Michael Allman wrote:
Hello,
Can someone with knowledge of such matters explain what
FileDispatcher.preClose() is supposed to do on Solaris/Linux. I mean,
I see the code, but I don't understand why it exists or what problem
it's supposed to avoid or something.
I ask because I'm trying to fix a file-locking problem on soylatte and
it seems the solution to that problem is to remove this code (on that
platform). But before I charge ahead, I need a better understanding
of why this code exists.
In particular, I'm really interested in the stuff that happens in
FileDispatcher.c, functions Java_sun_nio_ch_FileDispatcher_init and
Java_sun_nio_ch_FileDispatcher_preClose0. They're setting something
up that looks important, but I just don't get it.
In a multi-threaded application it is always difficult to know when you
can safely close and release a file descriptor (or other resource). If
one thread is using a file descriptor to read or write and another
thread releases (closes) it then it it possible for the first thread to
read or write to the wrong file or socket in the event that the file
descriptor is recycled quickly. The approach that we use in both classic
networking and NIO is to use a two-step process. In the first step we
duplicate (dup2) the file descriptor to another that is one end of a
half shutdown socket pair. Other threads that are reading or writing but
haven't called the read or write system calls yet will get an immediate
EOF or pipe error when they do so. As the threads complete the read or
write method then they examine their state. If there is a close pending
then the last one releases the file descriptor. Hopefully this brief
overview gives you some idea what this code is about. The
FileDescriptor#init method is where the socketpair is created, and that
preClose0 method does the dup2. I haven't been following the Soylatte
port very closely so I'm curious what problem you are seeing - when you
say "file locking" do you mean FileChannel#lock? If so then the issue
may be that the asynchronous close mechanism isn't completely extended
to FileChannel yet.
-Alan.
