libutil/mount: accept copy flags for bind path

Sometimes, `bindPath` will detect the source is a symlink and we are not
using the new mount API which support symlinks (kernel ≥ 5.12 IIRC?).

In those instances, we copy the inode to the target.

But some callers may want to follow the symlink in such circumstances,
we add a new default argument to the previous value and let caller
decide for themselves.

Change-Id: I8505b613fc614ce539eb89258fbbb7eaecebe23b
Signed-off-by: Raito Bezarius <raito@lix.systems>

lix/libutil/mount.cc

@@ -7,7 +7,8 @@
 
 namespace nix {
 
-void bindPath(const Path & source, const Path & target, bool optional) {
+void bindPath(const Path & source, const Path & target, bool optional, CopyFileFlags flags)
+{
     debug("bind mounting '%1%' to '%2%'", source, target);
 
     auto bindMount = [&]() {
@@ -30,14 +31,13 @@ void bindPath(const Path & source, const Path & target, bool optional) {
     } else if (S_ISLNK(st.st_mode)) {
         // Symlinks can (apparently) not be bind-mounted, so just copy it
         createDirs(dirOf(target));
-        copyFile(source, target, {});
+        copyFile(source, target, flags);
     } else {
         createDirs(dirOf(target));
         writeFile(target, "");
         bindMount();
     }
 }
-
 }
 
 #endif

lix/libutil/mount.hh

@@ -2,6 +2,7 @@
 ///@file
 
 #include "lix/libutil/types.hh"
+#include "lix/libutil/file-system.hh"
 
 #if __linux__
 namespace nix {
@@ -9,8 +10,14 @@ namespace nix {
 /**
  * Bind-mount file or directory from `source` to `destination`.
  * If source does not exist this will fail unless `optional` is set
+ *
+ * If `source` is a symlink, it will perform a copy instead of a bind mount
+ * because symlinks cannot be bind mounted on all versions of the Linux kernel.
+ *
+ * If a copy is performed, extra flags to the copy can be passed using `flags`.
  */
-void bindPath(const Path & source, const Path & target, bool optional = false);
-
+void bindPath(
+    const Path & source, const Path & target, bool optional = false, CopyFileFlags flags = {}
+);
 }
 #endif