go/analysis/passes/copylock: fix infinite recursion

Generalize the 'seenTParams' map to short-circuit all forms of infinite
recursion in the lockPath function, not just through type parameters.

For invalid code, it is possible to have infinite recursion through
named types.

Fixes golang/go#61678

Change-Id: I59d39c6fcaf3dd8bbd4f989516e2fb373b277889
Reviewed-on: https://go-review.googlesource.com/c/tools/+/514818
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

go/analysis/passes/copylock/copylock.go

@@ -242,29 +242,23 @@ func lockPathRhs(pass *analysis.Pass, x ast.Expr) typePath {
 // lockPath returns a typePath describing the location of a lock value
 // contained in typ. If there is no contained lock, it returns nil.
 //
-// The seenTParams map is used to short-circuit infinite recursion via type
-// parameters.
-func lockPath(tpkg *types.Package, typ types.Type, seenTParams map[*typeparams.TypeParam]bool) typePath {
-	if typ == nil {
+// The seen map is used to short-circuit infinite recursion due to type cycles.
+func lockPath(tpkg *types.Package, typ types.Type, seen map[types.Type]bool) typePath {
+	if typ == nil || seen[typ] {
 		return nil
 	}
+	if seen == nil {
+		seen = make(map[types.Type]bool)
+	}
+	seen[typ] = true
 
 	if tpar, ok := typ.(*typeparams.TypeParam); ok {
-		if seenTParams == nil {
-			// Lazily allocate seenTParams, since the common case will not involve
-			// any type parameters.
-			seenTParams = make(map[*typeparams.TypeParam]bool)
-		}
-		if seenTParams[tpar] {
-			return nil
-		}
-		seenTParams[tpar] = true
 		terms, err := typeparams.StructuralTerms(tpar)
 		if err != nil {
 			return nil // invalid type
 		}
 		for _, term := range terms {
-			subpath := lockPath(tpkg, term.Type(), seenTParams)
+			subpath := lockPath(tpkg, term.Type(), seen)
 			if len(subpath) > 0 {
 				if term.Tilde() {
 					// Prepend a tilde to our lock path entry to clarify the resulting
@@ -298,7 +292,7 @@ func lockPath(tpkg *types.Package, typ types.Type, seenTParams map[*typeparams.T
 	ttyp, ok := typ.Underlying().(*types.Tuple)
 	if ok {
 		for i := 0; i < ttyp.Len(); i++ {
-			subpath := lockPath(tpkg, ttyp.At(i).Type(), seenTParams)
+			subpath := lockPath(tpkg, ttyp.At(i).Type(), seen)
 			if subpath != nil {
 				return append(subpath, typ.String())
 			}
@@ -332,7 +326,7 @@ func lockPath(tpkg *types.Package, typ types.Type, seenTParams map[*typeparams.T
 	nfields := styp.NumFields()
 	for i := 0; i < nfields; i++ {
 		ftyp := styp.Field(i).Type()
-		subpath := lockPath(tpkg, ftyp, seenTParams)
+		subpath := lockPath(tpkg, ftyp, seen)
 		if subpath != nil {
 			return append(subpath, typ.String())
 		}

go/analysis/passes/copylock/testdata/src/a/issue61678.go

@@ -0,0 +1,30 @@
+package a
+
+import "sync"
+
+// These examples are taken from golang/go#61678, modified so that A and B
+// contain a mutex.
+
+type A struct {
+	a  A
+	mu sync.Mutex
+}
+
+type B struct {
+	a  A
+	b  B
+	mu sync.Mutex
+}
+
+func okay(x A) {}
+func sure()    { var x A; nop(x) }
+
+var fine B
+
+func what(x B)   {}                  // want `passes lock by value`
+func bad()       { var x B; nop(x) } // want `copies lock value`
+func good()      { nop(B{}) }
+func stillgood() { nop(B{b: B{b: B{b: B{}}}}) }
+func nope()      { nop(B{}.b) } // want `copies lock value`
+
+func nop(any) {} // only used to get around unused variable errors