=pod

=for comment $Id: Cookbook.pod 497 2014-03-17 23:44:36Z whynot $

=for comment $VERSION = qv v0.1.3;

=for comment Copyright 2009, 2010, 2014 Eric Pozharski <whynot@pozharski.name>

=for comment AS-IS, NO-WARRANTY, HOPE-TO-BE-USEFUL

=for comment CC-SA

=head1 NAME

File::AptFetch::Cookbook - Tips and Gotchas about APT Methods

=head1 DISCLAIMER

My understanding how APT methods work (and interact) is mostly experimental.
I've thoroughly read "APT Method Interface" (not big reading though).
And when you would read it, please note unsurprizing number on the very first
page -- C<1998>.
At least something.

Then I've read B<strace(1)> output -- very interesting reading.
And I made some conversations with methods themselves.
That's all.

I admit, I didn't dig the actual code.
(Once I'd got into B<apt-get(1)> -- it's C++;
is it possible to read C++ without debugger?)
I promise to do it later
(Read-The-Code seems to be Debian's mantra,
though that's the only authoritative place).

So, if in some next section I've said "I can't comment",
than that means that I didn't tortured methods yet.
I will.

=head1 INTRODUCTION

Briefly.
APT method is an executable that has no command-line interface at all.
All interaction is done through I<STDIN> and I<STDOUT>.
I<STDERR> is for side-effect messages
(pending to see any output yet).

Each B<message> is a sequence of newline (C<"\n">) terminated lines;
And is terminated by empty line
(lone newline works for sure;
I can't comment what would happen if "empty" line is set of spaces).

Each B<message> starts with B<message header>.
B<Message header> is a 3-digit number (B<Status Code>)
and B<informational string>
(for "visual debugging";
does it mean it's ignored?
what if it's empty?).
B<File::AptFetch> stores the B<Status Code> in I<$status> field and
B<informational string> in I<$Status> field.
I<$status> is read-only.

Then B<header field>s come.
B<Header field> is kind of RFC822 header:
colon (C<':'>) separates header name off header value.
(I can't comment on if header value wrapping is supported.
And what about any extra space?)
B<File::AptFetch> splits message, and stores it in obvious hash.
It's either I<%$capabilities> (I<$status> is C<100>) or
in I<%$message> (in any other case).
I<%$capabilities> is filled once and then stays unnoted
(that can change in future).
I<%$message> is overwritten each time new message comes (it's not refilled).

The list of B<message header>s and B<header field>s is
in "APT Method Interface" manual.
The list B<header field>s is incomplete;
that rises a question: is the list of B<message header>s complete?

=head1 MESSAGE HEADERS

Those are B<message header>s I have something to say about
(additions pending).
There's an asymmethry -- they are
either from an applicaton to a method (downstream) or
from a method to an application (upstream).

=over

=item 100 Capabilities

I<(upstream)>
That's the "Hello, world!" of the method.
That shows something about the method invoked.
It doesn't show what method is invoked
(you're supposed to know what you're doing).

=item 200 URI Start

I<(upstream)>
That informs application that the method started to process a request.
That's possible for method to skip this message.

=item 201 URI Done

I<(upstream)>
That marks a request completion.
I suppose that after this message a method forgets about just completed request
("Quotation Needed (tm)").
I<(v0.1.2)>
Sometimes method leaves remains (or fossils, if you like) of served request
what should have failed but succeeded;
KEE of F<file> method covers some details.

=item 400 URI Failure

I<(upstream)>
A named request can't be fulfilled.
Than goes for next or waits for request.

=item 600 URI Acquire

I<(downstream)>
That requests file.
One file -- one request.
Application isn't required to wait C<201> code before fileing next C<600> --
quite otherwise.
It's supposed that C<600>s will gone in at once.

=item 601 Configuration

I<(downstream)>
That's the "Hello, pal!" of the application.
There's a field I<$message{send_config}> (supposed to be in use with C<100>);
although C<601> is sent each time method is started.

=item 101 Log

=item 102 Status

=item 401 General Failure

=item 402 Authorization Required

=item 403 Media Failure

=item 602 Authorization Credentials

=item 603 Media Changed

Those are B<message header>s I didn't meet yet.
That's the problem with test-suite.
To fix this I need to do some work -- it's undone yet.

=back

As you can see, there's no message what would require status.
It's up to the method to report any progress.
(I<v.0.0.8>)
Alas, they don't.
Thus, that's up to application to count any progress.
Me wonders if they timeout at all.
However, there're signs that those of networking type properly detect if
underlying TCP/IP connection has been lost.

=head1 HEADER FIELDS

The same comment as for L</MESSAGE HEADERS> apply.
The subtle difference is this list mentions B<header filed>s missing in the
manual.

B<Header fields> are spelled first-capital (regular words) and
all-capital (abbreviations).
I can't comment what would happen otherwise.

