1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
|
---
title: Verifying "npm ci" reproducibility
date: 2018-08-01
layout: post
lang: en
ref: verifying-npm-ci-reproducibility
updated_at: 2019-05-22
---
When [npm@5](https://blog.npmjs.org/post/161081169345/v500) came bringing
[package-locks](https://docs.npmjs.com/files/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[^package-lock-message].
However the [addition of `npm ci`](https://blog.npmjs.org/post/171556855892/introducing-npm-ci-for-faster-more-reliable)
filled this gap: it's a stricter variation of `npm install` which
guarantees that "[subsequent installs are able to generate identical trees](https://docs.npmjs.com/files/package-lock.json)". 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](https://en.wikipedia.org/wiki/Merkle_tree)
implementation using `sha256sum` and a few piped commands at the
terminal:
```bash
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:
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
```bash
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
[Lerna](https://lernajs.io/)[^lerna-package-lock]:
```bash
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 I'd
[like to know](mailto:{{ site.author.email }}).
## *Edit*
2019-05-22: Fix spelling.
[^package-lock-message]: The
[documentation](https://docs.npmjs.com/cli/install#description) claims `npm
install` is driven by the existing `package-lock.json`, but that's actually
[a little bit tricky](https://github.com/npm/npm/issues/17979#issuecomment-332701215).
[^lerna-package-lock]: Finding a big known repo that actually committed the
`package-lock.json` file was harder than I expected.
|
