cmd/cgo: avoid exporting all symbols on windows buildmode=c-shared

Disable default symbol auto-export behaviour by marking exported
function with the __declspec(dllexport) attribute. Old behaviour can
still be used by setting -extldflags=-Wl,--export-all-symbols.

See https://sourceware.org/binutils/docs/ld/WIN32.html for more info.

This change cuts 50kb of a "hello world" dll.

Updates #6853
Fixes #30674

Change-Id: I9c7fb09c677cc760f24d0f7d199740ae73981413
Reviewed-on: https://go-review.googlesource.com/c/go/+/262797
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Trust: Alex Brainman <alex.brainman@gmail.com>

misc/cgo/testcshared/cshared_test.go

@@ -7,6 +7,8 @@ package cshared_test
 import (
 	"bytes"
 	"debug/elf"
+	"debug/pe"
+	"encoding/binary"
 	"flag"
 	"fmt"
 	"io/ioutil"
@@ -355,6 +357,100 @@ func TestExportedSymbols(t *testing.T) {
 	}
 }
 
+func checkNumberOfExportedFunctionsWindows(t *testing.T, exportAllSymbols bool) {
+	const prog = `
+package main
+
+import "C"
+
+//export GoFunc
+func GoFunc() {
+	println(42)
+}
+
+//export GoFunc2
+func GoFunc2() {
+	println(24)
+}
+
+func main() {
+}
+`
+
+	tmpdir := t.TempDir()
+
+	srcfile := filepath.Join(tmpdir, "test.go")
+	objfile := filepath.Join(tmpdir, "test.dll")
+	if err := ioutil.WriteFile(srcfile, []byte(prog), 0666); err != nil {
+		t.Fatal(err)
+	}
+	argv := []string{"build", "-buildmode=c-shared"}
+	if exportAllSymbols {
+		argv = append(argv, "-ldflags", "-extldflags=-Wl,--export-all-symbols")
+	}
+	argv = append(argv, "-o", objfile, srcfile)
+	out, err := exec.Command("go", argv...).CombinedOutput()
+	if err != nil {
+		t.Fatalf("build failure: %s\n%s\n", err, string(out))
+	}
+
+	f, err := pe.Open(objfile)
+	if err != nil {
+		t.Fatalf("pe.Open failed: %v", err)
+	}
+	defer f.Close()
+	section := f.Section(".edata")
+	if section == nil {
+		t.Error(".edata section is not present")
+	}
+
+	// TODO: deduplicate this struct from cmd/link/internal/ld/pe.go
+	type IMAGE_EXPORT_DIRECTORY struct {
+		_                 [2]uint32
+		_                 [2]uint16
+		_                 [2]uint32
+		NumberOfFunctions uint32
+		NumberOfNames     uint32
+		_                 [3]uint32
+	}
+	var e IMAGE_EXPORT_DIRECTORY
+	if err := binary.Read(section.Open(), binary.LittleEndian, &e); err != nil {
+		t.Fatalf("binary.Read failed: %v", err)
+	}
+
+	expectedNumber := uint32(2)
+
+	if exportAllSymbols {
+		if e.NumberOfFunctions <= expectedNumber {
+			t.Fatalf("missing exported functions: %v", e.NumberOfFunctions)
+		}
+		if e.NumberOfNames <= expectedNumber {
+			t.Fatalf("missing exported names: %v", e.NumberOfNames)
+		}
+	} else {
+		if e.NumberOfFunctions != expectedNumber {
+			t.Fatalf("too many exported functions: %v", e.NumberOfFunctions)
+		}
+		if e.NumberOfNames != expectedNumber {
+			t.Fatalf("too many exported names: %v", e.NumberOfNames)
+		}
+	}
+}
+
+func TestNumberOfExportedFunctions(t *testing.T) {
+	if GOOS != "windows" {
+		t.Skip("skipping windows only test")
+	}
+	t.Parallel()
+
+	t.Run("OnlyExported", func(t *testing.T) {
+		checkNumberOfExportedFunctionsWindows(t, false)
+	})
+	t.Run("All", func(t *testing.T) {
+		checkNumberOfExportedFunctionsWindows(t, true)
+	})
+}
+
 // test1: shared library can be dynamically loaded and exported symbols are accessible.
 func TestExportedSymbolsWithDynamicLoad(t *testing.T) {
 	t.Parallel()