roles/iso: allow squashfs compression to be overriden

modules/core/roles/iso/image/module.nix

@@ -5,23 +5,29 @@
   lib,
   ...
 }: let
-  inherit (lib) cleanSource mkImageMediaOverride;
+  inherit (builtins) substring;
+  inherit (lib.sources) cleanSource;
+  inherit (lib.modules) mkDefault mkImageMediaOverride;
 in {
   # the ISO image must be completely immutable in the sense that we do not
   # want the user to be able modify the ISO image after booting into it
   # the below option will disable rebuild switches (i.e nixos-rebuild switch)
   system.switch.enable = false;
 
   isoImage = let
-    # hostname will be set as a "top-level" attribute in hosts.nix, per-host.
-    # therefore we can use the networking.hostName to get the hostname of the live
-    # system without defining it explicitly in the system-agnostic ISO role module
+    # `hostname` will always be defined as a "top-level" attribute in hosts.nix
+    # while the host is being "constructed" so to speak. Therefore we can use
+    # networking.hostName (instead of `meta.hostname`) to get the hostname
+    # of the system we are building the ISO for without hardcoding anything
+    # in this role module, since this is meant to be system-agnostic. Though
+    # at the off chance that networking.hostName is not defined, we default to
+    # "nixos" as the hostname to avoid errors or empty strings.
     hostname = config.networking.hostName or "nixos";
 
     # if the system is built from a git repository, we want to include the git revision
     # in the ISO name. If the tree is dirty, we use the term "dirty" to make it explicit
     # and include the date in the ISO name.
-    rev = self.shortRev or "${builtins.substring 0 8 self.lastModifiedDate}-dirty";
+    rev = self.shortRev or "${substring 0 8 self.lastModifiedDate}-dirty";
 
     # the format of the iso will always be uniform:
     # $hostname-$release-$rev-$arch
@@ -36,8 +42,11 @@ in {
     # volumeID is used is used by stage 1 of the boot process, so it must be distintctive
     volumeID = mkImageMediaOverride "${name}";
 
-    # maximum compression, in exchange for build speed
-    squashfsCompression = "zstd -Xcompression-level 19"; # default uses gzip
+    # Maximum compression in exchange for speed, assuming ISOs to be built on GHA CI
+    # which has a 2GB limit for uploaded artifacts. Use mkDefault to allow hosts that
+    # import this role to override the compression options, potentially for faster builds
+    # or if the build host has space to spare.
+    squashfsCompression = mkDefault "zstd -Xcompression-level 19"; # default uses gzip
 
     # ISO image should be an EFI-bootable volume
     makeEfiBootable = true;
@@ -48,12 +57,13 @@ in {
     # is this supposed to make the ISO image bootable from *CD* instead of USB?
     makeUsbBootable = true;
 
-    # Get rid of "installer" suffix in boot menu
+    # Get rid of "installer" suffix in boot menu. Not all ISO images are
+    # installers.
     appendToMenuLabel = "";
 
     contents = [
       {
-        # my module system already contains an option to add memtest86+
+        # My module system already contains an option to add memtest86+
         # to the boot menu at will but in case our system is unbootable
         # lets include memtest86+ in the ISO image
         # so that we may test the memory of the system
@@ -62,10 +72,10 @@ in {
         target = "/boot/memtest.bin";
       }
       {
-        # link this flake to /etc/nixos/flake so that the user can
+        # Link system flake to /root/nyx so that the user can
         # can initiate a rebuild without having to clone and wait
         # useful if this installer is meant to be used on a system
-        # that cannot access github
+        # that cannot access GitHub.
         source = cleanSource self;
         target = "/root/nyx";
       }