[RFC] Pretty printers for NPTL lock types

[Tom Tromey]

>>>>> "Martin" == Martin Galvan <martin.galvan@tallertechnologies.com> writes:

Martin>         if self.lock == PTHREAD_MUTEX_UNLOCKED:
Martin>             print("* Status: Unlocked")
Martin>         else:  # Mutex is locked
Martin>             if self.lock & FUTEX_WAITERS:
Martin>                 print("* Status: Locked, possibly with waiters")
Martin>             else:
Martin>                 print("* Status: Locked, no waiters")

Despite their name, gdb pretty-printers shouldn't actually print
anything.  Really they are value-transformers and should either return a
string value (for scalar-like objects) or an iterable of some kind (for
aggregate objects).

See

  https://sourceware.org/gdb/onlinedocs/gdb/Pretty-Printing-API.html

for a description of how it's supposed to work.

The reason this matters is that plain prints won't do the right thing
with GUIs using gdb.  Also, I think even the CLI results will be weird
in some printing scenarios, like back-traces or nested structs.

Tom