Lossy numeric LayerContentMetadata

[joshwlewis]

When using any integer type as part of a Layer's Metadata, that metadata is deemed incompatible when reading from a previous layer. libcnb.rs seems to interpret these fields as Float.

For example, LayerMetadata like this:

#[derive(Deserialize, Serialize, Debug, Clone)]
struct CountingMetadata {
    counter_u64: u64,
    counter_i32: i32,
}

Will always hit the migrate_incompatible_metadata method, reporting the GenericMetadata as Some({"counter_i32": Float(1.0), "counter_u64": Float(1.0)}).

To reproduce:

1. Clone https://github.com/joshwlewis/sample-counter-buildpack
2. cargo libcnb package
3. pack build sample --buildpack target/buildpack/debug/sample-counter. It should build just fine.
4. pack build sample --buildpack target/buildpack/debug/sample-counter. It will fail to use the previously cached metadata, and that should show in the build logs.

[joshwlewis]

I dug into the cached docker volume. It looks like the values are stored as integers there:

$ docker run -it --rm -v=pack-cache-library_sample_latest-411aa02e5253.build:/tmp/build --entrypoint bash sample
heroku@85a7fa18b759:/workspace$ cat /tmp/build/committed/io.buildpacks.lifecycle.cache.metadata
{"sbom":{"sha":""},"buildpacks":[{"key":"sample-counter","version":"0.1.0","layers":{"counting":{"sha":"sha256:3748ee14800fba3519a14af54fffb9d0cbe8f20e8339f51cca422119dda0ecdf","data":{"counter_i32":1,"counter_u64":1},"build":true,"launch":true,"cache":true}}}]}

[edmorley]

Adding this to migrate_incompatible_metadata():

        let layer_file = std::fs::read_to_string(_ctx.layers_dir.join("counting.toml")).unwrap();
        println!("Layer file contents: {layer_file}");

...gives:

===> BUILDING
Layer file contents: [metadata]
  counter_i32 = 1.0
  counter_u64 = 1.0

Previous layer data invalid!!!

So the issue must either be on write, or a pack/lifecycle thing.

[edmorley]

Ok so this is a pack/lifecycle thing. If I read counting.toml to a string in build() before+after the handle_layer() call, I can see that it's written correctly, but on the next build it's been changed even before handle_layer() runs.

[edmorley]

This is enough to repro (create a skeleton buildpack using pack buildpack new testcase, drop the below in, then update api to 0.6 or newer and stacks to "*" in buildpack.toml):

#!/usr/bin/env bash

set -euo pipefail

layers_dir="${1}"

test_layer_dir="${layers_dir}/test"
test_layer_toml="${test_layer_dir}.toml"

mkdir -p "${test_layer_dir}"

if [[ -f "${test_layer_toml}" ]]; then
    echo "Existing layer TOML contents:"
    cat "${test_layer_toml}"
fi

cat > "${test_layer_toml}" << EOF
[types]
launch = true
build = true
cache = true

[metadata]
version = 1
EOF

...gives on the second (cached) build:

===> RESTORING
Restoring metadata for "testcase:test" from app image
Restoring data for "testcase:test" from cache
===> BUILDING
Existing layer TOML contents:
[metadata]
  version = 1.0

[schneems]

Here's a one stop reproduction script:

mkdir -p /tmp/fc3cf072c65938325d7541f1048f9f97
cd /tmp/fc3cf072c65938325d7541f1048f9f97

mkdir -p bin/

touch bin/build
touch bin/detect

cat > bin/detect <<'EOFF'
#!/usr/bin/env bash

set -euo pipefail

exit 0
EOFF


cat > bin/build <<'EOFF'
#!/usr/bin/env bash

set -euo pipefail

layers_dir="${1}"

test_layer_dir="${layers_dir}/test"
test_layer_toml="${test_layer_dir}.toml"

mkdir -p "${test_layer_dir}"

if [[ -f "${test_layer_toml}" ]]; then
    echo "Existing layer TOML contents:"
    cat "${test_layer_toml}"
fi

cat > "${test_layer_toml}" << EOF
[types]
launch = true
build = true
cache = true

[metadata]
version = 1
EOF
EOFF

cat > buildpack.toml <<'EOFF'
api = "0.6"

[buildpack]
id = "sample-counter"
version = "0.1.0"
name = "Sample Counter"
clear-env = true

[[buildpack.licenses]]
type = "BSD-3-Clause"

[[stacks]]
id = "*"
EOFF



chmod +x bin/build
chmod +x bin/detect

Running it twice demonstrates the error ed was seeing:

$ pack build sample --buildpack ./ --path ./
20: Pulling from heroku/buildpacks
Digest: sha256:c49ebf7c333d099ee765a1841acea6d5e5aa0a81a3375e96d7af41c4abc53344
Status: Image is up to date for heroku/buildpacks:20
20-cnb: Pulling from heroku/heroku
Digest: sha256:cb6e1902c1bd7c10e0e0d81d1a6c67a070e7820326b3cd22b11d0847b155ca5a
Status: Image is up to date for heroku/heroku:20-cnb
Previous image with name "sample" not found
===> DETECTING
sample-counter 0.1.0
===> RESTORING
===> BUILDING
===> EXPORTING
Adding layer 'sample-counter:test'
Adding 1/1 app layer(s)
Adding layer 'launcher'
Adding layer 'config'
Adding label 'io.buildpacks.lifecycle.metadata'
Adding label 'io.buildpacks.build.metadata'
Adding label 'io.buildpacks.project.metadata'
no default process type
Saving sample...
*** Images (12e0340bd3e0):
      sample
Adding cache layer 'sample-counter:test'
Successfully built image sample
⛄️ 3.1.2 🚀 /tmp/fc3cf072c65938325d7541f1048f9f97
$ pack build sample --buildpack ./ --path ./
20: Pulling from heroku/buildpacks
Digest: sha256:c49ebf7c333d099ee765a1841acea6d5e5aa0a81a3375e96d7af41c4abc53344
Status: Image is up to date for heroku/buildpacks:20
20-cnb: Pulling from heroku/heroku
Digest: sha256:cb6e1902c1bd7c10e0e0d81d1a6c67a070e7820326b3cd22b11d0847b155ca5a
Status: Image is up to date for heroku/heroku:20-cnb
===> DETECTING
sample-counter 0.1.0
===> RESTORING
Restoring metadata for "sample-counter:test" from app image
Restoring data for "sample-counter:test" from cache
===> BUILDING
Existing layer TOML contents:
[metadata]
  version = 1.0
===> EXPORTING
Reusing layer 'sample-counter:test'
Reusing 1/1 app layer(s)
Reusing layer 'launcher'
Reusing layer 'config'
Adding label 'io.buildpacks.lifecycle.metadata'
Adding label 'io.buildpacks.build.metadata'
Adding label 'io.buildpacks.project.metadata'
no default process type
Saving sample...
*** Images (12e0340bd3e0):
      sample
Reusing cache layer 'sample-counter:test'
Successfully built image sample

Note the version = 1.0 instead of version = 1 as expected.

According to https://toml.io/en/ integers are supported types. I say this is for sure a bug in lifecycle. I'll move to make this bash example an https://codetriage.com/example_app and open an issue against the lifecycle.

[schneems]

Bash example repro https://github.com/schneems/cnb-metadata-integer-bug