plugin: noopt builder broken by plugin change

[bradfitz]

The noopt builder is broken by the 03da269 (plugin versioning)

https://build.golang.org/log/857ba03fe3293b11a67a4391292aff2d3b29c28e

##### ../misc/cgo/testplugin
2016/11/15 16:27:25 plugin.Open failed: plugin.Open: plugin was built with a different version of package sync/atomic
2016/11/15 16:27:25 Failed: exit status 1
2016/11/15 16:27:25 FAILED

[wendigo]

Is it related to #17928 ?

[crawshaw]

It looks like the noopt builder found a potentially interesting bug.

Code included in plugins is all compiled with the cmd/compile arg -shared. However binaries that load the plugin do not (by default on linux) build with -shared. As the dynamic linker will favor loaded code, that means plugins sometimes get different code when they are loaded. This trips over the package-hashing versioning check with noopt.

I can't think of any cases where this causes a program to fail. The -shared flag changes the way code references other symbols and restricts its register use. But the register use shouldn't matter across function calls, and as the binary is compiled as not-shared, the code it has loaded has to be able to reach other symbols already.

Unfortunately because of the way the go/build package works, fixing this in cmd/go is a bit of a kludge. It's hard to justify a kludge like this to get a builder working again and fix a potential bug. But I'll send the CL anyway.

[gopherbot]

Change https://golang.org/cl/61071 mentions this issue: cmd/go: build binaries that use plugins with -shared