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, 22 insertions, 6 deletions

diff --git a/src/randomx_pow.cpp b/src/randomx_pow.cpp
index 1d058d4..9c59364 100644
--- a/src/randomx_pow.cpp
+++ b/src/randomx_pow.cpp
@@ -8,6 +8,7 @@
 #include <thread>
 #include <vector>
 #include <mutex>
+#include <system_error>
 
 // Fixed key (seed) that determines the RandomX cache/dataset. Keeping it
 // constant avoids per-epoch "reseed" logic; for the CPU+RAM fairness goal
@@ -101,13 +102,28 @@ bool RandomXInitDataset(int nThreads)
 
     std::vector<std::thread> workers;
     unsigned long per = total / nThreads;
-    for (int i = 0; i < nThreads; i++)
+    try
     {
-        unsigned long start = i * per;
-        unsigned long count = (i == nThreads - 1) ? (total - start) : per;
-        workers.emplace_back([start, count]() {
-            randomx_init_dataset(g_dataset, g_cache, start, count);
-        });
+        for (int i = 0; i < nThreads; i++)
+        {
+            unsigned long start = i * per;
+            unsigned long count = (i == nThreads - 1) ? (total - start) : per;
+            workers.emplace_back([start, count]() {
+                randomx_init_dataset(g_dataset, g_cache, start, count);
+            });
+        }
+    }
+    catch (const std::system_error& e)
+    {
+        // Same refusal as the allocation failure above: light mode is slow
+        // but alive, std::terminate is neither. Join the workers that did
+        // start before releasing the dataset out from under them.
+        for (auto& w : workers) w.join();
+        randomx_release_dataset(g_dataset);
+        g_dataset = NULL;
+        printf("RandomX: cannot spawn dataset init threads (%s), staying in light mode\n",
+               e.what());
+        return false;
     }
     for (auto& w : workers) w.join();