Commit 5fccfd83ec, with the fix for #6806,
switched eval to buffer its output (like other builtins do). But this
prevents using eval with commands that wants to see the tty, especially
fzf. So only buffer the output if the output is piped to the next process.
This will solve #6955 (which needs to go into a point release).
Prior to this fix, builtin_eval would direct output to the io_chain of the
job. The problem is with pipes: `builtin_eval` might happily attempt to
write unlimited output to the write end of a pipe, but the corresponding
reading process has not yet been launched. This results in deadlock.
The fix is to buffer all the output from `builtin_eval`. This is not fun
but the best that can be done until we have real concurrent processes.
Fixes#6806
Prior to this fix, fish was rather inconsistent in when $status gets set
in response to an error. For example, a failed expansion like "$foo["
would not modify $status.
This makes the following inter-related changes:
1. String expansion now directly returns the value to set for $status on
error. The value is always used.
2. parser_t::eval() now directly returns the proc_status_t, which cleans
up a lot of call sites.
3. We expose a new function exec_subshell_for_expand() which ignores
$status but returns errors specifically related to subshell expansion.
4. We reify the notion of "expansion breaking" errors. These include
command-not-found, expand syntax errors, and others.
The upshot is we are more consistent about always setting $status on
errors.
parser_t::eval indicates whether there was a parse error. It can be
easily confused with the status of the execution. Use a real type to
make it more clear.
I did not realize builtins could safely call into the parser and inject
jobs during execution. This is much cleaner than hacking around the
required shape of a plain_statement.
