Predicate locations

Predicate locations

A predicate location name starts with '$' and is interpreted as a
string evaluated at runtime, and its value is used to route the
request into the location.  A predicate location matches if the
string evaluates to a non-empty value not equal to "0".  At each
nesting level, the first matching predicate location in the
configuration order is chosen.  Predicate locations are matched
after prefix and regex locations.  They are not matched if an exact
or a regex location matched, or if the longest matching prefix location
has the "^~" modifier.  Predicate locations can be nested inside
other location types and can contain nested locations of their own.

Example:

map $arg_foo $pred {
    bar  1;
    qux  1;
}

location $pred {
    root html;
}