build: allow the cleanup step to be non-fatal

3e8962c (Clean up dep-info files from OUT_DIR, 2024-10-22)
made the cleanup step a fatal operation when cleanup fails.
Some filesystems (e.g. NFS v3) can run into timing issues
under load where .nfs* dummy files can still exist for a short
window of time after the probe command completes. This window
is just large enough for a failure to occur in remove_all().

Add an environment variable that allows users to make the
cleanup operation non-fatal.

Author: davvid

build.rs

@@ -203,10 +203,14 @@ fn compile_probe(rustc_bootstrap: bool) -> bool {
     // Clean up to avoid leaving nondeterministic absolute paths in the dep-info
     // file in OUT_DIR, which causes nonreproducible builds in build systems
     // that treat the entire OUT_DIR as an artifact.
+    // Set the PROC_MACRO2_IGNORE_CLEANUP environment variable to make errors from
+    // the cleanup process non-fatal.
     if let Err(err) = fs::remove_dir_all(&out_subdir) {
         if err.kind() != ErrorKind::NotFound {
             eprintln!("Failed to clean up {}: {}", out_subdir.display(), err);
-            process::exit(1);
+            if env::var_os("PROC_MACRO2_IGNORE_CLEANUP").is_none() {
+                process::exit(1);
+            }
         }
     }