Re-write insert-derivation-inputs in a more memory efficient manor

Previously it would compute a long list of strings, potentially more than
100,000 elements long, then split this string up and insert it in chunks. Only
then could memory be freed.

This new approach builds the strings in batches for the insertion query, then
moves on to the next batch. This should mean that more memory can be freed and
reused along the way.

guix-data-service/model/derivation.scm

@@ -1329,48 +1329,51 @@ WHERE derivation_source_files.store_path = $1"
         #f)))
 
 (define (insert-derivation-inputs conn derivation-ids derivations)
-  (let ((data
-         (append-map
-          (lambda (derivation-id derivation)
-            (append-map
-             (match-lambda
-               (($ <derivation-input> derivation-or-path sub-derivations)
-                (let ((path
-                       (match derivation-or-path
-                         ((? derivation? d)
-                          ;; The first field changed to a derivation (from the file
-                          ;; name) in 5cf4b26d52bcea382d98fb4becce89be9ee37b55
-                          (derivation-file-name d))
-                         ((? string? s)
-                          s))))
-                  (map (lambda (sub-derivation)
-                         (string-append "("
-                                        (number->string derivation-id)
-                                        ", '" path
-                                        "', '" sub-derivation "')"))
-                       sub-derivations))))
-             (derivation-inputs derivation)))
-          derivation-ids
-          derivations)))
+  (define (process-chunk derivation-ids derivations)
+    (let ((query-parts
+           (append-map!
+            (lambda (derivation-id derivation)
+              (append-map!
+               (match-lambda
+                 (($ <derivation-input> derivation-or-path sub-derivations)
+                  (let ((path
+                         (match derivation-or-path
+                           ((? derivation? d)
+                            ;; The first field changed to a derivation (from the file
+                            ;; name) in 5cf4b26d52bcea382d98fb4becce89be9ee37b55
+                            (derivation-file-name d))
+                           ((? string? s)
+                            s))))
+                    (map (lambda (sub-derivation)
+                           (string-append "("
+                                          (number->string derivation-id)
+                                          ", '" path
+                                          "', '" sub-derivation "')"))
+                         sub-derivations))))
+               (derivation-inputs derivation)))
+            derivation-ids
+            derivations)))
 
-    (unless (null? data)
-      (for-each
-       (lambda (chunk)
-         (exec-query
-          conn
-          (string-append
-           "
+      (unless (null? query-parts)
+        (exec-query
+         conn
+         (string-append
+          "
 INSERT INTO derivation_inputs (derivation_id, derivation_output_id)
 SELECT vals.derivation_id, derivation_outputs.id
 FROM (VALUES "
-           (string-join chunk ", ")
-           ") AS vals (derivation_id, file_name, output_name)
+          (string-join query-parts ", ")
+          ") AS vals (derivation_id, file_name, output_name)
 INNER JOIN derivations
   ON derivations.file_name = vals.file_name
 INNER JOIN derivation_outputs
   ON derivation_outputs.derivation_id = derivations.id
- AND vals.output_name = derivation_outputs.name")))
-       (chunk! data 1000)))))
+ AND vals.output_name = derivation_outputs.name")))))
+
+  (chunk-map! process-chunk
+              1000
+              (list-copy derivation-ids)
+              (list-copy derivations)))
 
 (define (select-from-derivation-source-files store-paths)
   (string-append