hashFiles silently generating empty hash

[rbtcollins]

I'm not sure whats going on yet, but I had inappropriate cache hits, and tracked it down to hashFiles giving the same signature when file content had changed.

Concretely, I write a number of strings to a file:

String hashStrings(List strings) {
    // need a java extension perhaps - the sandbox is blocking the
    // obvious .digest('SHA-1') method.
    // Manual salt to allow rekeying if needed
    input = ["serial-1"] + strings
    temp_path = pwd(tmp: true)
    hash_path = "${temp_path}/cargo-cache-key-vars"
    writeFile(file: hash_path, text: input.join('\n'))
    echo "hashing ${input.join('\n')}"
    hash = sh(script: "md5sum < ${hash_path}", returnStdout: true).split(' ')[0]
    hash_files_hash = hashFiles(hash_path)
    echo "${hash} ${hash_files_hash}"
    return hash
}

Which echos e973e42cfa03aa8c5a826ecffb41431a d41d8cd98f00b204e9800998ecf8427e

Now that hashFiles hash is a well known hash - a completely empty hash:

echo -n | md5sum
d41d8cd98f00b204e9800998ecf8427e

This code Seems fine IFF it is choosing the Path override.

So I'm not sure. Perhaps something to do with the k8s plugin :/.