Hi, thanks for the PR!

I don't know if you had seen #55500, which had to be reverted? I haven't followed through and re-updated, so that's a great help.

Though, noted there:

 # This is syslinux-6.04-pre3^1; syslinux-6.04-pre3 fails to run.
 # Same issue here https://www.syslinux.org/archives/2019-February/026330.html

I had to go with b40487005223a78c3bb4c300ef6c436b3f6ec1f7, as the next commit didn't boot once built.

diff --git a/pkgs/os-specific/linux/syslinux/default.nix b/pkgs/os-specific/linux/syslinux/default.nix
index 0e0f4c945b6..e36a9274b65 100644
--- a/pkgs/os-specific/linux/syslinux/default.nix
+++ b/pkgs/os-specific/linux/syslinux/default.nix
@@ -3,10 +3,12 @@
 stdenv.mkDerivation {
   name = "syslinux-2019-02-06";
 
+  # This is syslinux-6.04-pre3^1; syslinux-6.04-pre3 fails to run.
+  # Same issue here https://www.syslinux.org/archives/2019-February/026330.html
   src = fetchFromRepoOrCz {
     repo = "syslinux";
-    rev = "05ac953c23f90b2328d393f7eecde96e41aed067";
-    sha256 = "07a9jfm5957daziq28vdnb21gnlfsvc85aqyf3fz8hxbpx1krl1l";
+    rev = "b40487005223a78c3bb4c300ef6c436b3f6ec1f7";
+    sha256 = "1qrxl1114sr2i2791z9rf8v53g200aq30f08808d7i8qnmgvxl2w";
   };
 
   patches = let

This can be tested using:

nix-build nixos/tests/boot.nix -A biosCdrom

Which ends hanging at:

machine # Booting from DVD/CD...
machine #
machine # ISOLINUX 6.04  ETCD Copyright (C) 1994-2015 H. Peter Anvin et al
machine #
machine # Failed to load ldlinux.c32
machine # Boot failed: press a key to retry...

I missed #55500. With b40487005223a78c3bb4c300ef6c436b3f6ec1f7 and debian patches:
nix-build nixos/tests/boot.nix -A biosCdrom
works just fine.

I think this is good to go for staging.

Question: why staging? This is has a really limited rebuild impact. Additionally it doesn't have a wide-ranging impact that needs to be tested with other packages, mainly bootloader tests and the isos.

Am I missing something?

@GrahamcOfBorg eval

Something's weird with the eval. Likely the branch switch-a-roo.

I accidentally pushed the old branch, now it should be ok.

Made a small whitespace correction. Didn't have the heart to "reject" the PR for only a byte :).

@vcunat I still don't know what happened the last time we bumped syslinux that needed it to be reverted. If things break again, this PR updated syslinux.

Last time syslinux itself failed to build. Now it does, so hopefully it will hold :-)