cmd/cgo: discard trailing zero-sized fields in a non-empty C struct

In order to fix issue #9401 the compiler was changed to add a padding
byte to any non-empty Go struct that ends in a zero-sized field.  That
causes the Go version of such a C struct to have a different size than
the C struct, which can considerable confusion.  Change cgo so that it
discards any such zero-sized fields, so that the Go and C structs are
the same size.

This is a change from previous releases, in that it used to be
possible to refer to a zero-sized trailing field (by taking its
address), and with this change it no longer is.  That is unfortunate,
but something has to change.  It seems better to visibly break
programs that do this rather than to silently break programs that rely
on the struct sizes being the same.

Update #9401.
Fixes #11925.

Change-Id: I3fba3f02f11265b3c41d68616f79dedb05b81225
Reviewed-on: https://go-review.googlesource.com/12864
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/cgo/gcc.go

@@ -1544,11 +1544,13 @@ func (c *typeConv) intExpr(n int64) ast.Expr {
 }
 
 // Add padding of given size to fld.
-func (c *typeConv) pad(fld []*ast.Field, size int64) []*ast.Field {
+func (c *typeConv) pad(fld []*ast.Field, sizes []int64, size int64) ([]*ast.Field, []int64) {
 	n := len(fld)
 	fld = fld[0 : n+1]
 	fld[n] = &ast.Field{Names: []*ast.Ident{c.Ident("_")}, Type: c.Opaque(size)}
-	return fld
+	sizes = sizes[0 : n+1]
+	sizes[n] = size
+	return fld, sizes
 }
 
 // Struct conversion: return Go and (gc) C syntax for type.
@@ -1559,6 +1561,7 @@ func (c *typeConv) Struct(dt *dwarf.StructType, pos token.Pos) (expr *ast.Struct
 	var buf bytes.Buffer
 	buf.WriteString("struct {")
 	fld := make([]*ast.Field, 0, 2*len(dt.Field)+1) // enough for padding around every field
+	sizes := make([]int64, 0, 2*len(dt.Field)+1)
 	off := int64(0)
 
 	// Rename struct fields that happen to be named Go keywords into
@@ -1594,7 +1597,7 @@ func (c *typeConv) Struct(dt *dwarf.StructType, pos token.Pos) (expr *ast.Struct
 	anon := 0
 	for _, f := range dt.Field {
 		if f.ByteOffset > off {
-			fld = c.pad(fld, f.ByteOffset-off)
+			fld, sizes = c.pad(fld, sizes, f.ByteOffset-off)
 			off = f.ByteOffset
 		}
 
@@ -1652,6 +1655,8 @@ func (c *typeConv) Struct(dt *dwarf.StructType, pos token.Pos) (expr *ast.Struct
 			ident[name] = name
 		}
 		fld[n] = &ast.Field{Names: []*ast.Ident{c.Ident(ident[name])}, Type: tgo}
+		sizes = sizes[0 : n+1]
+		sizes[n] = size
 		off += size
 		buf.WriteString(t.C.String())
 		buf.WriteString(" ")
@@ -1662,9 +1667,22 @@ func (c *typeConv) Struct(dt *dwarf.StructType, pos token.Pos) (expr *ast.Struct
 		}
 	}
 	if off < dt.ByteSize {
-		fld = c.pad(fld, dt.ByteSize-off)
+		fld, sizes = c.pad(fld, sizes, dt.ByteSize-off)
 		off = dt.ByteSize
 	}
+
+	// If the last field in a non-zero-sized struct is zero-sized
+	// the compiler is going to pad it by one (see issue 9401).
+	// We can't permit that, because then the size of the Go
+	// struct will not be the same as the size of the C struct.
+	// Our only option in such a case is to remove the field,
+	// which means that it can not be referenced from Go.
+	for off > 0 && sizes[len(sizes)-1] == 0 {
+		n := len(sizes)
+		fld = fld[0 : n-1]
+		sizes = sizes[0 : n-1]
+	}
+
 	if off != dt.ByteSize {
 		fatalf("%s: struct size calculation error off=%d bytesize=%d", lineno(pos), off, dt.ByteSize)
 	}