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/13672
Reviewed-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>

diff --git a/src/cmd/go/note_test.go b/src/cmd/go/note_test.go
index fb25f94ec31c91ce73c98b0d15884ea634baff62..cbb3db8a17b88c427245503ece7c68f88fed264d 100644 (file)
--- a/src/cmd/go/note_test.go
+++ b/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)
+               }
+       }
 }

diff --git a/src/cmd/go/pkg.go b/src/cmd/go/pkg.go
index 0317536bce6aec8c0e6ea145e027a588275fef74..5dd23526068173f80d6e279a2940c0a349a3bdfb 100644 (file)
--- a/src/cmd/go/pkg.go
+++ b/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