Allow pyraise to include backtrace from Julia

Instead of using `catch_backtrace` in `showerror_string`, which may not always
be called from a catch block with a current exception set, I have added an
optional argument to `pyraise` which can be used to pass backtraces to
`showerror_string`.

Backtraces are only used if exception is not a PyError exception. By default, no
backtrace is shown.

All try-catch blocks in PyCall attempt to pass the backtrace for the current
exception to `pyraise`.

src/callback.jl

@@ -39,7 +39,7 @@ function _pyjlwrap_call(f, args_::PyPtr, kw_::PyPtr)
 
         return pyreturn(ret)
     catch e
-        pyraise(e)
+        pyraise(e, catch_backtrace())
     finally
         args.o = PyPtr_NULL # don't decref
     end

src/exception.jl

@@ -123,16 +123,21 @@ function pyexc_initialize()
     pyexc[PyIOError] = @pyglobalobjptr :PyExc_IOError
 end
 
+_showerror_string(io::IO, e, ::Nothing) = showerror(io, e)
+_showerror_string(io::IO, e, bt) = showerror(io, e, bt)
+
+# bt argument defaults to nothing, to delay dispatching on the presence of a
+# backtrace until after the try-catch block
 """
     showerror_string(e) :: String
 
 Convert output of `showerror` to a `String`.  Since this function may
 be called via Python C-API, it tries to not throw at all cost.
 """
-function showerror_string(e::T) where {T}
+function showerror_string(e::T, bt = nothing) where {T}
     try
         io = IOBuffer()
-        showerror(io, e, catch_backtrace())
+        _showerror_string(io, e, bt)
         return String(take!(io))
     catch
         try
@@ -163,14 +168,16 @@ function showerror_string(e::T) where {T}
     end
 end
 
-function pyraise(e)
+# If no backtrace is passed pyraise,
+function pyraise(e, bt = nothing)
     eT = typeof(e)
     pyeT = haskey(pyexc::Dict, eT) ? pyexc[eT] : pyexc[Exception]
     ccall((@pysym :PyErr_SetString), Cvoid, (PyPtr, Cstring),
-          pyeT, string("Julia exception: ", showerror_string(e)))
+          pyeT, string("Julia exception: ", showerror_string(e, bt)))
 end
 
-function pyraise(e::PyError)
+# Second argument allows for backtraces passed to `pyraise` to be ignored.
+function pyraise(e::PyError, ::Vector = [])
     ccall((@pysym :PyErr_Restore), Cvoid, (PyPtr, PyPtr, PyPtr),
           e.T, e.val, e.traceback)
     e.T.o = e.val.o = e.traceback.o = C_NULL # refs were stolen

src/pyiterator.jl

@@ -68,7 +68,7 @@ const jlWrapIteratorType = PyTypeObject()
                 return pyreturn(item)
             end
         catch e
-            pyraise(e)
+            pyraise(e, catch_backtrace())
         end
         return PyPtr_NULL
     end
@@ -83,7 +83,7 @@ else
                 return pyreturn(item)
             end
         catch e
-            pyraise(e)
+            pyraise(e, catch_backtrace())
         end
         return PyPtr_NULL
     end
@@ -96,7 +96,7 @@ function pyjlwrap_getiter(self_::PyPtr)
         self = unsafe_pyjlwrap_to_objref(self_)
         return pystealref!(jlwrap_iterator(self))
     catch e
-        pyraise(e)
+        pyraise(e, catch_backtrace())
     end
     return PyPtr_NULL
 end

src/pytype.jl

@@ -342,7 +342,7 @@ function pyjlwrap_repr(o::PyPtr)
         return pyreturn(o != C_NULL ? string("<PyCall.jlwrap ",unsafe_pyjlwrap_to_objref(o),">")
                         : "<PyCall.jlwrap NULL>")
     catch e
-        pyraise(e)
+        pyraise(e, catch_backtrace())
         return PyPtr_NULL
     end
 end
@@ -389,7 +389,7 @@ function pyjlwrap_getattr(self_::PyPtr, attr__::PyPtr)
             end
         end
     catch e
-        pyraise(e)
+        pyraise(e, catch_backtrace())
     finally
         attr_.o = PyPtr_NULL # don't decref
     end