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

@@ -1329,10 +1329,11 @@ WHERE derivation_source_files.store_path = $1"
         #f)))
 
 (define (insert-derivation-inputs conn derivation-ids derivations)
-  (let ((data
-         (append-map
+  (define (process-chunk derivation-ids derivations)
+    (let ((query-parts
+           (append-map!
             (lambda (derivation-id derivation)
-            (append-map
+              (append-map!
                (match-lambda
                  (($ <derivation-input> derivation-or-path sub-derivations)
                   (let ((path
@@ -1353,9 +1354,7 @@ WHERE derivation_source_files.store_path = $1"
             derivation-ids
             derivations)))
 
-    (unless (null? data)
-      (for-each
-       (lambda (chunk)
+      (unless (null? query-parts)
         (exec-query
          conn
          (string-append
@@ -1363,14 +1362,18 @@ WHERE derivation_source_files.store_path = $1"
 INSERT INTO derivation_inputs (derivation_id, derivation_output_id)
 SELECT vals.derivation_id, derivation_outputs.id
 FROM (VALUES "
-           (string-join chunk ", ")
+          (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

guix-data-service/utils.scm

@@ -32,7 +32,8 @@
             letpar&
 
             chunk
-            chunk!))
+            chunk!
+            chunk-map!))
 
 (define (call-with-time-logging action thunk)
   (simple-format #t "debug: Starting ~A\n" action)
@@ -175,3 +176,28 @@
           (cons first-lst
                 (chunk! rest max-length))))
       (list lst)))
+
+(define* (chunk-map! proc chunk-size #:rest lsts)
+  (define (do-one-iteration lsts)
+    (if (> (length (car lsts))
+           chunk-size)
+        (let ((chunks-and-rest
+               (map (lambda (lst)
+                      (call-with-values (lambda ()
+                                          (split-at! lst chunk-size))
+                        (lambda (first-lst rest)
+                          (cons first-lst
+                                rest))))
+                    lsts)))
+          (apply proc
+                 (map car chunks-and-rest))
+          (do-one-iteration
+           (map cdr chunks-and-rest)))
+        (apply proc lsts)))
+
+  (unless (eq? 1
+               (length (delete-duplicates
+                        (map length lsts))))
+    (error "lists not equal length"))
+
+  (do-one-iteration lsts))