store: Add `store-path-package-name'.

* guix/store.scm (store-path-package-name): New procedure.
* tests/utils.scm ("store-path-package-name"): New test.

guix/store.scm

@@ -29,6 +29,7 @@
   #:use-module (ice-9 match)
   #:use-module (ice-9 rdelim)
   #:use-module (ice-9 ftw)
+  #:use-module (ice-9 regex)
   #:export (nix-server?
             nix-server-major-version
             nix-server-minor-version
@@ -55,7 +56,8 @@
 
             %store-prefix
             store-path?
-            derivation-path?))
+            derivation-path?
+            store-path-package-name))
 
 (define %protocol-version #x10b)
 
@@ -446,3 +448,12 @@ file name.  Return #t on success."
 (define (derivation-path? path)
   "Return #t if PATH is a derivation path."
   (and (store-path? path) (string-suffix? ".drv" path)))
+
+(define (store-path-package-name path)
+  "Return the package name part of PATH, a file name in the store."
+  (define store-path-rx
+    (make-regexp (string-append "^.*" (regexp-quote (%store-prefix))
+                                "/[^-]+-(.+)$")))
+
+  (and=> (regexp-exec store-path-rx path)
+         (cut match:substring <> 1)))

tests/utils.scm

@@ -19,6 +19,7 @@
 
 (define-module (test-utils)
   #:use-module (guix utils)
+  #:use-module ((guix store) #:select (store-path-package-name))
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-11)
   #:use-module (srfi srfi-26)
@@ -162,6 +163,12 @@
           (match b (($ <foo> 1 2) #t))
           (equal? b c)))))
 
+;; This is actually in (guix store).
+(test-equal "store-path-package-name"
+  "bash-4.2-p24"
+  (store-path-package-name
+   "/nix/store/qvs2rj2ia5vci3wsdb7qvydrmacig4pg-bash-4.2-p24"))
+
 (test-end)