go: x/mobile: gomobile build fails for iOS targets

I did some digging and found that the answer to my own question is: nothing changed, but gomobile build mode broke with my bitcode-enabling CL168062. Presumably I tested gomobile bind but failed to test gomobile build when preparing that CL.

gomobile build builds an iOS executable, while gomobile bind builds a C archive (using buildmode=c-archive). Building an iOS executable with -fembed-bitcode is not compatible with the flags cmd/link pass to the host linker; I’ve found -headerpad, -pagezero_size and -fno_pie to be incompatible.

It seems to me the fix is to figure out why cmd/link needs to pass the incompatible flags, and if it doesn’t, remove or replace them with bitcode compatible flags. Removing the incomptable flags succeeded in producing a binary but I haven’t run it nor tried to upload it to the App Store for verification.

The next best fix is to work around the issue and switch gomobile build to use the buildmode=c-archive like gomobile bind. As a side effect, gomobile build will be much more similar to gomobile bind, decreasing the likelyhood of future breaks.

The easiest workaround is for gomobile build to omit -fembed-bitcode. The downside is that executables built this way will not have bitcode enabled. Since bitcode is not (yet) a requirement for iOS, I don’t think missing bitcode matters.

CL 189857 implements the third option, except that it also omits the flag for gomobile bind which is wrong.

I don’t know how to create a PR for gomobile, so here is a consolidated patch file instead…

gomobile.diff.txt

Aha, the problem seems to be that the go tools we are using force -headerpad and that bitcode is now enabled by default and these conflict. The following patch to gomobile fixed it for me 😎.

diff --git a/cmd/gomobile/build_iosapp.go b/cmd/gomobile/build_iosapp.go
index 9cc22ab..79b2c3f 100644
--- a/cmd/gomobile/build_iosapp.go
+++ b/cmd/gomobile/build_iosapp.go
@@ -426,6 +426,7 @@ const projPbxproj = `// !$*UTF8*$!
         SDKROOT = iphoneos;
         TARGETED_DEVICE_FAMILY = "1,2";
         VALIDATE_PRODUCT = YES;
+        ENABLE_BITCODE = NO;
       };
       name = Release;
     };
diff --git a/cmd/gomobile/env.go b/cmd/gomobile/env.go
index cb42d1c..a707965 100644
--- a/cmd/gomobile/env.go
+++ b/cmd/gomobile/env.go
@@ -140,7 +140,6 @@ func envInit() (err error) {
                default:
                        panic(fmt.Errorf("unknown GOARCH: %q", arch))
                }
-               cflags += " -fembed-bitcode"
                if err != nil {
                        return err
                }