Fix undefined behaviour in write() call

write()ing zero bytes to not a regular file (e.g. stdout) is undefined behaviour
and can generate a SIGHUP under certain circumstances, e.g. on AIX when running
with unbuffered stdout within sudo, screen, tmux:

    $ sudo python3 -u -c 'print(""); print("foo")'
    Hangup

Python's print() implementation in this case calls write() twice:

    # truss python3 -u -c 'print("")'
    [...]
    kwrite(1, 0x08001000A001B910, 0)                = 0

    kwrite(1, "\n", 1)                              = 1

However, the first write() is undefined behaviour.

    ssize_t write(int fildes, const void *buf, size_t nbyte);

    The write() function shall attempt to write nbyte bytes from the buffer
    pointed to by buf to the file associated with the open file descriptor,
    fildes.

    Before any action described below is taken, and if nbyte is zero and the
    file is a regular file, the write() function may detect and return errors as
    described below. In the absence of errors, or if error detection is not
    performed, the write() function shall return zero and have no other results.
    If nbyte is zero and the file is not a regular file, the results are
    unspecified.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html

Author: sideeffect42

Python/fileutils.c

@@ -1923,7 +1923,7 @@ _Py_read(int fd, void *buf, size_t count)
 static Py_ssize_t
 _Py_write_impl(int fd, const void *buf, size_t count, int gil_held)
 {
-    Py_ssize_t n;
+    Py_ssize_t n = 0;
     int err;
     int async_err = 0;
 
@@ -1970,7 +1970,11 @@ _Py_write_impl(int fd, const void *buf, size_t count, int gil_held)
                 c /= 2;
             } while (c > 0);
 #else
-            n = write(fd, buf, count);
+            /* only call write() if there is something to write.
+             * writing 0 bytes to not a regular file is undefined behaviour. */
+            if (count > 0) {
+                n = write(fd, buf, count);
+            }
 #endif
             /* save/restore errno because PyErr_CheckSignals()
              * and PyErr_SetFromErrno() can modify it */
@@ -1996,7 +2000,11 @@ _Py_write_impl(int fd, const void *buf, size_t count, int gil_held)
                 c /= 2;
             } while (c > 0);
 #else
-            n = write(fd, buf, count);
+            /* only call write() if there is something to write.
+             * writing 0 bytes to not a regular file is undefined behaviour. */
+            if (count > 0) {
+                n = write(fd, buf, count);
+            }
 #endif
             err = errno;
         } while (n < 0 && err == EINTR);