cmd/go: fix corner case missed rebuild of binary

If the only thing changing in the binary is the embedded main.a action ID,
go install was declining to install the binary, but go list could see that the
binary needed reinstalling (was stale).

Fixes #22531.

Change-Id: I4a53b0ebd4c34aad907bab7da571fada545f3c6f
Reviewed-on: https://go-review.googlesource.com/76014
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/go/go_test.go

@@ -4740,7 +4740,34 @@ func TestBuildCache(t *testing.T) {
 
 	tg.run("build", "-o", os.DevNull, "-x", "complex")
 	tg.grepStderr(`[\\/]link|gccgo`, "did not run linker")
+}
 
+func TestIssue22531(t *testing.T) {
+	if strings.Contains(os.Getenv("GODEBUG"), "gocacheverify") {
+		t.Skip("GODEBUG gocacheverify")
+	}
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.tempdir)
+	tg.setenv("GOCACHE", filepath.Join(tg.tempdir, "cache"))
+	tg.tempFile("src/m/main.go", "package main /* c1 */; func main() {}\n")
+	tg.run("install", "-x", "m")
+	tg.run("list", "-f", "{{.Stale}}", "m")
+	tg.grepStdout("false", "reported m as stale after install")
+	tg.run("tool", "buildid", filepath.Join(tg.tempdir, "bin/m"+exeSuffix))
+
+	// The link action ID did not include the full main build ID,
+	// even though the full main build ID is written into the
+	// eventual binary. That caused the following install to
+	// be a no-op, thinking the gofmt binary was up-to-date,
+	// even though .Stale could see it was not.
+	tg.tempFile("src/m/main.go", "package main /* c2 */; func main() {}\n")
+	tg.run("install", "-x", "m")
+	tg.run("list", "-f", "{{.Stale}}", "m")
+	tg.grepStdout("false", "reported m as stale after reinstall")
+	tg.run("tool", "buildid", filepath.Join(tg.tempdir, "bin/m"+exeSuffix))
 }
 
 func TestTestCache(t *testing.T) {

src/cmd/go/internal/work/exec.go

@@ -690,6 +690,11 @@ func (b *Builder) linkActionID(a *Action) cache.ActionID {
 				}
 				fmt.Fprintf(h, "packagefile %s=%s\n", p1.ImportPath, contentID(buildID))
 			}
+			// Because we put package main's full action ID into the binary's build ID,
+			// we must also put the full action ID into the binary's action ID hash.
+			if p1.Name == "main" {
+				fmt.Fprintf(h, "packagemain %s\n", a1.buildID)
+			}
 			if p1.Shlib != "" {
 				fmt.Fprintf(h, "pakageshlib %s=%s\n", p1.ImportPath, contentID(b.buildID(p1.Shlib)))
 			}