[PATCH v2] Cygwin: Have tmpfile(3) use O_TMPFILE

[Mark Geisert <mark@maxrnd.com>]

Per discussion on cygwin-developers, a Cygwin tmpfile(3) implementation
has been added to syscalls.cc.  This overrides the one supplied by
newlib.  Then the open(2) flag O_TMPFILE was added to the open call that
tmpfile internally makes.

This v2 patch removes O_CREAT from open() call as O_TMPFILE obviates it.
Note that open() takes a directory's path but returns an fd to a file.
---
 winsup/cygwin/release/3.2.0 |  4 ++++
 winsup/cygwin/syscalls.cc   | 17 +++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/winsup/cygwin/release/3.2.0 b/winsup/cygwin/release/3.2.0
index f748a9bc8..d02d16863 100644
--- a/winsup/cygwin/release/3.2.0
+++ b/winsup/cygwin/release/3.2.0
@@ -19,6 +19,10 @@ What changed:
 
 - A few FAQ updates.
 
+- Have tmpfile(3) make use of Win32 FILE_ATTRIBUTE_TEMPORARY via open(2)
+  flag O_TMPFILE.
+  Addresses: https://cygwin.com/pipermail/cygwin/2021-January/247304.html
+
 
 Bug Fixes
 ---------
diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
index 52a020f07..4cda69033 100644
--- a/winsup/cygwin/syscalls.cc
+++ b/winsup/cygwin/syscalls.cc
@@ -5225,3 +5225,20 @@ pipe2 (int filedes[2], int mode)
   syscall_printf ("%R = pipe2([%d, %d], %y)", res, read, write, mode);
   return res;
 }
+
+extern "C" FILE *
+tmpfile (void)
+{
+  char *dir = getenv ("TMPDIR");
+  if (!dir)
+    dir = P_tmpdir;
+  int fd = open (dir, O_RDWR | O_BINARY | O_TMPFILE, S_IRUSR | S_IWUSR);
+  if (fd < 0)
+    return NULL;
+  FILE *fp = fdopen (fd, "wb+");
+  int e = errno;
+  if (!fp)
+    close (fd); // ..will remove tmp file
+  set_errno (e);
+  return fp;
+}
-- 
2.30.0