http2: keep session objects alive during Http2Scope

Keep a local handle as a reference to the JS `Http2Session`
object so that it will not be garbage collected
when inside an `Http2Scope`, because the presence of the
latter usually indicates that further actions on
the session object are expected.

Strictly speaking, storing the `session_handle_` as a
property on the scope object is not necessary, but
this is not very costly and makes the code more
obviously correct.

Fixes: #17840

Backport-PR-URL: #18050
Backport-PR-URL: #20456
PR-URL: #17863
Fixes: #17840
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node_http2.cc

@@ -71,6 +71,11 @@ Http2Scope::Http2Scope(Http2Session* session) {
   }
   session->flags_ |= SESSION_STATE_HAS_SCOPE;
   session_ = session;
+
+  // Always keep the session object alive for at least as long as
+  // this scope is active.
+  session_handle_ = session->object();
+  CHECK(!session_handle_.IsEmpty());
 }
 
 Http2Scope::~Http2Scope() {
@@ -512,6 +517,7 @@ void Http2Session::Unconsume() {
 }
 
 Http2Session::~Http2Session() {
+  CHECK_EQ(flags_ & SESSION_STATE_HAS_SCOPE, 0);
   if (!object().IsEmpty())
     ClearWrap(object());
   persistent().Reset();
@@ -1365,6 +1371,7 @@ void Http2Session::OnStreamReadImpl(ssize_t nread,
                                     void* ctx) {
   Http2Session* session = static_cast<Http2Session*>(ctx);
   Http2Scope h2scope(session);
+  CHECK_NE(session->stream_, nullptr);
   DEBUG_HTTP2SESSION2(session, "receiving %d bytes", nread);
   if (nread < 0) {
     uv_buf_t tmp_buf;

src/node_http2.h

@@ -444,6 +444,7 @@ class Http2Scope {
 
  private:
   Http2Session* session_ = nullptr;
+  Local<Object> session_handle_;
 };
 
 // The Http2Options class is used to parse the options object passed in to

test/parallel/test-http2-createwritereq.js

@@ -1,5 +1,7 @@
 'use strict';
 
+// Flags: --expose-gc
+
 const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
@@ -62,6 +64,15 @@ server.listen(0, common.mustCall(function() {
       }
     }));
 
+    // Ref: https://github.com/nodejs/node/issues/17840
+    const origDestroy = req.destroy;
+    req.destroy = function(...args) {
+      // Schedule a garbage collection event at the end of the current
+      // MakeCallback() run.
+      process.nextTick(global.gc);
+      return origDestroy.call(this, ...args);
+    };
+
     req.end();
   });
 }));