relay: move LF forwarder ownership to the publisher exec, off the registry

[afrind]

In LocalForwarder mode the publisher forwarder is owned by tlForwarders_ on the publisher exec; clear the registry's ref once that ownership transfers. Callers resolve the forwarder by FullTrackName from the publisher exec rather than holding a registry ref, and duplicate-publisher eviction drains the old forwarder on its own exec (via the displaced same-exec ref or tlForwarders_).

---

This change is 

[afrind]

@afrind made 7 comments.
Reviewable status: 0 of 6 files reviewed, 4 unresolved discussions.

---

src/MoqxRelay.h line 490 at r1 (raw file):

    // Subsequent-subscriber seed (set only when !ownsRelayChain): the live publisher
    // forwarder's largest/extensions, read on the publisher exec.
    std::optional<moxygen::AbsoluteLocation> seedLargest;

I don't like "seed" here

---

src/MoqxRelay.cpp line 651 at r1 (raw file):

    // Null handle => previous publisher already terminated and onPublishDone() tore it down; skip.
    if (evicted.handle && mode() == Mode::LocalForwarder) {
      // The old forwarder lives on its own exec (registry no longer holds it); unsubscribe and

We don't need comments explaining what we no longer do. Also "its own exec" "its publisher's exec"

---

src/MoqxRelay.cpp line 660 at r1 (raw file):

            auto oldFwd = displacedLocalFwd;
            if (!oldFwd) {
              auto* localReg = tlForwarders_.get();

Add a one line comment explaining this case in a bit more detail. You can remove "using the displaced ref (same-exec) or tlForwarders_ (cross-exec)." above, in favor of a bit more detail here.

---

src/MoqxRelay.cpp line 885 at r1 (raw file):

  }
  if (!forwarder) {
    return false;

Does this case warrant an error log?

---

src/MoqxRelay.cpp line 919 at r1 (raw file):

        publisherExec,
        [pf = std::move(publisherFwd), ex = subscriberExec, re = relayExec]() noexcept {
          // lock on the pub exec: resolves the exact wired forwarder or null if it is gone.

You can add another line to help explain why it can be gone

---

src/MoqxRelay.cpp line 2163 at r1 (raw file):

  } else {
    // Subsequent subscriber, first on this iothread: seed from the live publisher
    // forwarder so the OK carries the established largest, not this fresh forwarder's

Remove ", not this fresh forwarder's
// unseeded value a client reads as a track restart.
"

---

src/MoqxRelay.cpp line 2748 at r1 (raw file):

        .sourceAddress = sourceAddr,
    };
    // LF mode clears the registry forwarder (owned by tlForwarders_); stats unavailable here.

FYI - this breaks some components of the /state endpoint that were never really safe in MT mode anyways. We can file an issue to circle back and fix