[PATCH] Gate boolean protocol options on filename.

[João Taveira Araújo]

That's what we do now, and it sucks. You still end up with embedding
BIRD syntax outside of BIRD. If you use no wildcards in the include
statement, the file must always be present otherwise the config is
invalid, so you now shifted the responsibility of managing "disabled
on|off" statements elsewhere.

So you have the added complexity of having to read in the file, parse
it to see if it matches your understanding of the world, and then
rewrite it back out if not. The presence or absence of a file is a
much more explicit signalling mechanism.


On Tue, May 24, 2016 at 6:00 AM, Baptiste Jonglez
<baptiste at bitsofnetworks.org> wrote:
> Hi,
>
> On Sun, May 22, 2016 at 04:50:12PM -0700, João Taveira Araújo wrote:
>> Toggling session parameters currently requires editing the configuration
>> file and reloading BIRD. Doing so programatically requires keeping up
>> with the syntax and layout of a particular configuration.
>>
>> This commit introduces an alternative method for gating configuration of
>> boolean protocol properties. In addition to the keywords on/off and
>> yes/no, a user can instead provide a filename on which to predicate
>> whether the flag is set.
>>
>> In particular, this makes toggling protocols programatically much simpler,
>> since we can decouple state from configuration more cleanly, i.e:
>>
>> protocol kernel {
>>   disabled filename "/etc/disable-kernel";
>> }
>
> Can't you achieve the same effect with an include?  The included file
> would just need to contain either nothing, or "disabled yes".
>
>> ---
>>  conf/cf-lex.l   | 23 +++++++++++++++++++++++
>>  conf/conf.h     |  1 +
>>  conf/confbase.Y |  3 ++-
>>  doc/bird.sgml   |  5 +++--
>>  4 files changed, 29 insertions(+), 3 deletions(-)
>>
>> diff --git a/conf/cf-lex.l b/conf/cf-lex.l
>> index 5a2a4d6..af85291 100644
>> --- a/conf/cf-lex.l
>> +++ b/conf/cf-lex.l
>> @@ -693,6 +693,29 @@ cf_symbol_class_name(struct symbol *sym)
>>      }
>>  }
>>
>> +/**
>> + * cf_file_exists - check if expanded filename exists
>> + * @arg: filename
>> + *
>> + * This function expands relative filenames and returns
>> + * whether file is present.
>> + */
>> +int
>> +cf_file_exists(char *arg)
>> +{
>> +  char *patt = arg;
>> +
>> +  if (*arg != '/')
>> +    {
>> +      int dlen = strlen(ifs->file_name);
>> +      char *dir = alloca(dlen + 1);
>> +      patt = alloca(dlen + strlen(arg) + 2);
>> +      memcpy(dir, ifs->file_name, dlen + 1);
>> +      sprintf(patt, "%s/%s", dirname(dir), arg);
>> +    }
>> +
>> +  return access(patt, F_OK) != -1;
>> +}
>>
>>  /**
>>   * DOC: Parser
>> diff --git a/conf/conf.h b/conf/conf.h
>> index 89a2c5b..6f6a33d 100644
>> --- a/conf/conf.h
>> +++ b/conf/conf.h
>> @@ -160,6 +160,7 @@ char *cf_symbol_class_name(struct symbol *sym);
>>  static inline int cf_symbol_is_constant(struct symbol *sym)
>>  { return (sym->class & 0xff00) == SYM_CONSTANT; }
>>
>> +int cf_file_exists(char *arg);
>>
>>  /* Parser */
>>
>> diff --git a/conf/confbase.Y b/conf/confbase.Y
>> index 5f487c1..7ee26f3 100644
>> --- a/conf/confbase.Y
>> +++ b/conf/confbase.Y
>> @@ -88,7 +88,7 @@ CF_DECLS
>>  %left '!'
>>  %nonassoc '.'
>>
>> -CF_KEYWORDS(DEFINE, ON, OFF, YES, NO, S, MS, US, PORT)
>> +CF_KEYWORDS(DEFINE, ON, OFF, YES, NO, S, MS, US, PORT, FILENAME)
>>
>>  CF_GRAMMAR
>>
>> @@ -143,6 +143,7 @@ bool:
>>   | YES { $$ = 1; }
>>   | OFF { $$ = 0; }
>>   | NO { $$ = 0; }
>> + | FILENAME text { $$ = cf_file_exists($2); }
>>   | /* Silence means agreement */ { $$ = 1; }
>>   ;
>>
>> diff --git a/doc/bird.sgml b/doc/bird.sgml
>> index a0b45f4..03682cb 100644
>> --- a/doc/bird.sgml
>> +++ b/doc/bird.sgml
>> @@ -493,8 +493,9 @@ protocol (see sections talking about the protocols).
>>  <p>Several options use a <m/switch/ argument. It can be either <cf/on/,
>>  <cf/yes/ or a numeric expression with a non-zero value for the option to be
>>  enabled or <cf/off/, <cf/no/ or a numeric expression evaluating to zero to
>> -disable it. An empty <m/switch/ is equivalent to <cf/on/ ("silence means
>> -agreement").
>> +disable it. Alternatively you can gate the value of <m/switch/ on the presence
>> +of a file through the <cf/filename/ "<m/name/" directive. An empty <m/switch/
>> +is equivalent to <cf/on/ ("silence means agreement").
>>
>>  <descrip>
>>       <tag>preference <m/expr/</tag>