nixos/iso-image: extremely cursed performance optimization for Hydra

Right now the worst case chain of events for building an ISO on Hydra is

    - copy everything to squashfs builder
    - run squashfs builder
    - download squashfs from builder
    - compress squashfs
    - upload squashfs to S3
    - copy squashfs to ISO builder
    - run ISO builder
    - download ISO from builder
    - compress ISO
    - upload ISO to S3

This inlines the squashfs build into the ISO build, which makes it

    - copy everything to ISO builder
    - run ISO builder
    - download ISO from builder
    - compress ISO
    - upload ISO to S3

Which should reduce queue runner load by $alot per ISO, which we have four of on small channels
(one release, one test per arch) and a lot more than four of on large channels (with various desktops)

1 files changed, 23 insertions, 2 deletions

diff --git a/nixos/lib/make-iso9660-image.nix b/nixos/lib/make-iso9660-image.nix
index 2f7dcf519a16f..ec520f5706822 100644
--- a/nixos/lib/make-iso9660-image.nix
+++ b/nixos/lib/make-iso9660-image.nix
@@ -1,4 +1,4 @@
-{ stdenv, closureInfo, xorriso, syslinux, libossp_uuid
+{ lib, stdenv, callPackage, closureInfo, xorriso, syslinux, libossp_uuid, squashfsTools
 
 , # The file name of the resulting ISO image.
   isoName ? "cd.iso"
@@ -16,6 +16,17 @@
   # symlink to `object' that will be added to the CD.
   storeContents ? []
 
+, # In addition to `contents', the closure of the store paths listed
+  # in `squashfsContents' is compressed as squashfs and the result is
+  # placed in /nix-store.squashfs on the CD.
+  # FIXME: This is a performance optimization to avoid Hydra copying
+  # the squashfs between builders and should be removed when Hydra
+  # is smarter about scheduling.
+  squashfsContents ? []
+
+, # Compression settings for squashfs
+  squashfsCompression ? "xz -Xdict-size 100%"
+
 , # Whether this should be an El-Torito bootable CD.
   bootable ? false
 
@@ -45,12 +56,20 @@ assert bootable -> bootImage != "";
 assert efiBootable -> efiBootImage != "";
 assert usbBootable -> isohybridMbrImage != "";
 
+let
+  needSquashfs = squashfsContents != [];
+  makeSquashfsDrv = callPackage ./make-squashfs.nix {
+    storeContents = squashfsContents;
+    comp = squashfsCompression;
+  };
+in
 stdenv.mkDerivation {
   name = isoName;
   __structuredAttrs = true;
 
   buildCommandPath = ./make-iso9660-image.sh;
-  nativeBuildInputs = [ xorriso syslinux zstd libossp_uuid ];
+  nativeBuildInputs = [ xorriso syslinux zstd libossp_uuid ]
+    ++ lib.optionals needSquashfs makeSquashfsDrv.nativeBuildInputs;
 
   inherit isoName bootable bootImage compressImage volumeID efiBootImage efiBootable isohybridMbrImage usbBootable;
 
@@ -60,6 +79,8 @@ stdenv.mkDerivation {
   objects = map (x: x.object) storeContents;
   symlinks = map (x: x.symlink) storeContents;
 
+  squashfsCommand = lib.optionalString needSquashfs makeSquashfsDrv.buildCommand;
+
   # For obtaining the closure of `storeContents'.
   closureInfo = closureInfo { rootPaths = map (x: x.object) storeContents; };
 }