Allow multiplexed upstream servers to half close the stream before the downstream

changelogs/current.yaml

@@ -10,6 +10,13 @@ behavior_changes:
   change: |
     Change ``OnLogDownstreamStart``, ``OnLogDownstreamPeriodic`` and ``OnLog`` methods so that user can get the request/response's
     headers and trailers when producing access log.
+- area: http
+  change: |
+    Allow HTTP/2 (and HTTP/3) upstream servers to half close the stream before the downstream. This enables bidirectional
+    gRPC streams where server completes streaming before the client. Behavior of HTTP/1 or TCP proxy upstream servers is
+    unchanged and the stream is reset if the upstream server completes response before the downstream. The stream is also
+    reset if the upstream server responds with an error status before the downstream. This behavior is disabled by default
+    and can be enabled by setting the ``envoy.reloadable_features.allow_multiplexed_upstream_half_close`` runtime key to true.
 - area: http
   change: |
     Added HTTP1-safe option for :ref:`max_connection_duration

docs/root/configuration/observability/access_log/usage.rst

@@ -428,6 +428,10 @@ The following command operators are supported:
     * ``us``: Microsecond precision.
     * ``ns``: Nanosecond precision.
 
+    NOTE: enabling independent half-close behavior for H/2 and H/3 protocols can produce
+    ``*_TX_END`` values lower than ``*_RX_END`` values, in cases where upstream peer has half-closed
+    its stream before downstream peer. In these cases ``COMMON_DURATION`` value will become negative.
+
   TCP/UDP
     Not implemented ("-").
 

docs/root/intro/arch_overview/http/http_connection_management.rst

@@ -36,6 +36,26 @@ In the case of HTTP/1.1, the codec translates the serial/pipelining capabilities
 that looks like HTTP/2 to higher layers. This means that the majority of the code does not need to
 understand whether a stream originated on an HTTP/1.1, HTTP/2, or HTTP/3 connection.
 
+HTTP lifecycle
+--------------
+
+Proxying of the request begins when the downstream HTTP codec has successfully decoded request header map.
+
+The point at which the proxying completes and the stream is destroyed depends on the upstream protocol and
+whether the independent half close is enabled.
+
+If independent half-close is enabled and the upstream protocol is either HTTP/2 or HTTP/3 protocols the stream
+is destroyed after both request and response are complete i.e. reach their respective end-of-stream,
+by receiving trailers or the header/body with end-stream set in both directions AND response has
+success (2xx) status code.
+
+For HTTP/1 upstream protocol or if independent half-close is disabled the stream is destroyed when the response
+is complete and reaches its end-of-stream, i.e. when trailers or the response header/body with
+end-stream set are received, even if the request has not yet completed. If request was incomplete at response
+completion, the stream is reset.
+
+Note that proxying can stop early when an error or timeout occurred or when a peer reset HTTP/2 or HTTP/3 stream.
+
 HTTP header sanitizing
 ----------------------
 

docs/root/intro/life_of_a_request.rst

@@ -212,8 +212,11 @@ A brief outline of the life cycle of a request and response using the example co
     :ref:`opposite order <arch_overview_http_filters_ordering>` from the request, starting at the
     codec filter, traversing any upstream HTTP filters, then going through the router filter and passing
     through CustomFilter, before being sent downstream.
-12. When the response is complete, the stream is destroyed. Post-request processing will update
-    stats, write to the access log and finalize trace spans.
+12. If independent half-close is enabled the stream is destroyed after both request and response are
+    complete (END_STREAM for the HTTP/2 stream is observed in both directions) AND response has success
+    (2xx) status code. Otherwise the stream is destroyed when the response is complete, even if the
+    request has not yet completed. Post-request processing will update stats, write to the access log
+    and finalize trace spans.
 
 We elaborate on each of these steps in the sections below.
 
@@ -533,8 +536,15 @@ directions during a request.
 :ref:`Outlier detection <arch_overview_outlier_detection>` status for the endpoint is revised as the
 request progresses.
 
-A request completes when the upstream response reaches its end-of-stream, i.e. when trailers or the
-response header/body with end-stream set are received. This is handled in
+The point at which the proxying completes and the stream is destroyed for HTTP/2 and HTTP/3 protocols
+is determined by the independent half-close option. If independent half-close is enabled the stream
+is destroyed after both request and response are complete i.e. reach their respective end-of-stream,
+by receiving trailers or the header/body with end-stream set in both directions AND response has
+success (2xx) status code. This is handled in ``FilterManager::checkAndCloseStreamIfFullyClosed()``.
+
+For HTTP/1 protocol or if independent half-close is disabled the stream is destroyed when the response
+is complete and reaches its end-of-stream, i.e. when trailers or the response header/body with
+end-stream set are received, even if the request has not yet completed. This is handled in
 ``Router::Filter::onUpstreamComplete()``.
 
