src: add method to compute storage in WriteWrap

[addaleax]

WriteWrap instances may contain extra storage space.
self_size() returns the size of the entire struct, member fields as
well as storage space, so it is not an accurate measure for the
storage space available.

Add a method ExtraSize() (like the existing Extra() for accessing
the storage memory) that yields the wanted value, and use it
in the HTTP2 impl to fix a crash.

Ref: #16669

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included (they crashe without this)
• commit message follows commit guidelines

Affected core subsystem(s)

http2

@nodejs/http2 @jasnell Probably want to fast-track this :)

[addaleax]

CI: https://ci.nodejs.org/job/node-test-commit/13717/

[jasnell]

Lgtm... Thanks for catching

[apapirovski]

Should land soon to unbreak the CI

[jasnell]

Looks like the fix is good, just waiting on CI to finish then will get it landed (CI is a bit bound up by a couple prior tests, taking longer than it should)

[jasnell]

Ci is green, landing

[jasnell]

Landed in a5f3b3a.
thank you @addaleax :-D

[gibfahn]

@addaleax conflicts for v8.x, mind raising a backport?

diff --cc src/node_http2_core-inl.h
index fa365fee05,c78e5d673f..0000000000
--- a/src/node_http2_core-inl.h
+++ b/src/node_http2_core-inl.h
@@@ -501,6 -501,11 +501,20 @@@ inline void Nghttp2Session::SendPending
  
    // While srcLength is greater than zero
    while ((srcLength = nghttp2_session_mem_send(session_, &src)) > 0) {
++<<<<<<< HEAD
++||||||| parent of a5f3b3a6da... src: add method to compute storage in WriteWrap
++    if (req == nullptr) {
++      req = AllocateSend();
++      destRemaining = req->self_size();
++      dest = req->Extra();
++    }
++=======
+     if (req == nullptr) {
+       req = AllocateSend();
+       destRemaining = req->ExtraSize();
+       dest = req->Extra();
+     }
++>>>>>>> a5f3b3a6da... src: add method to compute storage in WriteWrap
      DEBUG_HTTP2("Nghttp2Session %s: nghttp2 has %d bytes to send\n",
                  TypeName(), srcLength);
      size_t srcRemaining = srcLength;
@@@ -519,7 -524,9 +533,17 @@@
        destLength = 0;
        srcRemaining -= destRemaining;
        srcOffset += destRemaining;
++<<<<<<< HEAD
 +      destRemaining = dest.len;
++||||||| parent of a5f3b3a6da... src: add method to compute storage in WriteWrap
++      req = AllocateSend();
++      destRemaining = req->self_size();
++      dest = req->Extra();
++=======
+       req = AllocateSend();
+       destRemaining = req->ExtraSize();
+       dest = req->Extra();
++>>>>>>> a5f3b3a6da... src: add method to compute storage in WriteWrap
      }
  
      if (srcRemaining > 0) {

[addaleax]

@gibfahn I think that might just be because #16669 hasn’t landed there yet, right?