The "narinfo-string" procedure is expensive in term of IO operations and can
take a while under IO pressure, such a GC collecting. Defer its call to a new
thread created in the http-write procedure.
Fixes: <https://issues.guix.gnu.org/48468>
Partially fixes: <https://issues.guix.gnu.org/49089>
* guix/scripts/publish.scm (render-narinfo): Defer the narinfo string creation
to the http-write procedure.
(compression->sexp, sexp->compression): New procedures.
("X-Nar-Compression"): Use them.
("X-Narinfo-Compressions"): New custom header.
(strip-headers): Add the x-nar-path header.
(http-write): Add narinfo on-the-fly creation support. It happens in a
separated thread to prevent blocking the main thread.
