cmd/dist: skip rebuilding packages during builder testing

Since packages in "std" no longer have install targets, checking them
for staleness is somewhat meaningless: if they are not cached they
will be rebuilt anyway, and without installed archives against which
we can compare them the staleness check will not detect builder skew.

It would still be meaningful to check "cmd" for staleness, but
(especially on sharded VM-based builders) that is a fairly expensive
operation relative to its benefit. If we are really interested in
detecting builder skew and/or build reproducibility, we could instead
add a "misc" test (similar to "misc/reboot", or perhaps even a part of
that test) that verifies that bootstrapped binaries are reproducible.

For #57734.
Updates #47257.
Updates #56896.

Change-Id: I8683ee81aefe8fb59cce9484453df9729bdc587c
Reviewed-on: https://go-review.googlesource.com/c/go/+/452775
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/dist/test.go

@@ -154,41 +154,21 @@ func (t *tester) run() {
 
 	if !t.listMode {
 		if builder := os.Getenv("GO_BUILDER_NAME"); builder == "" {
-			// Complete rebuild bootstrap, even with -no-rebuild.
+			// Ensure that installed commands are up to date, even with -no-rebuild,
+			// so that tests that run commands end up testing what's actually on disk.
 			// If everything is up-to-date, this is a no-op.
-			// If everything is not up-to-date, the first checkNotStale
-			// during the test process will kill the tests, so we might
-			// as well install the world.
-			// Now that for example "go install cmd/compile" does not
-			// also install runtime (you need "go install -i cmd/compile"
-			// for that), it's easy for previous workflows like
-			// "rebuild the compiler and then run run.bash"
-			// to break if we don't automatically refresh things here.
-			// Rebuilding is a shortened bootstrap.
+			// We first build the toolchain twice to allow it to converge,
+			// as when we first bootstrap.
 			// See cmdbootstrap for a description of the overall process.
+			//
+			// On the builders, we skip this step: we assume that 'dist test' is
+			// already using the result of a clean build, and because of test sharding
+			// and virtualization we usually start with a clean GOCACHE, so we would
+			// end up rebuilding large parts of the standard library that aren't
+			// otherwise relevant to the actual set of packages under test.
 			goInstall(toolenv, gorootBinGo, toolchain...)
 			goInstall(toolenv, gorootBinGo, toolchain...)
 			goInstall(toolenv, gorootBinGo, "cmd")
-			goInstall(nil, gorootBinGo, "std")
-		} else {
-			// The Go builder infrastructure should always begin running tests from a
-			// clean, non-stale state, so there is no need to rebuild the world.
-			// Instead, we can just check that it is not stale, which may be less
-			// expensive (and is also more likely to catch bugs in the builder
-			// implementation).
-			// The cache used by dist when building is different from that used when
-			// running dist test, so rebuild (but don't install) std and cmd to make
-			// sure packages without install targets are cached so they are not stale.
-			goCmd(toolenv, gorootBinGo, "build", "cmd") // make sure dependencies of targets are cached
-			goCmd(nil, gorootBinGo, "build", "std")
-			checkNotStale(nil, gorootBinGo, "std")
-			if builder != "aix-ppc64" {
-				// The aix-ppc64 builder for some reason does not have deterministic cgo
-				// builds, so "cmd" is stale. Fortunately, most of the tests don't care.
-				// TODO(#56896): remove this special case once the builder supports
-				// determistic cgo builds.
-				checkNotStale(toolenv, gorootBinGo, "cmd")
-			}
 		}
 	}
 
@@ -1361,7 +1341,6 @@ func (t *tester) registerCgoTests() {
 // running in parallel with earlier tests, or if it has some other reason
 // for needing the earlier tests to be done.
 func (t *tester) runPending(nextTest *distTest) {
-	checkNotStale(nil, gorootBinGo, "std")
 	worklist := t.worklist
 	t.worklist = nil
 	for _, w := range worklist {
@@ -1419,7 +1398,6 @@ func (t *tester) runPending(nextTest *distTest) {
 			log.Printf("Failed: %v", w.err)
 			t.failed = true
 		}
-		checkNotStale(nil, gorootBinGo, "std")
 	}
 	if t.failed && !t.keepGoing {
 		fatalf("FAILED")