qubes: Stream page data in real time

Stream page data back to the caller, immediately after we read them from
pdftoppm. This way, we have more accurate progress reports and timeouts.

Fixes #557

dangerzone/conversion/doc_to_pixels_qubes_wrapper.py

@@ -54,6 +54,27 @@ async def write_int(num: int, file: TextIO = sys.stdout) -> None:
     return await asyncio.to_thread(_write_int, num, file=file)
 
 
+class QubesDocumentToPixels(DocumentToPixels):
+    # Override the write_page_* functions to stream data back to the caller, instead of
+    # writing it to separate files. This way, we have more accurate progress reports and
+    # client-side timeouts. See also:
+    #
+    # https://github.com/freedomofpress/dangerzone/issues/443
+    # https://github.com/freedomofpress/dangerzone/issues/557
+
+    async def write_page_count(self, count: int) -> None:
+        return await write_int(count)
+
+    async def write_page_width(self, width: int, filename: str) -> None:
+        return await write_int(width)
+
+    async def write_page_height(self, height: int, filename: str) -> None:
+        return await write_int(height)
+
+    async def write_page_data(self, data: bytes, filename: str) -> None:
+        return await write_bytes(data)
+
+
 async def main() -> None:
     out_dir = Path("/tmp/dangerzone")
     if out_dir.exists():
@@ -69,7 +90,7 @@ async def main() -> None:
         f.write(data)
 
     try:
-        converter = DocumentToPixels()
+        converter = QubesDocumentToPixels()
         await converter.convert()
     except errors.ConversionException as e:
         await write_bytes(str(e).encode(), file=sys.stderr)
@@ -79,20 +100,6 @@ async def main() -> None:
         error_code = errors.UnexpectedConversionError.error_code
         sys.exit(error_code)
 
-    num_pages = len(list(out_dir.glob("*.rgb")))
-    await write_int(num_pages)
-    for num_page in range(1, num_pages + 1):
-        page_base = out_dir / f"page-{num_page}"
-        with open(f"{page_base}.width", "r") as width_file:
-            width = int(width_file.read())
-        with open(f"{page_base}.height", "r") as height_file:
-            height = int(height_file.read())
-        await write_int(width)
-        await write_int(height)
-        with open(f"{page_base}.rgb", "rb") as rgb_file:
-            rgb_data = rgb_file.read()
-            await write_bytes(rgb_data)
-
     # Write debug information
     await write_bytes(converter.captured_output, file=sys.stderr)