Commits on Dec 29, 2018

1. Check that the first page can be successfully loaded, to try and asce… …

   …rtain the validity of the XRef table (issue 7496, issue 10326)
   
   For PDF documents with sufficiently broken XRef tables, it's usually quite obvious when you need to fallback to indexing the entire file. However, for certain kinds of corrupted PDF documents the XRef table will, for all intents and purposes, appear to be valid. It's not until you actually try to fetch various objects that things will start to break, which is the case in the referenced issues[1].
   
   Since there's generally a real effort being in made PDF.js to load even corrupt PDF documents, this patch contains a suggested approach to attempt to do a bit more validation of the XRef table during the initial document loading phase.
   
   Here the choice is made to attempt to load the *first* page, as a basic sanity check of the validity of the XRef table. Please note that attempting to load a more-or-less arbitrarily chosen object without any context of what it's supposed to be isn't a very useful, which is why this particular choice was made.
   Obviously, just because the first page can be loaded successfully that doesn't guarantee that the *entire* XRef table is valid, however if even the first page fails to load you can be reasonably sure that the document is *not* valid[2].
   
   Even though this patch won't cause any significant increase in the amount of parsing required during initial loading of the document[3], it will require loading of more data upfront which thus delays the initial `getDocument` call.
   Whether or not this is a problem depends very much on what you actually measure, please consider the following examples:
   
   ```javascript
   console.time('first');
   getDocument(...).promise.then((pdfDocument) => {
     console.timeEnd('first');
   });
   
   console.time('second');
   getDocument(...).promise.then((pdfDocument) => {
     pdfDocument.getPage(1).then((pdfPage) => { // Note: the API uses `pageNumber >= 1`, the Worker uses `pageIndex >= 0`.
       console.timeEnd('second');
     });
   });
   ```
   
   The first case is pretty much guaranteed to show a small regression, however the second case won't be affected at all since the Worker caches the result of `getPage` calls. Again, please remember that the second case is what matters for the standard PDF.js use-case which is why I'm hoping that this patch is deemed acceptable.
   
   ---
   [1] In issue 7496, the problem is that the document is edited without the XRef table being correctly updated.
   In issue 10326, the generator was sorting the XRef table according to the offsets rather than the objects.
   
   [2] The idea of checking the first page in particular came from the "standard" use-case for the PDF.js library, i.e. the default viewer, where a failure to load the first page basically means that nothing will work; note how `{BaseViewer, PDFThumbnailViewer}.setDocument` depends completely on being able to fetch the *first* page.
   
   [3] The only extra parsing is caused by, potentially, having to traverse *part* of the `Pages` tree to find the first page.

   

   Snuffleupagus committed Dec 29, 2018
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