[bobsingor]

Follow-up to #95. That PR took the font tests through layer mode. This one does the same for every other fork test that builds its document in memory with FPDF_LoadMemDocument*, FPDF_CreateNewDocument or CreateEmptyDocument(). Until now, those tests never ran on a layer with the read-only check and the layer DCHECKs on.

What changes

• Tests moved onto the layer-mode loaders, about 250 call sites in 26 fpdfsdk/epdf_*embeddertest.cpp files:
  • OpenDocumentFromMemory(EmptyPdf()) replaces CreateEmptyDocument() (22 sites);
  • the new NewTestDocument() (in epdf_edit_test_util) replaces FPDF_CreateNewDocument() (124 sites). It is EmptyPdf() through LoadMemDocumentForTest();
  • LoadMemDocumentForTest() with ScopedTestDocument covers reopened saves and in-memory PDFs, directly or through the files' helpers:
    • SaveAndReopen in ppo and page labels;
    • OpenRaw/OpenBytes in signatures;
    • Open in outline and page residency;
    • the stamp and rich-text fixtures.
• Left plain on purpose, each with a comment saying why:
  • the plain half of tests that compare a plain document with a layer (redact, catalog, content rewrite, annot identity, save, outline);
  • the stock reopens of a materialized file in epdf_layer;
  • tests whose subject is a plain document: transactions, chosen numbers, captures, WritesInPlaceAreNotSeen;
  • the signature tests whose plain document is the reference (OpenPlainBytes).
• Fix, rule reads-dont-write: taking a stamp's drawing from another document's page (EPDFDoc_CanonicalDrawing, EPDFAnnot_SetAppearanceFromPage) wrote to that source document. It took the page through GetMutablePageDictionary() and added a temporary form object, then deleted it. On a layer source, the page was copied into the layer and two object numbers were used up. ImportPageAsDrawing() now reads the page const and builds the temporary form as a stream no document holds.
  • New test: EPDFStampResizeEmbedderTest.ADrawingFromALayersPageLeavesTheLayerAlone. Before the fix it fails with 1 promoted object, and the last number goes from 4 to 6.
• Test fixes:
  • EPDFPageResidencyEmbedderTest.CountsWhileParsing built its page through GetMutablePageDictionary(). It now builds it as FPDF_LoadPage() does.
  • AStampFromAPageLeavesTheDocumentInfoAlone relied on the /Info that FPDF_CreateNewDocument() makes. It now writes /Info with EPDF_SetMetaText().
• Writers list: the 199 tests that write on purpose are added to testing/embedpdf_layer_mode_writers.txt by hand. They are new documents, fixtures that write, and reopened saves the tests write into. For every test that closes more than one document, I checked each document's count: the documents a test only reads leave 0.
• Ledger: the layer-mode rule names NewTestDocument() and says when a test stays plain. reads-dont-write lists the new guard test.

Testing (macOS arm64, debug)

• run_layer_mode.py over all 1760 tests: no new errors.
  • Every test that fails only in layer mode matches testing/embedpdf_layer_mode_expectations.txt. Almost all of these are the form-fill suites.
  • The one error is EPDFSignatureEmbedderTest.SignEncryptedDocument, which is local-only: its fixture is untracked, so the test skips on CI. It is left out of the writers list.
• Plain pdfium_embeddertests: 1742 pass, 13 fail.
  • 12 are the macOS render pixel goldens: FPDFProgressiveRenderEmbedderTest ×8, FPDFViewEmbedderTest.Render* ×2, FPDFEditEmbedderTest.SetTextRenderMode and EPDFMeasureEmbedderTest.CaptionAppearanceGolden.
  • The 13th is EPDFSlicedLoadEmbedderTest.SlicesKeepToTheirBudget, a timing bound that missed while the machine's load average was about 200. In a plain run its document opens as before.
• pdfium_unittests: 1312 pass.
• check-fork-tags.py --strict and check-c-api.py pass.

The Linux Docker suites and layer mode run in CI.