train: the start's claim is released on every exit that doesn't hand it to the worker

tests/test-chat.cpp

@@ -7277,6 +7277,46 @@ static int test_train_control(const char * model_path, const char * output_dir)
     require(trainer.start(req).at("ok").get<bool>(), "baseline refused");
     const auto baseline = wait_for(done);
 
+    // what this catches (continuum card 48029d24): an exception thrown AFTER the claim. "yield"
+    // is read with body.value(..., true), so a string there throws json::type_error once the run
+    // is claimed and prepared. The claim must be released (state "error" under this out, and the
+    // next start accepted), never left "preparing" with every later start refused as "already in
+    // progress" until a restart.
+    {
+        json bad = req;
+        bad["out"] = "thrown.gguf";
+        bad["yield"] = "not a bool";
+        bool threw = false;
+        try {
+            trainer.start(bad);
+        } catch (const std::exception &) { // nlohmann's type_error is a std::exception; `json` here is common_json
+            threw = true;
+        }
+        require(threw, "a non-bool yield must throw after the claim (the case under test)");
+        const auto after = trainer.status();
+        require(after.value("state", "") == "error" && after.value("out", "") == "thrown.gguf",
+                "the thrown start leaves its error under its own out, not 'preparing'");
+        json next = req;
+        next["out"] = "after-throw.gguf";
+        require(trainer.start(next).at("ok").get<bool>(), "the claim was released: the next start is accepted");
+        wait_for(done);
+
+        // and the refusal path: an empty "examples" is refused by `refuse` AFTER the claim, which
+        // must release it the same way (and settle the guard only once released)
+        json refused = req;
+        refused.erase("text");
+        refused["examples"] = json::array();
+        refused["out"] = "refused.gguf";
+        require(!trainer.start(refused).at("ok").get<bool>(), "an empty examples array is refused");
+        const auto after_refusal = trainer.status();
+        require(after_refusal.value("state", "") == "error" && after_refusal.value("out", "") == "refused.gguf",
+                "the refusal leaves its error under its own out");
+        json again = req;
+        again["out"] = "after-refusal.gguf";
+        require(trainer.start(again).at("ok").get<bool>(), "the refused claim was released: the next start is accepted");
+        wait_for(done);
+    }
+
     // Block after two updates, then exercise manual pause separately from serving admission.
     calls.store(0);
     hold_at.store(3);

tools/server/server-train.cpp

@@ -498,13 +498,45 @@ json server_trainer::start(const json & body_in) {
     // /train answers "preparing" with this out within milliseconds of the POST arriving, so a
     // caller reconciles by the run's identity instead of guessing from a clock. A refusal after the
     // claim releases it and leaves the run's own error, under its out, in the state.
+    // The state a claim released by an exception leaves behind, built BEFORE the claim (`out`
+    // is validated above) so releasing it later allocates nothing and cannot throw.
+    json failed_after_claim = json::object({{"state", "error"}, {"out", body.at("out")},
+        {"error", "the start failed after it claimed the run (an exception); the claim is released"},
+        {"epochs", json::array()}});
     {
         std::lock_guard<std::mutex> claim_lock(mu);
         bool expected = false;
         if (!running.compare_exchange_strong(expected, true)) {
             return json::object({{"ok", false}, {"error", "a training run is already in progress"}});
         }
     }
+    // THE CLAIM IS RELEASED ON EVERY EXIT THAT DOES NOT HAND IT TO THE WORKER (continuum card
+    // 48029d24). `refuse` releases it on a refusal, but an exception thrown after the claim (a
+    // json::type_error from a body field, std::system_error from std::thread) used to escape with
+    // `running` still true: /train named this run "preparing" forever and refused every later
+    // start as "already in progress" until the server restarted. `settled` is set only AFTER a
+    // release (by `refuse`) or by the hand-off, so this guard never releases a later run's claim
+    // and never misses one. Its destructor cannot throw (it runs while unwinding; a throw there
+    // terminates the server): the state is moved in, not built, recording it is best-effort, and
+    // `running` is released whatever happens. Declared before `lock` below, so `lock` (on `mu`) is
+    // gone before this destructor takes `mu`.
+    struct claim_guard {
+        server_trainer & trainer;
+        json             failed_state;
+        bool             settled = false;
+        ~claim_guard() noexcept {
+            if (settled) {
+                return;
+            }
+            try {
+                std::lock_guard<std::mutex> claim_lock(trainer.mu);
+                trainer.state = std::move(failed_state);
+            } catch (...) {
+                // the diagnostic state is best-effort; releasing the claim is not
+            }
+            trainer.running.store(false);
+        }
+    } claim{*this, std::move(failed_after_claim)};
     if (worker.joinable()) {
         worker.join();
     }
@@ -518,6 +550,7 @@ json server_trainer::start(const json & body_in) {
             state = json::object({{"state", "error"}, {"out", body.at("out")}, {"error", why}, {"epochs", json::array()}});
         }
         running.store(false);
+        claim.settled = true; // only once released: a throw above leaves the guard to release it
         return json::object({{"ok", false}, {"error", why}});
     };
     if (body.contains("examples")) {
@@ -615,6 +648,7 @@ json server_trainer::start(const json & body_in) {
     window_ms_samples.clear();
     window_ms_max = 0;
     worker = std::thread([this, body, ex = std::move(prepared)]() mutable { run(std::move(body), std::move(ex)); });
+    claim.settled = true; // handed to the worker, which owns `running` from here
     lock.unlock();
     return json::object({{"ok", true}, {"status", status()}});
 }