file_utils: add same_file_lax()

src/lxc/file_utils.c

@@ -725,3 +725,26 @@ char *read_file_at(int dfd, const char *fnam,
 
 	return move_ptr(buf);
 }
+
+bool same_file_lax(int fda, int fdb)
+{
+	struct stat st_fda, st_fdb;
+
+
+        if (fda == fdb)
+                return true;
+
+        if (fstat(fda, &st_fda) < 0)
+                return false;
+
+        if (fstat(fdb, &st_fdb) < 0)
+                return false;
+
+	errno = EINVAL;
+	if ((st_fda.st_mode & S_IFMT) != (st_fdb.st_mode & S_IFMT))
+		return false;
+
+	errno = EINVAL;
+	return (st_fda.st_dev == st_fdb.st_dev) &&
+	       (st_fda.st_ino == st_fdb.st_ino);
+}

src/lxc/file_utils.h

@@ -96,4 +96,12 @@ __hidden extern char *read_file_at(int dfd, const char *fnam,
 __hidden extern ssize_t lxc_read_try_buf_at(int dfd, const char *path,
                                             void *buf, size_t count);
 
+/*
+ * Check if two fds refer to the same file.
+ * The function is "lax" in so far, as it doesn't care whether fda and fdb have
+ * the same flags or whether they share the same device context when they refer
+ * to devices.
+ */
+__hidden extern bool same_file_lax(int fda, int fdb);
+
 #endif /* __LXC_FILE_UTILS_H */