[PATCHv2] gdb: Fix instability in thread groups test

[Andrew Burgess <andrew.burgess@embecosm.com>]

Here's an updated version of the patch based on previous feedback.
The new version has additional comments explaining why the regexp
allows an empty core list.  I also only allow the empty core list
when we scan all processes, its only in this case we might hit an
exiting process.  When we scan two known PIDs, I don't allow an empty
core list.

---

gdb: Fix instability in thread groups test

In the test script gdb.mi/list-thread-groups-available.exp we ask GDB
to list all thread groups, and match the output against a
regexp. Occasionally, I would see this test fail.

The expected output is a list of entries, each entry looking roughly
like this:

  {id="<DECIMAL>",type="process",description="<STRING>",
   user="<STRING>",cores=["<DECIMAL>","<DECIMAL>",...]}

All the fields after 'id' and 'type' are optional, and the 'cores'
list can contain 1 or more "<DECIMAL>" entries.

On my machine (Running Fedora 27, kernel 4.17.3-100.fc27.x86_64)
usually the 'description' is a non-empty string, and the 'cores' list
has at least one entry in it.  But sometimes, very rarely, I'll see an
entry in the process group list where the 'description' is an empty
string, the 'user' is the string "?", and the 'cores' list is empty.
Such an entry looks like this:

   {id="19863",type="process",description="",user="?",cores=[]}

I believe that this is caused by the process exiting while GDB is
scanning /proc for process information.  The current code in
gdb/nat/linux-osdata.c is not (I think) resilient against exiting
processes.

This commit adjusts the regex that matches the 'cores' list so that an
empty list is acceptable, with this patch in place the test script
gdb.mi/list-thread-groups-available.exp never fails for me now.

I've only adjusted the cores regexp for the occasion when we have GDB
read information about all processes, its only in this case that we
might encounter an exiting process.  When we read information about
two known PIDs, that we know will not exit for the duration of the
test, we require that the core list be non-empty.

gdb/testsuite/ChangeLog:

	* gdb.mi/list-thread-groups-available.exp: Update test regexp.
---
 gdb/testsuite/ChangeLog                               |  4 ++++
 gdb/testsuite/gdb.mi/list-thread-groups-available.exp | 12 +++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/gdb/testsuite/gdb.mi/list-thread-groups-available.exp b/gdb/testsuite/gdb.mi/list-thread-groups-available.exp
index c4dab2a2c34..11eddd577a3 100644
--- a/gdb/testsuite/gdb.mi/list-thread-groups-available.exp
+++ b/gdb/testsuite/gdb.mi/list-thread-groups-available.exp
@@ -45,7 +45,11 @@ set id_re "id=\"$decimal\""
 set type_re "type=\"process\""
 set description_re "description=\"$string_re\""
 set user_re "user=\"$string_re\""
-set cores_re "cores=\\\[\"$decimal\"(,\"$decimal\")*\\\]"
+
+# The CORES_RE regexp allows a process to be running on zero or more
+# cores.  This can happen if a process exits while GDB is reading
+# information out of /proc.
+set cores_re "cores=\\\[(\"$decimal\"(,\"$decimal\")*)?\\\]"
 
 # List all available processes.
 set process_entry_re "{${id_re},${type_re}(,$description_re)?(,$user_re)?(,$cores_re)?}"
@@ -64,6 +68,12 @@ set spawn_id_2 [remote_spawn target $binfile]
 set pid_2 [spawn_id_get_pid $spawn_id_2]
 set id_re_2 "id=\"$pid_2\""
 
+# Unlike the earlier CORES_RE this list must contain at least one
+# core.  Given that we know these processes will not exit while GDB is
+# reading their information from /proc we can expect at least one core
+# for each process.
+set cores_re "cores=\\\[\"$decimal\"(,\"$decimal\")*\\\]"
+
 set process_entry_re_1 "{${id_re_1},${type_re}(,$description_re)?(,$user_re)?(,$cores_re)?}"
 set process_entry_re_2 "{${id_re_2},${type_re}(,$description_re)?(,$user_re)?(,$cores_re)?}"
 
-- 
2.14.4