Re: [gentoo-portage-dev] [PATCH] ebuild: set up bash compat levels

[Ulrich Mueller]

>>>>> On Fri, 13 Nov 2015, Michał Górny wrote:

> Ok, I recalled the issue I was particularly concerned about.

>   PV="1_beta2"
>   MY_PV="${PV/_/~}"

> This triggers different behavior between bash<=4.2 and bash>=4.3,
> with the latter requiring:

>   PV="1_beta2"
>   MY_PV="${PV/_/\~}"

> (which in turns breaks older). Now, if someone put a conditional on
> BASH_VERSINFO that worked around this discrepancy, introducing
> BASH_COMPAT suddenly breaks it by forcing the old behavior when
> BASH_VERSINFO indicates the new behavior is expected.

> Not saying this is very likely to happen.

Highly unlikely, IMHO. There are several workarounds that are simpler
than making the code conditional on the bash version. For example:

   tilde="~"; MY_PV="${PV/_/${tilde}}"

or:

   MY_PV="${PV/_/$'\x7e'}"

> However, we are clearly violating the idea behind PMS by allowing it
> to happen retroactively.

Not sure. PMS is (and always was) quite clear about which bash version
belongs to which EAPI, so ebuilds have no business of circumventing
this by doing their own version checks.

> However, enabling BASH_COMPAT in EAPI 6 is a good thing. For
> example, because people in EAPI 6 no longer need to be concerned
> about this discrepancy.

Enabling it for EAPI 6 only will lead to the paradoxical situation
that bash will be compatible to 4.2 in EAPI 6, as it should be.
Whereas earlier EAPIs (which should use 3.2 really) will get the
behaviour of bash 4.3 or later instead.

Ulrich