cmd/go: fix spurious rebuild of binaries using cgo on OS X

The text segment starts farther into the binary when using
external linking on the mac. Test and fix.

Fixes #12173.

Change-Id: I1f0c81814bf70cd9decfceac3022784f4608eeef
Reviewed-on: https://go-review.googlesource.com/13672Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/note_test.go

@@ -6,6 +6,7 @@ package main_test
 
 import (
 	"cmd/go"
+	"runtime"
 	"testing"
 )
 
@@ -22,4 +23,20 @@ func TestNoteReading(t *testing.T) {
 	if id != buildID {
 		t.Fatalf("buildID in hello binary = %q, want %q", id, buildID)
 	}
+
+	switch runtime.GOOS {
+	case "plan9":
+		// no external linking
+		t.Logf("no external linking - skipping linkmode=external test")
+
+	default:
+		tg.run("build", "-ldflags", "-buildid="+buildID+" -linkmode=external", "-o", tg.path("hello.exe"), tg.path("hello.go"))
+		id, err := main.ReadBuildIDFromBinary(tg.path("hello.exe"))
+		if err != nil {
+			t.Fatalf("reading build ID from hello binary (linkmode=external): %v", err)
+		}
+		if id != buildID {
+			t.Fatalf("buildID in hello binary = %q, want %q (linkmode=external)", id, buildID)
+		}
+	}
 }

src/cmd/go/pkg.go

@@ -1796,10 +1796,10 @@ func ReadBuildIDFromBinary(filename string) (id string, err error) {
 		return "", &os.PathError{Op: "parse", Path: filename, Err: errBuildIDUnknown}
 	}
 
-	// Read the first 8 kB of the binary file.
+	// Read the first 16 kB of the binary file.
 	// That should be enough to find the build ID.
 	// In ELF files, the build ID is in the leading headers,
-	// which are typically less than 4 kB, not to mention 8 kB.
+	// which are typically less than 4 kB, not to mention 16 kB.
 	// On other systems, we're trying to read enough that
 	// we get the beginning of the text segment in the read.
 	// The offset where the text segment begins in a hello
@@ -1807,7 +1807,7 @@ func ReadBuildIDFromBinary(filename string) (id string, err error) {
 	//
 	//	Plan 9: 0x20
 	//	Windows: 0x600
-	//	Mach-O: 0x1000
+	//	Mach-O: 0x2000
 	//
 	f, err := os.Open(filename)
 	if err != nil {
@@ -1815,7 +1815,7 @@ func ReadBuildIDFromBinary(filename string) (id string, err error) {
 	}
 	defer f.Close()
 
-	data := make([]byte, 8192)
+	data := make([]byte, 16*1024)
 	_, err = io.ReadFull(f, data)
 	if err == io.ErrUnexpectedEOF {
 		err = nil