Keep the shared caches unchanged by a candidate's step, under a throwaway writable layer (issue 1309, S18, ADR 0448)

A trunk build alone writes them; any other step writes into an overlay that dies with it.

go/buildstep/isolate.go

@@ -132,13 +132,18 @@ func (iso Isolation) command(env []string, name string, args ...string) ([]strin
 	return append(argv, args...), nil
 }
 
-// cacheArgs binds the shared caches: read-write for a trunk build, otherwise read-only (S18).
+// cacheArgs binds the shared warm caches (S18): for a trunk build read-write, so the next build is warm; for any other
+// read-only beneath an overlay whose writes go to a tmpfs of the step's own, so the step writes as a build does and
+// nothing it wrote outlives it (bubblewrap's --tmp-overlay, an overlay mount in the step's user namespace).
 func cacheArgs(caches []string, write bool) []string {
 	var argv []string
 	for _, c := range caches {
-		argv = append(argv, "--ro-bind", c, c)
+		if write {
+			argv = append(argv, "--bind", c, c)
+			continue
+		}
+		argv = append(argv, "--overlay-src", c, "--tmp-overlay", c)
 	}
-	_ = write
 	return argv
 }
 

go/buildstep/isolate_caches_test.go

@@ -0,0 +1,30 @@
+package buildstep
+
+import "testing"
+
+// The shared warm caches are read under a throwaway writable layer (novox/hq ADR 0448; issue 1309): a candidate's step
+// writes into them as a build does, and nothing it writes outlives it or reaches a later build; only a trunk build
+// writes them.
+func TestACandidatesStepWritesTheCachesOnlyIntoAThrowawayLayer(t *testing.T) {
+	fixedSystem(t)
+	iso := Isolation{Workspace: "/w", Caches: []string{"/var/cache/go-build", "/var/cache/npm"}}
+	argv, err := iso.Command("go", "test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, c := range iso.Caches {
+		if !holds(argv, "--overlay-src", c, "--tmp-overlay", c) {
+			t.Errorf("the cache %s is not under a throwaway layer: %v", c, argv)
+		}
+		if holds(argv, "--bind", c, c) {
+			t.Errorf("a candidate's step writes the cache %s: %v", c, argv)
+		}
+	}
+	iso.WriteCaches = true
+	argv, _ = iso.Command("go", "build")
+	for _, c := range iso.Caches {
+		if !holds(argv, "--bind", c, c) || holds(argv, "--tmp-overlay", c) {
+			t.Errorf("a trunk build does not write the cache %s: %v", c, argv)
+		}
+	}
+}