Safer pyjlwrap (#583)

* Make pyjlwrap_repr safe

Without this change, following script using PyJulia kills the whole
process with "fatal: error thrown and no exception handler available."
printed from libjulia runtime:

    from julia import Main
    Spam = Main.eval("""
    struct Spam end
    function Base.show(io::IO, ::Spam)
         error("show(::IO, ::Spam) called")
    end
    Spam
    """)
    print(Spam())

* Make pyraise (hence pyjlwrap_call) safe

src/exception.jl

@@ -123,11 +123,51 @@ function pyexc_initialize()
     pyexc[PyIOError] = @pyglobalobjptr :PyExc_IOError
 end
 
+"""
+    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}
+    try
+        io = IOBuffer()
+        showerror(io, e)
+        return String(take!(io))
+    catch
+        try
+            return """
+                   $e
+                   ERROR: showerror(::IO, ::$T) failed!"""
+        catch
+            try
+                return """
+                       $T
+                       ERROR: showerror(::IO, ::$T) failed!
+                       ERROR: string(::$T) failed!"""
+            catch
+                name = try
+                    io = IOBuffer()
+                    Base.show_datatype(io, T)
+                    String(take!(io))
+                catch
+                    "_UNKNOWN_TYPE_"
+                end
+                return """
+                       Unprintable error.
+                       ERROR: showerror(::IO, ::$name) failed!
+                       ERROR: string(::$name) failed!
+                       ERROR: string($name) failed!"""
+            end
+        end
+    end
+end
+
 function pyraise(e)
     eT = typeof(e)
     pyeT = haskey(pyexc::Dict, eT) ? pyexc[eT] : pyexc[Exception]
     ccall((@pysym :PyErr_SetString), Cvoid, (PyPtr, Cstring),
-          pyeT, string("Julia exception: ", e))
+          pyeT, string("Julia exception: ", showerror_string(e)))
 end
 
 function pyraise(e::PyError)

src/pytype.jl

@@ -337,9 +337,15 @@ end
 unsafe_pyjlwrap_to_objref(o::PyPtr) =
   unsafe_pointer_to_objref(unsafe_load(convert(Ptr{Ptr{Cvoid}}, o), 3))
 
-pyjlwrap_repr(o::PyPtr) =
-    pystealref!(PyObject(o != C_NULL ? string("<PyCall.jlwrap ",unsafe_pyjlwrap_to_objref(o),">")
-                                     : "<PyCall.jlwrap NULL>"))
+function pyjlwrap_repr(o::PyPtr)
+    try
+        return pyreturn(o != C_NULL ? string("<PyCall.jlwrap ",unsafe_pyjlwrap_to_objref(o),">")
+                        : "<PyCall.jlwrap NULL>")
+    catch e
+        pyraise(e)
+        return PyPtr_NULL
+    end
+end
 
 function pyjlwrap_hash(o::PyPtr)
     h = hash(unsafe_pyjlwrap_to_objref(o))

test/runtests.jl

@@ -637,4 +637,33 @@ end
     @test anonymous.sys != PyNULL()
 end
 
+struct Unprintable end
+Base.show(::IO, ::Unprintable) = error("show(::IO, ::Unprintable) called")
+Base.show(::IO, ::Type{Unprintable}) = error("show(::IO, ::Type{Unprintable}) called")
+
+py"""
+def try_repr(x):
+    try:
+        return repr(x)
+    except Exception as err:
+        return err
+"""
+
+py"""
+def try_call(f):
+    try:
+        return f()
+    except Exception as err:
+        return err
+"""
+
+@testset "throwing show" begin
+    unp = Unprintable()
+    @test_throws Exception show(unp)
+    @test py"try_repr"("printable") isa String
+    @test pyisinstance(py"try_repr"(unp), pybuiltin("Exception"))
+    @test pyisinstance(py"try_call"(() -> throw(Unprintable())),
+                       pybuiltin("Exception"))
+end
+
 include("test_pyfncall.jl")