Add utilities to test tap interface networks

Add an ArpResponder based on a generic PacketResponder class, and a
TestHttpServer based on NanoHttpd.

Test: tests based on these utilities
Bug: 160617623
Bug: 160656765
Change-Id: I50b872a8b23e8df997e8f62f0adc7c0256c4d74d
diff --git a/staticlibs/devicetests/com/android/testutils/TestHttpServer.kt b/staticlibs/devicetests/com/android/testutils/TestHttpServer.kt
new file mode 100644
index 0000000..7aae8e3
--- /dev/null
+++ b/staticlibs/devicetests/com/android/testutils/TestHttpServer.kt
@@ -0,0 +1,86 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * 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
+ *
+ *      http://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.
+ */
+
+package com.android.testutils
+
+import android.net.Uri
+import com.android.net.module.util.ArrayTrackRecord
+import fi.iki.elonen.NanoHTTPD
+
+/**
+ * A minimal HTTP server running on a random available port.
+ *
+ * @param host The host to listen to, or null to listen on all hosts
+ */
+class TestHttpServer(host: String? = null) : NanoHTTPD(host, 0 /* auto-select the port */) {
+    // Map of URL path -> HTTP response code
+    private val responses = HashMap<Request, Response>()
+
+    /**
+     * A record of all requests received by the server since it was started.
+     */
+    val requestsRecord = ArrayTrackRecord<Request>()
+
+    /**
+     * A request received by the test server.
+     */
+    data class Request(
+        val path: String,
+        val method: Method = Method.GET,
+        val queryParameters: String = ""
+    ) {
+        /**
+         * Returns whether the specified [Uri] matches parameters of this request.
+         */
+        fun matches(uri: Uri) = (uri.path ?: "") == path && (uri.query ?: "") == queryParameters
+    }
+
+    /**
+     * Add a response for GET requests with the path and query parameters of the specified [Uri].
+     */
+    fun addResponse(
+        uri: Uri,
+        statusCode: Response.IStatus,
+        locationHeader: String? = null,
+        content: String = ""
+    ) {
+        addResponse(Request(uri.path
+                ?: "", Method.GET, uri.query ?: ""),
+                statusCode, locationHeader, content)
+    }
+
+    /**
+     * Add a response for the given request.
+     */
+    fun addResponse(
+        request: Request,
+        statusCode: Response.IStatus,
+        locationHeader: String? = null,
+        content: String = ""
+    ) {
+        val response = newFixedLengthResponse(statusCode, "text/plain", content)
+        locationHeader?.let { response.addHeader("Location", it) }
+        responses[request] = response
+    }
+
+    override fun serve(session: IHTTPSession): Response {
+        val request = Request(session.uri
+                ?: "", session.method, session.queryParameterString ?: "")
+        requestsRecord.add(request)
+        // Default response is a 404
+        return responses[request] ?: super.serve(session)
+    }
+}
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