Properly handle Win32 paths of 'E:abc', which can be either absolute or

relative, by creating a function path_is_relative_and_below_cwd() to
check for specific requirements.  It is unclear if this fixes a security
problem or not but the new code is more robust.

src/port/path.c

@@ -358,6 +358,39 @@ path_contains_parent_reference(const char *path)
 	return false;
 }
 
+/*
+ * Detect whether a path is only in or below the current working directory.
+ * An absolute path that matches the current working directory should
+ * return false (we only want relative to the cwd).  We don't allow
+ * "/../" even if that would keep us under the cwd (it is too hard to
+ * track that).
+ */
+bool
+path_is_relative_and_below_cwd(const char *path)
+{
+	if (!is_absolute_path(path))
+		return false;
+	/* don't allow anything above the cwd */
+	else if (path_contains_parent_reference(path))
+		return false;
+#ifdef WIN32
+	/*
+	 *	On Win32, a drive letter _not_ followed by a slash, e.g. 'E:abc', is
+	 *	relative to the cwd on that drive, or the drive's root directory
+	 *	if that drive has no cwd.  Because the path itself cannot tell us
+	 *	which is the case, we have to assume the worst, i.e. that it is not
+	 *	below the cwd.  We could use GetFullPathName() to find the full path
+	 *	but that could change if the current directory for the drive changes
+	 *	underneath us, so we just disallow it.
+	 */
+	else if (isalpha((unsigned char) path[0]) && path[1] == ':' &&
+			!IS_DIR_SEP(path[2]))
+		return false;
+#endif
+	else
+		return true;	
+}
+
 /*
  * Detect whether path1 is a prefix of path2 (including equality).
  *