atomic requires struct field to be 64-bit aligned

[tpng]

fix gohugoio/hugo#1621

[CLAassistant]

All committers have accepted the CLA.

[mbertschler]

Interesting issue, thank you for the fix.

[moorereason]

@tpng, I'm impressed. How in the world did you come up with that? What lead you to this solution?

[tpng]

@moorereason I first encountered this issue with the new v0.15 hugo using the server command and found that only the 386 version has this issue.

Then I went adding log.Print in the source code to find where hugo hang and it ends up in (*InMemoryFile).Open().

After that I try to create a repro to see how atomic failed in this case, which crash with a nil pointer error on the call to atomic.StoreInt64(&f.readDirCount, 0).

Finally a google search of golang atomic 386 amd64 ended up with a result that stated that struct field has to be 64-bit aligned and thus the fix.

[moorereason]

That's the first time I've seen a "Bugs" section in the Go stdlib docs. The docs also say this affects non-Linux ARM platforms.

I'm not very familiar with AppVeyor, but it seems like afero could use AppVeyor to automate testing on the Windows x86 platform to catch this sneaky bug.