server: (router) fix stopping_thread potentially hang (ggml-org#24728)

* server: (router) fix stopping_thread potentially hang

* fix windows build

Author: ngxson

tools/server/server-models.cpp

@@ -54,7 +54,7 @@ extern char **environ;
 
 struct server_subproc {
     std::optional<subprocess_s> sproc; // empty while in DOWNLOADING state
-    std::atomic<bool> stop_download{false}; // flag to signal download cancellation
+    std::atomic<bool> stopped{false}; // set to cancel a download or signal child process exit
 
     subprocess_s & get() {
         GGML_ASSERT(sproc.has_value() && "subprocess not initialized");
@@ -64,6 +64,22 @@ struct server_subproc {
     bool is_alive() {
         return sproc.has_value() && subprocess_alive(&sproc.value());
     }
+
+    void terminate() {
+        if (!sproc.has_value()) {
+            return;
+        }
+#if defined(_WIN32)
+        if (sproc->hProcess == NULL) {
+            return;
+        }
+#else
+        if (sproc->child <= 0) {
+            return;
+        }
+#endif
+        subprocess_terminate(&sproc.value());
+    }
 };
 
 
@@ -902,50 +918,49 @@ void server_models::load(const std::string & name) {
         });
 
         std::thread stopping_thread([&]() {
-            // thread to monitor stopping signal OR child crash
+            // thread to monitor explicit stop requests; child crash is signalled via child_proc->stopped
             auto is_stopping = [this, &name]() {
                 return this->stopping_models.find(name) != this->stopping_models.end();
             };
-            auto should_wake = [&]() {
-                return is_stopping() || !child_proc->is_alive();
-            };
             {
                 std::unique_lock<std::mutex> lk(this->mutex);
-                this->cv_stop.wait(lk, should_wake);
+                this->cv_stop.wait(lk, [&]() {
+                    return is_stopping() || child_proc->stopped.load(std::memory_order_acquire);
+                });
             }
-            // child may have already exited (e.g. crashed) — skip shutdown sequence
-            if (!child_proc->is_alive()) {
+            // child crashed or finished on its own — skip graceful shutdown sequence
+            if (child_proc->stopped.load(std::memory_order_acquire)) {
                 return;
             }
             SRV_INF("stopping model instance name=%s\n", name.c_str());
-            // send interrupt to child process
             fprintf(stdin_file, "%s\n", CMD_ROUTER_TO_CHILD_EXIT);
             fflush(stdin_file);
-            // wait to stop gracefully or timeout
             int64_t start_time = ggml_time_ms();
             while (true) {
                 std::unique_lock<std::mutex> lk(this->mutex);
-                if (!is_stopping()) {
-                    return; // already stopped
+                if (!is_stopping() || child_proc->stopped.load(std::memory_order_acquire)) {
+                    return;
                 }
                 int64_t elapsed = ggml_time_ms() - start_time;
                 if (elapsed >= stop_timeout * 1000) {
-                    // timeout, force kill
+                    lk.unlock();
                     SRV_WRN("force-killing model instance name=%s after %d seconds timeout\n", name.c_str(), stop_timeout);
-                    subprocess_terminate(&child_proc->get());
+                    child_proc->terminate();
                     return;
                 }
-                this->cv_stop.wait_for(lk, std::chrono::seconds(1));
+                this->cv_stop.wait_for(lk, std::chrono::seconds(1), [&]() {
+                    return !is_stopping() || child_proc->stopped.load(std::memory_order_acquire);
+                });
             }
         });
 
-        // we reach here when the child process exits
+        // we reach here when the child process exits (stdout EOF)
         // note: we cannot join() prior to this point because it will close stdin_file
         if (log_thread.joinable()) {
             log_thread.join();
         }
 
-        // stop the timeout monitoring thread
+        child_proc->stopped.store(true, std::memory_order_release);
         {
             std::lock_guard<std::mutex> lk(this->mutex);
             stopping_models.erase(name);
@@ -971,7 +986,7 @@ void server_models::load(const std::string & name) {
         // old process should have exited already, but just in case, we clean it up here
         if (old_instance.subproc->is_alive()) {
             SRV_WRN("old process for model name=%s is still alive, this is unexpected\n", name.c_str());
-            subprocess_terminate(&old_instance.subproc->get()); // force kill
+            old_instance.subproc->terminate(); // force kill
         }
         if (old_instance.th.joinable()) {
             old_instance.th.join();
@@ -1039,7 +1054,7 @@ void server_models::download(common_params_model && model, common_download_opts
     dl->opts  = opts;  // copy
 
     dl->should_stop = [sp = inst.subproc]() {
-        return sp->stop_download.load(std::memory_order_relaxed);
+        return sp->stopped.load(std::memory_order_relaxed);
     };
 
     dl->on_progress = [this, name](const common_download_progress & p) {
@@ -1069,7 +1084,7 @@ void server_models::unload(const std::string & name) {
     if (it != mapping.end()) {
         if (it->second.meta.status == SERVER_MODEL_STATUS_DOWNLOADING) {
             SRV_INF("cancelling download for model name=%s\n", name.c_str());
-            it->second.subproc->stop_download.store(true, std::memory_order_relaxed);
+            it->second.subproc->stopped.store(true, std::memory_order_relaxed);
             // for convenience, we wait the status change here
             wait(lk, name, [](const server_model_meta & new_meta) {
                 return new_meta.status != SERVER_MODEL_STATUS_DOWNLOADING;
@@ -1080,7 +1095,7 @@ void server_models::unload(const std::string & name) {
             if (it->second.meta.status == SERVER_MODEL_STATUS_LOADING) {
                 // special case: if model is in loading state, unloading means force-killing it
                 SRV_WRN("model name=%s is still loading, force-killing\n", name.c_str());
-                subprocess_terminate(&it->second.subproc->get());
+                it->second.subproc->terminate();
             }
             cv_stop.notify_all();
             // status change will be handled by the managing thread
@@ -1097,7 +1112,7 @@ void server_models::unload_all() {
         for (auto & [name, inst] : mapping) {
             if (inst.meta.status == SERVER_MODEL_STATUS_DOWNLOADING) {
                 SRV_INF("cancelling download for model name=%s\n", name.c_str());
-                inst.subproc->stop_download.store(true, std::memory_order_relaxed);
+                inst.subproc->stopped.store(true, std::memory_order_relaxed);
             } else if (inst.meta.is_running()) {
                 SRV_INF("stopping model instance name=%s\n", name.c_str());
                 stopping_models.insert(name);