In some conditions, and only with some installations, the called GDB
ends up hanging and never returning. Worse, even if you use non-blocking
functions such as poll() or select() with a timeout, in order to wait
for the debugger output, these block anyway and never return.
We are currently unsure what exactly causes such problem, but not using
the thread command in gdb avoids it for now.
This is a bit of a regression, though most of the time anyway the useful
traces are the main thread ones (I think ever since I implemented this,
there must not have been more than 2 or 3 cases where we actually needed
the secondary traces). So it's acceptable, at least for now.
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