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

The Dockerfile runs make test 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. script/test
passes -count=1, so no test result is taken from the cache.

Model: opus-5-5

Dockerfile

@@ -50,7 +50,11 @@ ARG CHECK_EPOCH
 
 COPY . .
 
-RUN make test
+# Go's build cache is kept between builds in this cache mount, so make test
+# 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.
+RUN --mount=type=cache,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

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` compiles 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 +18,15 @@ 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 with
+  Go's build cache in a BuildKit cache mount, which docker keeps between builds,
+  locally and on the Gitea runner alike, so it compiles only what changed since
+  the last build. `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 uses 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 "$@"