[PDFBOX-4963] TTF file leakage in font cache

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 2.0.21
• Component/s: PDModel
• Assignee: Unassigned
• Reporter: Maison
• Created: 24/Sep/20 08:32
• Updated: 26/Oct/22 09:38

Description

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.