 It is possible for a request to terminate early. This may be due to (but not limited to):

envoy/http/stream_reset_handler.h

@@ -36,7 +36,9 @@ enum class StreamResetReason {
   // Received payload did not conform to HTTP protocol.
   ProtocolError,
   // If the stream was locally reset by the Overload Manager.
-  OverloadManager
+  OverloadManager,
+  // If stream was locally reset due to HTTP/1 upstream half closing before downstream.
+  Http1PrematureUpstreamHalfClose,
 };
 
 /**

source/common/http/async_client_impl.cc

@@ -149,13 +149,14 @@ void AsyncStreamImpl::encodeHeaders(ResponseHeaderMapPtr&& headers, bool end_str
   encoded_response_headers_ = true;
   stream_callbacks_.onHeaders(std::move(headers), end_stream);
   closeRemote(end_stream);
-  // At present, the router cleans up stream state as soon as the remote is closed, making a
-  // half-open local stream unsupported and dangerous. Ensure we close locally to trigger completion
-  // and keep things consistent. Another option would be to issue a stream reset here if local isn't
-  // yet closed, triggering cleanup along a more standardized path. However, this would require
-  // additional logic to handle the response completion and subsequent reset, and run the risk of
-  // being interpreted as a failure, when in fact no error has necessarily occurred. Gracefully
-  // closing seems most in-line with behavior elsewhere in Envoy for now.
+  // At present, the AsyncStream is always fully closed when the server half closes the stream.
+  //
+  // Always ensure we close locally to trigger completion. Another option would be to issue a stream
+  // reset here if local isn't yet closed, triggering cleanup along a more standardized path.
+  // However, this would require additional logic to handle the response completion and subsequent
+  // reset, and run the risk of being interpreted as a failure, when in fact no error has
+  // necessarily occurred. Gracefully closing seems most in-line with behavior elsewhere in Envoy
+  // for now.
   closeLocal(end_stream);
 }
 

source/common/http/conn_manager_impl.cc

@@ -126,8 +126,10 @@ ConnectionManagerImpl::ConnectionManagerImpl(ConnectionManagerConfigSharedPtr co
                                      /*node_id=*/local_info_.node().id(),
                                      /*server_name=*/config_->serverName(),
                                      /*proxy_status_config=*/config_->proxyStatusConfig())),
-      max_requests_during_dispatch_(runtime_.snapshot().getInteger(
-          ConnectionManagerImpl::MaxRequestsPerIoCycle, UINT32_MAX)) {
+      max_requests_during_dispatch_(
+          runtime_.snapshot().getInteger(ConnectionManagerImpl::MaxRequestsPerIoCycle, UINT32_MAX)),
+      allow_upstream_half_close_(Runtime::runtimeFeatureEnabled(
+          "envoy.reloadable_features.allow_multiplexed_upstream_half_close")) {
   ENVOY_LOG_ONCE_IF(
       trace, accept_new_http_stream_ == nullptr,
       "LoadShedPoint envoy.load_shed_points.http_connection_manager_decode_headers is not "

source/common/http/conn_manager_impl.h

@@ -283,7 +283,7 @@ class ConnectionManagerImpl : Logger::Loggable<Logger::Id::http>,
     OptRef<const Tracing::Config> tracingConfig() const override;
     const ScopeTrackedObject& scope() override;
     OptRef<DownstreamStreamFilterCallbacks> downstreamCallbacks() override { return *this; }
-    bool isHalfCloseEnabled() override { return false; }
+    bool isHalfCloseEnabled() override { return connection_manager_.allow_upstream_half_close_; }
 
     // DownstreamStreamFilterCallbacks
     void setRoute(Router::RouteConstSharedPtr route) override;
@@ -640,6 +640,18 @@ class ConnectionManagerImpl : Logger::Loggable<Logger::Id::http>,
   uint32_t requests_during_dispatch_count_{0};
   const uint32_t max_requests_during_dispatch_{UINT32_MAX};
   Event::SchedulableCallbackPtr deferred_request_processing_callback_;
+
+  // If independent half-close is enabled and the upstream protocol is either HTTP/2 or HTTP/3
+  // protocols the stream is destroyed after both request and response are complete i.e. reach their
+  // respective end-of-stream, by receiving trailers or the header/body with end-stream set in both
+  // directions AND response has success (2xx) status code.
+  //
+  // For HTTP/1 upstream protocol or if independent half-close is disabled the stream is destroyed
+  // when the response is complete and reaches its end-of-stream, i.e. when trailers or the response
+  // header/body with end-stream set are received, even if the request has not yet completed. If
+  // request was incomplete at response completion, the stream is reset.
+
+  const bool allow_upstream_half_close_{};
 };
 
 } // namespace Http

source/common/http/conn_pool_base.cc

@@ -177,6 +177,7 @@ void MultiplexedActiveClientBase::onStreamReset(Http::StreamResetReason reason)
   case StreamResetReason::RemoteRefusedStreamReset:
   case StreamResetReason::Overflow:
   case StreamResetReason::ConnectError:
+  case StreamResetReason::Http1PrematureUpstreamHalfClose:
     break;
   }
 }