Keep Go's build cache between builds (closes #124)

The Dockerfile runs make test and make build with Go's build cache in a
BuildKit cache mount, so a build compiles only what changed since the
last one instead of the standard library and every dependency from
nothing. The mount has an id of its own, so builds of other repositories,
compiled against other C headers, do not share it. script/test passes
-count=1, so no test result is taken from the cache.

Model: opus-5-5

Dockerfile

@@ -50,15 +50,26 @@ ARG CHECK_EPOCH
 
 COPY . .
 
-RUN make test
+# Go's build cache is kept between builds in this cache mount, which make test
+# and make build both use, so each compiles only what changed since the last
+# build, not the standard library and every dependency from nothing.
+# script/test passes -count=1, so test results are never taken from it.
+# The mount has its own id because the default id, its path, is shared with
+# other repositories' builds, and Go's cache does not notice changes to C
+# libraries: an entry compiled there against other C headers could be reused
+# here. It does not notice a change of this image's own C headers either.
+RUN --mount=type=cache,id=sneak/secret/go-build,target=/root/.cache/go-build \
+    make test
 
 # The version stamped into the binary: the VERSION build argument when one
 # is given, otherwise `git describe --tags --always` of the .git the build
 # context carries: the tag on a tagged commit, tag-N-gHASH on a commit after
 # one, the short commit when no tag is reachable. A context that carries .git
-# and still yields no version fails the build.
+# and still yields no version fails the build. make build uses the same Go
+# build cache mount as make test.
 ARG VERSION
-RUN version="${VERSION:-$(git describe --tags --always)}"; \
+RUN --mount=type=cache,id=sneak/secret/go-build,target=/root/.cache/go-build \
+    version="${VERSION:-$(git describe --tags --always)}"; \
     if [ -e .git ] && { [ -z "$version" ] || [ "$version" = dev ] || \
         [ "$version" = unknown ]; }; then \
         echo "no version could be derived although the build context carries .git" >&2; \

README.md

@@ -604,7 +604,8 @@ provide:
 - `script/build` — build the `secret` binary into the repo root, stamping the
   version (`VERSION` from the environment, else `git describe`) and the git
   commit
-- `script/test` — run `go vet` and the test suite (verbose rerun on failure)
+- `script/test` — run `go vet` and the test suite (verbose rerun on failure),
+  every test on every run, never a result from Go's test cache
 - `script/lint` — run `golangci-lint` in docker only: builds `Dockerfile.lint`,
   where the linter is a build step that runs on every call, also on an unchanged
   tree
@@ -624,7 +625,8 @@ provide:
 - `script/cibuild` — CI entrypoint: `docker build --ulimit memlock=-1:-1 .`
   (memguard needs mlock; the Dockerfile runs the checks), with a new
   `CHECK_EPOCH` build argument on every run so the checks run again on an
-  unchanged tree
+  unchanged tree; the `Dockerfile` keeps Go's build cache between builds, so
+  `make test` and `make build` compile only what changed
 - `script/precommit` — pre-commit checks: `go mod tidy` verification, then
   `script/check`
 - `script/install-precommit` — install the git pre-commit hook that runs

TODO.md

@@ -18,6 +18,16 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 2026-10-06: `make test` in `script/cibuild` no longer compiles the standard
+  library and every dependency from nothing on every build
+  (https://git.eeqj.de/sneak/secret/issues/124). The `Dockerfile` runs it and
+  `make build` with Go's build cache in a BuildKit cache mount, which docker
+  keeps between builds, locally and on the Gitea runner alike, so each compiles
+  only what changed since the last build. The mount has an id of its own, so
+  other repositories' builds do not share it. `script/test` passes `-count=1`,
+  so every test runs on every build and no result comes from Go's test cache. A
+  build with an empty cache, such as the first after docker's build cache is
+  cleared, compiles everything in `make test` as before.
 - 2026-10-06: The tests run quickly with the race detector on
   (https://git.eeqj.de/sneak/secret/issues/120). Most of their time went to
   deriving keys from passphrases with scrypt, which is slow on purpose. The new

script/cibuild

@@ -6,7 +6,8 @@
 # the lower limit of a plain `docker build .`.
 # A cached build checks nothing: a new CHECK_EPOCH on every run makes the
 # Dockerfile's check steps run again on an unchanged tree, while its base
-# images and module downloads stay cached.
+# images, module downloads and the Go build cache that make test and make
+# build use stay cached.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"

script/test

@@ -9,7 +9,9 @@ main() {
     # CGO is required (Makefile exports this too)
     export CGO_ENABLED=1
     go vet ./...
-    go test ./... || go test -v ./...
+    # -count=1: run every test, never take a result from Go's test cache,
+    # which the Dockerfile keeps between builds
+    go test -count=1 ./... || go test -count=1 -v ./...
 }
 
 main "$@"