Send signal as a string in ClientProcess.kill (fixes "missing signal in xpc message")

Sources/Services/ContainerAPIService/Client/ClientProcess.swift

@@ -80,7 +80,14 @@ struct ClientProcessImpl: ClientProcess, Sendable {
         let request = XPCMessage(route: .containerKill)
         request.set(key: .id, value: containerId)
         request.set(key: .processIdentifier, value: id)
-        request.set(key: .signal, value: Int64(signal))
+        // The server reads `.signal` as a string and resolves it against the Linux
+        // signal table, so send the canonical signal name rather than a raw integer.
+        // A bare Int64 is read back as an empty string and rejected with
+        // "missing signal in xpc message". Translating the host signal number to its
+        // name (instead of sending the number) also keeps signals whose numbers differ
+        // between macOS and Linux (e.g. SIGUSR1) correct. Fall back to the numeric
+        // string for signals with no platform name (e.g. real-time signals).
+        request.set(key: .signal, value: Signal.platformName(signal) ?? "\(signal)")
 
         try await xpcClient.send(request)
     }

Tests/ContainerAPIServiceTests/SignalWireFormatTests.swift

@@ -0,0 +1,64 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the container project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import Containerization
+import Darwin
+import Testing
+
+/// Guards the client -> server signal contract that `ClientProcess.kill(_:)`
+/// depends on. The client holds a raw host (macOS) signal number and must put a
+/// value on the wire that the server resolves back to the intended signal via
+/// `Signal(_:)` (default Linux table), matching `RuntimeService.kill`.
+///
+/// Regression coverage for the "missing signal in xpc message" bug where the
+/// client sent a bare integer that the server read as a string (issues #1941,
+/// #1876, #1747).
+struct SignalWireFormatTests {
+    /// Exactly the expression `ClientProcess.kill(_:)` puts on the wire.
+    private func wireValue(_ signal: Int32) -> String {
+        Signal.platformName(signal) ?? "\(signal)"
+    }
+
+    /// Exactly how the server resolves it (see `RuntimeService.kill`).
+    private func resolved(_ signal: Int32) throws -> Int32 {
+        try Signal(wireValue(signal)).rawValue
+    }
+
+    @Test func commonSignalsRoundTrip() throws {
+        // Signals that share a number across macOS and Linux.
+        #expect(try resolved(SIGINT) == 2)  // ^C  (#1876)
+        #expect(try resolved(SIGWINCH) == 28)  // terminal resize (#1747)
+        #expect(try resolved(SIGKILL) == 9)
+        #expect(try resolved(SIGTERM) == 15)
+        #expect(try resolved(SIGHUP) == 1)
+    }
+
+    @Test func platformDivergentSignalTranslatesByName() throws {
+        // macOS SIGUSR1 == 30, Linux SIGUSR1 == 10. Sending the name (not the
+        // raw number) is what keeps this correct: a raw "30" would land on the
+        // container as Linux signal 30 (SIGPWR).
+        #expect(SIGUSR1 == 30)
+        #expect(wireValue(SIGUSR1) == "USR1")
+        #expect(try resolved(SIGUSR1) == 10)
+    }
+
+    @Test func nonEmptyWireValue() {
+        // The original bug delivered an empty/absent string. Whatever we send
+        // must be a non-empty string the server can parse.
+        #expect(!wireValue(SIGINT).isEmpty)
+        #expect((try? Signal(wireValue(SIGINT))) != nil)
+    }
+}