Re: [PATCH v2] gdb: make "start" breakpoint inferior-specific

[Simon Marchi via Gdb-patches <gdb-patches@sourceware.org>]

On 11/10/22 11:45, Pedro Alves wrote:
> On 2022-11-08 9:20 p.m., Simon Marchi via Gdb-patches wrote:
>> New in v2:
>>
>>   - Change the test so it doesn't call the main function
>>
>> I saw this failure on a CI:
>>
>>     (gdb) add-inferior
>>     [New inferior 2]
>>     Added inferior 2
>>     (gdb) PASS: gdb.threads/vfork-multi-inferior.exp: method=non-stop: add-inferior
>>     inferior 2
>>     [Switching to inferior 2 [<null>] (<noexec>)]
>>     (gdb) PASS: gdb.threads/vfork-multi-inferior.exp: method=non-stop: inferior 2
>>     kill
>>     The program is not being run.
>>     (gdb) file /home/jenkins/workspace/binutils-gdb_master_linuxbuild/platform/jammy-amd64/target_board/unix/tmp/tmp.GYATAXR8Ku/gdb/testsuite/outputs/gdb.threads/vfork-multi-inferior/vfork-multi-inferior-sleep
>>     Reading symbols from /home/jenkins/workspace/binutils-gdb_master_linuxbuild/platform/jammy-amd64/target_board/unix/tmp/tmp.GYATAXR8Ku/gdb/testsuite/outputs/gdb.threads/vfork-multi-inferior/vfork-multi-inferior-sleep...
>>     (gdb) run &
>>     Starting program: /home/jenkins/workspace/binutils-gdb_master_linuxbuild/platform/jammy-amd64/target_board/unix/tmp/tmp.GYATAXR8Ku/gdb/testsuite/outputs/gdb.threads/vfork-multi-inferior/vfork-multi-inferior-sleep
>>     (gdb) PASS: gdb.threads/vfork-multi-inferior.exp: method=non-stop: run inferior 2
>>     inferior 1
>>     [Switching to inferior 1 [<null>] (<noexec>)]
>>     (gdb) PASS: gdb.threads/vfork-multi-inferior.exp: method=non-stop: inferior 1
>>     kill
>>     The program is not being run.
>>     (gdb) file /home/jenkins/workspace/binutils-gdb_master_linuxbuild/platform/jammy-amd64/target_board/unix/tmp/tmp.GYATAXR8Ku/gdb/testsuite/outputs/gdb.threads/vfork-multi-inferior/vfork-multi-inferior
>>     Reading symbols from /home/jenkins/workspace/binutils-gdb_master_linuxbuild/platform/jammy-amd64/target_board/unix/tmp/tmp.GYATAXR8Ku/gdb/testsuite/outputs/gdb.threads/vfork-multi-inferior/vfork-multi-inferior...
>>     (gdb) break should_break_here
>>     Breakpoint 1 at 0x11b1: file /home/jenkins/workspace/binutils-gdb_master_linuxbuild/platform/jammy-amd64/target_board/unix/src/binutils-gdb/gdb/testsuite/gdb.threads/vfork-multi-inferior.c, line 25.
>>     (gdb) PASS: gdb.threads/vfork-multi-inferior.exp: method=non-stop: break should_break_here
>>     [Thread debugging using libthread_db enabled]
>>     Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
>>     start
>>     Temporary breakpoint 2 at 0x11c0: -qualified main. (2 locations)
>>     Starting program: /home/jenkins/workspace/binutils-gdb_master_linuxbuild/platform/jammy-amd64/target_board/unix/tmp/tmp.GYATAXR8Ku/gdb/testsuite/outputs/gdb.threads/vfork-multi-inferior/vfork-multi-inferior
>>     [Thread debugging using libthread_db enabled]
>>     Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
>>
>>     Thread 2.1 "vfork-multi-inf" hit Temporary breakpoint 2, main () at /home/jenkins/workspace/binutils-gdb_master_linuxbuild/platform/jammy-amd64/target_board/unix/src/binutils-gdb/gdb/testsuite/gdb.threads/vfork-multi-inferior-sleep.c:23
>>     23	  sleep (30);
>>     (gdb) FAIL: gdb.threads/vfork-multi-inferior.exp: method=non-stop: start inferior 1
>>
>> What happens is:
>>
>>  1. We start inferior 2 with "run&", it runs very slowly, takes time to
>>     get to main
>>  2. We switch to inferior 1, and run "start"
>>  3. The temporary breakpoint inserted by "start" applies to all inferiors
>>  4. Inferior 2 hits that breakpoint and GDB reports that hit
>>
>> To avoid this, breakpoints inserted by "start" should be
>> inferior-specific.  However, we don't have a nice way to make
>> inferior-specific breakpoints yet.  It's possible to make
>> pspace-specific breakpoints (for example how the internal_breakpoint
>> constructor does) by creating a symtab_and_line manually.  However,
>> inferiors can share program spaces (usually on particular embedded
>> targets), so we could have a situation where two inferiors run the same
>> code in the same program space.  In that case, it would just not be
>> possible to insert a breakpoint in one inferior but not the other.
>>
>> A simple solution that should work all the time is to add a condition to
>> the breakpoint inserted by "start", to check the inferior reporting the
>> hit is the expected one.  This is what this patch implements.
>>
> 
> Even though this does work, it still sets the breakpoint on all the pspaces
> unnecessarily.  It would be nice if the breakpoint was pspace specific, in
> addition to inferior specific like you have (or some other way).  But, what you
> have is fine with me as is, as it is better than what we have today.
> Maybe just add a little comment suggesting that it would be even better
> to make the breakpoint apply to the current pspace only?

