HTTPCLIENT-2221 Closing a classic response/entity allows connection reuse

Previously, a partially consumed response body closed via
CloseableHttpResponse.close or HttpEntity.close would fully consume
remaining bytes (via close), however it would not release the
connection for reuse.
If CloseableHttpResponse.close was called, it would follow the close
with a discard/disconnect, however if only the entity was closed,
the connection would remain in a checked-out (leaked) state.

Now, we take advantage of the fact that closing a response stream
on any reusable connection is required to drain bytes on closure.
Failures are detected by writeTo and the stream returned by
getContent, so we can be confident that we will not return a
broken connection to the pool.

httpclient5-testing/src/test/java/org/apache/hc/client5/testing/sync/TestConnectionReuse.java

@@ -84,6 +84,8 @@ public class TestConnectionReuse extends LocalServerTestBase {
             }
         }
 
+        // Expect leased connections to be returned
+        Assertions.assertEquals(0, this.connManager.getTotalStats().getLeased());
         // Expect some connection in the pool
         Assertions.assertTrue(this.connManager.getTotalStats().getAvailable() > 0);
     }
@@ -120,6 +122,8 @@ public class TestConnectionReuse extends LocalServerTestBase {
             }
         }
 
+        // Expect leased connections to be returned
+        Assertions.assertEquals(0, this.connManager.getTotalStats().getLeased());
         // Expect some connection in the pool
         Assertions.assertTrue(this.connManager.getTotalStats().getAvailable() > 0);
     }
@@ -166,6 +170,8 @@ public class TestConnectionReuse extends LocalServerTestBase {
             }
         }
 
+        // Expect leased connections to be returned
+        Assertions.assertEquals(0, this.connManager.getTotalStats().getLeased());
         // Expect zero connections in the pool
         Assertions.assertEquals(0, this.connManager.getTotalStats().getAvailable());
     }
@@ -197,8 +203,10 @@ public class TestConnectionReuse extends LocalServerTestBase {
             }
         }
 
-        // Expect zero connections in the pool
-        Assertions.assertEquals(0, this.connManager.getTotalStats().getAvailable());
+        // Expect leased connections to be returned
+        Assertions.assertEquals(0, this.connManager.getTotalStats().getLeased());
+        // Expect some connections in the pool
+        Assertions.assertTrue(this.connManager.getTotalStats().getAvailable() > 0);
     }
 
     @Test
@@ -242,6 +250,8 @@ public class TestConnectionReuse extends LocalServerTestBase {
             return null;
         });
 
+        // Expect leased connections to be returned
+        Assertions.assertEquals(0, this.connManager.getTotalStats().getLeased());
         Assertions.assertEquals(1, this.connManager.getTotalStats().getAvailable());
     }
 

httpclient5/src/main/java/org/apache/hc/client5/http/impl/classic/ResponseEntityProxy.java

@@ -174,4 +174,18 @@ class ResponseEntityProxy extends HttpEntityWrapper implements EofSensorWatcher
             }
     }
 
+    @Override
+    public void close() throws IOException {
+        try {
+            // HttpEntity.close will close the underlying resource. Closing a reusable request stream results in
+            // draining remaining data, allowing for connection reuse.
+            super.close();
+            releaseConnection();
+        } catch (final IOException | RuntimeException ex) {
+            discardConnection();
+            throw ex;
+        } finally {
+            cleanup();
+        }
+    }
 }