Make unlink() work for junction points on Windows.

To support harmonization of Windows and Unix code, teach our unlink()
wrapper that junction points need to be unlinked with rmdir() on
Windows.

Tested-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/CA%2BhUKGLfOOeyZpm5ByVcAt7x5Pn-%3DxGRNCvgiUPVVzjFLtnY0w%40mail.gmail.com
(cherry picked from commit f357233c9d)

Author: Thomas Munro <tmunro@postgresql.org>
Author: Alexandra Wang <alexandra.wang.oss@gmail.com>

src/port/dirmod.c

@@ -99,6 +99,32 @@ int
 pgunlink(const char *path)
 {
 	int			loops = 0;
+	struct stat st;
+
+	/*
+	 * This function might be called for a regular file or for a junction
+	 * point (which we use to emulate symlinks).  The latter must be unlinked
+	 * with rmdir() on Windows.  Before we worry about any of that, let's see
+	 * if we can unlink directly, since that's expected to be the most common
+	 * case.
+	 */
+	if (unlink(path) == 0)
+		return 0;
+	if (errno != EACCES)
+		return -1;
+
+	/*
+	 * EACCES is reported for many reasons including unlink() of a junction
+	 * point.  Check if that's the case so we can redirect to rmdir().
+	 *
+	 * Note that by checking only once, we can't cope with a path that changes
+	 * from regular file to junction point underneath us while we're retrying
+	 * due to sharing violations, but that seems unlikely.  We could perhaps
+	 * prevent that by holding a file handle ourselves across the lstat() and
+	 * the retry loop, but that seems like over-engineering for now.
+	 */
+	if (lstat(path, &st) < 0)
+		return -1;
 
 	/*
 	 * We need to loop because even though PostgreSQL uses flags that allow
@@ -107,7 +133,7 @@ pgunlink(const char *path)
 	 * someone else to close the file, as the caller might be holding locks
 	 * and blocking other backends.
 	 */
-	while (unlink(path))
+	while ((S_ISLNK(st.st_mode) ? rmdir(path) : unlink(path)) < 0)
 	{
 		if (errno != EACCES)
 			return -1;