bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Francesco Potortì]

While tar handles compressed archives without regard to their file name,
Emacs cannot handle a compressed tar archive unless its file name
suggests the compression type.  To reproduce:

$ tar -Jcf test.tar any.file.name	# create a compressed tar file
$ tar -tvf test.tar			# verify that tar can read it
$ cp test.tar test.tar.xz		# create a copy

If you find the files test.tar and test.tar.gz from Emacs, it will
recognise the contents of the second file, but not those of the first.

By looking at the comments on top of tar-mode.el, I get that it does not
rely on tar to get the archive listing, but reads the archive itself, so
correcting this problem does not appear straightforward to me...


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bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Lars Ingebrigtsen]

Francesco Potortì <pot@gnu.org> writes:

> By looking at the comments on top of tar-mode.el, I get that it does not
> rely on tar to get the archive listing, but reads the archive itself, so
> correcting this problem does not appear straightforward to me...

(I'm going through old bug reports that unfortunately weren't resolved
at the time.)

If Emacs had code to automatically recognise compressed data, I guess
tar-mode could decompress the buffer before it started working on it.
(Does Emacs have that?  I guess we could add stuff to
`magic-fallback-mode-alist' or something in that region...)

But I think the question is -- do we want to support this?  I'm not
quite sure -- it would be pretty unusual for a mode to do something like
this, and it's not clear what the semantics should be.  That is, if
we're saving the tar buffer afterwards, should it be compressed or not?
Either option would surprise somebody.

And finally -- is this a thing that actually exists in the wild?  I
don't think I've ever seen a compressed tar file that didn't have a name
that indicated how it was compressed.

So due to both the unclear semantics and that I doubt the utility, I'm
inclined to close this as a "wontfix".  Anybody got any opinions?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Eli Zaretskii]

> Cc: 25458@debbugs.gnu.org
> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Fri, 13 May 2022 16:12:24 +0200
> 
> Francesco Potortì <pot@gnu.org> writes:
> 
> > By looking at the comments on top of tar-mode.el, I get that it does not
> > rely on tar to get the archive listing, but reads the archive itself, so
> > correcting this problem does not appear straightforward to me...
> 
> (I'm going through old bug reports that unfortunately weren't resolved
> at the time.)
> 
> If Emacs had code to automatically recognise compressed data, I guess
> tar-mode could decompress the buffer before it started working on it.
> (Does Emacs have that?  I guess we could add stuff to
> `magic-fallback-mode-alist' or something in that region...)

But decompressing means, for most compression types, running an
external program on the file.  Or does OP suggest that we link Emacs
against libbz2, liblz4, liblzma, etc. to have these decompression
methods available in core?

bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Francesco Potortì]

I am not saying that it's worth the trouble, and anyway I would not have the time to help working on it, sorry.  However, here is some food for thought.

>> By looking at the comments on top of tar-mode.el, I get that it does not
>> rely on tar to get the archive listing, but reads the archive itself, so
>> correcting this problem does not appear straightforward to me...
>
>(I'm going through old bug reports that unfortunately weren't resolved
>at the time.)
>
>If Emacs had code to automatically recognise compressed data, I guess
>tar-mode could decompress the buffer before it started working on it.
>(Does Emacs have that?  I guess we could add stuff to
>`magic-fallback-mode-alist' or something in that region...)
>
>But I think the question is -- do we want to support this?  I'm not
>quite sure -- it would be pretty unusual for a mode to do something like
>this, and it's not clear what the semantics should be.  That is, if
>we're saving the tar buffer afterwards, should it be compressed or not?
>Either option would surprise somebody.

Not really.  If I read a compressed file, Emacs uncompresses it when loading it to a buffer, then you can edit it and save it.  Compression and decompression are transparent.  I use this feature quite often, and so I am led to expect it of compressed archives too.  If it was compressed to begin with, it should be saved compressed too.

>And finally -- is this a thing that actually exists in the wild?  I
>don't think I've ever seen a compressed tar file that didn't have a name
>that indicated how it was compressed.

Hm, maybe it's only me, but I routinely produce tar files without any compression suffix, as tar automatically does the right thing with compressed archives (it has done for many years now).  I think that adding compression suffixes to tar files is (should be?) a thing of the past.

>So due to both the unclear semantics and that I doubt the utility, I'm
>inclined to close this as a "wontfix".  Anybody got any opinions?

I definitely think that it would be useful, and the semantics is clear at least to me (I may miss things, obviously).  However, that needs someone implementing it who thinks the same...

