Perform on-host pixels to PDF conversion

Extend the base isolation provider to immediately convert each page to
a PDF, and optionally use OCR. In contract with the way we did things
previously, there are no more two separate stages (document to pixels,
pixels to PDF). We now handle each page individually, for two main
reasons:

1. We don't want to buffer pixel data, either on disk or in memory,
   since they take a lot of space, and can potentially leave traces.
2. We can perform these operations in parallel, saving time. This is
   more evident when OCR is not used, where the time to convert a page
   to pixels, and then back to a PDF are comparable.

dangerzone/isolation_provider/base.py

@@ -10,12 +10,13 @@ from abc import ABC, abstractmethod
 from pathlib import Path
 from typing import IO, Callable, Iterator, Optional
 
+import fitz
 from colorama import Fore, Style
 
 from ..conversion import errors
-from ..conversion.common import INT_BYTES
+from ..conversion.common import DEFAULT_DPI, INT_BYTES
 from ..document import Document
-from ..util import replace_control_chars
+from ..util import get_tessdata_dir, replace_control_chars
 
 log = logging.getLogger(__name__)
 
@@ -111,8 +112,7 @@ class IsolationProvider(ABC):
             with tempfile.TemporaryDirectory() as t:
                 Path(f"{t}/pixels").mkdir()
                 with self.doc_to_pixels_proc(document) as conversion_proc:
-                    self.doc_to_pixels(document, t, conversion_proc)
-                self.pixels_to_pdf(document, t, ocr_lang)
+                    self._convert(document, t, ocr_lang, conversion_proc)
             document.mark_as_safe()
             if document.archive_after_conversion:
                 document.archive()
@@ -126,8 +126,42 @@ class IsolationProvider(ABC):
             self.print_progress(document, True, str(e), 0)
             document.mark_as_failed()
 
-    def doc_to_pixels(
-        self, document: Document, tempdir: str, p: subprocess.Popen
+    def _pixels_to_pdf(
+        self,
+        untrusted_data: bytes,
+        untrusted_width: int,
+        untrusted_height: int,
+        ocr_lang: Optional[str],
+    ) -> fitz.Document:
+        """Convert a byte array of RGB pixels into a PDF page, optionally with OCR."""
+        pixmap = fitz.Pixmap(
+            fitz.Colorspace(fitz.CS_RGB),
+            untrusted_width,
+            untrusted_height,
+            untrusted_data,
+            False,
+        )
+        pixmap.set_dpi(DEFAULT_DPI, DEFAULT_DPI)
+
+        if ocr_lang:  # OCR the document
+            page_pdf_bytes = pixmap.pdfocr_tobytes(
+                compress=True,
+                language=ocr_lang,
+                tessdata=get_tessdata_dir(),
+            )
+        else:  # Don't OCR
+            page_doc = fitz.Document()
+            page_doc.insert_file(pixmap)
+            page_pdf_bytes = page_doc.tobytes(deflate_images=True)
+
+        return fitz.open("pdf", page_pdf_bytes)
+
+    def _convert(
+        self,
+        document: Document,
+        tempdir: str,
+        ocr_lang: Optional[str],
+        p: subprocess.Popen,
     ) -> None:
         percentage = 0.0
         with open(document.input_filename, "rb") as f:
@@ -142,10 +176,13 @@ class IsolationProvider(ABC):
             n_pages = read_int(p.stdout)
             if n_pages == 0 or n_pages > errors.MAX_PAGES:
                 raise errors.MaxPagesException()
-            percentage_per_page = 49.0 / n_pages
+            step = 100 / n_pages / 2
+
+            safe_doc = fitz.Document()
 
             for page in range(1, n_pages + 1):
                 text = f"Converting page {page}/{n_pages} to pixels"
+                percentage += step
                 self.print_progress(document, False, text, percentage)
 
                 width = read_int(p.stdout)
@@ -161,21 +198,31 @@ class IsolationProvider(ABC):
                     num_pixels,
                 )
 
-                # Wrapper code
-                with open(f"{tempdir}/pixels/page-{page}.width", "w") as f_width:
-                    f_width.write(str(width))
-                with open(f"{tempdir}/pixels/page-{page}.height", "w") as f_height:
-                    f_height.write(str(height))
-                with open(f"{tempdir}/pixels/page-{page}.rgb", "wb") as f_rgb:
-                    f_rgb.write(untrusted_pixels)
+                if ocr_lang:
+                    text = (
+                        f"Converting page {page}/{n_pages} from pixels to"
+                        " searchable PDF"
+                    )
+                else:
+                    text = f"Converting page {page}/{n_pages} from pixels to PDF"
+                percentage += step
+                self.print_progress(document, False, text, percentage)
 
-                percentage += percentage_per_page
+                page_pdf = self._pixels_to_pdf(
+                    untrusted_pixels,
+                    width,
+                    height,
+                    ocr_lang,
+                )
+                safe_doc.insert_pdf(page_pdf)
 
         # Ensure nothing else is read after all bitmaps are obtained
         p.stdout.close()
 
+        safe_doc.save(document.output_filename)
+
         # TODO handle leftover code input
-        text = "Converted document to pixels"
+        text = "Converted document"
         self.print_progress(document, False, text, percentage)
 
         if getattr(sys, "dangerzone_dev", False):