[Bug server/20392] New: gdb "run" command hangs

["jan.kratochvil at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=20392

            Bug ID: 20392
           Summary: gdb "run" command hangs
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: server
          Assignee: unassigned at sourceware dot org
          Reporter: jan.kratochvil at redhat dot com
  Target Milestone: ---

https://bugzilla.redhat.com/show_bug.cgi?id=1176227

Description of problem:

The gdb "run" command hangs if it's invoked before the user has issued any
resume-execution commands ("step", "continue", "stepi", etc.).

Version-Release number of selected component (if applicable):

$ gdb -v
GNU gdb (GDB) Fedora 7.8.1-30.fc21

How reproducible: 100%

Steps to Reproduce:
1. In one shell run |gdbserver --remote-debug :1111 `which ls`|
2. In another run |gdb `which ls`|
3. In the second shell, run |target extended-remote :1111| to connect to the
gdbserver in the first shell
4. In the second shell, run |r| to "restart" the inferior

Actual results:

gdb hangs after the following protocol traffic is seen

getpkt ("vCont;c:p2e6.-1");  [no ack sent] 
putpkt ("$T0506:a0e5f*"7f0* ;07:b0e4f*"7f0* ;10:42e9ddf7ff7f0*
;thread:p2e6.2e6;core:3;#f0"); [noack mode]
getpkt ("g");  [no ack sent] 
putpkt ("$20e1fff7ff7f0* 20e1fff7ff7f0* a748dff7ff7f0*(d8020*@a0e5f*"7f0*
b0e4f*"7f0* 67040*:b0ddf7ff7f0*!6020*(98d9fff7ff7f0*048e1fff7ff7f0*
48e1fff7ff7f0* 42e9ddf7ff7f0* 46020* 330*"2b0*}0*}0* 7f030*(f*
0*Hff0*2ff0*"242424242424242424242424242424240*"ff0*2f*
0*2ff0**ff0*}0*}0*}0*o801f0* f*,0*}0*}0*}0*}0*}0*7#35"); [noack mode]
getpkt ("G[snip]");  [no ack sent] 
putpkt ("$OK#9a"); [noack mode]
getpkt ("m7ffff7dde941,1");  [no ack sent] 
putpkt ("$90#69"); [noack mode]
getpkt ("m7ffff7dde941,1");  [no ack sent] 
putpkt ("$90#69"); [noack mode]

What this traffic says is that the inferior hits an "internal" gdb breakpoint
(one set by gdb itself, not the user), and then gdb queries the memory at that
breakpoint address, but then doesn't resume execution of the inferior (for
whatever reason).  So the user-visible result is a "hang".

Expected results:

Execution is restarted normally.  A workaround is to follow the steps above,
but just before step (4), issue a "stepi" command.  With that extra step, the
"run" command works as expected.

Additional info:

I happily concede that this is an edge-case bug for normal gdb users.  However,
this bug bites the rr tool[1] quite hard.  Indeed, I found this bug by running
the rr regression tests on a fedora 21 installation.  We have a gross
workaround[2] in hand, but we would like to disabuse ourselves of it at some
point.

[1] http://rr-project.org/
[2] https://github.com/mozilla/rr/pull/1406

Confirmed:
FAIL: gdb-7.11.1-75.fc24.x86_64
FAIL: GNU gdb (GDB) 7.11.50.20160720-git

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