Core (LV::Video): Fix and clarify the various seeks in the PNG loader.

Author: kaixiong

libvisual/libvisual/private/lv_video_png.cpp

@@ -58,18 +58,24 @@ namespace LV {
 
   VideoPtr bitmap_load_png (std::istream& input)
   {
-      auto saved_stream_pos = input.tellg ();
+      auto start_stream_pos = input.tellg ();
 
-      png_byte signature[8];
-      input.read (reinterpret_cast<char*> (signature), sizeof (signature));
-      input.seekg (saved_stream_pos);
+      // Check PNG signature.
 
-      if (!input) {
+      png_byte signature[8];
+      if (!input.read (reinterpret_cast<char*> (signature), sizeof (signature))) {
+          input.clear ();
+          input.seekg (start_stream_pos);
           return nullptr;
       }
 
       bool is_png = !png_sig_cmp (signature, 0, sizeof (signature));
 
+      // Clean up test by rewinding to the beginning, like we have read nothing.
+      if (!input.seekg (start_stream_pos)) {
+          return nullptr;
+      }
+
       if (!is_png) {
           return nullptr;
       }
@@ -91,11 +97,18 @@ namespace LV {
           return nullptr;
       }
 
+      // Read PNG image data
+
+      // Skip to the first chunk, which comes right after the signature.
+      input.seekg (start_stream_pos + std::streampos {sizeof (signature)});
+
       uint8_t*  pixels = nullptr;
       uint8_t** pixel_row_ptrs = nullptr;
 
       if (setjmp (png_jmpbuf (png_ptr))) {
-          input.seekg (saved_stream_pos);
+          // Some error happened during reading. Rewind to the beginning, like we have read nothing.
+          input.clear ();
+          input.seekg (start_stream_pos);
 
           png_destroy_read_struct (&png_ptr, &info_ptr, &end_info);
 
@@ -105,8 +118,6 @@ namespace LV {
           return nullptr;
       }
 
-      input.seekg (sizeof (signature), std::ios::cur);
-
       png_set_read_fn (png_ptr, &input, handle_png_read);
 
       png_set_sig_bytes (png_ptr, sizeof (signature));