Better handle retries for jobs

This was broken when the guix_revisions entry started being added before the
final commit.

guix-data-service/jobs/load-new-guix-revision.scm

@@ -1316,8 +1316,21 @@ WHERE job_id = $1"
                                           store
                                           channel-for-commit
                                           fetch-with-authentication?)))
-    (let ((guix-revision-id
+
-           (insert-guix-revision conn git-repository-id commit)))
+    (with-time-logging
+        "acquiring advisory transaction lock: load-new-guix-revision-inserts"
+      ;; Wait until this is the only transaction inserting data, to avoid any
+      ;; concurrency issues
+      (obtain-advisory-transaction-lock conn
+                                        'load-new-guix-revision-inserts))
+    (let* ((existing-guix-revision-id
+            (git-repository-id-and-commit->revision-id conn
+                                                       git-repository-id
+                                                       commit))
+           (guix-revision-id
+            (or existing-guix-revision-id
+                (insert-guix-revision conn git-repository-id commit))))
+      (unless existing-guix-revision-id
         (insert-channel-instances conn
                                   guix-revision-id
                                   (filter-map
@@ -1328,8 +1341,7 @@ WHERE job_id = $1"
                                                   'manifest-entry-item)
                                        (lambda (drv)
                                          (cons system drv)))))
-                                 channel-derivations-by-system))
+                                   channel-derivations-by-system)))
-
       (simple-format
        (current-error-port)
        "guix-data-service: saving the channel instance derivations to the database\n")
@@ -1777,9 +1789,7 @@ SKIP LOCKED")
         (simple-format #t "Processing job ~A (commit: ~A, source: ~A)\n\n"
                        id commit source)
 
-        (if (or
+        (if (eq?
-             (guix-revision-exists? conn git-repository-id commit)
-             (eq?
              (with-time-logging (string-append "loading revision " commit)
                (setup-logging
                 id
@@ -1802,7 +1812,7 @@ SKIP LOCKED")
                                                      git-repository-id
                                                      commit))))))
                     #:unwind? #t))))
-              #t))
+             #t)
             (begin
               (record-job-succeeded conn id)
               (record-job-event conn id "success")

guix-data-service/model/guix-revision.scm

@@ -23,6 +23,7 @@
   #:export (count-guix-revisions
             most-recent-n-guix-revisions
             commit->revision-id
+            git-repository-id-and-commit->revision-id
             insert-guix-revision
             guix-commit-exists?
             guix-revision-exists?
@@ -46,6 +47,19 @@
      id)
     (() #f)))
 
+(define (git-repository-id-and-commit->revision-id conn git-repository-id commit)
+  (match (exec-query
+          conn
+          "
+SELECT id
+FROM guix_revisions
+WHERE commit = $1
+  AND git_repository_id = $2"
+          (list commit git-repository-id))
+    (((id))
+     id)
+    (() #f)))
+
 (define (insert-guix-revision conn git-repository-id commit)
   (define insert
     "