server: split HTTP into its own interface (#17216)

* server: split HTTP into its own interface

* move server-http and httplib to its own file

* add the remaining endpoints

* fix exception/error handling

* renaming

* missing header

* fix missing windows header

* fix error responses from http layer

* fix slot save/restore handler

* fix case where only one stream chunk is returned

* add NOMINMAX

* do not call sink.write on empty data

* use safe_json_to_str for SSE

* clean up

* add some comments

* improve usage of next()

* bring back the "server is listening on" message

* more generic handler

* add req.headers

* move the chat template print to init()

* add req.path

* cont : minor

---------

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

tools/server/server-http.h

@@ -0,0 +1,78 @@
+#pragma once
+
+#include <atomic>
+#include <functional>
+#include <map>
+#include <string>
+#include <thread>
+
+struct common_params;
+
+// generator-like API for HTTP response generation
+// this object response with one of the 2 modes:
+// 1) normal response: `data` contains the full response body
+// 2) streaming response: each call to next(output) generates the next chunk
+//    when next(output) returns false, no more data after the current chunk
+//    note: some chunks can be empty, in which case no data is sent for that chunk
+struct server_http_res {
+    std::string content_type = "application/json; charset=utf-8";
+    int status = 200;
+    std::string data;
+    std::map<std::string, std::string> headers;
+
+    // TODO: move this to a virtual function once we have proper polymorphism support
+    std::function<bool(std::string &)> next = nullptr;
+    bool is_stream() const {
+        return next != nullptr;
+    }
+
+    virtual ~server_http_res() = default;
+};
+
+// unique pointer, used by set_chunked_content_provider
+// httplib requires the stream provider to be stored in heap
+using server_http_res_ptr = std::unique_ptr<server_http_res>;
+
+struct server_http_req {
+    std::map<std::string, std::string> params; // path_params + query_params
+    std::map<std::string, std::string> headers; // reserved for future use
+    std::string path; // reserved for future use
+    std::string body;
+    const std::function<bool()> & should_stop;
+
+    std::string get_param(const std::string & key, const std::string & def = "") const {
+        auto it = params.find(key);
+        if (it != params.end()) {
+            return it->second;
+        }
+        return def;
+    }
+};
+
+struct server_http_context {
+    class Impl;
+    std::unique_ptr<Impl> pimpl;
+
+    std::thread thread; // server thread
+    std::atomic<bool> is_ready = false;
+
+    std::string path_prefix;
+    std::string hostname;
+    int port;
+
+    server_http_context();
+    ~server_http_context();
+
+    bool init(const common_params & params);
+    bool start();
+    void stop() const;
+
+    // note: the handler should never throw exceptions
+    using handler_t = std::function<server_http_res_ptr(const server_http_req & req)>;
+
+    void get(const std::string & path, const handler_t & handler) const;
+    void post(const std::string & path, const handler_t & handler) const;
+
+    // for debugging
+    std::string listening_address;
+};