build-fhs-userenv: add extraBindMounts support

pkgs/build-support/build-fhs-userenv/chroot-user.rb

@@ -1,18 +1,18 @@
 #!/usr/bin/env ruby
 
-# Bind mounts hierarchy: [from, to (relative)]
+# Bind mounts hierarchy: from => to (relative)
 # If 'to' is nil, path will be the same
-mounts = [ ['/nix/store', nil],
-           ['/dev', nil],
-           ['/proc', nil],
-           ['/sys', nil],
-           ['/etc', 'host-etc'],
-           ['/tmp', 'host-tmp'],
-           ['/home', nil],
-           ['/var', nil],
-           ['/run', nil],
-           ['/root', nil],
-         ]
+mounts = { '/nix/store' => nil,
+           '/dev' => nil,
+           '/proc' => nil,
+           '/sys' => nil,
+           '/etc' => 'host-etc',
+           '/tmp' => 'host-tmp',
+           '/home' => nil,
+           '/var' => nil,
+           '/run' => nil,
+           '/root' => nil,
+         }
 
 # Propagate environment variables
 envvars = [ 'TERM',
@@ -65,8 +65,22 @@ abort "Usage: chrootenv swdir program args..." unless ARGV.length >= 2
 swdir = Pathname.new ARGV[0]
 execp = ARGV.drop 1
 
+# Populate extra mounts
+if not ENV["CHROOTENV_EXTRA_BINDS"].nil?
+  for extra in ENV["CHROOTENV_EXTRA_BINDS"].split(':')
+    paths = extra.split('=')
+    if not paths.empty?
+      if paths.size <= 2
+        mounts[paths[0]] = paths[1]
+      else
+        $stderr.puts "Ignoring invalid entry in CHROOTENV_EXTRA_BINDS: #{extra}"
+      end
+    end
+  end
+end
+
 # Set destination paths for mounts
-mounts.map! { |x| [x[0], x[1].nil? ? x[0].sub(/^\/*/, '') : x[1]] }
+mounts = mounts.map { |k, v| [k, v.nil? ? k.sub(/^\/*/, '') : v] }.to_h
 
 # Create temporary directory for root and chdir
 root = Dir.mktmpdir 'chrootenv'
@@ -97,10 +111,10 @@ if $cpid == 0
   write_file '/proc/self/gid_map', "#{gid} #{gid} 1"
 
   # Do rbind mounts.
-  mounts.each do |x|
-    to = "#{root}/#{x[1]}"
+  mounts.each do |from, rto|
+    to = "#{root}/#{rto}"
     FileUtils.mkdir_p to
-    $mount.call x[0], to, nil, MS_BIND | MS_REC, nil
+    $mount.call from, to, nil, MS_BIND | MS_REC, nil
   end
 
   # Chroot!
@@ -108,7 +122,7 @@ if $cpid == 0
   Dir.chdir '/'
 
   # Symlink swdir hierarchy
-  mount_dirs = Set.new mounts.map { |x| Pathname.new x[1] }
+  mount_dirs = Set.new mounts.map { |_, v| Pathname.new v }
   link_swdir = lambda do |swdir, prefix|
     swdir.find do |path|
       rel = prefix.join path.relative_path_from(swdir)

pkgs/build-support/build-fhs-userenv/default.nix

@@ -1,4 +1,4 @@
-{ runCommand, writeText, writeScriptBin, stdenv, ruby } : { env, runScript ? "bash" } :
+{ runCommand, lib, writeText, writeScriptBin, stdenv, ruby } : { env, runScript ? "bash", extraBindMounts ? [] } :
 
 let
   name = env.pname;
@@ -27,6 +27,7 @@ in runCommand name {
   passthru.env =
     runCommand "${name}-shell-env" {
       shellHook = ''
+        export CHROOTENV_EXTRA_BINDS="${lib.makeSearchPath extraBindMounts}:$CHROOTENV_EXTRA_BINDS"
         exec ${chroot-user}/bin/chroot-user ${env} bash -l ${init "bash"} "$(pwd)"
       '';
     } ''