net: a refused thread spawn drops one peer, not the node

std::thread throws std::system_error when pthread_create is refused
(RLIMIT_NPROC, a container's pids cgroup, memory pressure), and nothing
caught it, so the node died by std::terminate. Threads are spawned per
peer connection, so the pressure that triggers this scales with peer
count, which is attacker-influenced. Observed killing a healthy
participant worker mid-mining.

Every spawn site in the node build, and what a failure now does:

- btftunnel.cpp: both tunnel pumps are guarded. The inbound pump starts
  first because it owns cleanup of the pump end and rendezvous socket;
  if the second spawn is refused, shutdown() wakes the first out of its
  read and it cleans up itself -- closing those descriptors from the
  failure path instead would race it onto reused descriptors. Both
  paths return INVALID_SOCKET, which every caller already treats as
  "this connection failed".
- btfrv.cpp: the relay's accept loop drops the connection it cannot
  serve and stops accepting for a moment -- longer after consecutive
  failures, capped at 5s -- instead of dying at an arbitrary one. The
  per-pair second pump now runs on the handling thread itself, leaving
  exactly one spawn to guard and nothing running yet to race.
- main.cpp: a participant that cannot spawn its hash workers stops the
  ones that did start, drops the pool connection, and returns to the
  reconnect loop, the same way an ordinary disconnect does.
- randomx_pow.cpp: dataset init joins the workers it got, releases the
  dataset, and stays in light mode -- the same trade the allocation
  failure path above it already makes.
- rpc.cpp, net.cpp: the remaining unchecked _beginthread calls (which
  report failure by value, they cannot throw) now say precisely what
  did not start. The pool's accept and payout threads failing silently
  left the pool running deaf -- nobody accepts, nobody gets paid, and
  nothing in the log says why.

Left alone deliberately: the per-relay accept spawns (rpc.cpp) already
check, log and retry on the next poll; startup-critical spawns in
net.cpp keep failing hard; the selftest-only std::thread uses may abort
a selftest run, which is acceptable where dying over one peer is not.

Verified: full node rebuilds clean and links; pool-stratum selftest
passes; a scratch harness under RLIMIT_NPROC=1 confirms _beginthread
reports -1 and std::thread throws system_error, the premises of every
path above.

1 files changed, 27 insertions, 3 deletions

diff --git a/src/main.cpp b/src/main.cpp
index 7b3e555..d154523 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -14,6 +14,7 @@
 #include <atomic>
 #include <mutex>
 #include <thread>
+#include <system_error>
 #include "bip32.h"
 #include "btfaddr.h"
 #include "btftunnel.h"
@@ -3899,9 +3900,32 @@ connected:
     std::vector<ParticipantShareHit> shareHits;
     std::mutex shareHitsMtx;
     std::vector<std::thread> hashThreads;
-    for (int i = 0; i < nWorkers; i++)
-        hashThreads.emplace_back(ParticipantHashWorker, &jobShared, i, nWorkers,
-                                 &shareHits, &shareHitsMtx);
+    try
+    {
+        for (int i = 0; i < nWorkers; i++)
+            hashThreads.emplace_back(ParticipantHashWorker, &jobShared, i, nWorkers,
+                                     &shareHits, &shareHitsMtx);
+    }
+    catch (const std::system_error& e)
+    {
+        // A refused pthread_create used to reach std::terminate here and
+        // kill the worker mid-session -- observed twice on an otherwise
+        // healthy participant. Stop the workers that did start the same way
+        // the disconnect path below does, drop this pool connection, and let
+        // the caller's reconnect loop try again once the pressure clears.
+        LogPrint("worker", "[worker] cannot spawn hash threads (%s)"
+                 " -- dropping pool connection\n", e.what());
+        SetParticipantMiningStatus("hash thread spawn failed");
+        {
+            std::lock_guard<std::mutex> lk(jobShared.mtx);
+            jobShared.stop = true;
+            jobShared.haveJob = false;
+        }
+        for (std::thread& t : hashThreads)
+            t.join();
+        closesocket(s);
+        return false;
+    }
     LogPrint("worker", "[worker] spawned %d hash threads on shared RandomX dataset\n",
              nWorkers);
     const uint64 sessionStartHashes = gParticipantHashes.load();