cmd/cgo: for godefs, don't let field prefix removal cause duplicates

Fixes #48396

Change-Id: Idd7cb66536ef513806c472d394a929bc271fc26b
Reviewed-on: https://go-review.googlesource.com/c/go/+/350159
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

misc/cgo/testgodefs/testdata/issue48396.go

@@ -0,0 +1,18 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+// +build ignore
+
+package main
+
+/*
+// from <linux/kcm.h>
+struct issue48396 {
+	int fd;
+	int bpf_fd;
+};
+*/
+import "C"
+
+type Issue48396 C.struct_issue48396

misc/cgo/testgodefs/testdata/main.go

@@ -28,6 +28,9 @@ var v7 = S{}
 // Test that #define'd type is fully defined
 var _ = issue38649{X: 0}
 
+// Test that prefixes do not cause duplicate field names.
+var _ = Issue48396{Fd: 1, Bpf_fd: 2}
+
 func main() {
 	pass := true
 

misc/cgo/testgodefs/testgodefs_test.go

@@ -25,6 +25,7 @@ var filePrefixes = []string{
 	"issue37621",
 	"issue38649",
 	"issue39534",
+	"issue48396",
 }
 
 func TestGoDefs(t *testing.T) {

src/cmd/cgo/gcc.go

@@ -3030,6 +3030,31 @@ func upper(s string) string {
 // so that all fields are exported.
 func godefsFields(fld []*ast.Field) {
 	prefix := fieldPrefix(fld)
+
+	// Issue 48396: check for duplicate field names.
+	if prefix != "" {
+		names := make(map[string]bool)
+	fldLoop:
+		for _, f := range fld {
+			for _, n := range f.Names {
+				name := n.Name
+				if name == "_" {
+					continue
+				}
+				if name != prefix {
+					name = strings.TrimPrefix(n.Name, prefix)
+				}
+				name = upper(name)
+				if names[name] {
+					// Field name conflict: don't remove prefix.
+					prefix = ""
+					break fldLoop
+				}
+				names[name] = true
+			}
+		}
+	}
+
 	npad := 0
 	for _, f := range fld {
 		for _, n := range f.Names {