ircd::png: Add acTL info detection to decide animation from buffer.

ircd/png.cc

@@ -25,7 +25,7 @@ ircd::png::version_api
 	#endif
 };
 
-/// Since libpng may not loaded prior to static initialization there is
+/// Since libpng may not have loaded prior to static initialization there is
 /// no linked library identification. libmagick takes care of loading libpng
 /// dynamically if it requires it for us.
 decltype(ircd::png::version_abi)
@@ -38,10 +38,71 @@ bool
 ircd::png::is_animated(const const_buffer &buf)
 #ifdef PNG_APNG_SUPPORTED
 {
+	// Cannot be a PNG
+	if(unlikely(size(buf) < 8))
+		return false;
+
+	png_infop info {nullptr};
+	png_structp handle {nullptr};
+	const unwind destroy
+	{
+		[&handle, &info]
+		{
+			png_destroy_read_struct(&handle, &info, nullptr);
+		}
+	};
+
+	if(unlikely(!(handle = png_create_read_struct(PNG_LIBPNG_VER_STRING, nullptr, nullptr, nullptr))))
+		return false;
+
+	if(unlikely(!(info = png_create_info_struct(handle))))
+		return false;
+
+	const_buffer src{buf};
+	png_set_read_fn(handle, &src, []
+	(auto handle, uint8_t *const ptr, size_t sz)
+	{
+		// Destination where libpng wants data
+		const mutable_buffer dst
+		{
+			reinterpret_cast<char *>(ptr), sz
+		};
+
+		// Get source buffer from our callback user data.
+		const_buffer &src
+		{
+			*reinterpret_cast<const_buffer *>(png_get_io_ptr(handle))
+		};
+
+		// Copy from our buffer to libpng buffer.
+		const size_t copied
+		{
+			copy(dst, src)
+		};
+
+		// Advance our source pointer for the amount copied.
+		const size_t consumed
+		{
+			consume(src, copied)
+		};
+
+		assert(copied == consumed);
+	});
+
+	// Invokes the read callback a few times to traverse the buffer.
+	png_read_info(handle, info);
+
+	// If there's a non-zero result there's acTL for animation.
+	uint32_t frames(0), plays(0);
+	if(png_get_acTL(handle, info, &frames, &plays) != 0)
+		return true;
+
 	return false;
 }
 #else
 {
+	// If there's no libpng there's no reason to decide APNG's right now
+	// because they won't be thumbnailed by magick anyway.
 	#warning "Upgrade your libpng version for animation detection."
 	return false;
 }