[Bug gdb/17223] New: cannot do background execution in scripts

["dje at google dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=17223

            Bug ID: 17223
           Summary: cannot do background execution in scripts
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: dje at google dot com

Detection of new threads is done in target-async using the event loop.
However, when running a script, or when running multiple commands passed via
-ex, etc., the event loop is not used (at least not until gdb gets the
keyboard).

Testcase: forever-threads from pr 17147

bash$ cat foo.gdb
set trace-commands on
set non-stop on
file forever-threads
run &
shell sleep 3
info threads
bash$ gdb -x foo.gdb

Note that gdb doesn't become aware of the new threads until after the script
completes and gdb gets the keyboard. It also happens that gdb doesn't become
aware of shared libs being loaded (including libthread_db) until gdb gets the
keyboard.

This happens because script execution is done in its own loop that doesn't
check for events (ref: top.c:command_loop).  Plus more would be required to
handle foo.gdb being split up into a bunch of -ex options.
Question is: Do we want to support this use-case?  It sure would be nice.

One (not well thought out) thought is to add a command that processes current
inferior events.  Maybe (maybe not) it could be built on the hypothetical
"wait" command.
ref: https://sourceware.org/gdb/wiki/BackgroundExecution

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