[bug#51416] [PATCH] profiles: Build the man database only if 'man-db' is in the profile.

[Ludovic Courtès]

This allows us to skip the expensive man-db profile hook in most cases.

Suggested by Liliana Marie Prikler <liliana.prikler@gmail.com>.

* guix/profiles.scm (manual-database/optional): New procedure.
(%default-profile-hooks): Use it instead of 'manual-database'.
---
 guix/profiles.scm | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

Hi!

As y’all know, computing the manual page database is CPU- and I/O-intensive
and thus a good candidate for removal, as discussed earlier¹.

I would prefer to provide a replacement for the ‘man -k’ functionality,
such as a Xapian-based full-text search².  Now, this hasn’t materialized
yet, but I feel it’s about time to remove that hook.

The other day Liliana came with a smart idea on IRC: to build the database
only when ‘man-db’ is also in the profile.  It’s a good way to not quite
make a decision :-), but it’s going to address the main pain point.

Thoughts?

Thanks,
Ludo’.

¹ https://lists.gnu.org/archive/html/guix-devel/2020-12/msg00052.html
  Thread continues in January and March.

² https://lists.gnu.org/archive/html/guix-devel/2021-04/msg00027.html

diff --git a/guix/profiles.scm b/guix/profiles.scm
index 9f30349c69..ffdd5f57f6 100644
--- a/guix/profiles.scm
+++ b/guix/profiles.scm
@@ -1682,6 +1682,16 @@ (define man-directory
                     `((type . profile-hook)
                       (hook . manual-database))))
 
+(define (manual-database/optional manifest)
+  "Return a derivation to build the manual database of MANIFEST, but only if
+MANIFEST contains the \"man-db\" package.  Otherwise, return #f."
+  ;; Building the man database (for "man -k") is expensive and rarely used.
+  ;; Build it only if the profile also contains "man-db".
+  (mlet %store-monad ((man-db (manifest-lookup-package manifest "man-db")))
+    (if man-db
+        (manual-database manifest)
+        (return #f))))
+
 (define (texlive-configuration manifest)
   "Return a derivation that builds a TeXlive configuration for the entries in
 MANIFEST."
@@ -1784,7 +1794,7 @@ (define %default-profile-hooks
   ;; This is the list of derivation-returning procedures that are called by
   ;; default when making a non-empty profile.
   (list info-dir-file
-        manual-database
+        manual-database/optional
         fonts-dir-file
         ghc-package-cache-file
         ca-certificate-bundle

base-commit: 0a42998a50e8bbe9e49142b21a570db00efe7491
-- 
2.33.0