internal/gcimporter: refine golang/go#76222 crash

This CL asserts that pkg is non-nil in exportWriter.pkg, which
will  help us distinguish a nil pointer from an invalid (i.e.
memory corrupted) value in future telemetry field reports.

Also, eliminate the mutable currPkg field of importReader
by plumbing it down as a parameter through the obj() and
signature() methods.

Updates golang/go#76222

Change-Id: I702219bc0a3bd7fecaaf379bef80e759ba05a9bf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/719260
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: adonovan

internal/gcimporter/iexport.go

@@ -943,6 +943,13 @@ func (w *exportWriter) posV0(pos token.Pos) {
 }
 
 func (w *exportWriter) pkg(pkg *types.Package) {
+	if pkg == nil {
+		// [exportWriter.typ] accepts a nil pkg only for types
+		// of constants, which cannot contain named objects
+		// such as fields or methods and thus should never
+		// reach this method (#76222).
+		panic("nil package")
+	}
 	// Ensure any referenced packages are declared in the main index.
 	w.p.allPkgs[pkg] = true
 
@@ -958,9 +965,11 @@ func (w *exportWriter) qualifiedType(obj *types.TypeName) {
 	w.pkg(obj.Pkg())
 }
 
-// TODO(rfindley): what does 'pkg' even mean here? It would be better to pass
-// it in explicitly into signatures and structs that may use it for
-// constructing fields.
+// typ emits the specified type.
+//
+// Objects within the type (struct fields and interface methods) are
+// qualified by pkg. It may be nil if the type cannot contain objects,
+// such as the type of a constant.
 func (w *exportWriter) typ(t types.Type, pkg *types.Package) {
 	w.data.uint64(w.p.typOff(t, pkg))
 }
@@ -990,6 +999,7 @@ func (w *exportWriter) startType(k itag) {
 	w.data.uint64(uint64(k))
 }
 
+// doTyp is the implementation of [exportWriter.typ].
 func (w *exportWriter) doTyp(t types.Type, pkg *types.Package) {
 	if trace {
 		w.p.trace("exporting type %s (%T)", t, t)
@@ -1063,7 +1073,7 @@ func (w *exportWriter) doTyp(t types.Type, pkg *types.Package) {
 
 	case *types.Signature:
 		w.startType(signatureType)
-		w.pkg(pkg)
+		w.pkg(pkg) // qualifies param/result vars
 		w.signature(t)
 
 	case *types.Struct:
@@ -1109,19 +1119,19 @@ func (w *exportWriter) doTyp(t types.Type, pkg *types.Package) {
 
 	case *types.Interface:
 		w.startType(interfaceType)
-		w.pkg(pkg)
+		w.pkg(pkg) // qualifies unexported method funcs
 
 		n := t.NumEmbeddeds()
 		w.uint64(uint64(n))
 		for i := 0; i < n; i++ {
 			ft := t.EmbeddedType(i)
-			tPkg := pkg
 			if named, _ := types.Unalias(ft).(*types.Named); named != nil {
 				w.pos(named.Obj().Pos())
 			} else {
+				// e.g. ~int
 				w.pos(token.NoPos)
 			}
-			w.typ(ft, tPkg)
+			w.typ(ft, pkg)
 		}
 
 		// See comment for struct fields. In shallow mode we change the encoding

internal/gcimporter/iimport.go

@@ -432,10 +432,10 @@ func (p *iimporter) doDecl(pkg *types.Package, name string) {
 		errorf("%v.%v not in index", pkg, name)
 	}
 
-	r := &importReader{p: p, currPkg: pkg}
+	r := &importReader{p: p}
 	r.declReader.Reset(p.declData[off:])
 
-	r.obj(name)
+	r.obj(pkg, name)
 }
 
 func (p *iimporter) stringAt(off uint64) string {
@@ -551,7 +551,6 @@ func canReuse(def *types.Named, rhs types.Type) bool {
 type importReader struct {
 	p          *iimporter
 	declReader bytes.Reader
-	currPkg    *types.Package
 	prevFile   string
 	prevLine   int64
 	prevColumn int64
@@ -565,7 +564,8 @@ type importReader struct {
 // for 1.24, but the fix was not worth back-porting).
 var markBlack = func(name *types.TypeName) {}
 
-func (r *importReader) obj(name string) {
+// obj decodes and declares the package-level object denoted by (pkg, name).
+func (r *importReader) obj(pkg *types.Package, name string) {
 	tag := r.byte()
 	pos := r.pos()
 
@@ -576,27 +576,27 @@ func (r *importReader) obj(name string) {
 			tparams = r.tparamList()
 		}
 		typ := r.typ()
-		obj := aliases.NewAlias(r.p.aliases, pos, r.currPkg, name, typ, tparams)
+		obj := aliases.NewAlias(r.p.aliases, pos, pkg, name, typ, tparams)
 		markBlack(obj) // workaround for golang/go#69912
 		r.declare(obj)
 
 	case constTag:
 		typ, val := r.value()
 
-		r.declare(types.NewConst(pos, r.currPkg, name, typ, val))
+		r.declare(types.NewConst(pos, pkg, name, typ, val))
 
 	case funcTag, genericFuncTag:
 		var tparams []*types.TypeParam
 		if tag == genericFuncTag {
 			tparams = r.tparamList()
 		}
-		sig := r.signature(nil, nil, tparams)
-		r.declare(types.NewFunc(pos, r.currPkg, name, sig))
+		sig := r.signature(pkg, nil, nil, tparams)
+		r.declare(types.NewFunc(pos, pkg, name, sig))
 
 	case typeTag, genericTypeTag:
 		// Types can be recursive. We need to setup a stub
 		// declaration before recursing.
-		obj := types.NewTypeName(pos, r.currPkg, name, nil)
+		obj := types.NewTypeName(pos, pkg, name, nil)
 		named := types.NewNamed(obj, nil, nil)
 
 		markBlack(obj) // workaround for golang/go#69912
@@ -616,7 +616,7 @@ func (r *importReader) obj(name string) {
 			for n := r.uint64(); n > 0; n-- {
 				mpos := r.pos()
 				mname := r.ident()
-				recv := r.param()
+				recv := r.param(pkg)
 
