cmd/go: use the last -linkmode flag to determine external linking

Current linkmode checking in determining package dependencies doesn't
take multiple -linkmode options into consideration, may lead to redundant
dependency on 'runtime/cgo'.

Fixes the problem and adds a testcase.

Change-Id: Iac5ea9fb3ca5ef931201afd0f3441f41f946c919
Reviewed-on: https://go-review.googlesource.com/c/go/+/263497
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 9 insertions, 5 deletions

diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
index c9665265e9..f07bd3e075 100644
--- a/src/cmd/go/internal/load/pkg.go
+++ b/src/cmd/go/internal/load/pkg.go
@@ -1978,16 +1978,20 @@ func externalLinkingForced(p *Package) bool {
 	// external linking mode, as of course does
 	// -ldflags=-linkmode=external. External linking mode forces
 	// an import of runtime/cgo.
+	// If there are multiple -linkmode options, the last one wins.
 	pieCgo := cfg.BuildBuildmode == "pie" && !sys.InternalLinkPIESupported(cfg.BuildContext.GOOS, cfg.BuildContext.GOARCH)
 	linkmodeExternal := false
 	if p != nil {
 		ldflags := BuildLdflags.For(p)
-		for i, a := range ldflags {
-			if a == "-linkmode=external" {
-				linkmodeExternal = true
-			}
-			if a == "-linkmode" && i+1 < len(ldflags) && ldflags[i+1] == "external" {
+		for i := len(ldflags) - 1; i >= 0; i-- {
+			a := ldflags[i]
+			if a == "-linkmode=external" ||
+				a == "-linkmode" && i+1 < len(ldflags) && ldflags[i+1] == "external" {
 				linkmodeExternal = true
+				break
+			} else if a == "-linkmode=internal" ||
+				a == "-linkmode" && i+1 < len(ldflags) && ldflags[i+1] == "internal" {
+				break
 			}
 		}
 	}