nghttpd: Cache fd

Implement fd caching for static files.  The response body for such as
404 was dynamically generated previously, but now it is written in
temporally file and its fd is cached.  Currently, cache is reference
counted and expired when count becomes 0.  This makes caching is not
effective other than "busy" period, but we don't need this feature if
we are not busy.

src/HttpServer.h

@@ -78,14 +78,27 @@ struct Config {
 
 class Http2Handler;
 
+struct FileEntry {
+  FileEntry(std::string path, int64_t length, int64_t mtime, int fd)
+      : path(std::move(path)), length(length), mtime(mtime), dlprev(nullptr),
+        dlnext(nullptr), fd(fd), usecount(1) {}
+  std::string path;
+  int64_t length;
+  int64_t mtime;
+  FileEntry *dlprev, *dlnext;
+  int fd;
+  int usecount;
+};
+
 struct Stream {
   Headers headers;
   Http2Handler *handler;
+  FileEntry *file_ent;
   ev_timer rtimer;
   ev_timer wtimer;
-  int64_t body_left;
+  int64_t body_length;
+  int64_t body_offset;
   int32_t stream_id;
-  int file;
   http2::HeaderIndex hdidx;
   Stream(Http2Handler *handler, int32_t stream_id);
   ~Stream();
@@ -160,14 +173,21 @@ private:
   int fd_;
 };
 
+struct StatusPage {
+  std::string status;
+  FileEntry file_ent;
+};
+
 class HttpServer {
 public:
   HttpServer(const Config *config);
   int listen();
   int run();
   const Config *get_config() const;
+  const StatusPage *get_status_page(int status) const;
 
 private:
+  std::vector<StatusPage> status_pages_;
   const Config *config_;
 };