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https://issues.apache.org/jira/browse/PDFBOX-4963?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17206600#comment-17206600
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Tilman Hausherr commented on PDFBOX-4963:
-----------------------------------------
https://stackoverflow.com/a/41379293/535646
The "update" segment mentions that get() will always return null, so it's
useless to expect the real object (I tried before finding this SO question).
Which takes us back to the SO answer I mentioned first.
I think I mostly understand now. And now I think this ReferenceQueue is useless
even if we export the Closeable because using this ReferenceQueue does not
solve the problem that we don't know WHEN the object will be finalized.
We'd probably have to do something completely different, like some LRU cache,
or no cache at all except the standard 14.
> TTF file leakage in font cache
> ------------------------------
>
> Key: PDFBOX-4963
> URL: https://issues.apache.org/jira/browse/PDFBOX-4963
> Project: PDFBox
> Issue Type: Bug
> Components: PDModel
> Affects Versions: 2.0.21
> Reporter: Maison
> Priority: Major
>
> We observe many TTF opened files in our production server, which result in
> exhausting file descriptors.
> We have checked and rechecked that every PDDocument is properly closed (try
> with resource everywhere).
> By looking at pdfbox source code, I suspect 2 problems in FontCache and in
> FileSystemFontProvider
> 1 - FontCache
> In FontCache, a map keeps SoftReference<FontBoxFont> as values.
> IIUC for TTF fonts, the values are instances of
> org.apache.fontbox.ttf.TrueTypeFont. Such instances have a TTFDataStream
> member, which is RAFDataStream (so there is an opened file).
> Problem is that if the soft reference is cleared by GC, we can suppose the
> TrueTypeFont objects are GCed (is that guaranteed?) ; but what about the
> RAFDataStream sub-object ? There is no RAFDataStream.close() in TrueTypeFont
> finalizer
> 2 - FileSystemFontProvider
> There seems to be a TOCTOU-like race condition when a font is needed. Code
> looks like below (simplified) :
> @Override
> public FontBoxFont getFont()
> {
> FontBoxFont cached = parent.cache.getFont(this);
> if (cached != null) {
> return cached;
> }
> FontBoxFont font = ... // instantiate font
> parent.cache.addFont(this, font); // <--- not thread safe ?
> return font;
> }
> The font, if not in cache, is instantiated and added into cache. But two
> threads can do that at the same time, and the last addFont() wins. So the
> first SoftReference<FontBoxFont> object is now eligible to GC; but the
> FontBoxFont has not been closed.
> This problem is probably less frequent.
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