parsers: do not cache RevlogError type (issue4451) Index lookups raise RevlogError when the lookup fails. The previous implementation was caching a reference to the RevlogError type in a static variable. This assumed that the "mercurial.error" module was only loaded once and there was only a single copy of it floating around in memory. Unfortunately, in some situations - including certain mod_wsgi configurations - this was not the case: the "mercurial.error" module could be reloaded. It was possible for a "RevlogError" reference from the first interpreter to be used by a second interpreter. While the underlying thing was a "mercurial.error.RevlogError," the object IDs were different, so the Python code in revlog.py was failing to catch the exception! This error has existed since the C index lookup code was implemented in changeset e8d37b78acfb, which was first released in Mercurial 2.2 in 2012. http://emptysqua.re/blog/python-c-extensions-and-mod-wsgi/#static-variables-are-shared contains more details. This patch removes the caching of the RevlogError type from the function. Since pretty much the entire function was refactored and the return value of the function wasn't used, I changed the function signature to not return anything. For reasons unknown to me, we were calling PyErr_SetObject() with the type of RevlogError and an instance of RevlogError. This was equivalent to the Python code "raise RevlogError(RevlogError)". This seemed wonky and completely unnecessary. The Python code only cares about the type of the exception, not its contents. So I got rid of this complexity. This is my first Python C extension patch. Please give extra scrutiny to it during review.

--- a/mercurial/parsers.c	Mon Jun 08 18:14:22 2015 +0900
+++ b/mercurial/parsers.c	Fri Jun 12 14:43:59 2015 -0700
@@ -1483,41 +1483,34 @@
 	return -2;
 }
 
-static PyObject *raise_revlog_error(void)
+static void raise_revlog_error(void)
 {
-	static PyObject *errclass;
-	PyObject *mod = NULL, *errobj;
-
-	if (errclass == NULL) {
-		PyObject *dict;
-
-		mod = PyImport_ImportModule("mercurial.error");
-		if (mod == NULL)
-			goto classfail;
+	PyObject *mod = NULL, *dict = NULL, *errclass = NULL;
 
-		dict = PyModule_GetDict(mod);
-		if (dict == NULL)
-			goto classfail;
-
-		errclass = PyDict_GetItemString(dict, "RevlogError");
-		if (errclass == NULL) {
-			PyErr_SetString(PyExc_SystemError,
-					"could not find RevlogError");
-			goto classfail;
-		}
-		Py_INCREF(errclass);
-		Py_DECREF(mod);
+	mod = PyImport_ImportModule("mercurial.error");
+	if (mod == NULL) {
+		goto cleanup;
 	}
 
-	errobj = PyObject_CallFunction(errclass, NULL);
-	if (errobj == NULL)
-		return NULL;
-	PyErr_SetObject(errclass, errobj);
-	return errobj;
+	dict = PyModule_GetDict(mod);
+	if (dict == NULL) {
+		goto cleanup;
+	}
+	Py_INCREF(dict);
 
-classfail:
+	errclass = PyDict_GetItemString(dict, "RevlogError");
+	if (errclass == NULL) {
+		PyErr_SetString(PyExc_SystemError,
+				"could not find RevlogError");
+		goto cleanup;
+	}
+
+	/* value of exception is ignored by callers */
+	PyErr_SetString(errclass, "RevlogError");
+
+cleanup:
+	Py_XDECREF(dict);
 	Py_XDECREF(mod);
-	return NULL;
 }
 
 static PyObject *index_getitem(indexObject *self, PyObject *value)