 				// If the receiver has any targs, set those as the
 				// rparams of the method (since those are the
@@ -630,9 +630,9 @@ func (r *importReader) obj(name string) {
 						rparams[i] = types.Unalias(targs.At(i)).(*types.TypeParam)
 					}
 				}
-				msig := r.signature(recv, rparams, nil)
+				msig := r.signature(pkg, recv, rparams, nil)
 
-				named.AddMethod(types.NewFunc(mpos, r.currPkg, mname, msig))
+				named.AddMethod(types.NewFunc(mpos, pkg, mname, msig))
 			}
 		}
 
@@ -644,12 +644,12 @@ func (r *importReader) obj(name string) {
 			errorf("unexpected type param type")
 		}
 		name0 := tparamName(name)
-		tn := types.NewTypeName(pos, r.currPkg, name0, nil)
+		tn := types.NewTypeName(pos, pkg, name0, nil)
 		t := types.NewTypeParam(tn, nil)
 
 		// To handle recursive references to the typeparam within its
 		// bound, save the partial type in tparamIndex before reading the bounds.
-		id := ident{r.currPkg, name}
+		id := ident{pkg, name}
 		r.p.tparamIndex[id] = t
 		var implicit bool
 		if r.p.version >= iexportVersionGo1_18 {
@@ -672,7 +672,7 @@ func (r *importReader) obj(name string) {
 	case varTag:
 		typ := r.typ()
 
-		v := types.NewVar(pos, r.currPkg, name, typ)
+		v := types.NewVar(pos, pkg, name, typ)
 		typesinternal.SetVarKind(v, typesinternal.PackageVar)
 		r.declare(v)
 
@@ -905,11 +905,11 @@ func (r *importReader) doType(base *types.Named) (res types.Type) {
 	case mapType:
 		return types.NewMap(r.typ(), r.typ())
 	case signatureType:
-		r.currPkg = r.pkg()
-		return r.signature(nil, nil, nil)
+		paramPkg := r.pkg()
+		return r.signature(paramPkg, nil, nil, nil)
 
 	case structType:
-		r.currPkg = r.pkg()
+		fieldPkg := r.pkg()
 
 		fields := make([]*types.Var, r.uint64())
 		tags := make([]string, len(fields))
@@ -932,7 +932,7 @@ func (r *importReader) doType(base *types.Named) (res types.Type) {
 			// discussed in iexport.go, this is not correct, but mostly works and is
 			// preferable to failing (for now at least).
 			if field == nil {
-				field = types.NewField(fpos, r.currPkg, fname, ftyp, emb)
+				field = types.NewField(fpos, fieldPkg, fname, ftyp, emb)
 			}
 
 			fields[i] = field
@@ -941,7 +941,7 @@ func (r *importReader) doType(base *types.Named) (res types.Type) {
 		return types.NewStruct(fields, tags)
 
 	case interfaceType:
-		r.currPkg = r.pkg()
+		methodPkg := r.pkg() // qualifies methods and their param/result vars
 
 		embeddeds := make([]types.Type, r.uint64())
 		for i := range embeddeds {
@@ -963,12 +963,12 @@ func (r *importReader) doType(base *types.Named) (res types.Type) {
 			// don't agree with this.
 			var recv *types.Var
 			if base != nil {
-				recv = types.NewVar(token.NoPos, r.currPkg, "", base)
+				recv = types.NewVar(token.NoPos, methodPkg, "", base)
 			}
-			msig := r.signature(recv, nil, nil)
+			msig := r.signature(methodPkg, recv, nil, nil)
 
 			if method == nil {
-				method = types.NewFunc(mpos, r.currPkg, mname, msig)
+				method = types.NewFunc(mpos, methodPkg, mname, msig)
 			}
 			methods[i] = method
 		}
@@ -1049,9 +1049,9 @@ func (r *importReader) objectPathObject() types.Object {
 	return obj
 }
 
-func (r *importReader) signature(recv *types.Var, rparams []*types.TypeParam, tparams []*types.TypeParam) *types.Signature {
-	params := r.paramList()
-	results := r.paramList()
+func (r *importReader) signature(paramPkg *types.Package, recv *types.Var, rparams []*types.TypeParam, tparams []*types.TypeParam) *types.Signature {
+	params := r.paramList(paramPkg)
+	results := r.paramList(paramPkg)
 	variadic := params.Len() > 0 && r.bool()
 	return types.NewSignatureType(recv, rparams, tparams, params, results, variadic)
 }
@@ -1070,19 +1070,19 @@ func (r *importReader) tparamList() []*types.TypeParam {
 	return xs
 }
 
-func (r *importReader) paramList() *types.Tuple {
+func (r *importReader) paramList(pkg *types.Package) *types.Tuple {
 	xs := make([]*types.Var, r.uint64())
 	for i := range xs {
-		xs[i] = r.param()
+		xs[i] = r.param(pkg)
 	}
 	return types.NewTuple(xs...)
 }
 
-func (r *importReader) param() *types.Var {
+func (r *importReader) param(pkg *types.Package) *types.Var {
 	pos := r.pos()
 	name := r.ident()
 	typ := r.typ()
-	return types.NewParam(pos, r.currPkg, name, typ)
+	return types.NewParam(pos, pkg, name, typ)
 }
 
 func (r *importReader) bool() bool {