bug#55499: excessively large manifests due to propagation

[Ricardo Wurmus]

Packages of some languages rely heavily on propagation.  R is one
example.  Since the generated “manifest” file of a Guix profile records
entries for all propagated packages, this can get really big really
quickly.

A profile consisting only of four R packages (r-seurat, r-cistopic,
r-monocle3, and r-cicero-monocle3) results in a “manifest” file that
weighs 7.1MB.  At the MDC I repeatedly encountered manifest files that
are exceeding 24MB.

Simply reading that big a file with

    (call-with-input-file "huge-manifest" read)

takes several seconds.  On the MDC cluster I observed a delay of about
27 seconds.

Disabling read positions with (read-disable 'positions) significantly
speeds this up (18s vs 27s), but it’s still very slow.

We may be able to speed things up by supporting definitions or
references in manifest files, so that we don’t need to repeat a sub-tree
when generating the file.

-- 
Ricardo