Send setParserPaused straight to the end of the codec filter chain

Summary:
Finishes the part of D120517427 that was dropped. Picks up the `setParserPaused` half of D122722345 by willwang1; the `getTransportDirection` half already landed in D120517427 through the cached codec traits.

`HTTPSession::processReadData` calls `codec_.setParserPaused(false)` before every `onIngress`, and the call used to walk every filter in the chain, each one just forwarding `call_->setParserPaused(paused)`. Now `HTTPCodecFilterChain::setParserPaused` calls the codec at the end of the chain directly.

To keep that correct, filters no longer take part in `setParserPaused`:
- `HTTPCodecFilter` is now a class on top of `GenericFilter` with an empty `final` `setParserPaused`. No filter can override it, whatever it derives from, and a filter called directly ignores it.
- `HTTPCodecFilterChain::add` and `addFilters` require `std::derived_from<Filter, HTTPCodecFilter>`, so a raw `GenericFilter` cannot get into the chain. `foreach` hands out `HTTPCodecFilter*`, so existing `FilterIteratorFn` callers are unchanged.
- `chainEnd_` changes only in `setDestination`, so the chain always reaches the current codec.

No filter in fbsource overrides `setParserPaused`, and nothing outside the chain calls it on a filter.

Perf (from D122722345): Strobelight puts stacks through `PassThroughHTTPCodecFilter::setParserPaused` at about $148K a year fleet-wide, about 96% of it the forwarding itself. On proxygen/olb_internet, `HTTPCodecFilterChain::setParserPaused` is about 0.165% of CPU.

Reviewed By: hanidamlaj

Differential Revision: D123003021

fbshipit-source-id: ed2aeafd7c22d4ad4ba35cd469d5f7ede217dc8b

Author: DenisYaroshevskiy

proxygen/lib/http/codec/HTTPCodecFilter.cpp

@@ -147,10 +147,6 @@ bool PassThroughHTTPCodecFilter::isBusy() const {
   return call_->isBusy();
 }
 
-void PassThroughHTTPCodecFilter::setParserPaused(bool paused) {
-  call_->setParserPaused(paused);
-}
-
 bool PassThroughHTTPCodecFilter::isParserPaused() const {
   return call_->isParserPaused();
 }

proxygen/lib/http/codec/HTTPCodecFilter.h

@@ -8,15 +8,26 @@
 
 #pragma once
 
+#include <concepts>
+#include <memory>
 #include <proxygen/lib/http/codec/HTTPCodec.h>
 #include <proxygen/lib/utils/FilterChain.h>
 
 namespace proxygen {
 
-using HTTPCodecFilter = GenericFilter<HTTPCodec,
-                                      HTTPCodec::Callback,
-                                      &HTTPCodec::setCallback,
-                                      true>;
+class HTTPCodecFilter
+    : public GenericFilter<HTTPCodec,
+                           HTTPCodec::Callback,
+                           &HTTPCodec::setCallback,
+                           true> {
+ public:
+  using GenericFilter::GenericFilter;
+
+  // Filters are not allowed to change this: HTTPCodecFilterChain sends it
+  // straight to the codec at the end of the chain.
+  void setParserPaused(bool /* paused */) final {
+  }
+};
 
 /**
  * An implementation of HTTPCodecFilter that passes through all calls. This is
@@ -107,8 +118,6 @@ class PassThroughHTTPCodecFilter : public HTTPCodecFilter {
 
   [[nodiscard]] bool isBusy() const override;
 
-  void setParserPaused(bool paused) override;
-
   [[nodiscard]] bool isParserPaused() const override;
 
   size_t onIngress(const folly::IOBuf& buf) override;
@@ -252,7 +261,7 @@ class HTTPCodecFilterChain {
   }
 
   void setParserPaused(bool paused) {
-    chain_->setParserPaused(paused);
+    chain_.getChainEndPtr()->setParserPaused(paused);
   }
 
   [[nodiscard]] const std::string& getUserAgent() const {
@@ -434,19 +443,20 @@ class HTTPCodecFilterChain {
     chain_.setCallback(callback);
   }
 
-  template <typename Filter, typename... Args>
+  template <std::derived_from<HTTPCodecFilter> Filter, typename... Args>
   void add(Args&&... args) {
     chain_.add<Filter>(std::forward<Args>(args)...);
   }
 
-  template <typename... Filters>
-  void addFilters(Filters&&... filters) {
-    chain_.addFilters(std::forward<Filters>(filters)...);
+  template <std::derived_from<HTTPCodecFilter>... Filters>
+  void addFilters(std::unique_ptr<Filters>... filters) {
+    chain_.addFilters(std::move(filters)...);
   }
 
   template <typename Fn>
   void foreach (Fn&& fn) {
-    chain_.foreach (std::forward<Fn>(fn));
+    chain_.foreach (
+        [&fn](auto* filter) { fn(static_cast<HTTPCodecFilter*>(filter)); });
   }
 
   std::unique_ptr<HTTPCodec> setDestination(std::unique_ptr<HTTPCodec> dest) {

proxygen/lib/http/codec/test/FilterTests.cpp

@@ -265,6 +265,15 @@ TEST_F(HTTPChecksTest, RecvTraceBody) {
   callbackStart_->onHeadersComplete(0, std::move(msg));
 }
 
+TEST_F(HTTPChecksTest, SetParserPausedReachesChainEnd) {
+  auto* newCodec = new MockHTTPCodec();
+  auto oldCodec = chain_.setDestination(std::unique_ptr<HTTPCodec>(newCodec));
+  EXPECT_CALL(*codec_, setParserPaused(_)).Times(0);
+  EXPECT_CALL(*newCodec, setParserPaused(true));
+
+  chain_.setParserPaused(true);
+}
+
 TEST_F(DebugFilterTest, NoError) {
   chain_.onIngress(*makeIOBuf("foo"));
   chain_.onIngressEOF();