gnu: commencement: Memoize 'linux-libre-headers-boot0'.

Fixes <https://bugs.gnu.org/30155>.

The effect can be seen in the package graph produced by:

  guix graph -e '(@@ (gnu packages commencement) static-bash-for-glibc)'

This reduces the number of "duplicate" nodes in this graph, i.e.,
distinct package objects that correspond to the same derivation (objects
that are not 'eq?' but semantically equal.)

* gnu/packages/commencement.scm (linux-libre-headers-boot0): Make an
'mlambda' instead of a 'lambda'.
(hurd-core-headers-boot0): Ditto.

gnu/packages/commencement.scm

@@ -47,6 +47,7 @@
   #:use-module (guix download)
   #:use-module (guix build-system gnu)
   #:use-module (guix build-system trivial)
+  #:use-module (guix memoization)
   #:use-module (guix utils)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-26)
@@ -371,10 +372,13 @@
                                    (current-source-location)
                                    #:guile %bootstrap-guile))))
 
-(define (linux-libre-headers-boot0)
+(define linux-libre-headers-boot0
+  (mlambda ()
     "Return Linux-Libre header files for the bootstrap environment."
     ;; Note: this is wrapped in a thunk to nicely handle circular dependencies
-  ;; between (gnu packages linux) and this module.
+    ;; between (gnu packages linux) and this module.  Additionally, memoize
+    ;; the result to play well with further memoization and code that relies
+    ;; on pointer identity; see <https://bugs.gnu.org/30155>.
     (package-with-bootstrap-guile
      (package (inherit linux-libre-headers)
               (arguments `(#:guile ,%bootstrap-guile
@@ -382,7 +386,7 @@
                            ,@(package-arguments linux-libre-headers)))
               (native-inputs
                `(("perl" ,perl-boot0)
-        ,@%boot0-inputs)))))
+                 ,@%boot0-inputs))))))
 
 (define gnumach-headers-boot0
   (package-with-bootstrap-guile
@@ -423,7 +427,8 @@
                                    (current-source-location)
                                    #:guile %bootstrap-guile))))
 
-(define (hurd-core-headers-boot0)
+(define hurd-core-headers-boot0
+  (mlambda ()
     "Return the Hurd and Mach headers as well as initial Hurd libraries for
 the bootstrap environment."
     (package-with-bootstrap-guile
@@ -434,7 +439,7 @@ the bootstrap environment."
                `(("gnumach-headers" ,gnumach-headers-boot0)
                  ("hurd-headers" ,hurd-headers-boot0)
                  ("hurd-minimal" ,hurd-minimal-boot0)
-               ,@%boot0-inputs)))))
+                 ,@%boot0-inputs))))))
 
 (define* (kernel-headers-boot0 #:optional (system (%current-system)))
   (match system

guix/packages.scm

@@ -620,6 +620,9 @@ itself.
 This is implemented as a breadth-first traversal such that INPUTS is
 preserved, and only duplicate propagated inputs are removed."
   (define (seen? seen item outputs)
+    ;; FIXME: We're using pointer identity here, which is extremely sensitive
+    ;; to memoization in package-producing procedures; see
+    ;; <https://bugs.gnu.org/30155>.
     (match (vhash-assq item seen)
       ((_ . o) (equal? o outputs))
       (_       #f)))