Will do.

>> Add a test that does:
>>
>>  - start in background inferior 1 that sleeps 3 seconds before reaching
>>    its main function (using a sleep in a global C++ object constructor)
>>  - start inferior 2, which also sleeps 3 seconds before reaching its m
>>    ain function, with the "start" command
>>  - validate that we hit the breakpoint in inferior 2
>>
>> Without the fix, we hit the breakpoint in inferior 1 pretty much all the
>> time.  There could be some unfortunate scheduling causing the test not
>> to catch the bug, for instance if the scheduler decides not to schedule
>> inferior 1 for a long time, but it would be really rare.  If the bug is
>> re-introduced, the test will catch it much more often than not, so it
>> will be noticed.
> 
> My thinking when I saw that both inferiors wait 3 seconds before reaching
> main (before reading the commit log), was, "???, I don't understand this."
> 
> So this is assuming that because the first inferior was started first, that
> its 3 seconds always finish before the second inferior's 3 seconds?  That
> seems a bit risky.  gdb and the other inferiors may all be running on
> different cores, and on a fast machine, gdb may be fast enough to spawn
> both processes roughtly at the same time, and then inferior 2 may end up
> reporting the hit first.
> 
> Why not make it so that inferior 3 takes like 3 seconds to reach main,
> but inferior 2 takes 4 or 5 seconds?  Or 2 vs 4.  Something like that.
> I.e., make sure that inferior 2's time is larger than inferior 1's.
> That could be done by tweaking the sleep calls in the programs, or
> adding a sleep call in the .exp file between "run&" and starting the
> second inferior.
> 
> But maybe I'm missing something?

Hmm no, you're right, there's no reason for the two sleep times to be
equal.  I'll do 2 seconds for inferior 1 and 4 seconds for inferior 2.

>> +proc do_test {} {
>> +    # With remote, to be able to start an inferior while another one is
>> +    # running, we need to use the non-stop variant of the protocol.
>> +    save_vars { ::GDBFLAGS } {
>> +	if { [target_info gdb_protocol] == "extended-remote"} {
>> +	    append ::GDBFLAGS " -ex \"maintenance set target-non-stop on\""
>> +	}
>> +
>> +	clean_restart ${::binfile_other}
>> +    }
>> +
>> +    gdb_test "run&" "Starting program: .*" "start background inferior"
> 
> I was going to point out that if the inferior prints something, this can
> timeout, as that output would appear after the prompt.  I then looked around
> the tree for "run&" uses, to confirm we are using gdb_test_multiple with that,
> and found that you just recently added "gdb_test -no-prompt-anchor", for exactly
> this scenario.  :-)  I think that should be used here.

Even if, in this case, we know the inferior won't print anything?

Simon