ClearCase::Argv README

- IMPORTANT NOTE
- README
- SUMMARY OF FEATURES
- CHRONOLOGY

- IPC::ClearTool
- Argv
- ClearCase::Argv

- GRAND UNIFICATION

# IMPORTANT NOTE

THIS module is called ``ClearCase::Argv''. There is a DIFFERENT module called ``Argv''. ClearCase::Argv depends on (requires) Argv. That is, you must download and install BOTH in order for ClearCase::Argv to work. This has confused quite a few people so I highlight it here.

# README

Though ClearCase::Argv is itself pretty small, it represents the ``Grand Unification'' of some of my other ClearCase-related modules. The module itself is fully documented in the standard POD format; this file is an accompanying overview and chronology.

# SUMMARY OF FEATURES

- Interoperability/Portability
Using ClearCase::Argv can enhance portability of scripts between UNIX and Windows by providing versions of system, exec, and qx (aka backquotes) which behave the same on Windows as on UNIX. Ok, almost the same. This includes automatically quoting arguments to protect them from the `cmd.exe` shell, automatically converting /-separated pathnames to \, etc. These features can keep a lot of hair out of your script.

- Optional Co-Process for Speed
For any script which uses ClearCase::Argv, the 'ipc' class method

            ClearCase::Argv->ipc;

will start cleartool as an IPC::ClearTool co-process and arrange to send all subsequent system/exec/qx commands to it instead of forking a new child process each time. This can speed up scripts by anywhere between 1 (i.e. not at all) and 10 times by my measurements.

- CtCmd Interface for Speed
Just as with IPC::ClearTool, ClearCase::Argv can also be told to process commands via ClearCase::CtCmd

- Convenience Features
Many convenience features are provided by the replacement system/exec/qx functions. These include:

        1. 'autochomp' mode (chomps lines automatically, natch)
        2. 'autofail' mode (exit on child process failure)
        3. 'noexec' mode (print cmds without executing, like make -n)
        4. 'xargs' mode (breaks up long cmd lines to avoid system limits)

plus a few more.

- No Major Investment
It's easy to convert an existing script to use ClearCase::Argv or back. Just change all instances of backquotes to `qv()` and remove any existing Win32-porting hackery. To go back to native style, either change `qv()` to `qx()` (backquotes are generally deprecated in favor of qx anyway), or add a line like:

            sub qv { qx(@_) }

And remove the 'use ClearCase::Argv' line of course.

- Option Processing
There's a great deal of option-processing power inherited from the base class, much more in fact than most users will care about. Look in the PODs for more.

# CHRONOLOGY

## IPC::ClearTool

I wrote IPC::ClearTool to manage cleartool as a co-process for reasons of speed. I.e. instead of doing a fork/exec for each cleartool command it forks just one process in the background and sends all cleartool commands down to it. This is much (possibly up to 10 times) faster. Unfortunately IPC::ClearTool suffered from a few paradigmatic flaws:

It didn't work on Windows, where there's no such thing as a fork or a true ``child'' process.

The interface was/is strange and a little clunky. Not too surprising considering that UNIX requires 3 different API's `(system, exec, and backquotes)` to handle child processes; jamming all that functionality into one API is awkward.

Due to the above, any script written to the IPC::ClearTool API was non-portable to Windows and hard to convert back to traditional system/exec/qx. Thus converting an existing tool to it required a substantial commitment of time, and using it at all meant a substantial commitment of faith.

I was able to ``port'' `IPC::ClearTool` to Windows by calling into the ClearCase Automation Library, a COM interface first available in ClearCase 4.0 (it was actually present in unsupported form in 3.2.1). But the other issues remained, until ...

## Argv

I also had a ClearCase/Perl module called ClearCase::Ct. This was a wrapper that ran on top of cleartool to extend its functionality and/or allow site policies to be established at the wrapper level. But it suffered from an ugly programming model too (do we sense a trend here?). In particular it was necessary to do lots of shifting, grepping, splicing, and quoting of @ARGV, leading to terribly spaghetti-like code in places, especially when you throw in the need for UNIX/Windows portability and different shell-quoting rules. So extensions written to the ClearCase::Ct ``API'' tended to resemble a nest of ifdefs.

So I set out to rewrite `ClearCase::Ct`. The first step was to write a support module (eventually called Argv) to hide all the @ARGV machinations under an OO interface. Argv has plenty of its own docs so I won't go into it here, but suffice it to say it provides lots of ways to slice and dice an arg vector. In fact it provides much more parsing power than almost anyone would ever need, so while this was its original reason for existence it's the least interesting to most.

However, Argv also has execution methods, i.e. you can execute your Argv object via `$obj-`system()> or `$obj-`qx()>. Handling platform differences (quoting, pathname separators, etc.) in Argv seemed like a natural extension, so I added that. This lead to convenience methods like `$obj-`autochomp> (should be obvious) and `$obj-`qxargs> (implements xargs-like behavior to ensure that system limits aren't exceeded), etc.

At this point I realized that though the parsing features had a tiny constituency, the portability abstraction of the execution methods might be of interest to more users. So in order to make that more accessible I added a functional interface, allowing the single line

        use Argv qw(system exec qv);

to overrride the Perl builtins with Argv's relatively platform- independent versions. Note: qv is used because Perl doesn't allow `qx()`, which is itself a synonym for backquotes, to be overridden. Bottom line, adding the above line - plus converting ``cmd`` to `qv("cmd")` - buys a lot of UNIX/Win32 portability.

I eventually did get around to rewriting ClearCase::Ct; the new module is called ClearCase::Wrapper.

## ClearCase::Argv

Note that Argv itself has nothing to do with ClearCase. So I made a little subclass of Argv to tune it for use with cleartool, since I write a lot of Perl/ClearCase code. Originally, ClearCase::Argv simply extended Argv to prepend the word ``cleartool'' to all arg vectors. Thus, while

        Argv->new('ls', -l');

represents an `"ls -l"` command,

        ClearCase::Argv->new('ls', -l');

would run `"cleartool ls -l"`, and it understands that the 'program' part of the command line is ``cleartool ci'' (or more properly qw(cleartool ci)).

The functional interface of Argv is exposed through ClearCase::Argv, and it's also extended to support methods called ctsystem(), ctexec(), and ctqx() which automatically prepend 'cleartool'. E.g.:

            my @views = ccqx(lsview -s);

Attributes can be set through the functional interface like this:

            my @views = ccqx({autochomp=>1, dbglevel=>0}, lsview -s);

# GRAND UNIFICATION

Then one day I got an email question from Mark Scandariato of Lucent:

        Do you have any plan to use IPC-ClearTool within ClearCase-Ct? (I'd
        hate to duplicate anything already underway.)

I replied that I didn't but it got me to thinking about whether `ClearCase::Argv` could be taught to send its commands to a co-process. A few days later I got a chance to play with it and it came together with surprising ease. This is the big connection that makes it all pretty neat, IMHO, since you get improved speed, portability, and ease of use in one package. Without having to make a major commitment of rewriting code.