bug#53224: Cookbook recipe about profile collisions

[zimoun <zimon.toutoune@gmail.com>]

Hi,

On Fri, 14 Jan 2022 at 10:02, Ludovic Courtès <ludo@gnu.org> wrote:

> Currently, on profile collisions, the error message shows where the
> collision originates and a hint on how to work around it.  Perhaps the
> hint is sometimes wrong (in which cases?), or perhaps it’s too terse?
> Can it be improved?

I do not know how it could be improved since it is often case per
case.  For instance, recently we had a collision for python-numpy and
the fix was to introduce python-numpy-next.  That's said, to me the
origin of the collision is often enough for resolving.  However,
indeed the user must have a clear understanding of Guix profile and
propagation.  Therefore, an entry to the Cookbook explaining what does
it mean and where to look appears to me a good idea.


> The definition of what a profile is is another topic.  Currently the
> term “profile” is defined in “Getting Started”:
>
>   https://guix.gnu.org/manual/en/html_node/Getting-Started.html#index-profile
>
> It’s very much defined in passing though.  How can we improve on that?
>
> In some early talks we had illustrations of the symlink forest of a
> profile borrowed from Eelco Dolstra’s own talks on the matter; see for
> instance p. 17 of <https://guix.gnu.org/guix-fosdem-20140201.pdf>.  I
> stopped using it because I think those symlinks are an implementation
> detail and it doesn’t help to focus on symlinks (and hashes and all
> that) when giving an overview of the tool.  Now, perhaps that
> illustration could be useful in the manual.

Well, I do not want to drift the initial message.  I am just keeping
the rolling ball. :-)

In the tutorial I am preparing for JRES, I am doing this way, as a
dialogue (below).  Basically, it is how I am somehow explaining to the
few users I have here, with various background.  This cannot go to the
manual but maybe it could be useful or worth for the Cookbook.

To me, what is an implementation detail is all the dance with /var/,
the generations, the garbage collection, etc.  But the fact that a
profile is symlink pointing a directory with the structure of /usr/
and containing all the environment variables is not an implementation
detail.  It appears to me a core concept to switch from traditional
distro with only one /usr/ to Guix and how to compose many /usr/.

Alice: Where are my packages?
Bob: Installing a package ends with this hint ...snippet...
Alice: Ah, I am confused
Bob: Well, $HOME/.guix-profile is thus the default profile
Alice: What does it mean?
Bob: Look at guix package --search-paths -p $HOME/.guix-profile
Alice: I see, but what is this $HOME/.guix-profile
Bob: Look at: file $HOME/.guix-profile
Alice: Ah, right.  My conclusion is:
  - a profile contains the environment variables
  - a profile is a symlink
Bob: Correct.
Alice: But a symlink pointing to where?
Bob: Look at: readlink -f $HOME/.guix-profile
Alice: It points to a store item, but what does it mean?
Bob: Look at: file $(readlink -f $HOME/.guix-profile)
Alice: Ah right.  My conclusion is:
  - a profile points to a directory item in the store
Bob: Correct.
Alice: What is inside this directory
Bob: Look at: ls $(readlink -f $HOME/.guix-profile)
Alice: Oh, it looks the same hierarchy as the usual and well-know /usr/
Bob: Correct.
Alice: What does it happen when I type python3 I just installed
Bob: Look at: which python3
Alice: Ah right.  Is the binary really in the directory item above?
Bob: Look at: readlink $(which python3)
Alice: Oh right.  A profile points to store item which points to other
store items.
Bob: Correct.
Alice: I guess these other store items have also an /usr/ hierarchy, right?
Bob: Correct.  Do you know how to check that?
Alice: ls $(readlink $(which python3))
Bob: Correct.
Alice: I summarize.  A profile is:
  - a symlink
  - which points to a store item
  - which points to many other store items
  - and all have the structure hierarchy of /usr/
Woo, it is a forest of symlinks!
... image ...


Cheers,
simon