dictionary)
     }
 }

Tests/VaporTests/HTTPHeaderTests.swift

@@ -439,4 +439,42 @@ final class HTTPHeaderTests: XCTestCase {
         XCTAssertEqual(cacheControl.serialize(), "immutable")
 
     }
+
+    /// Test that multiple same-named headers round-trip through Codable
+    func testCodableMultipleHeadersRountrip() throws {
+        let encoder = JSONEncoder()
+        encoder.dateEncodingStrategy = .iso8601
+        encoder.outputFormatting = [.withoutEscapingSlashes, .sortedKeys]
+        let decoder = JSONDecoder()
+        decoder.dateDecodingStrategy = .iso8601
+
+        var headers = HTTPHeaders()
+        headers.add(name: .date, value: "\(Date(timeIntervalSinceReferenceDate: 100.0))")
+        headers.add(name: .date, value: "\(Date(timeIntervalSinceReferenceDate: -100.0))")
+        headers.add(name: .connection, value: "be-strange")
+
+        let encodedHeaders = try encoder.encode(headers)
+
+        XCTAssertEqual(String(decoding: encodedHeaders, as: UTF8.self), #"[{"name":"date","value":"2001-01-01 00:01:40 +0000"},{"name":"date","value":"2000-12-31 23:58:20 +0000"},{"name":"connection","value":"be-strange"}]"#)
+
+        let decodedHeaders = try decoder.decode(HTTPHeaders.self, from: encodedHeaders)
+
+        XCTAssertEqual(decodedHeaders.count, headers.count)
+        for ((k1, v1), (k2, v2)) in zip(headers, decodedHeaders) {
+            XCTAssertEqual(k1, k2)
+            XCTAssertEqual(v1, v2)
+        }
+    }
+
+    /// Make sure the old HTTPHeaders encoding can still be decoded
+    func testOldHTTPHeadersEncoding() throws {
+        let decoder = JSONDecoder()
+        let json = #"{"connection":"fun","attention":"none"}"#
+        var headers = HTTPHeaders()
+
+        XCTAssertNoThrow(headers = try decoder.decode(HTTPHeaders.self, from: Data(json.utf8)))
+        XCTAssertEqual(headers.count, 2)
+        XCTAssertEqual(headers.first(name: "connection"), "fun")
+        XCTAssertEqual(headers.first(name: "attention"), "none")
+    }
 }