ADD with an outdated checksum succeeds using outdated content, where a build error should occur

Note: Created with the help of Claude AI

ADD --checksum with an outdated checksum can produce a build that appears to succeed while using
outdated file content. A build error should have occurred, because the content at the URL does not match
the declared checksum.

This happens when the URL is updated and the checksum is not updated with it. The URL is not part of the
cache key, so two URLs sharing a final path segment share a cache entry and the previously cached content
is reused. No request is made and nothing is compared.

Reproducer

https://github.com/k0pernikus/add-cached-file-even-after-content-change

• repro fails on the step Build 2 should invalidate the cache and get file content BRAVO:
  https://github.com/k0pernikus/add-cached-file-even-after-content-change/actions/runs/33623957687
• updated-checksum passes, because it updates the URL and the checksum together:
  https://github.com/k0pernikus/add-cached-file-even-after-content-change/actions/runs/33623957548

Two files with the same basename in different directories, served over python -m http.server:

# syntax=docker/dockerfile:1
FROM scratch
ARG SRC_URL
ARG SRC_SHA256
ADD --checksum=sha256:${SRC_SHA256} ${SRC_URL} /payload

Observed

Build 2 logs the reused layer:

#6 [1/1] ADD --checksum=sha256:1921b918... http://127.0.0.1:8099/folder_2/same_file /payload
#6 CACHED

Build 3 shows that URL serves different bytes. Build 2's content therefore came from the cache, not the
server. The HTTP server's access log records no request during build 2.

Mechanism

In source/http/source.go, resolveMetadata calls resolveMetadataStatic, which returns early when a
checksum is declared, without making a request:

if hs.src.Checksum != "" {
    return &Metadata{
        Digest:   hs.src.Checksum,
        Filename: getFileName(hs.src.URL, hs.src.Filename, nil),
    }, nil
}

CacheKey then returns formatCacheKey(md.Filename, md.Digest, md.LastModified). The URL is not among
those inputs. getFileName reduces it to path.Base(u.Path), so folder_1/same_file and
folder_2/same_file both reduce to same_file. LastModified is nil because nothing was fetched.

Environment

Reproduced on two BuildKit versions, both with the docker driver embedded in dockerd:

• GitHub Actions ubuntu-latest: BuildKit v0.20.2, Docker 28.0.4, buildx v0.19.0
• Local: BuildKit v0.32.2, Docker 29.7.2, buildx v0.36.1

linux/amd64, frontend docker/dockerfile:1.

Start in source/http/source.go, focusing on resolveMetadata, resolveMetadataStatic, and CacheKey. Run the linked reproducer to observe the cache reuse and HTTP access logs. Done means changing the behavior so an outdated checksum cannot produce a successful build from cached content, while matching content still builds successfully.