Verifying npm ci reproducibility

Posted on August 1, 2018

When npm@5 came bringing package-locks with it, I was confused about the benefits it provided, since running npm install more than once could resolve all the dependencies again and yield yet another fresh package-lock.json file. The message saying “you should add this file to version control” left me hesitant on what to do1.

However the addition of npm ci filled this gapped: it’s a stricter variation of npm install which guarantees that “subsequent installs are able to generate identical trees”. But are they really identical? I could see that I didn’t have the same problems of different installation outputs, but I didn’t know for sure if it was really identical.

Computing the hash of a directory’s content

I quickly searched for a way to check for the hash signature of an entire directory tree, but I couldn’t find one. I’ve made a poor man’s Merkle tree implementation using sha256sum and a few piped comands at the terminal:

Going through it line by line:

  • #1 we define a Bash function called merkle-tree;
  • #2 it accepts a single argument: the directory to compute the merkle tree from. If nothing is given, it runs on the current directory (.);
  • #3 we go to the directory, so we don’t get different prefixes in find’s output (like ../a/b);
  • #4 we get all files from the directory tree. Since we’re using sha256sum to compute the hash of the file contents, we need to filter out folders from it;
  • #5 we need to sort the output, since different filesystems and find implementations may return files in different orders;
  • #6 we use xargs to compute the hash of each file individually through sha256sum. Since a file may contain spaces we need to scape it with quotes;
  • #7 we compute the hash of the combined hashes. Since sha256sum output is formatted like <hash> <filename>, it produces a different final hash if a file ever changes name without changing it’s content;
  • #8 we get the final hash output, excluding the <filename> (which is - in this case, aka stdin).

Positive points:

  1. ignore timestamp: running more than once on different installation yields the same hash;
  2. the name of the file is included in the final hash computation.

Limitations:

  1. it ignores empty folders from the hash computation;
  2. the implementation’s only goal is to represent using a digest whether the content of a given directory is the same or not. Leaf presence checking is obviously missing from it.

Testing locally with sample data

It seems to work for this simple test case.

You can try copying and pasting it to verify the hash signatures.

Using merkle-tree to check the output of npm ci

I’ve done all of the following using Node.js v8.11.3 and npm@6.1.0.

In this test case I’ll take the main repo of Lerna2:

Good job npm ci :)

#6 and #9 take some time to run (21s in my machine), but this specific use case isn’t performance sensitive. The slowest step is computing the hash of each individual file.

Conclusion

npm ci really “generates identical trees”.

I’m not aware of any other existing solution for verifying the hash signature of a directory. If you know any I’d like to know.


  1. The documentation claims npm install is driven by the existing package-lock.json, but that’ actually a little bit tricky.

  2. Finding a big known repo that actually committed the package-lock.json file was harder than I expected.