https://bugs.freedesktop.org/show_bug.cgi?id=62525
Priority: medium
Bug ID: 62525
Assignee: [email protected]
Summary: Convert manual refcounting / copy-on-write
implementations to cow_wrapper
Severity: normal
Classification: Unclassified
OS: All
Reporter: [email protected]
Hardware: Other
Status: UNCONFIRMED
Version: unspecified
Component: Libreoffice
Product: LibreOffice
This is a medium-level EasyHack, and can be split up nicely into converting
individual classes one after another. If you work on one class, or one set of
classes inside a subsystem, please add a line to this bug stating so.
What is this about
==================
The LibreOffice codebase, at a number of occasions, employs the CopyOnWrite
pattern to make passing big objects around by-value acceptably cheap, see this
article for some more background:
http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Copy-on-write
Currently we have quite a number of ad-hoc implementations of that pattern,
with a number of known problems (getting it wrong in corner cases, different
refcount semantics (is zero refcount good for deletion? does it mean one client
left?), and a (hopefully generic) helper template for easy conversion of
already-pimpled classes:
http://cgit.freedesktop.org/libreoffice/core/tree/o3tl/inc/o3tl/cow_wrapper.hxx
The task at hand is to convert as many of those sprawling ad-hoc
implementations over to cow_wrapper
What this is not about
======================
This is not about whether COW is a useful pattern in the first place - we want
to consolidate the code here to avoid nasty corner cases. If you want to go for
a tangential read on that, this one is nice: http://www.gotw.ca/gotw/045.htm
Also note that cow_wrapper defaults to non-threadsafe mode (mimicking std
behaviour in LibO code, the application being inherently single-threaded
anyway).
Details
=======
If you want to find a victim, go grep the source tree for .hxx files containing
nRefCount (there are variations in that name, but the aforementioned one should
already give enough hits for the while).
If the code looks like implementing pimpl idiom
(http://www.gotw.ca/gotw/024.htm) and Copy-on-write semantics, try wedging it
into using cow_wrapper instead.
Example conversion from back in the day:
http://cgit.freedesktop.org/libreoffice/core/commit/?id=b6368dd527fd7ebe307cebe5c2275983df9ee851
and
http://cgit.freedesktop.org/libreoffice/core/commit/?id=776b7235e23735189dbeafa6de93a4a2eaa2bf30
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