-- fp

bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Francesco Potortì]

>> Cc: 25458@debbugs.gnu.org
>> From: Lars Ingebrigtsen <larsi@gnus.org>
>> Date: Fri, 13 May 2022 16:12:24 +0200
>> 
>> Francesco Potortì <pot@gnu.org> writes:
>> 
>> > By looking at the comments on top of tar-mode.el, I get that it does not
>> > rely on tar to get the archive listing, but reads the archive itself, so
>> > correcting this problem does not appear straightforward to me...
>> 
>> (I'm going through old bug reports that unfortunately weren't resolved
>> at the time.)
>> 
>> If Emacs had code to automatically recognise compressed data, I guess
>> tar-mode could decompress the buffer before it started working on it.
>> (Does Emacs have that?  I guess we could add stuff to
>> `magic-fallback-mode-alist' or something in that region...)
>
>But decompressing means, for most compression types, running an
>external program on the file.  Or does OP suggest that we link Emacs
>against libbz2, liblz4, liblzma, etc. to have these decompression
>methods available in core?

If Emacs parses the tar file itself, without relying on tar, in principle it should read compressed tar files too.  It could do that by using external compression tools, or more efficiently by natively handling compression.

However, I think that in general this requires uncompressing the whole tar file in memory or on disk, which is against one of the main purposes stated in the inital comments of tar-mode.el.

bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

> But decompressing means, for most compression types, running an
> external program on the file.  Or does OP suggest that we link Emacs
> against libbz2, liblz4, liblzma, etc. to have these decompression
> methods available in core?

No, it would work just like when you read a tar.gz file -- we use
external decompressors for that, too, even if we then parse the
resulting tar file ourselves. 

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Lars Ingebrigtsen]

Francesco Potortì <pot@gnu.org> writes:

>>But I think the question is -- do we want to support this?  I'm not
>>quite sure -- it would be pretty unusual for a mode to do something like
>>this, and it's not clear what the semantics should be.  That is, if
>>we're saving the tar buffer afterwards, should it be compressed or not?
>>Either option would surprise somebody.
>
> Not really.  If I read a compressed file, Emacs uncompresses it when
> loading it to a buffer, then you can edit it and save it.  Compression
> and decompression are transparent.  I use this feature quite often,
> and so I am led to expect it of compressed archives too.  If it was
> compressed to begin with, it should be saved compressed too.

But the file name is "foo.tar" -- there'd be no way to tell Emacs that
the user wants to save that uncompressed.

You can load bar.gz into a buffer, and write out bar, and Emacs won't
compress that.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: pot@gnu.org,  25458@debbugs.gnu.org
> Date: Fri, 13 May 2022 17:53:15 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > But decompressing means, for most compression types, running an
> > external program on the file.  Or does OP suggest that we link Emacs
> > against libbz2, liblz4, liblzma, etc. to have these decompression
> > methods available in core?
> 
> No, it would work just like when you read a tar.gz file -- we use
> external decompressors for that, too, even if we then parse the
> resulting tar file ourselves. 

Then I guess I don't understand what is the problem we are asked to
fix here.  Francesco, can you explain?

bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Francesco Potortì]

>>>But I think the question is -- do we want to support this?  I'm not
>>>quite sure -- it would be pretty unusual for a mode to do something like
>>>this, and it's not clear what the semantics should be.  That is, if
>>>we're saving the tar buffer afterwards, should it be compressed or not?
>>>Either option would surprise somebody.
>>
>> Not really.  If I read a compressed file, Emacs uncompresses it when
>> loading it to a buffer, then you can edit it and save it.  Compression
>> and decompression are transparent.  I use this feature quite often,
>> and so I am led to expect it of compressed archives too.  If it was
>> compressed to begin with, it should be saved compressed too.
>
>But the file name is "foo.tar" -- there'd be no way to tell Emacs that
>the user wants to save that uncompressed.

In fact, if the file was compressed, Emacs should save it compressed with the same method.

>You can load bar.gz into a buffer, and write out bar, and Emacs won't
>compress that.

- I load bar.gz
- Edit it
- C-x C-s saves it with the same name, compressed with the same method

Same with foo.tar.gz

It should be the same with foo.tar, if foo.tar was compressed

And no, there would be no way to tell Emacs to save foo.tar uncompressed.  I do not think that it would be important to be able to save it uncompressed.

bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Francesco Potortì]

>> From: Lars Ingebrigtsen <larsi@gnus.org>
>> Cc: pot@gnu.org,  25458@debbugs.gnu.org
>> Date: Fri, 13 May 2022 17:53:15 +0200
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> > But decompressing means, for most compression types, running an
>> > external program on the file.  Or does OP suggest that we link Emacs
>> > against libbz2, liblz4, liblzma, etc. to have these decompression
>> > methods available in core?
>> 
>> No, it would work just like when you read a tar.gz file -- we use
>> external decompressors for that, too, even if we then parse the
>> resulting tar file ourselves. 
>
>Then I guess I don't understand what is the problem we are asked to
>fix here.  Francesco, can you explain?

