Linker copies files as a fallback when ref-linking fails

[snowsignal]

Summary

Fixes #1444.

In situations where the installer fails to perform a reflink, a regular file copy is also attempted, as a fallback. This circumvents issues with linking files across filesystems or volumes.

Test Plan

N/A

[charliermarsh]

Oh nice, I didn't know this existed.

[konstin]

I wonder what the right strategy to test this is, can we create a ram disk or a virtual filesystem as non-root?

[charliermarsh]

I would suggest just stubbing out reflink with a method that always errors.

[konstin]

How you implement this rust-wise, without unittest.mock?

[charliermarsh]

@konstin - Oh sorry, I meant for manual testing (while implementing).

[charliermarsh]

Nice, thanks!

[charliermarsh]

Is this different than including it as an else in the Attempt::Initial arm?

E.g.:

diff --git a/crates/install-wheel-rs/src/linker.rs b/crates/install-wheel-rs/src/linker.rs
index 6275994f..69eff653 100644
--- a/crates/install-wheel-rs/src/linker.rs
+++ b/crates/install-wheel-rs/src/linker.rs
@@ -356,6 +356,9 @@ fn clone_recursive(
                     *attempt = Attempt::UseCopyFallback;
                     clone_recursive(site_packages, wheel, entry, attempt)?;
                 }
+            } else {
+                // If the first reflink succeeded, we'll use reflink for the rest of the install.
+                *attempt = Attempt::Subsequent;
             }
         }
         Attempt::Subsequent => {
@@ -390,9 +393,6 @@ fn clone_recursive(
         }
     }

-    if *attempt == Attempt::Initial {
-        *attempt = Attempt::Subsequent;
-    }
     Ok(())
 }

[snowsignal]

It would be, because we also want to update the attempt value in the case of an AlreadyExists error that we handle. For example, if we have to overwrite a file, but don't need to fall-back to copying, this else branch would not be hit.