contextmanager for ftrace

[ahgamut]

Python likes to make a bunch of function calls for anything, so ftrace-ing a Python script from start to end produces a TON of info. cosmo.ftrace() + cosmo.exit1() are nice, but I have to still structure my Python script to exit right after a particular line.

As mentioned here, This commit adds a simple contextmanager wrapper to cosmo.ftrace() so instead I can do things like this:

import cosmo

with cosmo.ftrace(): # everything inside this will be logged to stderr
    a = 2 + 3

# logging disabled, do other things without having to terminate
print("a =", a)

the above snippet produces 32 lines of ftrace in stderr, which is much easier to examine:

+       sigprocmask 15481145
+ meth_dealloc 11490
+ PyFrame_BlockSetup 29
+ object_dealloc 389
+ PyObject_Free 61
+ _PyObject_Free.isra.0 14
+ PyDict_SetItem 372
+ insertdict 72
+   lookdict_unicode_nodummy 141
+   dictresize 133
+     new_keys_object 198
+       PyObject_Malloc 99
+       PyMem_Malloc 14
+       _PyObject_Alloc.isra.0 14
+         dlmalloc 54
+           dlmalloc_impl 14
+             tmalloc_large 78
+       memset 372
+       memset_avx 44
+       bzero 43
+       bzero_avx 38
+     PyObject_Free 197
+     _PyObject_Free.isra.0 10
+   find_empty_slot.isra.0 448
+ PyFrame_BlockPop 96
+ PyObject_CallFunctionObjArgs 252
+   objargs_mkstack.constprop.0 31
+   _PyObject_FastCallDict 55
+     _PyCFunction_FastCallDict 40
+       PyTuple_New 116
+       TracerObject_exit 150
a =  5

[pkulchenko]

I wonder if something like this can be made available for redbean as well (or maybe it already is and just need to be exposed to the Lua layer)? @jart?

[jart]

@pkulchenko Absolutely. Lua doesn't require parens in function calls, and it lets you pass code to a function, so we could probably define the syntax as follows:

ftrace {
  some_code()
}

[jart]

LGTM. Cool change! So excited to see people using the function call tracing.

[pkulchenko]

@pkulchenko Absolutely. Lua doesn't require parens in function calls, and it lets you pass code to a function, so we could probably define the syntax as follows:

ftrace {
  some_code()
}

@jart, we may need to tweak this a bit, as it will call some_code and then pass the results. So, maybe either: ftrace(some_code) (to pass an existing function) or ftrace(function() some_code() some_other_code() end) to pass a closure).

If we're setting/removing a hook, then ftrace_start() and ftrace_stop() may be appropriate as well.

I've been also wondering if it's possible to keep the trace output internal and implement sort of a sliding window for it: ftrace_start(50) turns it on, but doesn't generate any output and always keeps the last (up to) 50 calls; I can then call ftrace_flush() and the collected info is then flushed to stderr. This way I don't need to go through the entire log and can add _flush to the places where I need it (and it may be better than start/stop, since I may come to the same place from multiple points and want to see which branch is being triggered).