Re-implement the recursive directory copy

The recursive copy from the stl doesn’t exactly do what we need because
1. It doesn’t delete things as we go
2. It doesn’t keep the mtime, which change the nars

So re-implement it ourselves. A bit dull, but that way we have what we want

1 files changed, 50 insertions, 2 deletions

diff --git a/src/libutil/filesystem.cc b/src/libutil/filesystem.cc
index cf99b848a..198db2832 100644
--- a/src/libutil/filesystem.cc
+++ b/src/libutil/filesystem.cc
@@ -43,6 +43,53 @@ void replaceSymlink(const Path & target, const Path & link,
     }
 }
 
+void setWriteTime(const fs::path & p, const struct stat & st)
+{
+    struct timeval times[2];
+    times[0] = {
+        .tv_sec = st.st_atime,
+        .tv_usec = 0,
+    };
+    times[1] = {
+        .tv_sec = st.st_mtime,
+        .tv_usec = 0,
+    };
+    warn("Setting the mtime of %s to %d", p.c_str(), st.st_mtim.tv_sec);
+    if (lutimes(p.c_str(), times) != 0)
+        throw SysError("changing modification time of '%s'", p);
+}
+
+void copy(const fs::directory_entry & from, const fs::path & to, bool andDelete)
+{
+    // TODO: Rewrite the `is_*` to use `symlink_status()`
+    auto statOfFrom = lstat(from.path().c_str());
+    auto fromStatus = from.symlink_status();
+
+    // Mark the directory as writable so that we can delete its children
+    if (andDelete && fs::is_directory(fromStatus)) {
+        fs::permissions(from.path(), fs::perms::owner_write, fs::perm_options::add | fs::perm_options::nofollow);
+    }
+
+
+    if (fs::is_symlink(fromStatus) || fs::is_regular_file(fromStatus)) {
+        fs::copy(from.path(), to, fs::copy_options::copy_symlinks | fs::copy_options::overwrite_existing);
+    } else if (fs::is_directory(fromStatus)) {
+        fs::create_directory(to);
+        for (auto & entry : fs::directory_iterator(from.path())) {
+            copy(entry, to / entry.path().filename(), andDelete);
+        }
+    } else {
+        throw Error("file '%s' has an unsupported type", from.path());
+    }
+
+    setWriteTime(to, statOfFrom);
+    if (andDelete) {
+        if (!fs::is_symlink(fromStatus))
+            fs::permissions(from.path(), fs::perms::owner_write, fs::perm_options::add | fs::perm_options::nofollow);
+        fs::remove(from.path());
+    }
+}
+
 void moveFile(const Path & oldName, const Path & newName)
 {
     auto oldPath = fs::path(oldName);
@@ -51,8 +98,9 @@ void moveFile(const Path & oldName, const Path & newName)
         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);
+            fs::remove(newPath);
+            warn("Copying %s to %s", oldName, newName);
+            copy(fs::directory_entry(oldPath), newPath, true);
         }
     }
 }