cmd/compile, runtime: make the go.itab.* symbols module-local

Otherwise, the way the ELF dynamic linker works means that you can end up with
the same itab being passed to additab twice, leading to the itab linked list
having a cycle in it. Add a test to additab in runtime to catch this when it
happens, not some arbitrary and surprsing time later.

Fixes #17594

Change-Id: I6c82edcc9ac88ac188d1185370242dc92f46b1ad
Reviewed-on: https://go-review.googlesource.com/32131
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testshared/src/depBase/dep.go

 package depBase
 
+import (
+	"os"
+	"reflect"
+)
+
 var V int = 1
 
 var HasMask []string = []string{"hi"}
@@ -13,6 +18,10 @@ type Dep struct {
 }
 
 func (d *Dep) Method() int {
+	// This code below causes various go.itab.* symbols to be generated in
+	// the shared library. Similar code in ../exe/exe.go results in
+	// exercising https://github.com/golang/go/issues/17594
+	reflect.TypeOf(os.Stdout).Elem()
 	return 10
 }
 

misc/cgo/testshared/src/exe/exe.go

@@ -2,11 +2,17 @@ package main
 
 import (
 	"depBase"
+	"os"
+	"reflect"
 	"runtime"
 )
 
 func main() {
 	defer depBase.ImplementedInAsm()
+	// This code below causes various go.itab.* symbols to be generated in
+	// the executable. Similar code in ../depBase/dep.go results in
+	// exercising https://github.com/golang/go/issues/17594
+	reflect.TypeOf(os.Stdout).Elem()
 	runtime.GC()
 	depBase.V = depBase.F() + 1
 }

src/cmd/compile/internal/gc/reflect.go

@@ -1395,11 +1395,11 @@ func dumptypestructs() {
 		o += len(imethods(i.itype)) * Widthptr // skip fun method pointers
 		// at runtime the itab will contain pointers to types, other itabs and
 		// method functions. None are allocated on heap, so we can use obj.NOPTR.
-		ggloblsym(i.sym, int32(o), int16(obj.DUPOK|obj.NOPTR))
+		ggloblsym(i.sym, int32(o), int16(obj.DUPOK|obj.NOPTR|obj.LOCAL))
 
 		ilink := Pkglookup(i.t.tconv(FmtLeft)+","+i.itype.tconv(FmtLeft), itablinkpkg)
 		dsymptr(ilink, 0, i.sym, 0)
-		ggloblsym(ilink, int32(Widthptr), int16(obj.DUPOK|obj.RODATA))
+		ggloblsym(ilink, int32(Widthptr), int16(obj.DUPOK|obj.RODATA|obj.LOCAL))
 	}
 
 	// process ptabs

src/runtime/iface.go

@@ -138,6 +138,10 @@ func additab(m *itab, locked, canfail bool) {
 		throw("invalid itab locking")
 	}
 	h := itabhash(inter, typ)
+	if m == hash[h] {
+		println("duplicate itab for", typ.string(), "and", inter.typ.string())
+		throw("duplicate itabs")
+	}
 	m.link = hash[h]
 	atomicstorep(unsafe.Pointer(&hash[h]), unsafe.Pointer(m))
 }