core: throw IOException when ProxySelector returns null or empty list (#12793)

ProxySelector.select(URI) is contractually required to return a non-null, non-empty list. Some implementations violate this, which previously caused an opaque crash in ProxyDetectorImpl:

```
java.lang.IndexOutOfBoundsException: Index: 0
    at java.util.Collections$EmptyList.get
    at io.grpc.internal.ProxyDetectorImpl.detectProxy
```

Detect this case explicitly and throw an IOException naming the offending ProxySelector class, so a broken implementation can be identified and fixed by its author rather than silently worked around in every caller.

Author: mfperminov

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

@@ -207,6 +207,14 @@ private ProxiedSocketAddress detectProxy(InetSocketAddress targetAddr) throws IO
     }
 
     List<Proxy> proxies = proxySelector.select(uri);
+    // ProxySelector.select(URI) is contractually required to return a non-null, non-empty list.
+    // Surface the offending implementation's class name so a broken ProxySelector can be fixed.
+    if (proxies == null || proxies.isEmpty()) {
+      throw new IOException(
+          "ProxySelector " + proxySelector.getClass().getName()
+              + " returned " + (proxies == null ? "null" : "an empty list")
+              + ", which violates the java.net.ProxySelector#select(URI) contract");
+    }
     if (proxies.size() > 1) {
       log.warning("More than 1 proxy detected, gRPC will select the first one");
     }

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

@@ -21,6 +21,7 @@
 import static org.junit.Assert.assertNotEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyInt;
@@ -33,13 +34,15 @@
 import io.grpc.HttpConnectProxiedSocketAddress;
 import io.grpc.ProxiedSocketAddress;
 import io.grpc.ProxyDetector;
+import java.io.IOException;
 import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.net.PasswordAuthentication;
 import java.net.Proxy;
 import java.net.ProxySelector;
 import java.net.SocketAddress;
 import java.net.URI;
+import java.util.Collections;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
@@ -191,4 +194,24 @@ public ProxySelector get() {
         authenticator);
     assertNull(proxyDetector.proxyFor(destination));
   }
+
+  @Test
+  public void throwsWhenProxySelectorReturnsEmptyList() throws Exception {
+    when(proxySelector.select(any(URI.class))).thenReturn(Collections.<Proxy>emptyList());
+
+    IOException e =
+        assertThrows(IOException.class, () -> proxyDetector.proxyFor(destination));
+    assertTrue(e.getMessage(), e.getMessage().contains("empty list"));
+    assertTrue(e.getMessage(), e.getMessage().contains(proxySelector.getClass().getName()));
+  }
+
+  @Test
+  public void throwsWhenProxySelectorReturnsNullList() throws Exception {
+    when(proxySelector.select(any(URI.class))).thenReturn(null);
+
+    IOException e =
+        assertThrows(IOException.class, () -> proxyDetector.proxyFor(destination));
+    assertTrue(e.getMessage(), e.getMessage().contains("null"));
+    assertTrue(e.getMessage(), e.getMessage().contains(proxySelector.getClass().getName()));
+  }
 }