[BP] PdfDocument.render(): Fix non-API implementation issues

docs/devel/changelog_staging.md

@@ -8,5 +8,7 @@
 This release backports some key fixes/improvements from the development branch:
 - [V8/XFA] Fixed XFA init. This issue was caused by a typo in a struct field. Thanks to Benoît Blanchon.
 - [V8/XFA] Expose V8/XFA exclusive members in the bindings file by passing ctypesgen the pre-processor defines in question.
+- `PdfDocument.render()`: Fixed non-API implementation issues:
+  * Avoid full state data transfer and object re-initialization for each job. Instead, use a pool initializer and exploit global variables. This also makes bytes input tolerable for parallel rendering.
 - Fixed sourcebuild with system libraries.
 - Attempt to fix automatic GH pages rebuild on release.

src/pypdfium2/_helpers/document.py

@@ -6,7 +6,6 @@
 import os
 import ctypes
 import logging
-import functools
 from pathlib import Path
 from collections import namedtuple
 from concurrent.futures import ProcessPoolExecutor
@@ -568,27 +567,6 @@ def get_toc(
             bookmark = pdfium_c.FPDFBookmark_GetNextSibling(self, bookmark)
 
 
-    @classmethod
-    def _process_page(cls, index, input_data, password, renderer, converter, pass_info, need_formenv, mk_formconfig, **kwargs):
-
-        pdf = cls(input_data, password=password)
-        if need_formenv:
-            pdf.init_forms(config=mk_formconfig()) if mk_formconfig else pdf.init_forms()
-
-        page = pdf[index]
-        bitmap = renderer(page, **kwargs)
-        info = bitmap.get_info()
-        result = converter(bitmap)
-
-        # NOTE We MUST NOT call bitmap.close() before the converted object is serialized to the main process, otherwise we would free the buffer of a foreign bitmap prematurely if the converted object references the buffer rather than owning a copy. Confirmed by POC.
-        # This is not an issue when freeing the bitmap on garbage collection, provided the converted object keeps the buffer alive.
-
-        for g in (page, pdf):
-            g.close()
-
-        return (result, info) if pass_info else result
-
-
     def render(
             self,
             converter,
@@ -600,6 +578,12 @@ def render(
             **kwargs
         ):
         """
+        .. deprecated:: 4.19
+            This method will be removed with the next major release (v5) due to serious conceptual problems. See the upcoming changelog for more info.
+        
+        .. versionchanged:: 4.19
+            Fixed major non-API implementation issues.
+        
         Render multiple pages in parallel, using a process pool executor.
         
         Hint:
@@ -622,13 +606,13 @@ def render(
                 Keyword arguments to the renderer.
         
         Yields:
-            :data:`typing.Any`: Parameter-dependent result.
+            :data:`typing.Any`: Result as returned by the given converter.
         """
 
         # TODO(apibreak) remove mk_formconfig parameter (bloat)
 
-        if not isinstance(self._input, (Path, str)):
-            raise ValueError("Can only render in parallel with file path input.")
+        if not isinstance(self._input, (Path, str, bytes)):
+            raise ValueError(f"Cannot render in parallel with input type '{type(self._input).__name__}'.")
 
         n_pages = len(self)
         if not page_indices:
@@ -639,20 +623,36 @@ def render(
             if len(page_indices) != len(set(page_indices)):
                 raise ValueError("Duplicate page indices are prohibited.")
 
-        invoke_renderer = functools.partial(
-            PdfDocument._process_page,
-            input_data = self._input,
-            password = self._password,
-            renderer = renderer,
-            converter = converter,
-            pass_info = pass_info,
-            need_formenv = bool(self.formenv),
-            mk_formconfig = mk_formconfig,
-            **kwargs
+        # TODO consider `mp_context = multiprocessing.get_context("spawn")`
+        pool_kwargs = dict(
+            initializer = _parallel_renderer_init,
+            initargs = (self._input, self._password, bool(self.formenv), mk_formconfig, renderer, converter, pass_info, kwargs),
         )
-
-        with ProcessPoolExecutor(n_processes) as pool:
-            yield from pool.map(invoke_renderer, page_indices)
+        with ProcessPoolExecutor(n_processes, **pool_kwargs) as pool:
+            yield from pool.map(_parallel_renderer_job, page_indices)
+
+
+def _parallel_renderer_init(input_data, password, need_formenv, mk_formconfig, renderer, converter, pass_info, kwargs):
+
+    pdf = PdfDocument(input_data, password=password)
+    if need_formenv:
+        pdf.init_forms(config=mk_formconfig()) if mk_formconfig else pdf.init_forms()
+
+    global _ParallelRenderObjs
+    _ParallelRenderObjs = (pdf, renderer, converter, pass_info, kwargs)
+
+
+def _parallel_renderer_job(index):
+
+    global _ParallelRenderObjs
+    pdf, renderer, converter, pass_info, kwargs = _ParallelRenderObjs
+
+    page = pdf[index]
+    bitmap = renderer(page, **kwargs)
+    info = bitmap.get_info()
+    result = converter(bitmap)
+
+    return (result, info) if pass_info else result
 
 
 class PdfFormEnv (pdfium_i.AutoCloseable):