store: Add `store-path-hash-part'.

* guix/store.scm (store-path-hash-part): New procedure.
* tests/store.scm ("store-path-hash-part", "store-path-hash-part #f"):
  New tests.

guix/store.scm

@@ -83,7 +83,8 @@
             %store-prefix
             store-path?
             derivation-path?
-            store-path-package-name))
+            store-path-package-name
+            store-path-hash-part))
 
 (define %protocol-version #x10c)
 
@@ -751,3 +752,12 @@ collected, and the number of bytes freed."
 
   (and=> (regexp-exec store-path-rx path)
          (cut match:substring <> 1)))
+
+(define (store-path-hash-part path)
+  "Return the hash part of PATH as a base32 string, or #f if PATH is not a
+syntactically valid store path."
+  (let ((path-rx (make-regexp
+                  (string-append"^" (regexp-quote (%store-prefix))
+                                "/([0-9a-df-np-sv-z]{32})-[^/]+$"))))
+    (and=> (regexp-exec path-rx path)
+           (cut match:substring <> 1))))

tests/store.scm

@@ -48,6 +48,18 @@
 
 (test-begin "store")
 
+(test-equal "store-path-hash-part"
+  "283gqy39v3g9dxjy26rynl0zls82fmcg"
+  (store-path-hash-part
+   (string-append (%store-prefix)
+                  "/283gqy39v3g9dxjy26rynl0zls82fmcg-guile-2.0.7")))
+
+(test-equal "store-path-hash-part #f"
+  #f
+  (store-path-hash-part
+   (string-append (%store-prefix)
+                  "/foo/bar/283gqy39v3g9dxjy26rynl0zls82fmcg-guile-2.0.7")))
+
 (test-skip (if %store 0 10))
 
 (test-assert "dead-paths"