On Sun, Jun 18, 2017 at 11:38:45AM +0200, Ricardo Wurmus wrote:
> 
> Ludovic Courtès <ludo@gnu.org> writes:
> 
> > BTW, should --only-substitutes filter out packages without a substitute,
> > or should it simply stop and report the list of missing substitutes
> > (after which the user could use --do-not-upgrade)?

I like making it return a list on stdout so it can be composed as
suggested.

> In my opinion “--only-substitutes” should stop and report a list.
> If it continued without complaining there could be problems:
> 
> * partial upgrades could leave the profile in an unusable state
> 
> * an attacker could use this to trick a user into thinking that they
>   have all available updates
> 
> On the other hand, it would make “--only-substitutes” less usable,
> because to actually perform work one would have to deal with the failure
> case.
> 
> I suppose it could download the substitutes but not build a new profile
> and report an error at that point.

Perhaps there could be an additional flag --partial-upgrade to make it
build a new profile.

I understand why people want --only-substitutes but I'm a bit wary of it
for the reasons you gave, and I think we should solve their complaint by
improving our build infrastructure.