fix: rewrite form flattening handler

Previously we used the form flattening method from PDF-Lib
but unfortunately when it encountered orphaned form items
or other PDF oddities it would throw an error.

Because of this certain documents would fail to seal and
be stuck in a pending state with no recourse available.
This change rewrites the form flattening handler to be
more lenient when coming across the unknown opting to skip
items it can't handle rather than abort.

packages/lib/server-only/pdf/flatten-form.ts

@@ -0,0 +1,112 @@
+import type { PDFField, PDFWidgetAnnotation } from 'pdf-lib';
+import { PDFCheckBox, PDFRadioGroup, PDFRef } from 'pdf-lib';
+import {
+  PDFDict,
+  type PDFDocument,
+  PDFName,
+  drawObject,
+  popGraphicsState,
+  pushGraphicsState,
+  rotateInPlace,
+  translate,
+} from 'pdf-lib';
+
+export const flattenForm = (document: PDFDocument) => {
+  const form = document.getForm();
+
+  form.updateFieldAppearances();
+
+  for (const field of form.getFields()) {
+    for (const widget of field.acroField.getWidgets()) {
+      flattenWidget(document, field, widget);
+    }
+
+    try {
+      form.removeField(field);
+    } catch (error) {
+      console.error(error);
+    }
+  }
+};
+
+const getPageForWidget = (document: PDFDocument, widget: PDFWidgetAnnotation) => {
+  const pageRef = widget.P();
+
+  let page = document.getPages().find((page) => page.ref === pageRef);
+
+  if (!page) {
+    const widgetRef = document.context.getObjectRef(widget.dict);
+
+    if (!widgetRef) {
+      return null;
+    }
+
+    page = document.findPageForAnnotationRef(widgetRef);
+
+    if (!page) {
+      return null;
+    }
+  }
+
+  return page;
+};
+
+const getAppearanceRefForWidget = (field: PDFField, widget: PDFWidgetAnnotation) => {
+  try {
+    const normalAppearance = widget.getNormalAppearance();
+    let normalAppearanceRef: PDFRef | null = null;
+
+    if (normalAppearance instanceof PDFRef) {
+      normalAppearanceRef = normalAppearance;
+    }
+
+    if (
+      normalAppearance instanceof PDFDict &&
+      (field instanceof PDFCheckBox || field instanceof PDFRadioGroup)
+    ) {
+      const value = field.acroField.getValue();
+      const ref = normalAppearance.get(value) ?? normalAppearance.get(PDFName.of('Off'));
+
+      if (ref instanceof PDFRef) {
+        normalAppearanceRef = ref;
+      }
+    }
+
+    return normalAppearanceRef;
+  } catch (error) {
+    console.error(error);
+
+    return null;
+  }
+};
+
+const flattenWidget = (document: PDFDocument, field: PDFField, widget: PDFWidgetAnnotation) => {
+  try {
+    const page = getPageForWidget(document, widget);
+
+    if (!page) {
+      return;
+    }
+
+    const appearanceRef = getAppearanceRefForWidget(field, widget);
+
+    if (!appearanceRef) {
+      return;
+    }
+
+    const xObjectKey = page.node.newXObject('FlatWidget', appearanceRef);
+
+    const rectangle = widget.getRectangle();
+    const operators = [
+      pushGraphicsState(),
+      translate(rectangle.x, rectangle.y),
+      ...rotateInPlace({ ...rectangle, rotation: 0 }),
+      drawObject(xObjectKey),
+      popGraphicsState(),
+    ].filter((op) => !!op);
+
+    page.pushOperators(...operators);
+  } catch (error) {
+    console.error(error);
+  }
+};