guix graph: Add '--path'.

* guix/scripts/graph.scm (display-path): New procedure.
(%options, show-help): Add '--path'.
(guix-graph): Handle it.
* tests/guix-graph.sh: Add tests.
* doc/guix.texi (Invoking guix graph): Document it.
(Invoking guix size): Mention it.

doc/guix.texi

@@ -10021,6 +10021,12 @@ In this example we see that the combination of the four packages takes
 102.3@tie{}MiB in total, which is much less than the sum of each closure
 since they have a lot of dependencies in common.
 
+When looking at the profile returned by @command{guix size}, you may
+find yourself wondering why a given package shows up in the profile at
+all.  To understand it, you can use @command{guix graph --path -t
+references} to display the shortest path between the two packages
+(@pxref{Invoking guix graph}).
+
 The available options are:
 
 @table @option
@@ -10081,8 +10087,9 @@ directly to the @command{dot} command of Graphviz.  It can also emit an
 HTML page with embedded JavaScript code to display a ``chord diagram''
 in a Web browser, using the @uref{https://d3js.org/, d3.js} library, or
 emit Cypher queries to construct a graph in a graph database supporting
-the @uref{https://www.opencypher.org/, openCypher} query language.
+the @uref{https://www.opencypher.org/, openCypher} query language.  With
-The general syntax is:
+@option{--path}, it simply displays the shortest path between two
+packages.  The general syntax is:
 
 @example
 guix graph @var{options} @var{package}@dots{}
@@ -10235,6 +10242,29 @@ collected.
 
 @end table
 
+@cindex shortest path, between packages
+Often, the graph of the package you are interested in does not fit on
+your screen, and anyway all you want to know is @emph{why} that package
+actually depends on some seemingly unrelated package.  The
+@option{--path} option instructs @command{guix graph} to display the
+shortest path between two packages (or derivations, or store items,
+etc.):
+
+@example
+$ guix graph --path emacs libunistring
+emacs@@26.3
+mailutils@@3.9
+libunistring@@0.9.10
+$ guix graph --path -t derivation emacs libunistring
+/gnu/store/@dots{}-emacs-26.3.drv
+/gnu/store/@dots{}-mailutils-3.9.drv
+/gnu/store/@dots{}-libunistring-0.9.10.drv
+$ guix graph --path -t references emacs libunistring
+/gnu/store/@dots{}-emacs-26.3
+/gnu/store/@dots{}-libidn2-2.2.0
+/gnu/store/@dots{}-libunistring-0.9.10
+@end example
+
 The available options are the following:
 
 @table @option
@@ -10255,6 +10285,20 @@ List the supported graph backends.
 
 Currently, the available backends are Graphviz and d3.js.
 
+@item --path
+Display the shortest path between two nodes of the type specified by
+@option{--type}.  The example below shows the shortest path between
+@code{libreoffice} and @code{llvm} according to the references of
+@code{libreoffice}:
+
+@example
+$ guix graph --path -t references libreoffice llvm
+/gnu/store/@dots{}-libreoffice-6.4.2.2
+/gnu/store/@dots{}-libepoxy-1.5.4
+/gnu/store/@dots{}-mesa-19.3.4
+/gnu/store/@dots{}-llvm-9.0.1
+@end example
+
 @item --expression=@var{expr}
 @itemx -e @var{expr}
 Consider the package @var{expr} evaluates to.

guix/scripts/graph.scm

@@ -455,6 +455,29 @@ package modules, while attempting to retain user package modules."
                       (graph-backend-description backend)))
             %graph-backends))
 
+
+;;;
+;;; Displaying a path.
+;;;
+
+(define (display-path node1 node2 type)
+  "Display the shortest path from NODE1 to NODE2, of TYPE."
+  (mlet %store-monad ((path (shortest-path node1 node2 type)))
+    (define node-label
+      (let ((label (node-type-label type)))
+        ;; Special-case derivations and store items to print them in full,
+        ;; contrary to what their 'node-type-label' normally does.
+        (match-lambda
+          ((? derivation? drv) (derivation-file-name drv))
+          ((? string? str) str)
+          (node (label node)))))
+
+    (if path
+        (format #t "~{~a~%~}" (map node-label path))
+        (leave (G_ "no path from '~a' to '~a'~%")
+               (node-label node1) (node-label node2)))
+    (return #t)))
+
 
 ;;;
 ;;; Command-line options.
@@ -465,6 +488,9 @@ package modules, while attempting to retain user package modules."
                  (lambda (opt name arg result)
                    (alist-cons 'node-type (lookup-node-type arg)
                                result)))
+         (option '("path") #f #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'path? #t result)))
          (option '("list-types") #f #f
                  (lambda (opt name arg result)
                    (list-node-types)
@@ -510,6 +536,8 @@ Emit a representation of the dependency graph of PACKAGE...\n"))
   -t, --type=TYPE        represent nodes of the given TYPE"))
   (display (G_ "
       --list-types       list the available graph types"))
+  (display (G_ "
+      --path             display the shortest path between the given nodes"))
   (display (G_ "
   -e, --expression=EXPR  consider the package EXPR evaluates to"))
   (display (G_ "
@@ -566,11 +594,19 @@ Emit a representation of the dependency graph of PACKAGE...\n"))
           (mlet %store-monad ((_     (set-grafting #f))
                               (nodes (mapm %store-monad
                                            (node-type-convert type)
-                                           items)))
+                                           (reverse items))))
-            (export-graph (concatenate nodes)
+            (if (assoc-ref opts 'path?)
-                          (current-output-port)
+                (match nodes
-                          #:node-type type
+                  (((node1 _ ...) (node2 _ ...))
-                          #:backend backend))
+                   (display-path node1 node2 type))
+                  (_
+                   (leave (G_ "'--path' option requires exactly two \
+nodes (given ~a)~%")
+                          (length nodes))))
+                (export-graph (concatenate nodes)
+                              (current-output-port)
+                              #:node-type type
+                              #:backend backend)))
           #:system (assq-ref opts 'system)))))
   #t)
 

tests/guix-graph.sh

@@ -1,5 +1,5 @@
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2015, 2016, 2019 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2015, 2016, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 # Copyright © 2019 Simon Tournier <zimon.toutoune@gmail.com>
 #
 # This file is part of GNU Guix.
@@ -82,3 +82,17 @@ then false; else true; fi
 
 # Try --load-path
 guix graph -L $module_dir dummy | grep 'label = "dummy'
+
+# Displaying shortest paths (or lack thereof).
+if guix graph --path emacs vim; then false; else true; fi
+
+path="\
+emacs
+gnutls
+guile
+libffi"
+test "`guix graph --path emacs libffi | cut -d '@' -f1`" = "$path"
+
+# At the derivation level, there's a direct path because libffi is propagated
+# via gtk+.
+test "`guix graph --path -t derivation emacs libffi | wc -l`" -ge 2