From 4879a963d4a2020f4c26f2583f3ac35ec8d6edfe Mon Sep 17 00:00:00 2001
From: Antoine Pitrou <solipsis@pitrou.net>
Date: Sat, 31 Aug 2013 00:26:02 +0200
Subject: [PATCH] Issue #18756: os.urandom() now uses a lazily-opened
 persistent file descriptor, so as to avoid using many file descriptors when
 run in parallel from multiple threads.

---
 Include/pythonrun.h |  1 +
 Misc/NEWS           |  4 ++++
 Python/pythonrun.c  |  1 +
 Python/random.c     | 55 ++++++++++++++++++++++++++++++++++-----------
 4 files changed, 48 insertions(+), 13 deletions(-)

diff --git a/Include/pythonrun.h b/Include/pythonrun.h
index aca591507e3..8fdb5b52108 100644
--- a/Include/pythonrun.h
+++ b/Include/pythonrun.h
@@ -246,6 +246,7 @@ PyAPI_FUNC(void) _PyGC_DumpShutdownStats(void);
 PyAPI_FUNC(void) _PyGC_Fini(void);
 PyAPI_FUNC(void) PySlice_Fini(void);
 PyAPI_FUNC(void) _PyType_Fini(void);
+PyAPI_FUNC(void) _PyRandom_Fini(void);
 
 PyAPI_DATA(PyThreadState *) _Py_Finalizing;
 #endif
diff --git a/Misc/NEWS b/Misc/NEWS
index dfba2cac0c9..7258ce1755f 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -51,6 +51,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #18756: os.urandom() now uses a lazily-opened persistent file
+  descriptor, so as to avoid using many file descriptors when run in
+  parallel from multiple threads.
+
 - Issue #18418: After fork(), reinit all threads states, not only active ones.
   Patch by A. Jesse Jiryu Davis.
 
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index 375bf34c82d..cbd62aa056f 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -625,6 +625,7 @@ Py_Finalize(void)
     PyDict_Fini();
     PySlice_Fini();
     _PyGC_Fini();
+    _PyRandom_Fini();
 
     /* Cleanup Unicode implementation */
     _PyUnicode_Fini();
diff --git a/Python/random.c b/Python/random.c
index 8cf2fef7c6a..1d470c703cc 100644
--- a/Python/random.c
+++ b/Python/random.c
@@ -90,6 +90,7 @@ vms_urandom(unsigned char *buffer, Py_ssize_t size, int raise)
 
 
 #if !defined(MS_WINDOWS) && !defined(__VMS)
+static int urandom_fd = -1;
 
 /* Read size bytes from /dev/urandom into buffer.
    Call Py_FatalError() on error. */
@@ -133,18 +134,30 @@ dev_urandom_python(char *buffer, Py_ssize_t size)
     if (size <= 0)
         return 0;
 
-    Py_BEGIN_ALLOW_THREADS
-    fd = _Py_open("/dev/urandom", O_RDONLY);
-    Py_END_ALLOW_THREADS
-    if (fd < 0)
-    {
-        if (errno == ENOENT || errno == ENXIO ||
-            errno == ENODEV || errno == EACCES)
-            PyErr_SetString(PyExc_NotImplementedError,
-                            "/dev/urandom (or equivalent) not found");
+    if (urandom_fd >= 0)
+        fd = urandom_fd;
+    else {
+        Py_BEGIN_ALLOW_THREADS
+        fd = _Py_open("/dev/urandom", O_RDONLY);
+        Py_END_ALLOW_THREADS
+        if (fd < 0)
+        {
+            if (errno == ENOENT || errno == ENXIO ||
+                errno == ENODEV || errno == EACCES)
+                PyErr_SetString(PyExc_NotImplementedError,
+                                "/dev/urandom (or equivalent) not found");
+            else
+                PyErr_SetFromErrno(PyExc_OSError);
+            return -1;
+        }
+        if (urandom_fd >= 0) {
+            /* urandom_fd was initialized by another thread while we were
+               not holding the GIL, keep it. */
+            close(fd);
+            fd = urandom_fd;
+        }
         else
-            PyErr_SetFromErrno(PyExc_OSError);
-        return -1;
+            urandom_fd = fd;
     }
 
     Py_BEGIN_ALLOW_THREADS
@@ -168,12 +181,20 @@ dev_urandom_python(char *buffer, Py_ssize_t size)
             PyErr_Format(PyExc_RuntimeError,
                          "Failed to read %zi bytes from /dev/urandom",
                          size);
-        close(fd);
         return -1;
     }
-    close(fd);
     return 0;
 }
+
+static void
+dev_urandom_close(void)
+{
+    if (urandom_fd >= 0) {
+        close(urandom_fd);
+        urandom_fd = -1;
+    }
+}
+
 #endif /* !defined(MS_WINDOWS) && !defined(__VMS) */
 
 /* Fill buffer with pseudo-random bytes generated by a linear congruent
@@ -271,3 +292,11 @@ _PyRandom_Init(void)
 #endif
     }
 }
+
+void
+_PyRandom_Fini(void)
+{
+#if !defined(MS_WINDOWS) && !defined(__VMS)
+    dev_urandom_close();
+#endif
+}