cmd/go: clear cmd cache to avoid duplicate loads errors

go get -u all command updates all packages including standard
commands. We need to get commands evicted from their cache to
avoid loading old versions of the packages evicted from the
packages cache.

Fixes #14444

Change-Id: Icd581a26e1db34ca634aba595fed62b097094c2f
Reviewed-on: https://go-review.googlesource.com/19899Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/22040Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/go/get.go

@@ -119,6 +119,14 @@ func runGet(cmd *Command, args []string) {
 		delete(packageCache, name)
 	}
 
+	// In order to rebuild packages information completely,
+	// we need to clear commands cache. Command packages are
+	// referring to evicted packages from the package cache.
+	// This leads to duplicated loads of the standard packages.
+	for name := range cmdCache {
+		delete(cmdCache, name)
+	}
+
 	args = importPaths(args)
 	packagesForBuild(args)
 

src/cmd/go/go_test.go

@@ -2759,3 +2759,15 @@ func TestParallelTest(t *testing.T) {
 	tg.setenv("GOPATH", tg.path("."))
 	tg.run("test", "-p=4", "p1", "p2", "p3", "p4")
 }
+
+// Issue 14444: go get -u .../ duplicate loads errors
+func TestGoGetUpdateAllDoesNotTryToLoadDuplicates(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.path("."))
+	tg.run("get", "-u", ".../")
+	tg.grepStderrNot("duplicate loads of", "did not remove old packages from cache")
+}