core: For Android, ignores DNS cache

There is a known issue that causes DNS lookup issue when network
siwtchover on android. This issue is tracked separately in #4962.
This change simply disables DNS cache to avoid the issue on Android.

core/src/main/java/io/grpc/internal/DnsNameResolver.java

@@ -98,6 +98,8 @@ final class DnsNameResolver extends NameResolver {
    * <p>Default value is -1 (cache forever) if security manager is installed. If security manager is
    * not installed, the ttl value is {@code null} which falls back to {@link
    * #DEFAULT_NETWORK_CACHE_TTL_SECONDS gRPC default value}.
+   *
+   * <p>For android, gRPC doesn't attempt to cache; this property value will be ignored.
    */
   @VisibleForTesting
   static final String NETWORKADDRESS_CACHE_TTL_PROPERTY = "networkaddress.cache.ttl";
@@ -143,7 +145,7 @@ final class DnsNameResolver extends NameResolver {
 
   DnsNameResolver(@Nullable String nsAuthority, String name, Attributes params,
       Resource<ExecutorService> executorResource, ProxyDetector proxyDetector,
-      Stopwatch stopwatch) {
+      Stopwatch stopwatch, boolean isAndroid) {
     // TODO: if a DNS server is provided as nsAuthority, use it.
     // https://www.captechconsulting.com/blogs/accessing-the-dusty-corners-of-dns-with-java
     this.executorResource = executorResource;
@@ -166,7 +168,7 @@ final class DnsNameResolver extends NameResolver {
       port = nameUri.getPort();
     }
     this.proxyDetector = proxyDetector;
-    this.resolveRunnable = new Resolve(this, stopwatch);
+    this.resolveRunnable = new Resolve(this, stopwatch, getNetworkAddressCacheTtlNanos(isAndroid));
   }
 
   @Override
@@ -196,10 +198,10 @@ final class DnsNameResolver extends NameResolver {
     private final long cacheTtlNanos;
     private ResolutionResults cachedResolutionResults = null;
 
-    Resolve(DnsNameResolver resolver, Stopwatch stopwatch) {
+    Resolve(DnsNameResolver resolver, Stopwatch stopwatch, long cacheTtlNanos) {
       this.resolver = resolver;
       this.stopwatch = Preconditions.checkNotNull(stopwatch, "stopwatch");
-      this.cacheTtlNanos = getNetworkAddressCacheTtlNanos();
+      this.cacheTtlNanos = cacheTtlNanos;
     }
 
     @Override
@@ -230,23 +232,6 @@ final class DnsNameResolver extends NameResolver {
           || (cacheTtlNanos > 0 && stopwatch.elapsed(TimeUnit.NANOSECONDS) > cacheTtlNanos);
     }
 
-    /** Returns value of network address cache ttl property. */
-    private static long getNetworkAddressCacheTtlNanos() {
-      String cacheTtlPropertyValue = System.getProperty(NETWORKADDRESS_CACHE_TTL_PROPERTY);
-      long cacheTtl = DEFAULT_NETWORK_CACHE_TTL_SECONDS;
-      if (cacheTtlPropertyValue != null) {
-        try {
-          cacheTtl = Long.parseLong(cacheTtlPropertyValue);
-        } catch (NumberFormatException e) {
-          logger.log(
-              Level.WARNING,
-              "Property({0}) valid is not valid number format({1}), fall back to default({2})",
-              new Object[] {NETWORKADDRESS_CACHE_TTL_PROPERTY, cacheTtlPropertyValue, cacheTtl});
-        }
-      }
-      return cacheTtl > 0 ? TimeUnit.SECONDS.toNanos(cacheTtl) : cacheTtl;
-    }
-
     @VisibleForTesting
     void resolveInternal(Listener savedListener) {
       InetSocketAddress destination =
@@ -483,6 +468,31 @@ final class DnsNameResolver extends NameResolver {
         ServiceConfigUtil.getList(serviceConfigChoice, SERVICE_CONFIG_CHOICE_CLIENT_HOSTNAME_KEY));
   }
 
