= Verifying "npm ci" reproducibility
:updatedat: 2019-05-22

:empty:
:npm-5: https://blog.npmjs.org/post/161081169345/v500
:package-locks-old: https://docs.npmjs.com/files/package-locks
:package-lock: https://docs.npmjs.com/files/package-lock.json
:add-npm-ci: https://blog.npmjs.org/post/171556855892/introducing-npm-ci-for-faster-more-reliable
:cli-docs: https://docs.npmjs.com/cli/install#description
:tricky-issue: https://github.com/npm/npm/issues/17979#issuecomment-332701215

When {npm-5}[npm@5] came bringing {package-locks-old}[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
do{empty}footnote:package-lock-message[
  {cli-docs}[documentation] claims `npm install` is driven by the existing
  `package-lock.json`, but that's actually {tricky-issue}[a little bit tricky].
].

However the {add-npm-ci}[addition of `npm ci`] filled this gap: it's a stricter
variation of `npm install` which guarantees that "{package-lock}[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

:merkle-tree: https://en.wikipedia.org/wiki/Merkle_tree

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}[Merkle tree] implementation using `sha256sum` and a few piped
commands at the terminal:

[source,sh]
----
merkle-tree () {
	dirname="${1-.}"
	pushd "$dirname"
	find . -type f |
		sort                      |
		xargs -I{} sha256sum "{}" |
		sha256sum                 |
		awk '{print $1}'
	popd
}
----

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 file systems 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 escape 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:

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

=== Limitations:

. it ignores empty folders from the hash computation;
. 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

[source,sh]
----
mkdir /tmp/merkle-tree-test/
cd /tmp/merkle-tree-test/
mkdir -p a/b/ a/c/ d/
echo "one"   > a/b/one.txt
echo "two"   > a/c/two.txt
echo "three" > d/three.txt
merkle-tree . # output is       be343bb01fe00aeb8fef14a3e16b1c3d1dccbf86d7e41b4753e6ccb7dc3a57c3
merkle-tree . # output still is be343bb01fe00aeb8fef14a3e16b1c3d1dccbf86d7e41b4753e6ccb7dc3a57c3
echo "four"  > d/four.txt
merkle-tree . # output is now   b5464b958969ed81815641ace96b33f7fd52c20db71a7fccc45a36b3a2ae4d4c
rm d/four.txt
merkle-tree . # output back to  be343bb01fe00aeb8fef14a3e16b1c3d1dccbf86d7e41b4753e6ccb7dc3a57c3
echo "hidden-five" > a/b/one.txt
merkle-tree . # output changed  471fae0d074947e4955e9ac53e95b56e4bc08d263d89d82003fb58a0ffba66f5
----

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
https://lernajs.io/[Lerna]footnote:lerna-package-lock[
  Finding a big known repo that actually committed the `package-lock.json` file
  was harder than I expected.
]:

[source,sh]
----
cd /tmp/
git clone https://github.com/lerna/lerna.git
cd lerna/
git checkout 57ff865c0839df75dbe1974971d7310f235e1109
npm ci
merkle-tree node_modules/ # outputs 11e218c4ac32fac8a9607a8da644fe870a25c99821167d21b607af45699afafa
rm -rf node_modules/
npm ci
merkle-tree node_modules/ # outputs 11e218c4ac32fac8a9607a8da644fe870a25c99821167d21b607af45699afafa
npm ci      # test if it also works with an existing node_modules/ folder
merkle-tree node_modules/ # outputs 11e218c4ac32fac8a9607a8da644fe870a25c99821167d21b607af45699afafa
----

Good job `npm ci` :)

#6 and #9 take some time to run (21 seconds 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, shoot me an email, as I'd like to know it.

== *Edit*

2019-05-22: Fix spelling.
