[dev.link] cmd/compile: fix data race on LSym.Pkg

LSym may be created concurrently. Reading/writing LSym.Pkg may
cause data race (see
https://build.golang.org/log/f0351c5cc7bf4c92e3aa5e78e294c2d009ebf118).
Fix this by setting LSym.Pkg only when holding the lock.

Change-Id: Ib3160ecf47c4ca530b09369e0e8284db6597cfd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/198492Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/compile/internal/types/sym.go

@@ -76,24 +76,22 @@ func (sym *Sym) LinksymName() string {
 	return sym.Pkg.Prefix + "." + sym.Name
 }
 
-func (sym *Sym) Linksym() (r *obj.LSym) {
+func (sym *Sym) Linksym() *obj.LSym {
 	if sym == nil {
 		return nil
 	}
-	if sym.Func() {
-		// This is a function symbol. Mark it as "internal ABI".
-		r = Ctxt.LookupABI(sym.LinksymName(), obj.ABIInternal)
-	} else {
-		r = Ctxt.Lookup(sym.LinksymName())
-	}
-	if r.Pkg == "" {
+	initPkg := func(r *obj.LSym) {
 		if sym.Linkname != "" {
 			r.Pkg = "_"
 		} else {
 			r.Pkg = sym.Pkg.Prefix
 		}
 	}
-	return
+	if sym.Func() {
+		// This is a function symbol. Mark it as "internal ABI".
+		return Ctxt.LookupABIInit(sym.LinksymName(), obj.ABIInternal, initPkg)
+	}
+	return Ctxt.LookupInit(sym.LinksymName(), initPkg)
 }
 
 // Less reports whether symbol a is ordered before symbol b.

src/cmd/internal/obj/sym.go

@@ -78,6 +78,13 @@ func (ctxt *Link) LookupStatic(name string) *LSym {
 // LookupABI looks up a symbol with the given ABI.
 // If it does not exist, it creates it.
 func (ctxt *Link) LookupABI(name string, abi ABI) *LSym {
+	return ctxt.LookupABIInit(name, abi, nil)
+}
+
+// LookupABI looks up a symbol with the given ABI.
+// If it does not exist, it creates it and
+// passes it to init for one-time initialization.
+func (ctxt *Link) LookupABIInit(name string, abi ABI, init func(s *LSym)) *LSym {
 	var hash map[string]*LSym
 	switch abi {
 	case ABI0:
@@ -94,6 +101,9 @@ func (ctxt *Link) LookupABI(name string, abi ABI) *LSym {
 		s = &LSym{Name: name}
 		s.SetABI(abi)
 		hash[name] = s
+		if init != nil {
+			init(s)
+		}
 	}
 	ctxt.hashmu.Unlock()
 	return s