Chore: Generate PDF thumbnails with pdftoppm and Pillow instead of ImageMagick

PDF thumbnails were produced by handing the original PDF to ImageMagick's
convert, which delegates PDF handling to Ghostscript. When that failed, a
fallback invoked gs directly and then ran convert a second time just to
encode the WebP, so a single thumbnail could take three subprocess calls
through two general purpose tools for what is only "render page one small".

The first page is now rasterized with Poppler's pdftoppm, which is already
installed for pdftotext, at a DPI computed from the page's own CropBox and
effective rotation as read by pikepdf. The DPI is chosen so the page fits
500x5000 pixels in a single render and is capped at 72 so small pages are
never enlarged, matching the previous shrink-only scale. Pillow then
flattens any alpha onto white, applies a no-enlarge safety clamp and saves
the WebP in process. If the geometry cannot be read, a fixed 150 DPI is
used and the clamp keeps the output in bounds.

The Ghostscript fallback is replaced with a qpdf repair and retry: the PDF
is copied, repaired in place with qpdf (treating its "repaired with
warnings" exit status as success), and rasterized again. If that also
fails, the default thumbnail is used as before.

src/paperless/tests/test_parser_utils.py

@@ -6,8 +6,10 @@ import codecs
 from pathlib import Path
 from typing import TYPE_CHECKING
 
+import pikepdf
 import pytest
 
+from paperless.parsers.utils import get_pdf_first_page_size_points
 from paperless.parsers.utils import is_tagged_pdf
 from paperless.parsers.utils import pdf_born_digital_text
 from paperless.parsers.utils import post_process_text
@@ -70,6 +72,151 @@ class TestIsTaggedPdf:
         assert is_tagged_pdf(bad) is False
 
 
+class TestGetPdfFirstPageSizePoints:
+    @staticmethod
+    def _write_pdf(
+        path: Path,
+        *,
+        media_box: tuple[float, float, float, float] = (0, 0, 600, 800),
+        crop_box: tuple[float, float, float, float] | None = None,
+        page_rotate: int | None = None,
+        inherited_rotate: int | None = None,
+    ) -> Path:
+        pdf = pikepdf.new()
+        pdf.add_blank_page(page_size=(media_box[2], media_box[3]))
+        page = pdf.pages[0]
+        page.obj.MediaBox = pikepdf.Array(media_box)
+        if crop_box is not None:
+            page.obj.CropBox = pikepdf.Array(crop_box)
+        if page_rotate is not None:
+            page.obj.Rotate = page_rotate
+        if inherited_rotate is not None:
+            pdf.Root.Pages.Rotate = inherited_rotate
+        pdf.save(path)
+        return path
+
+    def test_letter_sample(self) -> None:
+        """
+        GIVEN:
+            - A US Letter sample PDF with no CropBox and no rotation
+        WHEN:
+            - The first page size is requested
+        THEN:
+            - The MediaBox size in points is returned
+        """
+        assert get_pdf_first_page_size_points(SAMPLES / "simple-digital.pdf") == (
+            612.0,
+            792.0,
+        )
+
+    @pytest.mark.parametrize(
+        ("rotate", "expected"),
+        [
+            pytest.param(0, (600.0, 800.0), id="rotate-0"),
+            pytest.param(90, (800.0, 600.0), id="rotate-90"),
+            pytest.param(180, (600.0, 800.0), id="rotate-180"),
+            pytest.param(270, (800.0, 600.0), id="rotate-270"),
+            pytest.param(-90, (800.0, 600.0), id="rotate-negative-90"),
+        ],
+    )
+    def test_page_rotation_swaps_dimensions(
+        self,
+        tmp_path: Path,
+        rotate: int,
+        expected: tuple[float, float],
+    ) -> None:
+        """
+        GIVEN:
+            - A portrait PDF page with /Rotate set directly on the page
+        WHEN:
+            - The first page size is requested
+        THEN:
+            - Width and height are swapped for quarter-turn rotations only
+        """
+        pdf_path = self._write_pdf(tmp_path / "rotated.pdf", page_rotate=rotate)
+        assert get_pdf_first_page_size_points(pdf_path) == expected
+
+    def test_inherited_rotation_swaps_dimensions(self, tmp_path: Path) -> None:
+        """
+        GIVEN:
+            - A portrait PDF page whose /Rotate 90 is set on the /Pages node,
+              not on the page itself
+        WHEN:
+            - The first page size is requested
+        THEN:
+            - The inherited rotation is honored and width/height are swapped
+        """
+        pdf_path = self._write_pdf(tmp_path / "inherited.pdf", inherited_rotate=90)
+        assert get_pdf_first_page_size_points(pdf_path) == (800.0, 600.0)
+
+    def test_crop_box_preferred_over_media_box(self, tmp_path: Path) -> None:
+        """
+        GIVEN:
+            - A PDF page with a CropBox smaller than its MediaBox
+        WHEN:
+            - The first page size is requested
+        THEN:
+            - The CropBox dimensions are returned, matching what pdftoppm
+              renders with -cropbox
+        """
+        pdf_path = self._write_pdf(
+            tmp_path / "cropped.pdf",
+            crop_box=(50, 100, 350, 500),
+        )
+        assert get_pdf_first_page_size_points(pdf_path) == (300.0, 400.0)
+
+    def test_degenerate_box_returns_none(self, tmp_path: Path) -> None:
+        """
+        GIVEN:
+            - A PDF page whose box has zero width
+        WHEN:
+            - The first page size is requested
+        THEN:
+            - None is returned instead of a size that would break DPI math
+        """
+        pdf_path = self._write_pdf(
+            tmp_path / "degenerate.pdf",
+            media_box=(0, 0, 600, 800),
+            crop_box=(100, 0, 100, 800),
+        )
+        assert get_pdf_first_page_size_points(pdf_path) is None
+
+    def test_nonexistent_path_returns_none(self) -> None:
+        """
+        GIVEN:
+            - A path that does not exist
+        WHEN:
+            - The first page size is requested
+        THEN:
+            - None is returned and nothing is raised
+        """
+        assert get_pdf_first_page_size_points(Path("/nonexistent/file.pdf")) is None
+
+    def test_corrupt_pdf_returns_none(self, tmp_path: Path) -> None:
+        """
+        GIVEN:
+            - A file that is not a PDF
+        WHEN:
+            - The first page size is requested
+        THEN:
+            - None is returned and nothing is raised
+        """
+        bad = tmp_path / "bad.pdf"
+        bad.write_bytes(b"not a pdf")
+        assert get_pdf_first_page_size_points(bad) is None
+
+    def test_encrypted_pdf_returns_none(self) -> None:
+        """
+        GIVEN:
+            - A password protected PDF
+        WHEN:
+            - The first page size is requested
+        THEN:
+            - None is returned and nothing is raised
+        """
+        assert get_pdf_first_page_size_points(SAMPLES / "encrypted.pdf") is None
+
+
 class TestPostProcessText:
     @pytest.mark.parametrize(
         ("source", "expected"),