Re: [PATCH 2/2] hmp: Simplify qom_set

[Markus Armbruster]

"Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:

> * Markus Armbruster (armbru@redhat.com) wrote:
>> "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:
>> 
>> > * Philippe Mathieu-Daudé (philmd@redhat.com) wrote:
>> >> On 5/20/20 5:11 PM, Dr. David Alan Gilbert (git) wrote:
>> >> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>> >> > 
>> >> > Simplify qom_set by making it use qmp_qom_set and the JSON parser.
>> >> > 
>> >> > Note that qom-set likes JSON strings quoted with ' not ", e.g.:
>> >> > 
>> >> > (qemu) qom-get /machine smm
>> >> > "auto"
>> >> > (qemu) qom-set /machine smm 'auto'
>> >> 
>> >> Will I get this output using "?
>> >> 
>> >> (qemu) qom-set /machine smm "auto"
>> >> Error: Expecting a JSON value
>> >
>> > The error you get is:
>> >
>> > (qemu) qom-set /machine smm "auto"
>> > Error: JSON parse error, invalid keyword 'auto'
>> >
>> > I think, having seen alphanumerics, it's expecting a keyword;
>> > i.e. a true/false making a bool, or a null.
>> 
>> The command parses its argument as JSON.
>> 
>> Before we get there, the HMP core extracts the argument from the line of
>> input.  The extraction is guided by the command's .args_type, in this
>> case the 's' in "value:s" in
>> 
>>     {
>>         .name       = "qom-set",
>>         .args_type  = "path:s,property:s,value:s",
>>         [...]
>>     },
>> 
>> monitor/monitor-internal.h documents type code 's' as
>> 
>>  * 's'          string (accept optional quote)
>> 
>> The implementation boils down to:
>> 
>> 1. Skip whitespace.
>> 
>> 2. If looking at '"', get the string enclosed in '"', with C-like escape
>>    sequences \n, \r, \\, \', \".
>> 
>> 3. Else, get the string up to the next whitespace.
>> 
>> See get_str().
>> 
>> Therefore, argument "auto" is the same as auto.  Parsing auto as JSON
>> duly fails.
>> 
>> Argument 'auto' works, but only because qobject_from_json() recognizes
>> single-quoted strings.  This is as extension over RFC 8259.
>> 
>> Using single quotes falls apart when you want to pass something
>> containing whitespace.  Then you'd have to use
>> 
>>     "\"ugly and unintuitive\""
>> 
>> or, again relying on the extension
>> 
>>     "'ugly and unintuitive'"
>> 
>> There's a better way, and Paolo pointed it out in
>> 
>>     Subject: Re: [RFC PATCH] qom: Implement qom-get HMP command
>>     Date: Thu, 21 May 2020 16:24:12 +0200
>>     Message-ID: <2c148331-78ae-31f7-8702-d65c37a090b0@redhat.com>
>> 
>> Use argument type 'S'.  Documented as
>> 
>>  * 'S'          it just appends the rest of the string (accept optional 
>> quote)
>> 
>> but the parenthesis is confusing.  It really just skips whitespace, then
>> extracts the remainder of the line.  Can't do string with leading
>> whitespace, but that's just fine for us.
>
> Yep, thanks - I spotted Paolo's response but thanks for the deeper
> explanation.
> With that would you give me a Review-by?

Also fix the typo in the title "hmp: Simplify qom_set": it's qom-set.

Reviewed-by: Markus Armbruster <armbru@redhat.com>