cmd/cgo: use preprocessor macros to avoid prolog redefinitions

Avoid redefinition errors when a Go file uses a cgo comment to
There is no particularly good reason to do this, but there is also no
particularly good reason that it should fail.

Fixes #27019

Change-Id: Icd6f8197a89be4ee6b03ddae675667998a8b4189
Reviewed-on: https://go-review.googlesource.com/c/152079
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/testcshared/cshared_test.go

@@ -602,3 +602,46 @@ func copyFile(t *testing.T, dst, src string) {
 		t.Fatal(err)
 	}
 }
+
+func TestGo2C2Go(t *testing.T) {
+	t.Parallel()
+
+	tmpdir, err := ioutil.TempDir("", "cshared-TestGo2C2Go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	shlib := filepath.Join(tmpdir, "libtestgo2c2go."+libSuffix)
+	run(t, gopathEnv, "go", "build", "-buildmode=c-shared", "-o", shlib, "go2c2go/go")
+
+	cgoCflags := os.Getenv("CGO_CFLAGS")
+	if cgoCflags != "" {
+		cgoCflags += " "
+	}
+	cgoCflags += "-I" + tmpdir
+
+	cgoLdflags := os.Getenv("CGO_LDFLAGS")
+	if cgoLdflags != "" {
+		cgoLdflags += " "
+	}
+	cgoLdflags += "-L" + tmpdir + " -ltestgo2c2go"
+
+	goenv := append(gopathEnv[:len(gopathEnv):len(gopathEnv)], "CGO_CFLAGS="+cgoCflags, "CGO_LDFLAGS="+cgoLdflags)
+
+	ldLibPath := os.Getenv("LD_LIBRARY_PATH")
+	if ldLibPath != "" {
+		ldLibPath += ":"
+	}
+	ldLibPath += tmpdir
+
+	runenv := append(gopathEnv[:len(gopathEnv):len(gopathEnv)], "LD_LIBRARY_PATH="+ldLibPath)
+
+	bin := filepath.Join(tmpdir, "m1") + exeSuffix
+	run(t, goenv, "go", "build", "-o", bin, "go2c2go/m1")
+	runExe(t, runenv, bin)
+
+	bin = filepath.Join(tmpdir, "m2") + exeSuffix
+	run(t, goenv, "go", "build", "-o", bin, "go2c2go/m2")
+	runExe(t, runenv, bin)
+}

misc/cgo/testcshared/src/go2c2go/go/shlib.go

@@ -0,0 +1,12 @@
+// Copyright 2018 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.
+
+package main
+
+import "C"
+
+//export GoFunc
+func GoFunc() int { return 1 }
+
+func main() {}

misc/cgo/testcshared/src/go2c2go/m1/c.c

@@ -0,0 +1,9 @@
+// Copyright 2018 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.
+
+#include "libtestgo2c2go.h"
+
+int CFunc(void) {
+	return (GoFunc() << 8) + 2;
+}

misc/cgo/testcshared/src/go2c2go/m1/main.go

@@ -0,0 +1,22 @@
+// Copyright 2018 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.
+
+package main
+
+// extern int CFunc(void);
+import "C"
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	got := C.CFunc()
+	const want = (1 << 8) | 2
+	if got != want {
+		fmt.Printf("got %#x, want %#x\n", got, want)
+		os.Exit(1)
+	}
+}

misc/cgo/testcshared/src/go2c2go/m2/main.go

@@ -0,0 +1,22 @@
+// Copyright 2018 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.
+
+package main
+
+// #include "libtestgo2c2go.h"
+import "C"
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	got := C.GoFunc()
+	const want = 1
+	if got != want {
+		fmt.Printf("got %#x, want %#x\n", got, want)
+		os.Exit(1)
+	}
+}

src/cmd/cgo/out.go

@@ -1555,6 +1555,7 @@ const builtinProlog = `
 /* Define intgo when compiling with GCC.  */
 typedef ptrdiff_t intgo;
 
+#define GO_CGO_GOSTRING_TYPEDEF
 typedef struct { const char *p; intgo n; } _GoString_;
 typedef struct { char *p; intgo n; intgo c; } _GoBytes_;
 _GoString_ GoString(char *p);
@@ -1806,15 +1807,20 @@ void localCgoCheckResult(Eface val) {
 // because _cgo_export.h defines GoString as a struct while builtinProlog
 // defines it as a function. We don't change this to avoid unnecessarily
 // breaking existing code.
+// The test of GO_CGO_GOSTRING_TYPEDEF avoids a duplicate definition
+// error if a Go file with a cgo comment #include's the export header
+// generated by a different package.
 const builtinExportProlog = `
-#line 1 "cgo-builtin-prolog"
+#line 1 "cgo-builtin-export-prolog"
 
 #include <stddef.h> /* for ptrdiff_t below */
 
 #ifndef GO_CGO_EXPORT_PROLOGUE_H
 #define GO_CGO_EXPORT_PROLOGUE_H
 
+#ifndef GO_CGO_GOSTRING_TYPEDEF
 typedef struct { const char *p; ptrdiff_t n; } _GoString_;
+#endif
 
 #endif
 `
@@ -1823,6 +1829,19 @@ func (p *Package) gccExportHeaderProlog() string {
 	return strings.Replace(gccExportHeaderProlog, "GOINTBITS", fmt.Sprint(8*p.IntSize), -1)
 }
 
+// gccExportHeaderProlog is written to the exported header, after the
+// import "C" comment preamble but before the generated declarations
+// of exported functions. This permits the generated declarations to
+// use the type names that appear in goTypes, above.
+//
+// The test of GO_CGO_GOSTRING_TYPEDEF avoids a duplicate definition
+// error if a Go file with a cgo comment #include's the export header
+// generated by a different package. Unfortunately GoString means two
+// different things: in this prolog it means a C name for the Go type,
+// while in the prolog written into the start of the C code generated
+// from a cgo-using Go file it means the C.GoString function. There is
+// no way to resolve this conflict, but it also doesn't make much
+// difference, as Go code never wants to refer to the latter meaning.
 const gccExportHeaderProlog = `
 /* Start of boilerplate cgo prologue.  */
 #line 1 "cgo-gcc-export-header-prolog"
@@ -1852,7 +1871,9 @@ typedef double _Complex GoComplex128;
 */
 typedef char _check_for_GOINTBITS_bit_pointer_matching_GoInt[sizeof(void*)==GOINTBITS/8 ? 1:-1];
 
+#ifndef GO_CGO_GOSTRING_TYPEDEF
 typedef _GoString_ GoString;
+#endif
 typedef void *GoMap;
 typedef void *GoChan;
 typedef struct { void *t; void *v; } GoInterface;