Initially I wanted to pick a different name to avoid confusion with
process groups, but really job trees *are* process groups. So name them
to reflect that fact.
Also rename "placeholder" to "internal" which is clearer.
Job trees come in two flavors: “placeholders” for jobs which are only fish
functions, and non-placeholders which need to track a pgid. This adds
logic to allow a job to decide if its parent's job tree is appropriate,
and allocating a new tree if not.
Give string expansion an (optional) parent pgroup. This is threaded all
the way into eval(). This ensures that in a mixed pipeline like:
cmd | begin ; something (cmd2) ; end
that cmd2 and cmd have the same pgroup.
Add a test to ensure that command substitutions inherit pgroups
properly.
Fixes#6624
This commit recognizes an existing pattern: many operations need some
combination of a set of variables, a way to detect cancellation, and
sometimes a parser. For example, tab completion needs a parser to execute
custom completions, the variable set, should cancel on SIGINT. Background
autosuggestions don't need a parser, but they do need the variables and
should cancel if the user types something new. Etc.
This introduces a new triple operation_context_t that wraps these concepts
up. This simplifies many method signatures and argument passing.
