JSON output

Supported types:

any: a valid printf format, parameters to fmt_print() must match the number (and are not validated)
number: %llu
string: %s
string: str - backslash escaped special characters (0x08, 0x08, 0x0a, 0x0c, 0x0d, 0x0b), the rest of range from 0x00 to 0x1f as \uXXXX encoding
bool: bool - unquoted native json values true or false
qgroupid: qgroupid - split to 48/16 for level/subvolid
size: size - size with SI/IEC size suffix
size: size-or-none - same as size but for 0 it’s none
UUID: uuid - if all zeros then null (native json), or properly formatted UUID string
date + time: date-time - timestamp formatted as YYYY-MM-DD HH:MM:SS TIMEZONE
duration: duration - difference of two timestamps, like D days HH:MM:SS (days not show of 0)

Commands that support json output

Keys and formatting are defined as an array of struct rowspec.

key names
- should be unified with the printed value if they mean the same thing
- not abbreviated (e.g. generation instead of gen)
- referring to existing and well known names (qgroupid, devid, …)
- using - as word separator, or _ if it’s better to keep the same name of the value
values
- numbers without suffix or other transformation, i.e. no KiB
- formatted by the types if possible
- any printf format is possible but should be avoided or a new type should be defined
printing more data about an item is better than printing less, assuming the filtering is done on the user side
structure of json output may not reflect the way it’s printed in plain text, in that case do two separate printer functions
if plain and json output roughly follow the same style, e.g. line oriented that is easy to transform to a map, then both outputs should use the same rowspec
one value can be printed by multiple rowspecs that may do different formatting depending on the context