From d313d2bd51ee9754c3b719a9664c085ec24905ae Mon Sep 17 00:00:00 2001
From: Samuel Dionne-Riel <samuel@dionne-riel.com>
Date: Sat, 25 Aug 2018 23:18:47 -0400
Subject: [PATCH] Fixes channel going back in time due to incomplete change.

The change in d3d33d5b2518e4e6516b6f31270b406937c48ca2 changed how the
locally saved channel information was saved on the local machine.

 * https://github.com/NixOS/nixos-channel-scripts/commit/d3d33d5b2518e4e6516b6f31270b406937c48ca2#diff-682bf482f6dd273f1c3c49afd99b4660R192

The changeset did *not* change the guard clause, which now made it act
like this:

 1. Try readlink on the file
 2. (bail as it's undefined)

The new behaviour is as more or less expected:

 1. Reads the file if available
 2. Split the URL components
 3. Keep the last bit

This can give us an empty string, which will compare to `-1` thus
fulfilling the need.
---
 mirror-nixos-branch.pl | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/mirror-nixos-branch.pl b/mirror-nixos-branch.pl
index 9dc88b2..9c37fc6 100755
--- a/mirror-nixos-branch.pl
+++ b/mirror-nixos-branch.pl
@@ -72,13 +72,11 @@ my $rev = $evalInfo->{jobsetevalinputs}->{nixpkgs}->{revision} or die;
 print STDERR "release is ‘$releaseName’ (build $releaseId), eval is $evalId, prefix is $releasePrefix, Git commit is $rev\n";
 
 # Guard against the channel going back in time.
-my $curReleaseDir = readlink "$channelsDir/$channelName";
-if (defined $curReleaseDir) {
-    my $curRelease = basename($curReleaseDir);
-    my $d = `NIX_PATH= nix-instantiate --eval -E "builtins.compareVersions (builtins.parseDrvName \\"$curRelease\\").version (builtins.parseDrvName \\"$releaseName\\").version"`;
-    chomp $d;
-    die "channel would go back in time from $curRelease to $releaseName, bailing out\n" if $d == 1;
-}
+my @releaseUrl = split(/\//, read_file("$channelsDir/$channelName", err_mode => 'quiet') // "");
+my $curRelease = pop @releaseUrl;
+my $d = `NIX_PATH= nix-instantiate --eval -E "builtins.compareVersions (builtins.parseDrvName \\"$curRelease\\").version (builtins.parseDrvName \\"$releaseName\\").version"`;
+chomp $d;
+die "channel would go back in time from $curRelease to $releaseName, bailing out\n" if $d == 1;
 
 if ($bucket->head_key("$releasePrefix")) {
     print STDERR "release already exists\n";