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

* guix/store.scm (hash-part->path): New procedure.
* tests/store.scm ("hash-part->path"): New test.

guix/store.scm

@@ -57,6 +57,7 @@
             set-build-options
             valid-path?
             query-path-hash
+            hash-part->path
             add-text-to-store
             add-to-store
             build-derivations
@@ -501,6 +502,18 @@ encoding conversion errors."
   "Return the SHA256 hash of PATH as a bytevector."
   base16)
 
+(define hash-part->path
+  (let ((query-path-from-hash-part
+         (operation (query-path-from-hash-part (string hash))
+                    #f
+                    store-path)))
+   (lambda (server hash-part)
+     "Return the store path whose hash part is HASH-PART (a nix-base32
+string).  Raise an error if no such path exists."
+     ;; This RPC is primarily used by Hydra to reply to HTTP GETs of
+     ;; /HASH.narinfo.
+     (query-path-from-hash-part server hash-part))))
+
 (define add-text-to-store
   ;; A memoizing version of `add-to-store', to avoid repeated RPCs with
   ;; the very same arguments during a given session.

tests/store.scm

@@ -87,7 +87,12 @@
               (%store-prefix)
               "/283gqy39v3g9dxjy26rynl0zls82fmcg-guile-2.0.7/bin/guile")))))
 
-(test-skip (if %store 0 10))
+(test-skip (if %store 0 11))
+
+(test-assert "hash-part->path"
+  (let ((p (add-text-to-store %store "hello" "hello, world")))
+    (equal? (hash-part->path %store (store-path-hash-part p))
+            p)))
 
 (test-assert "dead-paths"
   (let ((p (add-text-to-store %store "random-text" (random-text))))