internal/ci: restore git modified times after cloning

On one hand, "git clone" creates every file anew,
with their modified times representing the current time.
On the other, Go's test caching tracks the attributes and contents of
the files opened by each test as part of its cache key hashes.
Unfortunately, the modified times are always part of that hash,
even when the files are small. See https://go.dev/issues/58571.

Due to those factors, the trybots always re-run nearly all tests.
In particular, we use testdata files in quite a few packages,
so none of those can realistically hit the test cache in CI.

Until Go's test caching can be made a bit smarter about modified times,
use a python script packaged as a GitHub Action to restore each file
tracked by git to the commit time when they were last modified.
Luckily, the action only runs a simple python script,
so it seems to consistently finish in under a second.

We also have to fetch the git repository with full history,
since actions/checkout defaults to a shallow clone and the python script
needs full git history to properly reconstruct modified times.
Luckily, GitHub seem to have optimized git cloning from Actions,
so the actions/checkout step still seems to finish in around one second.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I7f1decd2ad1484f1a30e092b530b93833f6099c2
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/551227
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

.github/workflows/trybot.yml

@@ -32,6 +32,9 @@ jobs:
         uses: actions/checkout@v3
         with:
           ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: 0
+      - name: Restore git file modification times
+        uses: chetan/git-restore-mtime-action@075f9bc9d159805603419d50f794bd9f33252ebe
       - name: Install Go
         uses: actions/setup-go@v3
         with:

internal/ci/base/base.cue

@@ -53,6 +53,16 @@ import (
 	uses: "actions/checkout@v3"
 }
 
+// #restoreGitModTimes works around test cache misses due to https://go.dev/issues/58571.
+// Note that this action requires actions/checkout to use a fetch-depth of 0.
+// Since this is a third-party action which runs arbitrary code,
+// we pin a commit hash for v2 to be in control of code updates.
+// TODO(mvdan): May be unnecessary once the Go bug above is fixed.
+#restoreGitModTimes: json.#step & {
+	name: "Restore git file modification times"
+	uses: "chetan/git-restore-mtime-action@075f9bc9d159805603419d50f794bd9f33252ebe"
+}
+
 #earlyChecks: json.#step & {
 	name: "Early git and code sanity checks"
 	run: """

internal/ci/github/trybot.cue

@@ -51,8 +51,12 @@ trybot: _base.#bashWorkflow & {
 					// testing the PR's HEAD merged on top of the master branch.
 					// For consistency with Gerrit, avoid that merge commit entirely.
 					// This doesn't affect "push" builds, which never used merge commits.
-					with: ref: "${{ github.event.pull_request.head.sha }}"
+					with: {
+						ref: "${{ github.event.pull_request.head.sha }}"
+						"fetch-depth": 0 // see #restoreGitModTimes's doc
+					}
 				},
+				_base.#restoreGitModTimes,
 				_base.#installGo,
 
 				// cachePre must come after installing Node and Go, because the cache locations