cmd/go: add LDFLAGS to cache ID when using cgo

The cgo tool records the value of the CGO_LDFLAGS environment variable
in the generated file, so that the linker can later read and use it.
Therefore, we must add CGO_LDFLAGS to the cache ID, as otherwise
changing CGO_LDFLAGS may cause a build result to be incorrectly read
from the cache, producing a different final program.

Change-Id: Ic89c1edc4069837451a36376710ca9b56fb87455
Reviewed-on: https://go-review.googlesource.com/122520
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -6300,6 +6300,30 @@ echo $* >>`+tg.path("pkg-config.out"))
 	}
 }
 
+func TestCgoCache(t *testing.T) {
+	if !canCgo {
+		t.Skip("no cgo")
+	}
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("src/x/a.go", `package main
+		// #ifndef VAL
+		// #define VAL 0
+		// #endif
+		// int val = VAL;
+		import "C"
+		import "fmt"
+		func main() { fmt.Println(C.val) }
+	`)
+	tg.setenv("GOPATH", tg.path("."))
+	exe := tg.path("x.exe")
+	tg.run("build", "-o", exe, "x")
+	tg.setenv("CGO_LDFLAGS", "-lnosuchlibraryexists")
+	tg.runFail("build", "-o", exe, "x")
+	tg.grepStderr(`nosuchlibraryexists`, "did not run linker with changed CGO_LDFLAGS")
+}
+
 // Issue 23982
 func TestFilepathUnderCwdFormat(t *testing.T) {
 	tg := testgo(t)

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

@@ -208,8 +208,8 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 	}
 	if len(p.CgoFiles)+len(p.SwigFiles) > 0 {
 		fmt.Fprintf(h, "cgo %q\n", b.toolID("cgo"))
-		cppflags, cflags, cxxflags, fflags, _, _ := b.CFlags(p)
-		fmt.Fprintf(h, "CC=%q %q %q\n", b.ccExe(), cppflags, cflags)
+		cppflags, cflags, cxxflags, fflags, ldflags, _ := b.CFlags(p)
+		fmt.Fprintf(h, "CC=%q %q %q %q\n", b.ccExe(), cppflags, cflags, ldflags)
 		if len(p.CXXFiles)+len(p.SwigFiles) > 0 {
 			fmt.Fprintf(h, "CXX=%q %q\n", b.cxxExe(), cxxflags)
 		}