Fix httpfs may return wrong received body length if http socket disco…

…nnected during body transfering (#254)

Signed-off-by: Coldwings <coldwings@me.com>

fs/httpfs/httpfs.cpp

@@ -229,8 +229,7 @@ class HttpFile : public fs::VirtualReadOnlyFile {
                              VALUE(url), VALUE(offset), VALUE(ret));
         }
         authorized = true;
-        headers.try_get("content-length", ret);
-        return ret;
+        return writer.written;
     }
 
     int fstat(struct stat* buf) override {

net/curl.h

@@ -145,9 +145,11 @@ class StringWriter {
 struct IOVWriter : public IOVector {
     using IOVector::IOVector;
     size_t drop = 0;
+    size_t written = 0;
     size_t write(const void* buf, size_t size) {
         auto actual_count = memcpy_from(
             buf, size);  // copy from (buf, size) to iovec[] in *this,
+        written += actual_count;
         extract_front(actual_count);  // and extract the portion just copied
         if (actual_count < size)      // means full
         {