Fix mg_http_upload()

docs/README.md

@@ -1319,6 +1319,54 @@ A diagram below shows how `mg_http_next_multipart()` in action:
 <img src="images/mg_http_next_multipart.svg" alt="Function mg_http_next_multipart()" />
 
 
+### mg\_http\_upload()
+
+```c
+
+int mg_http_upload(struct mg_connection *c, struct mg_http_message *hm,
+                   struct mg_fs *fs, const char *dir);
+```
+
+This is a helper utility function that is used to upload large files by small
+chunks.
+
+Append HTTP POST data to a file in a specified directory.  A file name and
+file offset are specified by the query string parameters:
+`POST /upload?name=firmware.bin&offset=2048 HTTP/1.1`. If the offset is 0, then the
+file is truncated. It is a client's responsibility to divide a file into a
+smaller chunks and send a sequence of POST requests that will be handled by
+this function.
+
+Parameters:
+- `c`- a connection
+- `hm` - a parsed HTTP message
+- `fs` - a filesystem where to write a file, e.g. `&mg_fs_posix`
+- `dir` - a directory name where a file should be stored
+
+Return value: number of bytes written
+
+Usage example:
+
+```c
+static void fn(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
+  if (ev == MG_EV_HTTP_MSG) {
+    struct mg_http_message *hm = (struct mg_http_message *) ev_data;
+    if (mg_http_match_uri(hm, "/upload")) {
+      mg_http_upload(c, hm, &mg_fs_posix, "/tmp");
+    } else {
+      struct mg_http_serve_opts opts = {.root_dir = "."};   // Serve
+      mg_http_serve_dir(c, ev_data, &opts);                 // static content
+    }
+  }
+}
+
+```
+
+See [app.js](https://github.com/cesanta/mongoose/blob/7.5/examples/file-upload/web_root/app.js)
+for the example Javascript HTML page code that takes a file and
+sends it by small chunks.
+
+
 ## Websocket
 
 ### struct mg\_ws\_message

mongoose.c

@@ -1859,7 +1859,9 @@ int mg_http_upload(struct mg_connection *c, struct mg_http_message *hm,
   if (name[0] == '\0') {
     mg_http_reply(c, 400, "", "%s", "name required");
     res = -1;
-  } else if (hm->body.len > 0) {
+  } else if (hm->body.len == 0) {
+    mg_http_reply(c, 200, "", "%d", res);  // Nothing to write
+  } else {
     struct mg_fd *fd;
     long oft = strtol(offset, NULL, 0);
     mg_snprintf(path, sizeof(path), "%s%c%s", dir, MG_DIRSEP, name);

src/http.c

@@ -865,7 +865,9 @@ int mg_http_upload(struct mg_connection *c, struct mg_http_message *hm,
   if (name[0] == '\0') {
     mg_http_reply(c, 400, "", "%s", "name required");
     res = -1;
-  } else if (hm->body.len > 0) {
+  } else if (hm->body.len == 0) {
+    mg_http_reply(c, 200, "", "%d", res);  // Nothing to write
+  } else {
     struct mg_fd *fd;
     long oft = strtol(offset, NULL, 0);
     mg_snprintf(path, sizeof(path), "%s%c%s", dir, MG_DIRSEP, name);