Trac #23162: Give a warning when using citation.get_systems() without…

… Cython profiling

`sage.misc.citation.get_systems()` is implemented by using `cProfile` to
look at which modules implement the functions called when executing the
code.

The problem is that this is totally unreliable when Cython is compiled
without profiling support (which is the default). This doctest
{{{
sage: from sage.misc.citation import get_systems
sage: get_systems('((x+1)^2).expand()')
['ginac']
}}}
only works because `Expression.expand()` is called by Python instead of
Cython. If that call would be inside some other Cython code, then
Python's profiler would not detect it:
{{{
sage: cython('def callexpand(x): return x.expand()')
sage: from sage.misc.citation import get_systems
sage: get_systems('callexpand(((x+1)^2))')
[]
}}}

There is a problem because #22747 will "break" profiling even further as
even the top-level call of `Expression.expand()` would not be detected
as something to be entered in the profiler.

So here I propose simply to give a warning whenever `get_systems()` is
used when profiling was not enabled.

URL: https://trac.sagemath.org/23162
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Sébastien Labbé

src/sage/misc/citation.pyx

@@ -49,20 +49,29 @@ systems['PolyBoRi'] = ['sage.rings.polynomial.pbori']
 
 def get_systems(cmd):
     """
-    Returns a list of the systems used in running the command
-    cmd.  Note that the results can sometimes include systems
-    that did not actually contribute to the computation. Due
-    to caching and the inability to follow all C calls, it
+    Returns a list of the systems used in running the command ``cmd``.
+    Note that the results can sometimes include systems that did not
+    actually contribute to the computation. Due to caching, it
     could miss some dependencies as well.
 
     INPUT:
 
     - ``cmd`` - a string to run
 
+    .. WARNING::
+
+        In order to properly support Cython code, this requires that
+        Sage was compiled with the environment variable
+        ``SAGE_PROFILE=yes``. If this was not the case, a warning will
+        be given when calling this function.
+
     EXAMPLES::
 
         sage: from sage.misc.citation import get_systems
-        sage: s = get_systems('integrate(x^2, x)'); #priming coercion model
+        sage: get_systems('print("hello")')  # random (may print warning)
+        []
+        sage: integrate(x^2, x)  # Priming coercion model
+        1/3*x^3
         sage: get_systems('integrate(x^2, x)')
         ['ginac', 'Maxima']
         sage: R.<x,y,z> = QQ[]
@@ -76,6 +85,13 @@ def get_systems(cmd):
     """
     import cProfile, pstats, re
 
+    if not cython_profile_enabled():
+        from warnings import warn
+        warn("get_systems() requires Cython profiling to be enabled, "
+             "otherwise the results will be very unreliable. "
+             "Rebuild Sage with the environment variable 'SAGE_PROFILE=yes' "
+             "to enable profiling.")
+
     if not isinstance(cmd, basestring):
         raise TypeError("command must be a string")
 
@@ -91,7 +107,7 @@ def get_systems(cmd):
     strings = [a[0].replace(SAGE_ROOT, "") + " " + a[2] for a in stats.stats.keys()]
 
     #Remove trivial functions
-    bad_res = [re.compile(r'is_.*Element')]
+    bad_res = [re.compile(r'is_.*Element'), re.compile("is_[a-z_]*_type")]
     for bad_re in bad_res:
         i = 0
         while i < len(strings):
@@ -106,3 +122,27 @@ def get_systems(cmd):
         if any([(r in s) or (r.replace('.','/') in s) for r in systems[system] for s in strings]):
             systems_used.append(system)
     return systems_used
+
+
+cdef extern from *:
+    int CYTHON_PROFILE """
+        #ifdef CYTHON_PROFILE
+        CYTHON_PROFILE
+        #else
+        0
+        #endif
+        """
+
+cpdef inline bint cython_profile_enabled():
+    """
+    Return whether Cython profiling is enabled.
+
+    EXAMPLES::
+
+        sage: from sage.misc.citation import cython_profile_enabled
+        sage: cython_profile_enabled()  # random
+        False
+        sage: type(cython_profile_enabled()) is bool
+        True
+    """
+    return CYTHON_PROFILE