cmd/go: stat the archive file only when executing the commands

Fixes #14944

Change-Id: I73e0997cb6ebaeced1045b0ddadac893319bd78f
Reviewed-on: https://go-review.googlesource.com/21065Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/cmd/go/build.go

@@ -2346,8 +2346,10 @@ func (gcToolchain) pack(b *builder, p *Package, objDir, afile string, ofiles []s
 
 	// The archive file should have been created by the compiler.
 	// Since it used to not work that way, verify.
-	if _, err := os.Stat(absAfile); err != nil {
-		fatalf("os.Stat of archive file failed: %v", err)
+	if !buildN {
+		if _, err := os.Stat(absAfile); err != nil {
+			fatalf("os.Stat of archive file failed: %v", err)
+		}
 	}
 
 	if buildN || buildX {

src/cmd/go/go_test.go

@@ -1982,6 +1982,27 @@ func TestCoverageUsesActualSettingToOverrideEvenForRace(t *testing.T) {
 	checkCoverage(tg, data)
 }
 
+func TestBuildDryRunWithCgo(t *testing.T) {
+	if !canCgo {
+		t.Skip("skipping because cgo not enabled")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.tempFile("foo.go", `package main
+
+/*
+#include <limits.h>
+*/
+import "C"
+
+func main() {
+        println(C.INT_MAX)
+}`)
+	tg.run("build", "-n", tg.path("foo.go"))
+	tg.grepStderrNot(`os.Stat .* no such file or directory`, "unexpected stat of archive file")
+}
+
 func TestCoverageWithCgo(t *testing.T) {
 	if !canCgo {
 		t.Skip("skipping because cgo not enabled")