[dev.boringcrypto] cmd/go: exclude SysoFiles when using -msan

There's no way for a *.syso file to be compiled to work both in
normal mode and in msan mode. Assume they are compiled in
normal mode and drop them in msan mode.

Packages with syso files currently fail in -msan mode because
the syso file calls out to a routine like memcmp which then
falsely reports uninitialized memory. After this CL, they will fail
in -msan with link errors, because the syso will not be used.
But then it will at least be possible for package authors to write
fallback code in the package that avoids the syso in -msan mode,
so that the package with the syso can at least run in both modes.
Without this CL, that's not possible.

See #21884.

Change-Id: I77340614c4711325032484e65fa9c3f8332741d5
Reviewed-on: https://go-review.googlesource.com/63917
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -3614,8 +3614,8 @@ func TestBinaryOnlyPackages(t *testing.T) {
 	tg.grepStdout("false", "did not see BinaryOnly=false for p4")
 }
 
-// Issue 16050.
-func TestAlwaysLinkSysoFiles(t *testing.T) {
+// Issue 16050 and 21884.
+func TestLinkSysoFiles(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()
 	tg.parallel()
@@ -3634,6 +3634,10 @@ func TestAlwaysLinkSysoFiles(t *testing.T) {
 	tg.setenv("CGO_ENABLED", "0")
 	tg.run("list", "-f", "{{.SysoFiles}}", "syso")
 	tg.grepStdout("a.syso", "missing syso file with CGO_ENABLED=0")
+
+	tg.setenv("CGO_ENABLED", "1")
+	tg.run("list", "-msan", "-f", "{{.SysoFiles}}", "syso")
+	tg.grepStdoutNot("a.syso", "unexpected syso file with -msan")
 }
 
 // Issue 16120.

src/cmd/go/internal/load/pkg.go

@@ -207,6 +207,12 @@ func (p *Package) copyBuild(pp *build.Package) {
 	p.SwigFiles = pp.SwigFiles
 	p.SwigCXXFiles = pp.SwigCXXFiles
 	p.SysoFiles = pp.SysoFiles
+	if cfg.BuildMSan {
+		// There's no way for .syso files to be built both with and without
+		// support for memory sanitizer. Assume they are built without,
+		// and drop them.
+		p.SysoFiles = nil
+	}
 	p.CgoCFLAGS = pp.CgoCFLAGS
 	p.CgoCPPFLAGS = pp.CgoCPPFLAGS
 	p.CgoCXXFLAGS = pp.CgoCXXFLAGS