Cache Apt and pacman package downloads with BuildKit cache mounts

CHANGES.md

@@ -2,6 +2,9 @@ unreleased
 ----------
 
 - Add Alpine 3.21, deprecate Alpine 3.20. (@MisterDA, #225)
+- Cache packages downloads for Apt (Debian, Ubuntu) and pacman (Arch
+  Linux) based distributions using BuildKit cache mounts.
+  (@MisterDA, #224)
 
 v8.2.4 2024-11-18
 -----------------

src-opam/linux.ml

@@ -86,14 +86,30 @@ end
 
 (** Debian rules *)
 module Apt = struct
+  (* https://docs.docker.com/reference/dockerfile/#example-cache-apt-packages *)
+  let cache_mounts =
+    [
+      mount_cache ~target:"/var/cache/apt" ~sharing:`Locked ();
+      mount_cache ~target:"/var/lib/apt" ~sharing:`Locked ();
+    ]
+
   let update =
-    run "apt-get -y update"
-    @@ run "DEBIAN_FRONTEND=noninteractive apt-get -y upgrade"
+    run
+      {|mv /etc/apt/apt.conf.d/docker-clean /tmp/; echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache|}
+    @@ run ~mounts:cache_mounts
+         "apt update && DEBIAN_FRONTEND=noninteractive apt-get -y upgrade"
+    @@ run
+         "mv /tmp/docker-clean /etc/apt/apt.conf.d/ && rm -f \
+          /etc/apt/apt.conf.d/keep-cache"
 
   let install fmt =
     ksprintf
       (fun s ->
-        update @@ run "DEBIAN_FRONTEND=noninteractive apt-get -y install %s" s)
+        update
+        @@ run ~mounts:cache_mounts
+             "DEBIAN_FRONTEND=noninteractive apt-get --no-install-recommends \
+              install -y %s"
+             s)
       fmt
 
   let dev_packages ?extra () =
@@ -242,11 +258,13 @@ end
 
 (** Pacman rules *)
 module Pacman = struct
-  let update = run "pacman -Syu --noconfirm && yes | pacman -Scc"
+  let cache_mount = mount_cache ~target:"/var/cache/pacman" ~sharing:`Locked ()
+  let update = run ~mounts:[ cache_mount ] "pacman -Syu --noconfirm --needed"
 
   let install fmt =
     ksprintf
-      (fun s -> run "pacman -Syu --noconfirm %s && yes | pacman -Scc" s)
+      (fun s ->
+        run ~mounts:[ cache_mount ] "pacman -Syu --noconfirm --needed %s" s)
       fmt
 
   let dev_packages ?extra () =

src-opam/linux.mli

@@ -62,11 +62,12 @@ end
 module Apt : sig
   val update : t
   (** [update] will run [apt-get update && apt-get upgrade] non-interactively.
-  *)
+      Requires [syntax=docker/dockerfile:1]. *)
 
   val install : ('a, unit, string, t) format4 -> 'a
   (** [install fmt] will [apt-get update && apt-get install] the packages
-      specified by the [fmt] format string. *)
+      specified by the [fmt] format string. Requires
+      [syntax=docker/dockerfile:1]. *)
 
   val add_user : ?uid:int -> ?gid:int -> ?sudo:bool -> string -> t
   (** [add_user username] will install a new user with name [username] and a
@@ -150,11 +151,12 @@ end
 (** Rules for Pacman-based distributions such as Archlinux *)
 module Pacman : sig
   val update : t
-  (** [update] will run [pacman -Syu] non-interactively. *)
+  (** [update] will run [pacman -Syu] non-interactively. Requires
+      [syntax=docker/dockerfile:1]. *)
 
   val install : ('a, unit, string, t) format4 -> 'a
   (** [install fmt] will [pacman -Syu] the packages specified by the [fmt]
-      format string. *)
+      format string. Requires [syntax=docker/dockerfile:1]. *)
 
   val add_user : ?uid:int -> ?gid:int -> ?sudo:bool -> string -> t
   (** [add_user username] will install a new user with name [username] and a