=over

=item Config-Item

I<(downstream)>
That's used with C<601> message.
The format of value is set to

    APT::Configuration::Item=value

That seems there should be no space inside
(neiter around equal sign (C<'='>) nor in I<$value>).
I can't comment what kind of dragons hide behind this.
OTOH, if spaces are escaped, then should equal sign be escaped too?
The set of items consists almost of "APT configuration space";
however there are (at least one: C<quiet>) undocumented items too.

=item Filename

I<(up/down-stream)>
The 2nd of two most used fields.
Designates the target for request.
It's local FS path and doesn't need and bear scheme.
Some methods seem to ignore it completely.
B<File::AptFetch> sets it anyway.

=item Last-Modified

I<(upstream)>
That's a time stamp of I<$message{uri}> mentioned file.
I can't comment what would be returned
if the time can't be retrieved before fetching.
The time is in F<RFC1123> format.
(I'm puzzled.
F<RFC1123> really touches in section I<5.2.14>
("RFC-822 Date and Time Specification: RFC-822 Section 5") date-n-time spec.
It covers Y2K and timezone issues.
It doesn't set the actual format.
So, I believe, that the format is in F<RFC822> format
with F<RFC1123> comments applied.
I don't know why it's this way.)
Meanwhile, B<File::AptFetch> exports returned value
(via I<$message{last_modified}>),
while provides no means for time checks.
(However, refering to symlink maintanance issue --
is mtime checks duty of method or application?)

=item MD5-Hash

I<(upstream)>
The obvious MD5sum of already fetched file.

=item MD5Sum-Hash

I<(upstream)>
I<(undocumented)>
Obvious.
I can't comment why it duplicates I<MD5-Hash>.

=item Message

I<(upstream)>
Some piece of diagnostic.
Comes with error messages (C<400> etc).

=item Send-Config

I<(upstream)>
Comes with C<100>.
But the config is sent anyway.
Probably the remains of old ages.
Or does it mean that such field can come asynchronously?

=item SHA1-Hash

=item SHA256-Hash

I<(upstream)>
I<(undocumented)>
Obvious.

=item Single-Instance

I<(upstream)>
Comes with C<100>, requires the method to be set up once.
I can't comment what would happen if that requirement is violated.

=item Size

I<(upstream)>
That's obvious -- the size of the source.
I can't comment what whould be returned
if the size can't be retrieved before fetching.

=item URI

I<(up/down-stream)>
The 1st of two most used fields.
Designates the source for request.
This field's value must start with B<scheme>;
OTOH, this scheme must exactly match the method name.
Otherwise the method denies such URI.
In this release, B<File::AptFetch> prepends requested URI
with scheme unconditionally.
That will be relaxed in the next release, meanwhile please strip.

=item Version

I<(upstream)>
Is the value always C<1.0>?

=item Index-File

That's missing in documentation.

=item Drive

=item Fail

=item IMS-Hit

=item Local

=item Media

=item Needs-Cleanup

=item Password

=item Pipeline

=item Resume-Point

=item Site

=item User

Those are fields I didn't meet yet.

=back

=head1 PROTOCOL

The conversation between application ("APP") and method ("MTD") is like that:

=over

=item APP <- "100 Capabilities" <- MTD

=item APP -> "601 Configuration" -> MTD

Those are the very first messages.
Both are required.
I don't think that C<601> could be sent before C<100> is received
(that C<100> states the method is up and running).
So does B<File::AptFetch>.

=item APP -> "600 URI Acquire" -> MTD

Requests should be filed as quick as possible (in sense --- with least pause).
I suppose, that application shouldn't wait for responces.
So here is no cycle actually.
File requests when you need something, and then check for completion.

=item APP <- "200 URI Start" <- MTD

That seems to be purely informational.
(I<v0.0.8>)
In fact that's not.
If connection is ruined somehow (monkey-wrench on ISP?)
then the method restarts transfer and manifests this this way.
Once, I've seen it five times in row.
I can't comment if method would give up ever.

=item APP <- "201 URI Done" <- MTD

=item APP <- "400 URI Failure" <- MTD

Only one of them should be sent (that's obvious).
Either marks completion of request -- successful or not.

=back

=head1 METHOD SPECIFIC NOTES

# FIXME: verification needed

One common behaviour should be mentioned.
Methods are either local or remote (that's obvious).
Local methods require I<scheme-specific-part> in I<$message{uri}> to be
absoulte and it must start with lone slash (C<'/'>).
Remote methods require double-slash (C<'//'>).
Either would complain otherwise.

And one more, that seems all I<$message{message}>s bear trailing space.
I've better note, in case I would ever spot one that doesn't.

And one piece of advice.
Don't mess with methods.
They ain't forgetting.
And they ain't forgiving.

