Patchwork parsers: fail fast if Python has wrong minor version (issue4110)

diff --git a/mercurial/parsers.c b/mercurial/parsers.c
--- a/mercurial/parsers.c
+++ b/mercurial/parsers.c
@@ -1941,6 +1941,25 @@ 
 	dirstate_unset = Py_BuildValue("ciii", 'n', 0, -1, -1);
 }
 
+static int check_python_version()
+{
+	PyObject *sys = PyImport_ImportModule("sys");
+	PyObject *hexversion = PyObject_GetAttrString(sys, "hexversion");
+	long version = PyInt_AsLong(hexversion);
+	/* sys.hexversion is a 32-bit number by default, so the -1 case
+	 * should only occur in unusual circumstances (e.g. if sys.hexversion
+	 * is manually set to an invalid value). */
+	if ((version == -1) || (version >> 16 != PY_VERSION_HEX >> 16)) {
+		PyErr_Format(PyExc_ImportError, "Python minor version mismatch: "
+			"The Mercurial extension modules were compiled with Python "
+			PY_VERSION ", but Mercurial is currently using Python with "
+			"sys.hexversion=%ld: Python %s\n at: %s", version,
+			Py_GetVersion(), Py_GetProgramFullPath());
+		return -1;
+	}
+	return 0;
+}
+
 #ifdef IS_PY3K
 static struct PyModuleDef parsers_module = {
 	PyModuleDef_HEAD_INIT,
@@ -1952,6 +1971,8 @@ 
 
 PyMODINIT_FUNC PyInit_parsers(void)
 {
+	if (check_python_version() == -1)
+		return;
 	PyObject *mod = PyModule_Create(&parsers_module);
 	module_init(mod);
 	return mod;
@@ -1959,6 +1980,8 @@ 
 #else
 PyMODINIT_FUNC initparsers(void)
 {
+	if (check_python_version() == -1)
+		return;
 	PyObject *mod = Py_InitModule3("parsers", methods, parsers_doc);
 	module_init(mod);
 }
diff --git a/tests/test-parseindex2.py b/tests/test-parseindex2.py
--- a/tests/test-parseindex2.py
+++ b/tests/test-parseindex2.py
@@ -1,6 +1,8 @@ 
 from mercurial import parsers
 from mercurial.node import nullid, nullrev
 import struct
+import subprocess
+import sys
 
 # This unit test compares the return value of the original Python
 # implementation of parseindex and the new C implementation for
@@ -97,7 +99,62 @@ 
     index, chunkcache = parsers.parse_index2(data, inline)
     return list(index), chunkcache
 
+def importparsers(hexversion):
+    """Import mercurial.parsers with the given sys.hexversion."""
+    # The file parsers.c inspects sys.hexversion to determine the version
+    # of the currently-running Python interpreter, so we monkey-patch
+    # sys.hexversion to simulate using different versions.
+    code = ("import sys; sys.hexversion=%s; "
+            "import mercurial.parsers" % hexversion)
+    cmd = "python -c \"%s\"" % code
+    # We need to do these tests inside a subprocess because parser.c's
+    # version-checking code happens inside the module init function, and
+    # when using reload() to reimport an extension module, "The init function
+    # of extension modules is not called a second time"
+    # (from http://docs.python.org/2/library/functions.html?#reload).
+    p = subprocess.Popen(cmd, shell=True,
+                         stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
+    return p.communicate()  # returns stdout, stderr
+
+def printhexfail(testnumber, hexversion, msg):
+    try:
+        hexstring = hex(hexversion)
+    except TypeError:
+        hexstring = None
+    print ("%s) using Python %s and patched sys.hexversion %r (%r): %s" %
+           (testnumber, sys.version_info, hexversion, hexstring, msg))
+
+def testversionokay(testnumber, hexversion):
+    stdout, stderr = importparsers(hexversion)
+    if stdout:
+        printhexfail(testnumber, hexversion,
+                     "Expected no stdout but got: %r" % stdout)
+
+def testversionfail(testnumber, hexversion):
+    stdout, stderr = importparsers(hexversion)
+    if not "ImportError: Python minor version mismatch" in stdout:
+        printhexfail(testnumber, hexversion,
+                     "Expected stdout to contain %r but got: %r" %
+                     (errstring, stdout))
+
+def makehex(major, minor, micro):
+    return int("%x%02x%02x00" % (major, minor, micro), 16)
+
+def runversiontests():
+    """Test importing parsers using different Python versions."""
+    info = sys.version_info
+    major, minor, micro = info[0], info[1], info[2]
+    # Test same major-minor versions.
+    testversionokay(1, makehex(major, minor, micro))
+    testversionokay(2, makehex(major, minor, micro + 1))
+    # Test different major-minor versions.
+    testversionfail(3, makehex(major + 1, minor, micro))
+    testversionfail(4, makehex(major, minor + 1, micro))
+    testversionfail(5, "'foo'")
+
 def runtest() :
+    runversiontests()
+
     # Check that parse_index2() raises TypeError on bad arguments.
     try:
         parse_index2(0, True)