cmd/cgo: fix use of unsafe argument in new deferred function

The combination of https://golang.org/cl/23650 and
https://golang.org/cl/23675 did not work--they were tested separately
but not together.

The problem was that 23650 introduced deferred argument checking, and
the deferred function loses the type that 23675 started requiring. The
fix is to go back to using an empty interface type in a deferred
argument check.

No new test required--fixes broken build.

Change-Id: I5ea023c5aed71d70e57b11c4551242d3ef25986d
Reviewed-on: https://go-review.googlesource.com/23961
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/cgo/gcc.go

@@ -657,7 +657,7 @@ func (p *Package) rewriteCall(f *File, call *Call, name *Name) {
 		// Instead we use a local variant of _cgoCheckPointer.
 
 		var arg ast.Expr
-		if n := p.unsafeCheckPointerName(param.Go); n != "" {
+		if n := p.unsafeCheckPointerName(param.Go, call.Deferred); n != "" {
 			c.Fun = ast.NewIdent(n)
 			arg = c
 		} else {
@@ -939,20 +939,31 @@ func (p *Package) isType(t ast.Expr) bool {
 // assertion to unsafe.Pointer in our copy of user code. We return
 // the name of the _cgoCheckPointer function we are going to build, or
 // the empty string if the type does not use unsafe.Pointer.
-func (p *Package) unsafeCheckPointerName(t ast.Expr) string {
+//
+// The deferred parameter is true if this check is for the argument of
+// a deferred function. In that case we need to use an empty interface
+// as the argument type, because the deferred function we introduce in
+// rewriteCall will use an empty interface type, and we can't add a
+// type assertion. This is handled by keeping a separate list, and
+// writing out the lists separately in writeDefs.
+func (p *Package) unsafeCheckPointerName(t ast.Expr, deferred bool) string {
 	if !p.hasUnsafePointer(t) {
 		return ""
 	}
 	var buf bytes.Buffer
 	conf.Fprint(&buf, fset, t)
 	s := buf.String()
-	for i, t := range p.CgoChecks {
+	checks := &p.CgoChecks
+	if deferred {
+		checks = &p.DeferredCgoChecks
+	}
+	for i, t := range *checks {
 		if s == t {
-			return p.unsafeCheckPointerNameIndex(i)
+			return p.unsafeCheckPointerNameIndex(i, deferred)
 		}
 	}
-	p.CgoChecks = append(p.CgoChecks, s)
-	return p.unsafeCheckPointerNameIndex(len(p.CgoChecks) - 1)
+	*checks = append(*checks, s)
+	return p.unsafeCheckPointerNameIndex(len(*checks)-1, deferred)
 }
 
 // hasUnsafePointer returns whether the Go type t uses unsafe.Pointer.
@@ -980,7 +991,10 @@ func (p *Package) hasUnsafePointer(t ast.Expr) bool {
 
 // unsafeCheckPointerNameIndex returns the name to use for a
 // _cgoCheckPointer variant based on the index in the CgoChecks slice.
-func (p *Package) unsafeCheckPointerNameIndex(i int) string {
+func (p *Package) unsafeCheckPointerNameIndex(i int, deferred bool) string {
+	if deferred {
+		return fmt.Sprintf("_cgoCheckPointerInDefer%d", i)
+	}
 	return fmt.Sprintf("_cgoCheckPointer%d", i)
 }