cmd/link: pass flags to external linker in deterministic order

Currently we may pass C linker flags in nondeterministic order,
as on ELf systems we pass --export-dynamic-symbol for symbols from
a map. This is usally not a big problem because even if the flags
are passed in nondeterministic order the resulting binary is
probably still deterministic. This CL makes it pass them in a
deterministic order to be extra sure. This also helps build
systems where e.g. there is a build cache for the C linking action.

Change-Id: I930524dd2c3387f49d62be7ad2cef937cb2c2238
Reviewed-on: https://go-review.googlesource.com/c/go/+/509215
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>

src/cmd/link/internal/ld/lib.go

@@ -44,6 +44,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"runtime"
+	"sort"
 	"strings"
 	"sync"
 
@@ -1670,9 +1671,12 @@ func (ctxt *Link) hostlink() {
 		if ctxt.DynlinkingGo() || ctxt.BuildMode == BuildModeCShared || !linkerFlagSupported(ctxt.Arch, argv[0], altLinker, "-Wl,--export-dynamic-symbol=main") {
 			argv = append(argv, "-rdynamic")
 		} else {
+			var exports []string
 			ctxt.loader.ForAllCgoExportDynamic(func(s loader.Sym) {
-				argv = append(argv, "-Wl,--export-dynamic-symbol="+ctxt.loader.SymExtname(s))
+				exports = append(exports, "-Wl,--export-dynamic-symbol="+ctxt.loader.SymExtname(s))
 			})
+			sort.Strings(exports)
+			argv = append(argv, exports...)
 		}
 	}
 	if ctxt.HeadType == objabi.Haix {

src/cmd/link/link_test.go

@@ -1167,6 +1167,86 @@ func TestUnlinkableObj(t *testing.T) {
 	}
 }
 
+func TestExtLinkCmdlineDeterminism(t *testing.T) {
+	// Test that we pass flags in deterministic order to the external linker
+	testenv.MustHaveGoBuild(t)
+	testenv.MustHaveCGO(t) // this test requires -linkmode=external
+	t.Parallel()
+
+	// test source code, with some cgo exports
+	testSrc := `
+package main
+import "C"
+//export F1
+func F1() {}
+//export F2
+func F2() {}
+//export F3
+func F3() {}
+func main() {}
+`
+
+	tmpdir := t.TempDir()
+	src := filepath.Join(tmpdir, "x.go")
+	if err := os.WriteFile(src, []byte(testSrc), 0666); err != nil {
+		t.Fatal(err)
+	}
+	exe := filepath.Join(tmpdir, "x.exe")
+
+	// Use a deterministc tmp directory so the temporary file paths are
+	// deterministc.
+	linktmp := filepath.Join(tmpdir, "linktmp")
+	if err := os.Mkdir(linktmp, 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	// Link with -v -linkmode=external to see the flags we pass to the
+	// external linker.
+	ldflags := "-ldflags=-v -linkmode=external -tmpdir=" + linktmp
+	var out0 []byte
+	for i := 0; i < 5; i++ {
+		cmd := testenv.Command(t, testenv.GoToolPath(t), "build", ldflags, "-o", exe, src)
+		out, err := cmd.CombinedOutput()
+		if err != nil {
+			t.Fatalf("build failed: %v, output:\n%s", err, out)
+		}
+		if err := os.Remove(exe); err != nil {
+			t.Fatal(err)
+		}
+
+		// extract the "host link" invocaton
+		j := bytes.Index(out, []byte("\nhost link:"))
+		if j == -1 {
+			t.Fatalf("host link step not found, output:\n%s", out)
+		}
+		out = out[j+1:]
+		k := bytes.Index(out, []byte("\n"))
+		if k == -1 {
+			t.Fatalf("no newline after host link, output:\n%s", out)
+		}
+		out = out[:k]
+
+		// filter out output file name, which is passed by the go
+		// command and is nondeterministic.
+		fs := bytes.Fields(out)
+		for i, f := range fs {
+			if bytes.Equal(f, []byte(`"-o"`)) && i+1 < len(fs) {
+				fs[i+1] = []byte("a.out")
+				break
+			}
+		}
+		out = bytes.Join(fs, []byte{' '})
+
+		if i == 0 {
+			out0 = out
+			continue
+		}
+		if !bytes.Equal(out0, out) {
+			t.Fatalf("output differ:\n%s\n==========\n%s", out0, out)
+		}
+	}
+}
+
 // TestResponseFile tests that creating a response file to pass to the
 // external linker works correctly.
 func TestResponseFile(t *testing.T) {