cmd/cgo: make sure we FinishType everything

Ensure that we call FinishType on all the types added to the ptrs map.
We only add a key to ptrKeys once. Once we FinishType for that key,
we'll never look at that key again. But we can add a new type under that
key later, and we'll never finish it.

Make sure we add the key to the ptrKeys list every time we make the list
of types for that key non-empty.

This makes sure we FinishType each pointer type exactly once.

Fixes #26517

Change-Id: Iad86150d516fcfac167591daf5a26c38bec7d143
Reviewed-on: https://go-review.googlesource.com/126275
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/cgo/gcc.go

@@ -1735,6 +1735,7 @@ 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 contains exactly the keys in ptrs.
 	ptrKeys []dwarf.Type
 
 	// Type names X for which there exists an XGetTypeID function with type func() CFTypeID.
@@ -1877,14 +1878,15 @@ func (c *typeConv) FinishType(pos token.Pos) {
 	for len(c.ptrKeys) > 0 {
 		dtype := c.ptrKeys[0]
 		c.ptrKeys = c.ptrKeys[1:]
+		ptrs := c.ptrs[dtype]
+		delete(c.ptrs, dtype)
 
 		// Note Type might invalidate c.ptrs[dtype].
 		t := c.Type(dtype, pos)
-		for _, ptr := range c.ptrs[dtype] {
+		for _, ptr := range ptrs {
 			ptr.Go.(*ast.StarExpr).X = t.Go
 			ptr.C.Set("%s*", t.C)
 		}
-		c.ptrs[dtype] = nil // retain the map key
 	}
 }