fix #7173: config: drop empty tag list when writing

When the last tag of a container gets removed, for example in the web
UI, the API gets an empty 'tags' value, which then ends up as an empty
'tags:' line in the config and in the output of 'pct config'. As the
config parser reads that line back as a single space, it also gets
written out again on every following config change.

Skip a tag list without any tag when writing the config, which also
cleans up existing configs with such a line on their next change. Keep
it in the pending section, where it is still needed for removing all
tags with a pending change.

Add a basic test for parsing and writing container configs, similar to
the one in qemu-server, to cover this.

Signed-off-by: Michal Fox <me@dualfroz.com>

src/PVE/LXC/Config.pm

@@ -1243,7 +1243,7 @@ sub write_pct_config {
     }
 
     my $generate_raw_config = sub {
-        my ($conf) = @_;
+        my ($conf, $pending) = @_;
 
         my $raw = '';
 
@@ -1265,6 +1265,8 @@ sub write_pct_config {
             my $value = $conf->{$key};
             die "detected invalid newline or carriage return inside property '$key'\n"
                 if $value =~ m/[\r\n]/;
+            # an empty tag list is only needed as pending change for removing all tags
+            next if $key eq 'tags' && !$pending && $value =~ m/^\s*$/;
             $raw .= "$key: $value\n";
         }
 
@@ -1293,7 +1295,7 @@ sub write_pct_config {
 
     if (scalar(keys %{ $conf->{pending} })) {
         $raw .= "\n[pve:pending]\n";
-        $raw .= &$generate_raw_config($conf->{pending});
+        $raw .= &$generate_raw_config($conf->{pending}, 1);
     }
 
     foreach my $snapname (sort keys %{ $conf->{snapshots} }) {

src/test/Makefile

@@ -2,7 +2,7 @@ RUN_USERNS := lxc-usernsexec -m "u:0:`id -u`:1" -m "g:0:`id -g`:1" --
 
 all: test
 
-test: test_setup test_snapshot test_bindmount test_idmap
+test: test_setup test_snapshot test_bindmount test_idmap test_parse_config
 
 test_setup: run_setup_tests.pl
 	if test -e /run/lock/sbuild; then \
@@ -24,5 +24,8 @@ test_bindmount: bindmount_test.pl
 test_idmap: run_idmap_tests.pl
 	./run_idmap_tests.pl
 
+test_parse_config: run_parse_config_tests.pl
+	./run_parse_config_tests.pl
+
 clean:
 	rm -rf tmprootfs

src/test/parse-config-expected/empty-tags.conf

@@ -0,0 +1,8 @@
+# empty tag list
+arch: amd64
+cores: 1
+hostname: notags
+memory: 512
+ostype: debian
+rootfs: local-lvm:vm-8006-disk-0,size=8G
+unprivileged: 1

src/test/parse-config-input/empty-tags.conf

@@ -0,0 +1,9 @@
+# empty tag list
+arch: amd64
+cores: 1
+hostname: notags
+memory: 512
+ostype: debian
+rootfs: local-lvm:vm-8006-disk-0,size=8G
+tags: 
+unprivileged: 1

src/test/parse-config-input/plain.conf

@@ -0,0 +1,11 @@
+# plain container
+arch: amd64
+cores: 1
+hostname: plain
+memory: 512
+net0: name=eth0,bridge=vmbr0,hwaddr=BC:24:11:2C:69:EC,ip=dhcp,type=veth
+ostype: debian
+rootfs: local-lvm:vm-8006-disk-0,size=8G
+swap: 512
+tags: bar;foo
+unprivileged: 1

src/test/run_parse_config_tests.pl

@@ -0,0 +1,47 @@
+#!/usr/bin/perl
+
+# Tests parsing and writing container configuration files.
+#
+# If no expected file exists, the input is assumed to be equal to the expected output.
+
+use strict;
+use warnings;
+
+use lib qw(..);
+
+use Test::More;
+
+use PVE::LXC::Config;
+use PVE::Tools;
+
+my $INPUT_DIR = './parse-config-input';
+my $EXPECTED_DIR = './parse-config-expected';
+
+PVE::Tools::dir_glob_foreach(
+    $INPUT_DIR,
+    '.*\.conf',
+    sub {
+        my ($file) = @_;
+
+        my $fake_config_fn = "$file/lxc/8006.conf";
+        my $input = PVE::Tools::file_get_contents("${INPUT_DIR}/${file}");
+
+        my $output = eval {
+            my $conf = PVE::LXC::Config::parse_pct_config($fake_config_fn, $input, 1);
+            PVE::LXC::Config::write_pct_config($fake_config_fn, $conf);
+        };
+        if (my $err = $@) {
+            note("got unexpected error '$err'");
+            fail($file);
+            return;
+        }
+
+        my $expected_file = "${EXPECTED_DIR}/${file}";
+        my $expected =
+            -f $expected_file ? PVE::Tools::file_get_contents($expected_file) : $input;
+
+        is($output, $expected, $file);
+    },
+);
+
+done_testing();