Unix System Administration Handbook 1997 October/usah_oct97.iso/news/cnews.tar/notebook/locking

Text File | 1994-08-16 | 7KB | 226 lines

.DA "16 Aug 1994"
.TL
Locking in C News
.Ix locking
.SH
Introduction
.LP
Several parts of
C News
need some way of locking parts of the news
subsystem against concurrent execution.
Various system-specific locking system calls exist,
but none of them
is truly portable,
and most of them provide far more functionality than
we need.
.LP
C News
.Ix link
locking uses the \fIlink\fR(2) system call and pre-agreed names.
\fILink\fR has the necessary characteristic for safe locking:
it is an atomic test-and-set operation.
Furthermore,
it exists in all Unixes.
.SH
Mechanism
.LP
.Ix lock names
All locks are created in the NEWSCTL directory
(see \fIConfiguration Mechanisms in C News\fR for where this directory
is to be found and how programs can determine this)
and have names starting with `LOCK'.
To acquire a lock,
.Ix lock acquiring
.Ix LOCK
.Ix files LOCK
first create a temporary file in NEWSCTL with a name
of the form `L.\fIn\fR', where \fIn\fR is your process id.
Write your process id,
in decimal ASCII,
into this file.
Then attempt to link the temporary file to `LOCK\fIx\fR',
where \fIx\fR is chosen based
on what sort of locking you wish to do.
Existing lock names are:
.IP LOCK \w'LOCKexplode'u+2n
additions to article tree, changes to control files.
Normally held mostly by \fIrelaynews\fR and
administrative programs;
grabbed briefly by \fIexpire\fR at the end of its run.
.IP LOCKinput
processing of spooled input.
Held by \fInewsrun\fR to prevent multiple simultaneous \fInewsrun\fRs.
.IP LOCKexplode
changes to batch files.
Held by the exploder when writing them,
and grabbed briefly by the batcher when moving them aside.
.IP LOCKoverview
changes to overview files.
.IP LOCKbatch
preparation of batches.
Held by the batcher (in the normal configuration)
to prevent multiple simultaneous batching runs.
.IP LOCKexpire
rebuilding of history file, removal/archiving of articles.
Held by \fIdoexpire\fR to prevent multiple simultaneous \fIexpire\fRs.
.LP
If the link fails,
sleep briefly and try again.
If it succeeds,
remove the temporary file and proceed
(some old code may defer removal of the temporary file
until the lock itself is removed).
Programs are expected to make a determined effort to remove lock files
when they terminate,
normally or as a result of signals
(however, caution is indicated:
race conditions should be resolved in favor of
a small probability of leaving a dead lock around,
\fInot\fR in favor of a small probability of removing
a lock owned by someone else!).
.LP
Although there are various thorny questions associated with breaking
locks by dead programs,
.Ix lock clearing
reboot is a time when surviving locks are
definitely invalid.
(Although there are problems even here if a networked group of systems
are not rebooted as a unit.)
For this and other reasons,
a system running C News should execute
.Ix newsboot
.Ix /etc/rc
NEWSCTL/bin/newsboot at reboot time (e.g. from \fI/etc/rc\fR).
.SH
C Interface
.LP
.Ix locking in C
It's obviously desirable to package this mechanism up in a convenient form,
to avoid having to reinvent it.
It happens that the main lock,
LOCK,
is the only one that current C code needs\(emeverything else is done
only from shell files.
The C interface consists of three functions:
.IP newslock \w'newsunlock'u+2n
.Ix newslock
acquire the main lock, and remember the fact.
.IP newsunlock
.Ix newsunlock
release the main lock \fIif\fR I acquired it.
.IP nolock
.Ix nolock
remember that I did \fInot\fR acquire the main lock
(used by children of lock-holding processes).
.LP
In addition, there are two commonly-used utility functions
that tie into the locking subsystem:
.IP errunlock \w'errunlock'u+2n
.Ix errunlock
.Ix error
.Ix warning
invoke the \fIwarning\fR function
to print an error message,
call \fInewsunlock\fR,
and do \fIexit(1)\fR.
.IP nemalloc
.Ix nemalloc
.Ix malloc
like \fImalloc\fR, except on failure, call \fIerrunlock\fR
instead of returning NULL.
.SH
Shell Interface
.LP
.Ix locking in shell scripts
The shell interface is a bit more problematic.
Ideally, one would like to hide the locking machinery inside a pair
of shell commands,
analogous to the C functions,
so that the locking mechanism could be changed easily.
The trouble is that some kinds of mechanism require a single process
to acquire the lock and then release it, and
death of such a process automatically releases its lock(s).
It's much more natural for a shell program to invoke separate ``lock''
and ``unlock'' commands;
an attempt at a single-process interface for locking in the C News
Performance Release produced a rather odd shell programming style
that did not lend itself well to complex cases.
.LP
The bottom line is:
we're using a two-command model.
If a single-process locking mechanism is used, it will be necessary to
create a lock-holder background process as part of the locking operation,
and kill it as part of the unlocking operation.
The standard mechanism, using links, doesn't have this problem.
.LP
To acquire a lock, use the \fIlock\fR command,
.Ix lock command
found in NEWSBIN.
The syntax is:
.DS
\fBlock\fR [\fB\-o\fR] [\fB\-v\fR] lockname pid [sleeptime]
.DE
\fILockname\fR is the name of the lock, e.g. ``LOCKinput''.
\fIPid\fR is the process ID of the lock-holding process,
normally written ``$$'' to have the shell supply it.
.LP
Normally, \fIlock\fR persists until it gets the lock,
sleeping \fIsleeptime\fR (default 30) seconds after each failure.
If the \fB\-o\fR option is used,
\fIlock\fR tries only once, and returns exit status 0 for success
and 1 for failure.
If the \fB\-v\fR option is used,
\fIlock\fR prints
``news system locked, waiting..''
to standard output just before sleeping after a failure.
.LP
To release a lock, invoke \fIunlock\fR
.Ix unlock command
(found in NEWSBIN)
with the lock name(s) as arguments.
You must be the lock holder\(em\fIunlock\fR does not check this.
.LP
A lock that has been sitting around for some time is naturally an
object of suspicion.
It can be useful for a program to indicate that it is \fIdoing\fR
something by updating the date on the lock.
To update a lock, invoke \fItouchlock\fR
.Ix touchlock command
(found in NEWSBIN)
with the lock name(s) as arguments.
You must be the lock holder\(em\fItouchlock\fR does not check this.
.LP
As an additional feature, potentially of use in special situations,
if the
\fIlockname\fR given to \fIlock\fR, \fItouchlock\fR, or \fIunlock\fR
contains ``/'',
it is understood to be the pathname of the lock,
and \fIlockname\fB.\fIpid\fR is used as the pathname of the temporary.
This permits use of locks that do not reside in NEWSCTL.
.LP
.Ix Unix "System V"
.Ix "System V" breakage
.Ix ln
The underlying implementation of our standard locking mechanism
in shell has an additional problem in that System V has broken
\fIln\fR(1) so that it removes a pre-existing destination file.
C News
therefore provides a pure,
simple locking program under
the name NEWSBIN/newslock
.Ix newslock
(if the recommendations in
\fIDirectory Layout and PATH in C News\fR are followed,
this will
automatically be in the search path of shell programs).
Syntax is:
.DS
\fBnewslock\fR tempfile lockfile
.DE
Exit status is 0 for success,
1 for failure,
2 for wrong number of arguments.
No messages are printed for normal failure,
so no redirection of output is needed.