Description

FSCompressedFileSuffixes (fs.go:456) is an exported mutable map[string]string. It is read during FSHandler initialization inside a sync.Once, but being exported, any caller can concurrently read/write this map while the Once executes.

var FSCompressedFileSuffixes = map[string]string{
    "br":   ".br",
    "gzip": ".gz",
}

If a user modifies this map concurrently with FS initialization, it's a data race on the map (concurrent map read/write causes a fatal panic in Go).

Impact

• Fatal crash (concurrent map read and map write) if modified after import while FS is being initialized concurrently.
• Low probability in typical usage but high severity when it occurs.

Suggested Fix

Either:

1. Unexport the map and provide a SetCompressedFileSuffixes() function that copies.
2. Copy the map once at init time in the sync.Once block, so the original can be freely modified.