cmd/cgo: avoid worklist nondeterminism.

+ Regression test.

Fixes #9026.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/162490043

misc/cgo/test/cgo_test.go

@@ -62,5 +62,6 @@ func Test8517(t *testing.T)                  { test8517(t) }
 func Test8811(t *testing.T)                  { test8811(t) }
 func TestReturnAfterGrow(t *testing.T)       { testReturnAfterGrow(t) }
 func TestReturnAfterGrowFromGo(t *testing.T) { testReturnAfterGrowFromGo(t) }
+func Test9026(t *testing.T)                  { test9026(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

misc/cgo/test/issue9026.go

+package cgotest
+
+/*
+typedef struct {} git_merge_file_input;
+
+typedef struct {} git_merge_file_options;
+
+int git_merge_file(
+        git_merge_file_input *in,
+        git_merge_file_options *opts) {}
+*/
+import "C"
+import (
+	"fmt"
+	"testing"
+)
+
+func test9026(t *testing.T) {
+	var in C.git_merge_file_input
+	var opts *C.git_merge_file_options
+	C.git_merge_file(&in, opts)
+
+	// Test that the generated type names are deterministic.
+	// (Previously this would fail about 10% of the time.)
+	//
+	// Brittle: the assertion may fail spuriously when the algorithm
+	// changes, but should remain stable otherwise.
+	got := fmt.Sprintf("%T %T", in, opts)
+	want := "cgotest._Ctype_struct___12 *cgotest._Ctype_struct___13"
+	if got != want {
+		t.Errorf("Non-deterministic type names: got %s, want %s", got, want)
+	}
+}

src/cmd/cgo/gcc.go

@@ -944,6 +944,8 @@ type typeConv struct {
 
 	// Map from types to incomplete pointers to those types.
 	ptrs map[dwarf.Type][]*Type
+	// Keys of ptrs in insertion order (deterministic worklist)
+	ptrKeys []dwarf.Type
 
 	// Predeclared types.
 	bool                                   ast.Expr
@@ -1061,16 +1063,17 @@ func (tr *TypeRepr) Set(repr string, fargs ...interface{}) {
 func (c *typeConv) FinishType(pos token.Pos) {
 	// Completing one pointer type might produce more to complete.
 	// Keep looping until they're all done.
-	for len(c.ptrs) > 0 {
-		for dtype := range c.ptrs {
+	for len(c.ptrKeys) > 0 {
+		dtype := c.ptrKeys[0]
+		c.ptrKeys = c.ptrKeys[1:]
+
 		// Note Type might invalidate c.ptrs[dtype].
 		t := c.Type(dtype, pos)
 		for _, ptr := range c.ptrs[dtype] {
 			ptr.Go.(*ast.StarExpr).X = t.Go
 			ptr.C.Set("%s*", t.C)
 		}
-			delete(c.ptrs, dtype)
-		}
+		c.ptrs[dtype] = nil // retain the map key
 	}
 }
 
@@ -1237,6 +1240,9 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 		// Placeholder initialization; completed in FinishType.
 		t.Go = &ast.StarExpr{}
 		t.C.Set("<incomplete>*")
+		if _, ok := c.ptrs[dt.Type]; !ok {
+			c.ptrKeys = append(c.ptrKeys, dt.Type)
+		}
 		c.ptrs[dt.Type] = append(c.ptrs[dt.Type], t)
 
 	case *dwarf.QualType: