Guard against nested transactions

Not sure how to do this in PostgreSQL, so use a parameter.

guix-data-service/database.scm

@@ -19,6 +19,7 @@
   #:use-module (system foreign)
   #:use-module (ice-9 match)
   #:use-module (ice-9 threads)
+  #:use-module (ice-9 exceptions)
   #:use-module (squee)
   #:use-module (prometheus)
   #:use-module (guix-data-service config)
@@ -31,6 +32,7 @@
             open-postgresql-connection
             close-postgresql-connection
 
+            %postgresql-in-transaction?
             with-postgresql-transaction
 
             check-test-database!
@@ -205,8 +207,16 @@
 (define %postgresql-connections-name
   (make-parameter #f))
 
+(define %postgresql-in-transaction?
+  (make-parameter #f))
+
 (define* (with-postgresql-transaction conn f
                                       #:key always-rollback?)
+  (when (%postgresql-in-transaction?)
+    (raise-exception
+     (make-exception-with-message
+      "nested transaction detected")))
+
   (exec-query conn "BEGIN;")
 
   (with-exception-handler
@@ -219,7 +229,10 @@
         ;; TODO Include the stack in the exception via knots
         (raise-exception exn))
     (lambda ()
-      (let ((result (f conn)))
+      (let ((result
+             (parameterize
+                 ((%postgresql-in-transaction? #t))
+               (f conn))))
         (exec-query conn (if always-rollback?
                              "ROLLBACK;"
                              "COMMIT;"))

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

@@ -2562,6 +2562,8 @@ SELECT store_path FROM derivation_source_files WHERE id = $1"
     (define guix-revision-id-promise
       (fibers-delay
        (lambda ()
+         (parameterize
+             ((%postgresql-in-transaction? #f))
            (retry-on-missing-store-item
             (lambda ()
               (let ((guix-source
@@ -2574,7 +2576,7 @@ SELECT store_path FROM derivation_source_files WHERE id = $1"
                                         channel-derivations-by-system)))
             #:on-exception
             (lambda ()
-            (fibers-promise-reset channel-derivations-by-system-promise))))))
+              (fibers-promise-reset channel-derivations-by-system-promise)))))))
 
     ;; Prompt getting the guix-revision-id as soon as possible
     (spawn-fiber
@@ -3032,6 +3034,9 @@ SKIP LOCKED")
     (make-channel))
 
   (define result
+    (parameterize
+        ;; Mimic the behaviour of with-postgresql-transaction
+        ((%postgresql-in-transaction? #t))
       (with-postgresql-connection
        (simple-format #f "load-new-guix-revision ~A" id)
        (lambda (conn)
@@ -3120,7 +3125,7 @@ SKIP LOCKED")
            (()
             (exec-query conn "ROLLBACK")
             (simple-format #t "job ~A not found to be processed\n"
-                         id))))))
+                           id)))))))
 
   (when result
     (fibers-parallel