Re: [PATCH] configure: Check bzip2 is available

From: Eric Blake <eblake@redhat.com>
To: "Laszlo Ersek" <lersek@redhat.com>,
	"Philippe Mathieu-Daudé" <philmd@redhat.com>,
	qemu-devel@nongnu.org
Cc: qemu-trivial@nongnu.org, Thomas Huth <thuth@redhat.com>,
	"Daniel P . Berrange" <berrange@redhat.com>,
	Wainer dos Santos Moschetta <wainersm@redhat.com>
Subject: Re: [PATCH] configure: Check bzip2 is available
Date: Fri, 8 Nov 2019 09:43:42 -0600	[thread overview]
Message-ID: <00cca0a5-7a51-f2d1-5120-821c335954b8@redhat.com> (raw)
In-Reply-To: <862eb773-609d-4250-b46b-d922fc5a86a7@redhat.com>

On 11/8/19 5:01 AM, Laszlo Ersek wrote:

>>
>> Add a check in ./configure to warn the user if bzip2 is missing.
> 
> We've come full circle. Let me explain:
> 
>>
>> Fixes: 536d2173b2b
> 
> So this makes me kinda grumpy. If you look at the v3 posting of the patch that would later become commit 536d2173b2b:
> 
>    http://mid.mail-archive.com/20190321113408.19929-8-lersek@redhat.com
> 
> you see the following note in the changelog:
> 
>      - compress FD files with bzip2 rather than xz, so that decompression at
>        "make install" time succeed on older build OSes too [Peter]
> 
> So I couldn't use xz because that was "too new" for some build OSes, but now we also can't take bzip2 for granted because that's "too old" for some other build OSes? This is ridiculous.
> 
> To be clear, my disagreement is only with the "Fixes" tag. For me, "Fixes" stands for something that, in retrospect, can be proven to have been a bug at the time the code was *originally* committed. But, at the time, taking "bzip2" for granted was *not* a bug. The conditions / circumstances have changed more recently, and the assumption about bzip2 has been invalidated *after* adding a dependency on bzip2.
> 
> Nonetheless, thank you for adapting the code to the potential absence of bzip2. Can you perhaps go in some details in the commit message, near "not included in default installations" and "freshly installed systems"? If we can, we should identify the exact distro release where this problem has been encountered (and I wouldn't mind a link to the BZ or ticket under which people agreed to remove bzip2 from the default package set).

bzip2 is no longer a favored compression.  If we are trying to pick a 
compression that is most likely to be present on any system, go with 
gzip.  If we are trying to pick a compression that packs tighter and 
uncompresses faster, pick xz or zstd.  But bzip2 does neither: it packs 
slightly tighter than gzip but has slower performance in doing so, and 
thus is no longer used as a default compression.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org