From 6f89fb60088c4bc1513a005f0350c2bc13068892 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?= <theophane@hufschmitt.net>
Date: Thu, 17 Mar 2022 16:13:29 +0100
Subject: [PATCH] rename: Fallback to a copy if the filesystems mismatch

In `nix::rename`, if the call to `rename` fails with `EXDEV` (failure
because the source and the destination are in a different filesystems)
switch to copying and removing the source.

To avoid having to re-implement the copy manually, I switched the
function to use the c++17 `filesystem` library (which has a `copy`
function that should do what we want).

Fix #6262
---
 src/libutil/filesystem.cc | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/src/libutil/filesystem.cc b/src/libutil/filesystem.cc
index 33a8d81a6..cf99b848a 100644
--- a/src/libutil/filesystem.cc
+++ b/src/libutil/filesystem.cc
@@ -1,8 +1,11 @@
 #include <sys/time.h>
+#include <filesystem>
 
 #include "util.hh"
 #include "types.hh"
 
+namespace fs = std::filesystem;
+
 namespace nix {
 
 void createSymlink(const Path & target, const Path & link,
@@ -42,8 +45,16 @@ void replaceSymlink(const Path & target, const Path & link,
 
 void moveFile(const Path & oldName, const Path & newName)
 {
-    if (::rename(oldName.c_str(), newName.c_str()))
-        throw SysError("renaming '%1%' to '%2%'", oldName, newName);
+    auto oldPath = fs::path(oldName);
+    auto newPath = fs::path(newName);
+    try {
+        fs::rename(oldPath, newPath);
+    } catch (fs::filesystem_error & e) {
+        if (e.code().value() == EXDEV) {
+            fs::copy(oldName, newName, fs::copy_options::copy_symlinks);
+            fs::remove_all(oldName);
+        }
+    }
 }
 
 }