=head2 copy:

Local.
It's marked as C<(internal)> in doucmentation.
I can't comment if that means that you can't have C<copy:> scheme in your
F<sources.list>.
When succeedes than returns a set of usual informational B<message headers>
(hashes, mtime, size).

Another issue (that, BTW, clearly shows B<File::AptFetch> bad state) if is it
possible to create symlinks with F<copy> method.
There is an APT configuration parameter I<Acquire::Source-Symlinks>.
I suppose it's set to C<true> by default, although it's missing in
C<apt-config dump> output.
I can't comment what exactly it affects
(C<apt-get>, F<copy> method, or F<file> method).
Maybe there's an undocumented B<message header> that would force F<copy> method
to symlink instead of copying.
However, if there's such header, then B<File::AptFetch> should have means
substitute right value for I<Acquire::Source-Symlinks> parameter
(or whatever else).
Right now it doesn't.

Known Easter Eggs are:

=over

=item *

Inspite of Etch'es (S<v. 0.6.46.4-0.1>) and Lenny's (S<v. 0.7.20>) APT packages
both have a F<copy> method, and they both set I<$message{version}> to C<1.0> in
C<100>, Etch'es version doesn't return hashes at all.
Beware.

=item *

It does B<not> reget!
If a source and a target match, then it will silently truncate the source.
Then happily return size and hashes of now gone file.
(That's for pre-Squeeze's APT.)

=item *

(I<v0.0.9>)
It does B<not> reget!
However, the Squeeze's B<copy> B<unlinks> the target now.
Thus it needs write permissions in the targt directory.
The permissions of just created target are affected by umask.
And, as you already guessed, I<$message{version}> is C<1.0> still.
(OMG, THAT'S RACE!)

=item *

While I<$message{uri}> should be absolute, the I<$message{filename}> can be
relative.

=item *

However, if I<$message{uri}> isn't absolute, then the message is
(in contrary with F<file> method)

    Failed to stat - stat (2 No such file or directory) 

(Does it really stats F<->?
Should check it.)

=item *

However, if I<$message{uri}> starts with double-slash,
then the I<$message{message}> is the same "S<Failed to stat...>".

=item *

And I<$message{filename}> can start with double-slash.

=item *

(I<v0.1.6>, C<Wheezy>)
After target's transfer is finished, the method reads it back.
I can't comment if it's for hash-set calculation or verification.
If target's media is slow then it's huge performance loss.

=back

=head2 file:

Local.
It doesn't fetch anything (kind of fake method).
It provides usual set of properties for I<$message{uri}> --
hash-sums, mtime, size.

Known Easter Eggs are:

=over

=item *

Etch'es version of APT method doesn't returns I<$message{md5sum_hash}> and
I<$message{sha256_hash}>.
Since both (Etch'es and Lenny's) versions has I<$message{version}> set to
C<1.0> off C<100> message, you can find out what version of APT you have only
by experimenting.

=item *

If I<$message{uri}> is unabsolute, then the I<$message{message}> is

    Invalid URI, local URIS must not start with // 

=item *

If I<$message{uri}> has permissions set a way that prohibits read access,
then the method surprisingly succeedes, but hashes are of empty file (puzled).

=item *

I<(v0.1.2)>
EE that laid just here is retracted.
At the time the TS was lazy (units reused samples).
At present I can't verify what was going on in pre-Squeeze.

=item *

I<(v0.1.2)>
(Following route sounds really weird and there isn't RL scenario when it could
possibly happen but that's the way it is.)
As of Wheezy:
(1) file F<foo> isn't readable;
(2) access it;
(3) access all right file F<bar> with leading double-slash URI.
Then get this I<$message{message}>:

    Could not open file %s - open (13 Permission denied) 

But!
It refers F<foo>;
while I<$message{uri}> refers F<bar>.
F<t/file/fail.t> (around C<ftagaab5>) really does it.
No comments.

=item *

If I<$message{message}> points to a directory,
then the behaviour is the same as for the unreadable file.
Even if I<scheme-specific-part> ends with slash.
However, if there are leading double-slash in I<$message{message}>,
then the method complains about invalid URI.

=back

=head2 networking methods

Thorough testing of networking methods pending.
However, I'm pretty sure that default timeout of 2min is enough for now.
Methods itself seem to timeout itself within that frame.
However, I can't say how many times networking methods would reconnect before
giving up.

=head1 SEE ALSO

L<File::AptFetch>,
S<"APT Method Itnerface"> in B<libapt-pkg-doc> package,
B<apt-config(1)>

=head1 AUTHOR

Eric Pozharski, <whynot@cpan.org>

=head1 COPYRIGHT & LICENSE

Copyright 2009, 2010, 2014 by Eric Pozharski

This overview is free in sense: AS-IS, NO-WARANRTY, HOPE-TO-BE-USEFUL.
This overview is released under CC-SA.

=cut

1;

