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Add two new system callbacks Module:system_get_state/1 and
Module:system_replace_state/2 to allow sys:get_state/1,2 and
sys:replace_state/2,3 to operate correctly even if a process is sys
suspended. Modify gen_server, gen_fsm, and gen_event to support the new
callbacks.
If a callback module does not export these functions, then by default the
Misc value (the same as that passed as the final argument to
sys:handle_system_msg/6, and returned as part of the return value of
sys:get_status/1,2) is treated as the callback module's state.
The previous behaviour of intercepting the system message and passing a
tuple of size 2 as the last argument to sys:handle_system_msg/6 is no
longer supported.
Add tests to verify the correctness of sys:get_state/1,2 and
sys:replace_state/2,3 when processes are sys suspended. Add two tests for
modules that implement special processes, one that exports
system_get_state/1 and system_replace_state/2 and one that doesn't.
Much of the credit for this patch goes to James Fish, who reported the
initial problem and implemented much of the fix.
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At Erlang Factory 2013 there was discussion during one of the talks about
the sys:get_status functions and how useful they were for debugging. Geoff
Cant mentioned it would be very useful if the sys module also provided
functions to use while debugging to get just the state of a process and
also to be able to replace the state of a process, and many others in the
audience appeared to agree.
The sys:get_state/1,2 functions return the state of a gen_server, gen_fsm,
or gen_event process. The return value varies depending on the process
type: process state for a gen_server, state name and state data for a
gen_fsm, and handler module, handler id, and handler state for each handler
registered in a gen_event process.
The sys:replace_state/2,3 functions allow the state of a gen_server,
gen_fsm, or gen_event process to be replaced with a new state. These
functions take a function argument that updates or replaces the process
state; using a function to change the state eliminates the race condition
of first reading the state via sys:get_state/1 or sys:get_state/2, using
its return value to create a new state, and then replacing the old state
with the new state, since during that time the process might have received
other calls or messages that could have changed its state.
* For a gen_server process, the state replacement function takes the
process state as an argument and returns a new state.
* For a gen_fsm process, the state replacement function gets a tuple of
{StateName, StateData} and returns a similar tuple that specifies a new
state name, new state data, or both.
* For a gen_event process, the state replacement function is called for
each registered event handler. It gets a tuple {Module, Id, HandlerState}
and returns a similar tuple that specifies the same Module and Id values
but may specify a different value for HandlerState.
If the state replacement function crashes or results in an error, the
original state of a gen_server or gen_fsm process is maintained; if such a
crash occurs for a gen_event process, the original state of the event
handler for which the state replacement function was called is maintained,
but the states of other event handlers of the same gen_event process may
still be updated if no errors or crashes occur while replacing their
states.
Add documentation for sys:get_state/1,2 and sys:replace_state/2,3. The
documentation explicitly notes that the functions are intended for use
during debugging.
Add new tests for these functions to gen_server_SUITE, gen_fsm_SUITE, and
gen_event_SUITE.
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Modify the documentation for the sys:get_status/2,3 functions to reflect
that they also work on gen_event processes, and add a cross reference for
gen_event:format_status/2 to go along with the existing cross references
for gen_server and gen_fsm.
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Use Erlang specs and types for documentation
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* sv/sys_get_status:
Teach sys:get_status/1,2 to call Mod:format_status/2
gen_fsm: Fix format_status/2 to handle Pids
OTP-8324 The ability for the gen_server and gen_fsm callback modules to
format their own state for display under the sys:get_status/1,2
calls has been restored and documented. (Thanks to Steve
Vinoski.)
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Restore the ability for gen_server and gen_fsm callback
modules to format their own state for display under the
sys:get_status/1,2 calls.
This ability is extremely useful for new behavior modules
based on gen_server or gen_fsm, so that they can display
their status in a more meaningful way than just dumping
the state record. It is also generally useful for applications
wanting to display their gen_server or gen_fsm callback module
state in something other than the default manner.
Also document the previously undocumented the
gen_server:format_status/2 and gen_fsm:format_status/2 optional
callback functions that, if exported by the callback module, are
invoked when sys:get_status/1,2 are called.
Add unit tests to ensure that format_status/2 functions exported
from a gen_fsm callback module and a gen_server callback module
are called when sys:get_status/1,2 are called.
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