Introduce the CancellationScheduler interface (line#5220)

Motivation:

While working on line#5212, I realized
that `noopResponseCancellationScheduler` is an already completed
scheduler.
When we create a `DefaultClientRequestContext` for the upgrade request,
we use this scheduler.

https://github.com/line/armeria/blob/8e139d972f5137f9d1805ddda4a7aeae60fecd40/core/src/main/java/com/linecorp/armeria/client/HttpClientPipelineConfigurator.java#L550

However, actually what we want is a scheduler which never completes
instead of an already completed scheduler.
For this reason, I propose that we separate a
- `CancellationScheduler#noop`: signifies a scheduler which never
completes
- `CancellationScheduler#finished`: signifies a scheduler which already
completed

Note that the upgrade request is already bound by `connectTimeout`, so
it is safe to use a `CancellationScheduler` which never completes.

Modifications:

- Rename `CancellationScheduler` to `DefaultCancellationScheduler`, and
reduce visibility
- Introduce an interface `CancellationScheduler`
- Introduce factory methods `CancellationScheduler#of`,
`CancellationScheduler#noop`, `CancellationScheduler#finished`

Result:

- It is easier to handle line#5212.
- Cleaner code overall.

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core/src/main/java/com/linecorp/armeria/client/ClientRequestContextBuilder.java

@@ -15,6 +15,7 @@
  */
 package com.linecorp.armeria.client;
 
+import static com.linecorp.armeria.internal.common.CancellationScheduler.noopCancellationTask;
 import static java.util.Objects.requireNonNull;
 
 import java.net.InetSocketAddress;
@@ -35,12 +36,10 @@
 import com.linecorp.armeria.common.util.SystemInfo;
 import com.linecorp.armeria.internal.client.DefaultClientRequestContext;
 import com.linecorp.armeria.internal.common.CancellationScheduler;
-import com.linecorp.armeria.internal.common.CancellationScheduler.CancellationTask;
 
 import io.micrometer.core.instrument.MeterRegistry;
 import io.netty.buffer.ByteBufAllocator;
 import io.netty.channel.EventLoop;
-import io.netty.util.concurrent.ImmediateEventExecutor;
 
 /**
  * Builds a new {@link ClientRequestContext}. Note that it is not usually required to create a new context by
@@ -49,27 +48,6 @@
  */
 public final class ClientRequestContextBuilder extends AbstractRequestContextBuilder {
 
-    private static final CancellationTask noopCancellationTask = new CancellationTask() {
-        @Override
-        public boolean canSchedule() {
-            return true;
-        }
-
-        @Override
-        public void run(Throwable cause) { /* no-op */ }
-    };
-
-    /**
-     * A cancellation scheduler that has been finished.
-     */
-    static final CancellationScheduler noopResponseCancellationScheduler = new CancellationScheduler(0);
-
-    static {
-        noopResponseCancellationScheduler
-                .init(ImmediateEventExecutor.INSTANCE, noopCancellationTask, 0, /* server */ false);
-        noopResponseCancellationScheduler.finishNow();
-    }
-
     @Nullable
     private Endpoint endpoint;
     private ClientOptions options = ClientOptions.of();
@@ -136,9 +114,9 @@ public ClientRequestContext build() {
 
         final CancellationScheduler responseCancellationScheduler;
         if (timedOut()) {
-            responseCancellationScheduler = noopResponseCancellationScheduler;
+            responseCancellationScheduler = CancellationScheduler.finished(false);
         } else {
-            responseCancellationScheduler = new CancellationScheduler(0);
+            responseCancellationScheduler = CancellationScheduler.of(0);
             final CountDownLatch latch = new CountDownLatch(1);
             eventLoop().execute(() -> {
                 responseCancellationScheduler.init(eventLoop(), noopCancellationTask, 0, /* server */ false);

core/src/main/java/com/linecorp/armeria/client/HttpClientPipelineConfigurator.java

@@ -17,7 +17,6 @@
 package com.linecorp.armeria.client;
 
 import static com.google.common.base.MoreObjects.firstNonNull;
-import static com.linecorp.armeria.client.ClientRequestContextBuilder.noopResponseCancellationScheduler;
 import static com.linecorp.armeria.common.SessionProtocol.H1;
 import static com.linecorp.armeria.common.SessionProtocol.H1C;
 import static com.linecorp.armeria.common.SessionProtocol.H2;
@@ -57,6 +56,7 @@
 import com.linecorp.armeria.internal.client.UserAgentUtil;
 import com.linecorp.armeria.internal.common.ArmeriaHttp2HeadersDecoder;
 import com.linecorp.armeria.internal.common.ArmeriaHttpUtil;
+import com.linecorp.armeria.internal.common.CancellationScheduler;
 import com.linecorp.armeria.internal.common.ReadSuppressingHandler;
 import com.linecorp.armeria.internal.common.TrafficLoggingHandler;
 import com.linecorp.armeria.internal.common.util.ChannelUtil;
@@ -547,7 +547,7 @@ public void onComplete() {}
                     com.linecorp.armeria.common.HttpMethod.OPTIONS,
                     RequestTarget.forClient("*"), ClientOptions.of(),
                     HttpRequest.of(com.linecorp.armeria.common.HttpMethod.OPTIONS, "*"),
-                    null, REQUEST_OPTIONS_FOR_UPGRADE_REQUEST, noopResponseCancellationScheduler,
+                    null, REQUEST_OPTIONS_FOR_UPGRADE_REQUEST, CancellationScheduler.noop(),
                     System.nanoTime(), SystemInfo.currentTimeMicros());
 
             // NB: No need to set the response timeout because we have session creation timeout.

core/src/main/java/com/linecorp/armeria/internal/client/DefaultClientRequestContext.java

@@ -226,7 +226,7 @@ private DefaultClientRequestContext(
                 responseTimeoutMillis = options().responseTimeoutMillis();
             }
             this.responseCancellationScheduler =
-                    new CancellationScheduler(TimeUnit.MILLISECONDS.toNanos(responseTimeoutMillis));
+                    CancellationScheduler.of(TimeUnit.MILLISECONDS.toNanos(responseTimeoutMillis));
         } else {
             this.responseCancellationScheduler = responseCancellationScheduler;
         }
@@ -504,7 +504,7 @@ private DefaultClientRequestContext(DefaultClientRequestContext ctx,
         log = RequestLog.builder(this);
         log.startRequest();
         responseCancellationScheduler =
-                new CancellationScheduler(TimeUnit.MILLISECONDS.toNanos(ctx.responseTimeoutMillis()));
+                CancellationScheduler.of(TimeUnit.MILLISECONDS.toNanos(ctx.responseTimeoutMillis()));
         writeTimeoutMillis = ctx.writeTimeoutMillis();
         maxResponseLength = ctx.maxResponseLength();