Re: Extracting a reachability path out of a (package) DAG

[Nils Gillmann <ng0@n0.is>]

Björn Höfling transcribed 74K bytes:
> Hi Guix,
> 
> we have this nice `guix graph` tool which outputs DAGs of the packages
> (or even derivations,bags, ...). This is cool if you look at simple
> packages like the "hello" package with little to no dependencies. If
> you look at "real" packages like qt or maven the information is just
> overwhelming and you are scrolling around the image for just getting a
> headache. 
> 
> Often the only think I want to know is something like: Why is "goodbye"
> a dependency of "hello"?
> 
> To answer this, I want to extract from hello's package graph the path
> (more precisely: the sub-DAG) leading from the root "hello" to the
> target node (or even nodes) "goodbye".
> 
> After several attempts and failures, I wrote a script for gvpr from
> the GraphViz suite that does the job.
> 
> Example 1: In bug #30710 Hartmut Goebel asked why qt depends on two
> different autoconf-wrapper packages. To answer that, you can find out
> the two node names from the .dot file and then call:
> 
> gvpr -f markpath.g -a "ex 64168128 64167936" < qt-thing/qt.package.dot
> >qt-acw.dot
> 
> This extracts (ex) the path (sub-DAG) to the two seed nodes and outputs
> it in a new graph. This result is quite compact with only 12 nodes (attached).
> 
> Example 2: How/Why is glib a dependency of maven? The extracted graph
> has about 50 nodes, so I don't attach it here. You will see that
> java-logback-classic depends on a groovy-cluster that finally mounts
> into antlr. That depends on a gtk/pango/cairo-cluster that finally
> sinks into glib.
> 
> Hope that is useful to someone else,
> 
> Björn
> 
> 

To add to this idea:
abyayala$ nix why-depends --help
Usage: nix why-depends <FLAGS>... <PACKAGE> <DEPENDENCY>

Summary: show why a package has another package in its closure.

Flags:
  -a, --all                     show all edges in the dependency graph leading from 'package' to 'dependency', rather than just a shortest path
  --arg <NAME> <EXPR>       argument to be passed to Nix functions
  --argstr <NAME> <STRING>  string-valued argument to be passed to Nix functions
  -f, --file <FILE>             evaluate FILE rather than the default
  -I, --include <PATH>          add a path to the list of locations used to look up <...> file names

Examples:

  To show one path through the dependency graph leading from Hello to Glibc:
    $ nix why-depends nixpkgs.hello nixpkgs.glibc

  To show all files and paths in the dependency graph leading from Thunderbird to libX11:
    $ nix why-depends --all nixpkgs.thunderbird nixpkgs.xorg.libX11

  To show why Glibc depends on itself:
    $ nix why-depends nixpkgs.glibc nixpkgs.glibc

Note: this program is EXPERIMENTAL and subject to change.



Would it be useful to have a guix package subcommand which replicates this?