vcpkg binary caching miss: slow rebuild every run in CI

vcpkg builds ports from source and only skips the build when a matching binary is in a cache. With no persisted binary cache, every CI run recompiles every dependency, so builds are slow but do not error. The fix is to enable and persist binary caching.

What this error means

vcpkg install spends many minutes "Building X" on every run for dependencies that never changed. Logs show no "Restored" or cache hit lines.

Building openssl:x64-linux...
Building boost-system:x64-linux...
Building curl:x64-linux...
-- Installing ... (0 restored from cache)

Common causes

No binary cache is configured

vcpkg defaults to a local cache under the home directory, which is empty on a fresh runner, so every port builds from source.

The cache is not persisted between runs

Even with a local cache, if the directory is not saved and restored across jobs, each run starts cold.

How to fix it

Use the GitHub Actions binary cache

Point vcpkg at the Actions cache so built binaries persist across runs.

env:
  VCPKG_BINARY_SOURCES: 'clear;x-gha,readwrite'
# and enable the GHA cache backend in the setup step

Cache the binary directory yourself

Restore and save the vcpkg binary cache path with actions/cache keyed on the manifest and baseline.

- uses: actions/cache@v4
  with:
    path: ~/.cache/vcpkg/archives
    key: vcpkg-${{ hashFiles('vcpkg.json') }}

How to prevent it

• Enable a persisted vcpkg binary cache in every CI job.
• Key the cache on the manifest and baseline so it invalidates correctly.
• Pin the vcpkg baseline so cache keys stay stable between runs.

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