Ensure user-provieded metadata has lowercase keys

Motivation:

HTTP/2 requires header field names to be lowercased. Metadata keys
should be lowercased automatically.

Modifications:

Lowercase user-provided metadata keys.

Results:

Fewer bugs

Sources/GRPCNIOTransportCore/GRPCStreamStateMachine.swift

@@ -659,7 +659,8 @@ extension GRPCStreamStateMachine {
     }
 
     for metadataPair in customMetadata {
-      headers.add(name: metadataPair.key, value: metadataPair.value.encoded())
+      // Lowercase the field names for user-provided metadata.
+      headers.add(name: metadataPair.key.lowercased(), value: metadataPair.value.encoded())
     }
 
     return headers
@@ -1248,7 +1249,8 @@ extension GRPCStreamStateMachine {
     }
 
     for metadataPair in customMetadata {
-      headers.add(name: metadataPair.key, value: metadataPair.value.encoded())
+      // Lowercase the field names for user-provided metadata.
+      headers.add(name: metadataPair.key.lowercased(), value: metadataPair.value.encoded())
     }
   }
 
@@ -1827,7 +1829,8 @@ extension HPACKHeaders {
     }
 
     for (key, value) in metadata {
-      trailers.add(name: key, value: value.encoded())
+      // Lowercase the field names for user-provided metadata.
+      trailers.add(name: key.lowercased(), value: value.encoded())
     }
   }
 }

Tests/GRPCNIOTransportHTTP2Tests/ControlMessages.swift

@@ -64,6 +64,12 @@ struct ControlInput: Codable {
   /// removed (":path" should become "echo-path").
   var echoMetadataInTrailers: Bool = false
 
+  /// Key-value pairs to add to the initial metadata.
+  var initialMetadataToAdd: [String: String] = [:]
+
+  /// Key-value pairs to add to the trailing metadata.
+  var trailingMetadataToAdd: [String: String] = [:]
+
   struct Status: Codable {
     var code: GRPCCore.Status.Code
     var message: String

Tests/GRPCNIOTransportHTTP2Tests/ControlService.swift

@@ -102,7 +102,10 @@ extension ControlService {
 
     // Check if the request is for a trailers-only response.
     if let status = message.status, message.isTrailersOnly {
-      let trailers = message.echoMetadataInTrailers ? request.metadata.echo() : [:]
+      var trailers = message.echoMetadataInTrailers ? request.metadata.echo() : [:]
+      for (key, value) in message.trailingMetadataToAdd {
+        trailers.addString(value, forKey: key)
+      }
       let code = Status.Code(rawValue: status.code.rawValue).flatMap { RPCError.Code($0) }
 
       if let code = code {
@@ -117,7 +120,10 @@ extension ControlService {
     }
 
     // Not a trailers-only response. Should the metadata be echo'd back?
-    let metadata = message.echoMetadataInHeaders ? request.metadata.echo() : [:]
+    var metadata = message.echoMetadataInHeaders ? request.metadata.echo() : [:]
+    for (key, value) in message.initialMetadataToAdd {
+      metadata.addString(value, forKey: key)
+    }
 
     // The iterator needs to be transferred into the response. This is okay: we won't touch the
     // iterator again from the current concurrency domain.
@@ -174,10 +180,13 @@ extension ControlService {
 
     // Check whether the RPC should be finished (i.e. the input `hasStatus`).
     guard let status = input.status else {
-      if input.echoMetadataInTrailers {
+      if input.echoMetadataInTrailers || !input.trailingMetadataToAdd.isEmpty {
         // There was no status in the input, but echo metadata in trailers was set. This is an
         // implicit 'ok' status.
-        let trailers = input.echoMetadataInTrailers ? metadata.echo() : [:]
+        var trailers = input.echoMetadataInTrailers ? metadata.echo() : [:]
+        for (key, value) in input.trailingMetadataToAdd {
+          trailers.addString(value, forKey: key)
+        }
         return .return(trailers)
       } else {
         // No status, and not echoing back metadata. Continue consuming the input stream.
@@ -186,7 +195,10 @@ extension ControlService {
     }
 
     // Build the trailers.
-    let trailers = input.echoMetadataInTrailers ? metadata.echo() : [:]
+    var trailers = input.echoMetadataInTrailers ? metadata.echo() : [:]
+    for (key, value) in input.trailingMetadataToAdd {
+      trailers.addString(value, forKey: key)
+    }
 
     if status.code == .ok {
       return .return(trailers)

Tests/GRPCNIOTransportHTTP2Tests/HTTP2TransportTests.swift

@@ -1446,6 +1446,38 @@ final class HTTP2TransportTests: XCTestCase {
       }
     }
   }
+
+  func testUppercaseClientMetadataKey() async throws {
+    try await self.forEachTransportPair { control, _, _ in
+      let request = ClientRequest<ControlInput>(
+        message: .with {
+          $0.echoMetadataInHeaders = true
+          $0.numberOfMessages = 1
+        },
+        metadata: ["UPPERCASE-KEY": "value"]
+      )
+      try await control.unary(request: request) { response in
+        // Keys will be lowercase before being sent over the wire.
+        XCTAssertEqual(Array(response.metadata["echo-uppercase-key"]), ["value"])
+      }
+    }
+  }
+
+  func testUppercaseServerMetadataKey() async throws {
+    try await self.forEachTransportPair { control, _, _ in
+      let request = ClientRequest<ControlInput>(
+        message: .with {
+          $0.initialMetadataToAdd["UPPERCASE-KEY"] = "initial"
+          $0.trailingMetadataToAdd["UPPERCASE-KEY"] = "trailing"
+          $0.numberOfMessages = 1
+        }
+      )
+      try await control.unary(request: request) { response in
+        XCTAssertEqual(Array(response.metadata["uppercase-key"]), ["initial"])
+        XCTAssertEqual(Array(response.trailingMetadata["uppercase-key"]), ["trailing"])
+      }
+    }
+  }
 }
 
 extension [HTTP2TransportTests.Transport] {