If I read foo.tar into Emacs, where foo.tar was created with

$ tar -Jcf foo.tar any.file.name

then Emacs does not recognise it as a tar file.  Instead, it should look at it, understand that it is a compressed tar file, uncompress and parse it.  If asked to save it after modification, it should compress it before saving, using the same compression method that was used in the first place.

Compression would be transparent to Emacs if it relied on Tar for parsing the tar file, because Tar manages compressed files transparently.  But since Emacs parses the tar file itself, it should behave as Tar does, that is, uncompress the file if it was compressed in the first place, and recompress when saving it after editing.

bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Lars Ingebrigtsen]

Francesco Potortì <pot@gnu.org> writes:

> And no, there would be no way to tell Emacs to save foo.tar
> uncompressed.  I do not think that it would be important to be able to
> save it uncompressed.

I understand that that's your use case.  I'm saying that it would be
surprising to many, and that that's something that others might well
want to.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Eli Zaretskii]

> From: Francesco Potortì <pot@gnu.org>
> Date: Sat, 14 May 2022 00:25:36 +0200
> Cc: 25458@debbugs.gnu.org,
> 	Lars Ingebrigtsen <larsi@gnus.org>
> 
> >Then I guess I don't understand what is the problem we are asked to
> >fix here.  Francesco, can you explain?
> 
> If I read foo.tar into Emacs, where foo.tar was created with
> 
> $ tar -Jcf foo.tar any.file.name
> 
> then Emacs does not recognise it as a tar file.  Instead, it should look at it, understand that it is a compressed tar file, uncompress and parse it.  If asked to save it after modification, it should compress it before saving, using the same compression method that was used in the first place.

You could do that by using magic-mode-alist, I guess?  It's just a
matter of writing the FUNCTION to put into the association for the
magic signatures of the compression methods you want to support.

FWIW, I don't think Emacs should do this by default, because looking
at the file's contents before looking at its file-name extension will
slow down visiting files, and having a .tar file that is actually a
.tar.xz file is quite unusual in the wild.  So this sounds like your
personal preference, and therefore customizing magic-mode-alist for
you sounds like TRT to fulfill your personal needs.

> Compression would be transparent to Emacs if it relied on Tar for parsing the tar file, because Tar manages compressed files transparently.  But since Emacs parses the tar file itself, it should behave as Tar does, that is, uncompress the file if it was compressed in the first place, and recompress when saving it after editing.

I don't think it's a good idea for Emacs to require Tar for visiting
archives.  For starters, Tar is not always available, and when it is
available, it doesn't always support all the compression methods
people could use.  We could use bsdtar (from libarchive), which
supports every archive format and compression method under the sun,
but its availability is even less than that of Tar.

So I think what we have now hits a good balance between functionality
in the popular use cases and reliability.

bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Francesco Potortì]

>Francesco Potortì <pot@gnu.org> writes:
>
>> And no, there would be no way to tell Emacs to save foo.tar
>> uncompressed.  I do not think that it would be important to be able to
>> save it uncompressed.
>
>I understand that that's your use case.  I'm saying that it would be
>surprising to many, and that that's something that others might well
>want to.

I'll try one last time to illustrate my point of view.

From Emacs:
1. I read foo.tar and I write it out as foo.tar (I don't care if it's compressed or not)
2. I read foo.tar.gz and write it out a foo.tar.gz
3. I read foo.tar.xx and write it out as foo.tar.yy, that is, with a different compression method
4. I read foo.tar.xx and write it out as foo.tar (I explicitly remove compression)
5. I read foo.tar and write it out as foo.tar.xx (I explicitly add compression)

In my opinion, 1 and 2 are by far the most common use cases.  My proposal is to make tar-mode.el to behave as tar does in case 1: it does not care about the name suffix, and it does the right thing whether the file is compressed or not, which has been true since 2008.

You are objecting that if I read foo.tar and it's compressed, I cannot write it out uncompressed using Emacs.  I think that this use case is definitely exotic.  Especially with respect with people wanting to just read tar files and writing them without worrying whether they are compressed or not.