From 6d4cde98b67349fbd8342c8334e17794e91cdd7c Mon Sep 17 00:00:00 2001
From: Matthias Valvekens <matthias@mvalvekens.be>
Date: Thu, 21 Mar 2024 20:32:24 +0100
Subject: [PATCH] Tolerate AcroForms without a Fields entry

Fixes #403
---
 pyhanko/sign/fields.py       | 2 +-
 pyhanko_tests/test_fields.py | 9 +++++----
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/pyhanko/sign/fields.py b/pyhanko/sign/fields.py
index 6919b808..737ac53e 100644
--- a/pyhanko/sign/fields.py
+++ b/pyhanko/sign/fields.py
@@ -1473,7 +1473,7 @@ def prepare_sig_field(
         try:
             fields = form['/Fields']
         except KeyError:
-            raise PdfError('/AcroForm has no /Fields')
+            fields = form['/Fields'] = generic.ArrayObject()
 
         candidates = enumerate_sig_fields_in(
             fields, with_name=sig_field_name, refs_seen=set()
diff --git a/pyhanko_tests/test_fields.py b/pyhanko_tests/test_fields.py
index 3f80dea1..a431e990 100644
--- a/pyhanko_tests/test_fields.py
+++ b/pyhanko_tests/test_fields.py
@@ -439,8 +439,6 @@ def test_append_sig_field_acro_update():
 
 def test_append_acroform_no_fields():
     w = PdfFileWriter()
-    # Technically, this is not standards-compliant, but our routine
-    # shouldn't care
     w.root['/AcroForm'] = generic.DictionaryObject()
     w.insert_page(simple_page(w, 'Hello world'))
     out = BytesIO()
@@ -449,8 +447,11 @@ def test_append_acroform_no_fields():
 
     sp = fields.SigFieldSpec('InvisibleSig')
     w = IncrementalPdfFileWriter(out)
-    with pytest.raises(PdfError, match="has no /Fields"):
-        fields.append_signature_field(w, sp)
+    fields.append_signature_field(w, sp)
+    w.write_in_place()
+
+    r = PdfFileReader(out)
+    assert len(r.root['/AcroForm']['/Fields']) == 1
 
 
 def test_append_acroform_reference_broken_nonstrict():