osutil: add a function to unblock signals

osutil: add a function to unblock signals

Signals could be blocked by something like:

  #include <unistd.h>
  #include <signal.h>
  int main(int argc, char * const argv[]) {
    sigset_t set;
    sigfillset(&set);
    sigprocmask(SIG_BLOCK, &set, NULL);
    execv("/bin/hg", argv);
    return 0;
  }

One of the problems is if SIGCHLD is blocked, chgserver would not reap
zombie workers since it depends on SIGCHLD handler entirely.

While it's the parent process to blame but it seems a good idea to just
unblock the signal from hg. FWIW git does that for SIGPIPE already [1].

Unfortunately Python 2 does not reset or provide APIs to change signal
masks. Therefore let's add one in osutil. Note: Python 3.3 introduced
`signal.pthread_sigmask` which solves the problem.

`sigprocmask` is part of POSIX [2] so there is no feature testing in
`setup.py`.

[1]: https://github.com/git/git/commit/7559a1be8a0afb10df41d25e4cf4c5285a5faef1
[2]: http://pubs.opengroup.org/onlinepubs/7908799/xsh/sigprocmask.html

Differential Revision: https://phab.mercurial-scm.org/D1736

--- a/mercurial/cext/osutil.c	Mon Dec 18 21:15:53 2017 +0900
+++ b/mercurial/cext/osutil.c	Wed Dec 20 02:13:35 2017 -0800
@@ -20,6 +20,7 @@
 #include <windows.h>
 #else
 #include <dirent.h>
+#include <signal.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/types.h>
@@ -1111,6 +1112,25 @@
 }
 #endif /* defined(HAVE_LINUX_STATFS) || defined(HAVE_BSD_STATFS) */
 
+static PyObject *unblocksignal(PyObject *self, PyObject *args)
+{
+	int sig = 0;
+	int r;
+	if (!PyArg_ParseTuple(args, "i", &sig))
+		return NULL;
+	sigset_t set;
+	r = sigemptyset(&set);
+	if (r != 0)
+		return PyErr_SetFromErrno(PyExc_OSError);
+	r = sigaddset(&set, sig);
+	if (r != 0)
+		return PyErr_SetFromErrno(PyExc_OSError);
+	r = sigprocmask(SIG_UNBLOCK, &set, NULL);
+	if (r != 0)
+		return PyErr_SetFromErrno(PyExc_OSError);
+	Py_RETURN_NONE;
+}
+
 #endif /* ndef _WIN32 */
 
 static PyObject *listdir(PyObject *self, PyObject *args, PyObject *kwargs)
@@ -1291,6 +1311,8 @@
 	{"getfstype", (PyCFunction)getfstype, METH_VARARGS,
 	 "get filesystem type (best-effort)\n"},
 #endif
+	{"unblocksignal", (PyCFunction)unblocksignal, METH_VARARGS,
+	 "change signal mask to unblock a given signal\n"},
 #endif /* ndef _WIN32 */
 #ifdef __APPLE__
 	{
@@ -1301,7 +1323,7 @@
 	{NULL, NULL}
 };
 
-static const int version = 1;
+static const int version = 2;
 
 #ifdef IS_PY3K
 static struct PyModuleDef osutil_module = {

--- a/mercurial/policy.py	Mon Dec 18 21:15:53 2017 +0900
+++ b/mercurial/policy.py	Wed Dec 20 02:13:35 2017 -0800
@@ -74,7 +74,7 @@
     (r'cext', r'bdiff'): 1,
     (r'cext', r'diffhelpers'): 1,
     (r'cext', r'mpatch'): 1,
-    (r'cext', r'osutil'): 1,
+    (r'cext', r'osutil'): 2,
     (r'cext', r'parsers'): 4,
 }
 

--- a/mercurial/util.py	Mon Dec 18 21:15:53 2017 +0900
+++ b/mercurial/util.py	Wed Dec 20 02:13:35 2017 -0800
@@ -163,6 +163,10 @@
     setprocname = osutil.setprocname
 except AttributeError:
     pass
+try:
+    unblocksignal = osutil.unblocksignal
+except AttributeError:
+    pass
 
 # Python compatibility