Enable HTTP connection reuse for okhttp

app/src/androidTest/kotlin/at/bitfire/davdroid/network/HttpClientBuilderTest.kt

@@ -49,6 +49,14 @@ class HttpClientBuilderTest {
     }
 
 
+    @Test
+    fun testBuild_SharesConnectionPoolAndDispatcher() {
+        val client1 = httpClientBuilder.get().build()
+        val client2 = httpClientBuilder.get().build()
+        assertEquals(client1.connectionPool, client2.connectionPool)
+        assertEquals(client1.dispatcher, client2.dispatcher)
+    }
+
     @Test
     fun testBuildKtor_CreatesWorkingClient() = runTest {
         server.enqueue(MockResponse()

app/src/main/kotlin/at/bitfire/davdroid/network/HttpClientBuilder.kt

@@ -73,6 +73,23 @@ class HttpClientBuilder @Inject constructor(
             // make sure Conscrypt is available when the HttpClientBuilder class is loaded the first time
             ConscryptIntegration().initialize()
         }
+
+        /**
+         * According to [OkHttpClient] documentation, [OkHttpClient]s should be shared, which allows it to use a
+         * shared connection and thread pool.
+         *
+         * We need custom settings for each actual client, but we can use a shared client as a base. This also
+         * enables sharing resources like connection and thread pool.
+         *
+         * The shared client is available for the lifetime of the application and must not be shut down or
+         * closed (which is not necessary, according to its documentation).
+         */
+        val sharedOkHttpClient = OkHttpClient.Builder()
+            .connectTimeout(15, TimeUnit.SECONDS)
+            .writeTimeout(30, TimeUnit.SECONDS)
+            .readTimeout(120, TimeUnit.SECONDS)
+            .pingInterval(45, TimeUnit.SECONDS)     // avoid cancellation because of missing traffic; only works for HTTP/2
+            .build()
     }
 
     /**
@@ -206,21 +223,21 @@ class HttpClientBuilder @Inject constructor(
      *
      * However in this case the configuration of `client1` is still in `builder` and would be reused for `client2`,
      * which is usually not desired.
+     *
+     * Closing/shutting down the client is not necessary.
      */
     fun build(): OkHttpClient {
         if (alreadyBuilt)
             logger.warning("build() should only be called once; use Provider<HttpClientBuilder> instead")
 
-        val builder = OkHttpClient.Builder()
+        val builder = sharedOkHttpClient.newBuilder()
         configureOkHttp(builder)
 
         alreadyBuilt = true
         return builder.build()
     }
 
     private fun configureOkHttp(builder: OkHttpClient.Builder) {
-        buildTimeouts(builder)
-
         // don't allow redirects by default because it would break PROPFIND handling
         builder.followRedirects(followRedirects)
 
@@ -353,19 +370,6 @@ class HttpClientBuilder @Inject constructor(
         }
     }
 
-    /**
-     * Set timeouts for the connection.
-     *
-     * **Note:** According to [android.content.AbstractThreadedSyncAdapter], when there is no network
-     * traffic within a minute, a sync will be cancelled.
-     */
-    private fun buildTimeouts(builder: OkHttpClient.Builder) {
-        builder.connectTimeout(15, TimeUnit.SECONDS)
-            .writeTimeout(30, TimeUnit.SECONDS)
-            .readTimeout(120, TimeUnit.SECONDS)
-            .pingInterval(45, TimeUnit.SECONDS)     // avoid cancellation because of missing traffic; only works for HTTP/2
-    }
-
 
     // Ktor builder