+  /**
+   * Returns value of network address cache ttl property if not Android environment. For android,
+   * DnsNameResolver does not cache the dns lookup result.
+   */
+  private static long getNetworkAddressCacheTtlNanos(boolean isAndroid) {
+    if (isAndroid) {
+      // on Android, ignore dns cache.
+      return 0;
+    }
+
+    String cacheTtlPropertyValue = System.getProperty(NETWORKADDRESS_CACHE_TTL_PROPERTY);
+    long cacheTtl = DEFAULT_NETWORK_CACHE_TTL_SECONDS;
+    if (cacheTtlPropertyValue != null) {
+      try {
+        cacheTtl = Long.parseLong(cacheTtlPropertyValue);
+      } catch (NumberFormatException e) {
+        logger.log(
+            Level.WARNING,
+            "Property({0}) valid is not valid number format({1}), fall back to default({2})",
+            new Object[] {NETWORKADDRESS_CACHE_TTL_PROPERTY, cacheTtlPropertyValue, cacheTtl});
+      }
+    }
+    return cacheTtl > 0 ? TimeUnit.SECONDS.toNanos(cacheTtl) : cacheTtl;
+  }
+
   /**
    * Determines if a given Service Config choice applies, and if so, returns it.
    *

core/src/main/java/io/grpc/internal/DnsNameResolverProvider.java

@@ -19,6 +19,7 @@ package io.grpc.internal;
 import com.google.common.base.Preconditions;
 import com.google.common.base.Stopwatch;
 import io.grpc.Attributes;
+import io.grpc.InternalServiceProviders;
 import io.grpc.NameResolverProvider;
 import java.net.URI;
 
@@ -54,7 +55,8 @@ public final class DnsNameResolverProvider extends NameResolverProvider {
           params,
           GrpcUtil.SHARED_CHANNEL_EXECUTOR,
           GrpcUtil.getDefaultProxyDetector(),
-          Stopwatch.createUnstarted());
+          Stopwatch.createUnstarted(),
+          InternalServiceProviders.isAndroid(getClass().getClassLoader()));
     } else {
       return null;
     }

core/src/test/java/io/grpc/internal/DnsNameResolverTest.java

@@ -118,18 +118,34 @@ public class DnsNameResolverTest {
     return newResolver(name, port, GrpcUtil.NOOP_PROXY_DETECTOR, Stopwatch.createUnstarted());
   }
 
+  private DnsNameResolver newResolver(String name, int port, boolean isAndroid) {
+    return
+        newResolver(
+            name, port, GrpcUtil.NOOP_PROXY_DETECTOR, Stopwatch.createUnstarted(), isAndroid);
+  }
+
   private DnsNameResolver newResolver(
       String name,
       int port,
       ProxyDetector proxyDetector,
       Stopwatch stopwatch) {
+    return newResolver(name, port, proxyDetector, stopwatch, false);
+  }
+
+  private DnsNameResolver newResolver(
+      String name,
+      int port,
+      ProxyDetector proxyDetector,
+      Stopwatch stopwatch,
+      boolean isAndroid) {
     DnsNameResolver dnsResolver = new DnsNameResolver(
         null,
         name,
         Attributes.newBuilder().set(NameResolver.Factory.PARAMS_DEFAULT_PORT, port).build(),
         fakeExecutorResource,
         proxyDetector,
-        stopwatch);
+        stopwatch,
+        isAndroid);
     return dnsResolver;
   }
 
@@ -200,14 +216,30 @@ public class DnsNameResolverTest {
     }
   }
 
+  @Test
+  public void resolve_androidIgnoresPropertyValue() throws Exception {
+    System.setProperty(DnsNameResolver.NETWORKADDRESS_CACHE_TTL_PROPERTY, Long.toString(2));
+    resolveNeverCache(true);
+  }
+
+  @Test
+  public void resolve_androidIgnoresPropertyValueCacheForever() throws Exception {
+    System.setProperty(DnsNameResolver.NETWORKADDRESS_CACHE_TTL_PROPERTY, Long.toString(-1));
+    resolveNeverCache(true);
+  }
+
   @Test
   public void resolve_neverCache() throws Exception {
     System.setProperty(DnsNameResolver.NETWORKADDRESS_CACHE_TTL_PROPERTY, "0");
+    resolveNeverCache(false);
+  }
+
+  private void resolveNeverCache(boolean isAndroid) throws Exception {
     final List<InetAddress> answer1 = createAddressList(2);
     final List<InetAddress> answer2 = createAddressList(1);
     String name = "foo.googleapis.com";
 
-    DnsNameResolver resolver = newResolver(name, 81);
+    DnsNameResolver resolver = newResolver(name, 81, isAndroid);
     AddressResolver mockResolver = mock(AddressResolver.class);
     when(mockResolver.resolveAddress(Matchers.anyString())).thenReturn(answer1).thenReturn(answer2);
     resolver.setAddressResolver(mockResolver);
@@ -241,7 +273,7 @@ public class DnsNameResolverTest {
       }
     });
 
-    new DnsNameResolver.Resolve(nrf, Stopwatch.createUnstarted()).resolveInternal(mockListener);
+    new DnsNameResolver.Resolve(nrf, Stopwatch.createUnstarted(), 0).resolveInternal(mockListener);
 
     ArgumentCaptor<Status> ac = ArgumentCaptor.forClass(Status.class);
     verify(mockListener).onError(ac.capture());