[RFC PATCH common] section config: correctly handle required options in write_config

[Daniel Kral <d.kral@proxmox.com>]

parse_config(...) warns about and skips sections, which do not specify
all required properties according to their schema, if $allow_unknown is
not set.

Meanwhile, write_config(...) will not write the config line for some
required options, as format_config_line(...) does return an empty string
for empty array properties or other empty non-boolean properties.

To ensure that required options are always written to the config to not
be skipped by parse_config(...), check whether format_config_line(...)
produces any output.

Signed-off-by: Daniel Kral <d.kral@proxmox.com>
---
This came up while fixing [0] as I noticed that at least for required
options with the '*-list' format, the parameter can still be an empty
string / list. AFAICT this is because we check for definedness of the
property value only.

Even though I considered changing this in PVE::JSONSchema first, it
could break existing code where this behavior is currently expected.
Essentially, the check would need to be specified for types, e.g., to
ensure that setting '0' for booleans, integer and numbers is still
interpreted as a set required property.

The behavior is much more apparent for section configs, where
parse_config(...) expects these properties to be written in the config,
while write_config(...) will drop these for empty strings and will
result in warn log messages for any subsequent reads with $allow_unknown
set. Therefore I propose the change only for section config for now.

[0] https://bugzilla.proxmox.com/show_bug.cgi?id=7399

 src/PVE/SectionConfig.pm | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/PVE/SectionConfig.pm b/src/PVE/SectionConfig.pm
index 84ff81a..d0332f9 100644
--- a/src/PVE/SectionConfig.pm
+++ b/src/PVE/SectionConfig.pm
@@ -1572,11 +1572,12 @@ sub write_config {
             next if $opts->{$k}->{optional};
             $done_hash->{$k} = 1;
             my $v = $scfg->{$k};
-            die "section '$sectionId' - missing value for required option '$k'\n"
-                if !defined($v);
             $v = $class->encode_value($type, $k, $v);
             my $prop = $class->get_property_schema($type, $k);
-            $data .= format_config_line($prop, $k, $v);
+            my $cfg_line = format_config_line($prop, $k, $v);
+            die "section '$sectionId' - missing value for required option '$k'\n"
+                if !$cfg_line;
+            $data .= $cfg_line;
         }
 
         for my $k (@option_keys) {
-- 
2.47.3