Replace getAll with getKeys in PartialEvaluator_hasBlendModes to speed up loading of badly generated PDF files (issue 6961)

Some bad PDF generators, in particular "Scribus PDF", duplicates resources *a lot* at various levels of the PDF files. This can lead to `PartialEvaluator_hasBlendModes` taking an unreasonable amount of time to complete.
The reason is that the current code is using `Dict_getAll`, which recursively dereferences *all* indirect objects, which can be really slow. This patch instead uses `Dict_getKeys`, and then manually looks up only the necessary indirect objects.

I've added the PDF file as a `load` test. The most important thing here is probably to ensure that the file remains available in the repo, and the comment should help reduced the chance of regressions. (Note that locally, the `load` test times out without this patch, but we cannot really assume that that always happens.)

Fixes 6961.

src/core/evaluator.js

@@ -151,17 +151,19 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
         processed[resources.objId] = true;
       }
 
-      var nodes = [resources];
+      var nodes = [resources], xref = this.xref;
       while (nodes.length) {
-        var key;
+        var key, i, ii;
         var node = nodes.shift();
         // First check the current resources for blend modes.
         var graphicStates = node.get('ExtGState');
         if (isDict(graphicStates)) {
-          graphicStates = graphicStates.getAll();
-          for (key in graphicStates) {
-            var graphicState = graphicStates[key];
-            var bm = graphicState['BM'];
+          var graphicStatesKeys = graphicStates.getKeys();
+          for (i = 0, ii = graphicStatesKeys.length; i < ii; i++) {
+            key = graphicStatesKeys[i];
+
+            var graphicState = graphicStates.get(key);
+            var bm = graphicState.get('BM');
             if (isName(bm) && bm.name !== 'Normal') {
               return true;
             }
@@ -172,9 +174,20 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
         if (!isDict(xObjects)) {
           continue;
         }
-        xObjects = xObjects.getAll();
-        for (key in xObjects) {
-          var xObject = xObjects[key];
+        var xObjectsKeys = xObjects.getKeys();
+        for (i = 0, ii = xObjectsKeys.length; i < ii; i++) {
+          key = xObjectsKeys[i];
+
+          var xObject = xObjects.getRaw(key);
+          if (isRef(xObject)) {
+            if (processed[xObject.toString()]) {
+              // The XObject has already been processed, and by avoiding a
+              // redundant `xref.fetch` we can *significantly* reduce the load
+              // time for badly generated PDF files (fixes issue6961.pdf).
+              continue;
+            }
+            xObject = xref.fetch(xObject);
+          }
           if (!isStream(xObject)) {
             continue;
           }

test/pdfs/.gitignore

@@ -18,6 +18,7 @@
 !issue5972.pdf
 !issue5874.pdf
 !issue6782.pdf
+!issue6961.pdf
 !filled-background.pdf
 !ArabicCIDTrueType.pdf
 !ThuluthFeatures.pdf

test/test_manifest.json

@@ -2760,5 +2760,13 @@
        "md5": "8961cb55149495989a80bf0487e0f076",
        "rounds": 1,
        "type": "load"
+    },
+    {  "id": "issue6961",
+       "file": "pdfs/issue6961.pdf",
+       "md5": "a80e4357a8fda758d96c2c76f2980b03",
+       "link": false,
+       "rounds": 1,
+       "lastPage": 1,
+       "type": "load"
     }
 ]