From 7f2818958eac40750bf9740fc1863c0b913553cd Mon Sep 17 00:00:00 2001
From: Seongmin Cho <seongmin.cho@genon.ai>
Date: Tue, 9 Jun 2026 10:54:46 +0900
Subject: [PATCH] =?UTF-8?q?fix:=20prevent=20O(N=C2=B2)=20XObject=20reloadi?=
 =?UTF-8?q?ng=20in=20do=5Fform()=20causing=20OOM=20on=20complex=20PDFs?=
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When a PDF page contains many Form XObjects sharing the same resource
dictionary (e.g. 2296 scatter plot data points each referencing 2301
XObjects), do_form() was calling set() on fonts, graphics states, and
XObjects on every invocation — reloading all N resources N times,
resulting in O(N²) memory and CPU usage that caused OOM kills.

Fix: before calling set(), check whether all keys in the resource
dictionary are already present in the parent chain. If so, skip the
set() call entirely. This reduces the redundant reloading to O(1) per
do_form() call when resources are already inherited.

Fixes: https://github.com/docling-project/docling/issues/2109

Signed-off-by: Seongmin Cho <seongmin.cho@genon.ai>
---
 src/parse/pdf_decoders/stream.h | 50 +++++++++++++++++++++++++++++++--
 1 file changed, 47 insertions(+), 3 deletions(-)

diff --git a/src/parse/pdf_decoders/stream.h b/src/parse/pdf_decoders/stream.h
index 804fe23f..e46df24a 100644
--- a/src/parse/pdf_decoders/stream.h
+++ b/src/parse/pdf_decoders/stream.h
@@ -1,5 +1,6 @@
 //-*-C++-*-
 
+
 #ifndef PDF_STREAM_DECODER_H
 #define PDF_STREAM_DECODER_H
 
@@ -385,19 +386,62 @@ namespace pdflib
       if(xobj.has_fonts())
         {
           QPDFObjectHandle xobj_fonts = xobj.get_fonts();
-          page_fonts_->set(xobj_fonts, timings);
+          // Skip set() if all fonts are already loaded in the parent chain to avoid O(N²) reloading
+          auto font_keys = xobj_fonts.getKeys();
+          bool fonts_all_loaded = !font_keys.empty();
+          for(auto& k : font_keys)
+            {
+              if(page_fonts_->count(k) == 0)
+                {
+                  fonts_all_loaded = false;
+                  break;
+                }
+            }
+          if(!fonts_all_loaded)
+            {
+              page_fonts_->set(xobj_fonts, timings);
+            }
         }
 
       if(xobj.has_grphs())
         {
           QPDFObjectHandle xobj_grphs = xobj.get_grphs();
-          page_grphs_->set(xobj_grphs, timings);
+          // Skip set() if all graphics states are already loaded in the parent chain to avoid O(N²) reloading
+          auto grph_keys = xobj_grphs.getKeys();
+          bool grphs_all_loaded = !grph_keys.empty();
+          for(auto& k : grph_keys)
+            {
+              if(page_grphs_->count(k) == 0)
+                {
+                  grphs_all_loaded = false;
+                  break;
+                }
+            }
+          if(!grphs_all_loaded)
+            {
+              page_grphs_->set(xobj_grphs, timings);
+            }
         }
 
       if(xobj.has_xobjects())
         {
           QPDFObjectHandle xobj_xobjects = xobj.get_xobjects();
-          page_xobjects_->set(xobj_xobjects, timings);
+          // Skip set() if all XObjects are already loaded in the parent chain.
+          // Prevents O(N²) reloading when do_form() is called N times and each call reloads all N resources.
+          auto keys = xobj_xobjects.getKeys();
+          bool all_already_loaded = !keys.empty();
+          for(auto& k : keys)
+            {
+              if(!page_xobjects_->has(k))
+                {
+                  all_already_loaded = false;
+                  break;
+                }
+            }
+          if(!all_already_loaded)
+            {
+              page_xobjects_->set(xobj_xobjects, timings);
+            }
         }
     }