chunking: Support exclusive chunks defined via xattrs

[ckyrouac]

This adds a new "specific_components" parameter to process_mapping to enable mapping individual files to specific layers. This will be used by the rechunker in rpm-ostree.

Related to: coreos/rpm-ostree#5431

[gemini-code-assist]

Code Review

This pull request introduces support for exclusive chunks, which allows specific components to be placed in their own layers. The implementation is well-structured, and includes a comprehensive set of new unit tests. Suggestions have been provided to improve code clarity, reduce test code duplication, and strengthen a test assertion.

[cgwalters]

This is all very complex code (mostly my fault!) and I find it hard to get it all in my head, but overall this looks reasonable to me.

I think what would really help is seeing this more "end to end" in combination with coreos/rpm-ostree#5431 - especially via a Dockerfile build that sets xattrs and verifies the chunking.

Maybe we could try standing up a CI job that clones your rpm-ostree PR and runs a test job from there? But optional from my PoV right now.

[ckyrouac]

This is all very complex code (mostly my fault!) and I find it hard to get it all in my head

Yep, took me awhile to wrap my head around. Might take a stab at refactoring it a bit after this gets in.

I think what would really help is seeing this more "end to end" in combination with coreos/rpm-ostree#5431 - especially via a Dockerfile build that sets xattrs and verifies the chunking.

Maybe we could try standing up a CI job that clones your rpm-ostree PR and runs a test job from there? But optional from my PoV right now.

I've been testing locally using this Containerfile (thanks claude) and using my local checkout of bootc in the rpm-ostree Cargo.toml. I can take a stab at doing this in our CI.

FROM quay.io/fedora/fedora-bootc:42

# Install attr package to set extended attributes
RUN dnf install -y attr

# Create some test files with different content
RUN echo "Database component file 1" > /usr/local/db-config.conf
RUN echo "Database component file 2" > /usr/local/db-schema.sql
RUN echo "Web server component file 1" > /usr/local/nginx.conf
RUN echo "Web server component file 2" > /usr/local/web-static.html
RUN echo "Monitoring component file" > /usr/local/metrics.json
RUN echo "Cache component file 1" > /usr/local/redis.conf
RUN echo "Cache component file 2" > /usr/local/cache-policy.json

# Set user.component extended attributes for different components (using user namespace since system may not be supported)
RUN setfattr -n user.component -v "database" /usr/local/db-config.conf
RUN setfattr -n user.component -v "database" /usr/local/db-schema.sql
RUN setfattr -n user.component -v "webserver" /usr/local/nginx.conf
RUN setfattr -n user.component -v "webserver" /usr/local/web-static.html
RUN setfattr -n user.component -v "monitoring" /usr/local/metrics.json
RUN setfattr -n user.component -v "cache" /usr/local/redis.conf
RUN setfattr -n user.component -v "cache" /usr/local/cache-policy.json

# Verify the extended attributes are set correctly
RUN echo "=== Verifying extended attributes ===" && \
    getfattr -n user.component /usr/local/db-config.conf && \
    getfattr -n user.component /usr/local/db-schema.sql && \
    getfattr -n user.component /usr/local/nginx.conf && \
    getfattr -n user.component /usr/local/web-static.html && \
    getfattr -n user.component /usr/local/metrics.json && \
    getfattr -n user.component /usr/local/redis.conf && \
    getfattr -n user.component /usr/local/cache-policy.json

# Also create some files without component xattrs for comparison
RUN echo "No component file 1" > /usr/local/no-component-1.txt
RUN echo "No component file 2" > /usr/local/no-component-2.txt

# Display file listing with extended attributes
RUN echo "=== Final file listing with xattrs ===" && \
    ls -la /usr/local/ && \
    echo "=== Extended attributes summary ===" && \
    find /usr/local/ -name "*.conf" -o -name "*.sql" -o -name "*.html" -o -name "*.json" | \
    xargs -I {} sh -c 'echo "File: {}"; getfattr -n user.component {} 2>/dev/null || echo "  No user.component xattr"'

[cgwalters]

I can take a stab at doing this in our CI.

I think it could also live in the rpm-ostree CI and then we just run that job from here, but ultimately it's also fine if it just lives in rpm-ostree I think. Hmm, maybe what could actually make sense is to move the chunking code there instead since there's 0% chance anything else uses it.