Support returning build information by output.

Being able to take a derivation and query the build information is useful, but
in cases where there are multiple derivations that produce the same outputs,
the probability of getting the data back from Cuirass is reduced.

This is because Cuirass might not have build the exact derivation you have,
but a different derivation that produces the same outputs (this can commonly
happen when a related fixed output derivation changes).

Cuirass doesn't store derivations if they produce the same outputs as a
derivation it already knows about, so it can't determine if this is the
case. Therefore, provide a way of querying build results by output, rather
than derivation.

The motivation behind this is to make it easier to import build information in
to the Guix Data Service.

* src/cuirass/database.scm (db-get-output): New procedure.
* src/cuirass/http.scm (respond-output-not-found): New procedure.
(request-path-components): Handle /output/… requests.
* doc/cuirass.texi (Build information): Mention that you can get build
information by output.

doc/cuirass.texi

@@ -477,6 +477,10 @@ It is possible to query Cuirass web server for build informations. The
 dedicated API is "/build/@var{build-id}" where @var{build-id} is the
 unique id associated to the build in database.
 
+The build information can also be queried by output. For example,
+@samp{/output/kg9mirg6xbvzcp0a98v7326n1nvvwgsj-hello-2.10} will return
+the details of the output, along with the build if available.
+
 For instance, querying a local Cuirass web server can be done with
 @code{curl} and @code{jq} to format the JSON response :
 

src/cuirass/database.scm

@@ -47,6 +47,7 @@
             build-status
             db-add-build
             db-update-build-status!
+            db-get-output
             db-get-build
             db-get-builds
             db-get-builds-by-search
@@ -588,6 +589,20 @@ log file for DRV."
                             (#:event      . ,(assq-ref status-names
                                                        status)))))))))
 
+(define (db-get-output path)
+  "Retrieve the OUTPUT for PATH."
+  (with-db-critical-section db
+    ;; There isn't a unique index on path, but because Cuirass avoids adding
+    ;; derivations which introduce the same outputs, there should only be one
+    ;; result.
+    (match (sqlite-exec db "SELECT derivation, name FROM Outputs
+WHERE path =" path "
+LIMIT 1;")
+      (() #f)
+      ((#(derivation name))
+       `((#:derivation . ,derivation)
+         (#:name . ,name))))))
+
 (define (db-get-outputs derivation)
   "Retrieve the OUTPUTS of the build identified by DERIVATION in the
 database."

src/cuirass/http.scm

@@ -226,6 +226,11 @@ Hydra format."
      404
      (format #f "Build with ID ~a doesn't exist." build-id)))
 
+  (define (respond-output-not-found output-id)
+    (respond-json-with-error
+     404
+     (format #f "Output with ID ~a doesn't exist." output-id)))
+
   (define (respond-html-eval-not-found eval-id)
     (respond-html
      (html-page "Page not found"
@@ -331,6 +336,16 @@ Hydra format."
              (#f
               (respond-build-not-found build-id)))
            (respond-build-not-found build-id))))
+    (('GET "output" id)
+     (let ((output (db-get-output
+                    (string-append (%store-prefix) "/" id))))
+       (if output
+           (let ((build (db-get-build (assq-ref output #:derivation))))
+             (respond-json
+              (object->json-string
+               (append output
+                       `((#:build . ,(or build #nil)))))))
+           (respond-output-not-found id))))
     (('GET "api" "evaluations")
      (let* ((params (request-parameters request))
             ;; 'nr parameter is mandatory to limit query size.