PEP 466: backport persistent urandom fd (closes #21305)

Patch from Alex Gaynor.

Include/pythonrun.h

@@ -145,6 +145,7 @@ PyAPI_FUNC(void) PyInt_Fini(void);
 PyAPI_FUNC(void) PyFloat_Fini(void);
 PyAPI_FUNC(void) PyOS_FiniInterrupts(void);
 PyAPI_FUNC(void) PyByteArray_Fini(void);
+PyAPI_FUNC(void) _PyRandom_Fini(void);
 
 PyAPI_DATA(PyThreadState *) _Py_Finalizing;
 

Misc/NEWS

@@ -19,6 +19,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #21305: os.urandom now caches a fd to /dev/urandom. This is a PEP 466
+  backport from Python 3.
+
 - Issue #21307: As part of PEP 466, backport hashlib.algorithms_guaranteed and
   hashlib.algorithms_available.
 

Python/pythonrun.c

@@ -536,6 +536,7 @@ Py_Finalize(void)
     PyInt_Fini();
     PyFloat_Fini();
     PyDict_Fini();
+    _PyRandom_Fini();
 
 #ifdef Py_USING_UNICODE
     /* Cleanup Unicode implementation */

Python/random.c

@@ -118,10 +118,16 @@ vms_urandom(unsigned char *buffer, Py_ssize_t size, int raise)
 
 #if !defined(MS_WINDOWS) && !defined(__VMS)
 
+static struct {
+    int fd;
+    dev_t st_dev;
+    ino_t st_ino;
+} urandom_cache = { -1 };
+
 /* Read size bytes from /dev/urandom into buffer.
    Call Py_FatalError() on error. */
 static void
-dev_urandom_noraise(char *buffer, Py_ssize_t size)
+dev_urandom_noraise(unsigned char *buffer, Py_ssize_t size)
 {
     int fd;
     Py_ssize_t n;
@@ -156,22 +162,56 @@ dev_urandom_python(char *buffer, Py_ssize_t size)
 {
     int fd;
     Py_ssize_t n;
+    struct stat st;
 
     if (size <= 0)
         return 0;
 
-    Py_BEGIN_ALLOW_THREADS
-    fd = 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_cache.fd >= 0) {
+        /* Does the fd point to the same thing as before? (issue #21207) */
+        if (fstat(urandom_cache.fd, &st)
+            || st.st_dev != urandom_cache.st_dev
+            || st.st_ino != urandom_cache.st_ino) {
+            /* Something changed: forget the cached fd (but don't close it,
+               since it probably points to something important for some
+               third-party code). */
+            urandom_cache.fd = -1;
+        }
+    }
+    if (urandom_cache.fd >= 0)
+        fd = urandom_cache.fd;
+    else {
+        Py_BEGIN_ALLOW_THREADS
+        fd = 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_cache.fd >= 0) {
+            /* urandom_fd was initialized by another thread while we were
+               not holding the GIL, keep it. */
+            close(fd);
+            fd = urandom_cache.fd;
+        }
+        else {
+            if (fstat(fd, &st)) {
+                PyErr_SetFromErrno(PyExc_OSError);
+                close(fd);
+                return -1;
+            }
+            else {
+                urandom_cache.fd = fd;
+                urandom_cache.st_dev = st.st_dev;
+                urandom_cache.st_ino = st.st_ino;
+            }
+        }
     }
 
     Py_BEGIN_ALLOW_THREADS
@@ -195,12 +235,21 @@ 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_cache.fd >= 0) {
+        close(urandom_cache.fd);
+        urandom_cache.fd = -1;
+    }
+}
+
+
 #endif /* !defined(MS_WINDOWS) && !defined(__VMS) */
 
 /* Fill buffer with pseudo-random bytes generated by a linear congruent
@@ -305,8 +354,21 @@ _PyRandom_Init(void)
 # ifdef __VMS
         vms_urandom((unsigned char *)secret, secret_size, 0);
 # else
-        dev_urandom_noraise((char*)secret, secret_size);
+        dev_urandom_noraise((unsigned char*)secret, secret_size);
 # endif
 #endif
     }
 }
+
+void
+_PyRandom_Fini(void)
+{
+#ifdef MS_WINDOWS
+    if (hCryptProv) {
+        CryptReleaseContext(hCryptProv, 0);
+        hCryptProv = 0;
+    }
+#else
+    dev_urandom_close();
+#endif
+}