Fix alignment bug on 32-bit platforms in new rackAwareRR code

[satta]

When updating Debian packaging to 1.3.0, we noticed that tests were failing on 32-bit systems e.g. on 32-bit ARM [1] or i386 [2]:

=== RUN   TestHostPolicy_RackAwareRR
--- FAIL: TestHostPolicy_RackAwareRR (0.00s)
panic: unaligned 64-bit atomic operation [recovered]
	panic: unaligned 64-bit atomic operation

goroutine 96 [running]:
testing.tRunner.func1.2({0x27b5b8, 0x30a408})
	/usr/lib/go-1.19/src/testing/testing.go:1396 +0x27c
testing.tRunner.func1()
	/usr/lib/go-1.19/src/testing/testing.go:1399 +0x3f4
panic({0x27b5b8, 0x30a408})
	/usr/lib/go-1.19/src/runtime/panic.go:884 +0x23c
runtime/internal/atomic.panicUnaligned()
	/usr/lib/go-1.19/src/runtime/internal/atomic/unaligned.go:8 +0x24
runtime/internal/atomic.Xadd64(0x181267c, 0x1)
	/usr/lib/go-1.19/src/runtime/internal/atomic/atomic_arm.s:258 +0x14
github.com/gocql/gocql.(*rackAwareRR).Pick(0x1812660, {0x0, 0x0})
	/tmp/autopkgtest-lxc.7ok3wgk4/downtmp/autopkgtest_tmp/obj-arm-linux-gnueabihf/src/github.com/gocql/gocql/policies.go:956 +0x34
github.com/gocql/gocql.TestHostPolicy_RackAwareRR(0x11a052c0)
	/tmp/autopkgtest-lxc.7ok3wgk4/downtmp/autopkgtest_tmp/obj-arm-linux-gnueabihf/src/github.com/gocql/gocql/policies_test.go:682 +0x64c
testing.tRunner(0x11a052c0, 0x2c02f0)
	/usr/lib/go-1.19/src/testing/testing.go:1446 +0x118
created by testing.(*T).Run
	/usr/lib/go-1.19/src/testing/testing.go:1493 +0x3a0
FAIL	github.com/gocql/gocql	1.787s

Further investigation on affected platforms showed that this issue was related to a Go problem regarding the use of atomic.*64* functions [3]:

On ARM, 386, and 32-bit MIPS, it is the caller's responsibility to arrange for 64-bit alignment of 64-bit words accessed atomically via the primitive atomic functions (types Int64 and Uint64 are automatically aligned). The first word in an allocated struct, array, or slice; in a global variable; or in a local variable (because the subject of all atomic operations will escape to the heap) can be relied upon to be 64-bit aligned.

This problem surfaces in https://github.com/gocql/gocql/blob/fe55d80b186e16ef11ea79c0cd98065d85886cbe/policies.go#L956 where the last item in a struct is accessed, triggering the unaligned access.

An obvious workaround is moving lastUsedHostIdx to the beginning of the rackAwareRR struct (also see [4]), which indeed solves the problem for me. Tests complete on such these platforms now as well. Hence I am proposing this PR as a low-impact solution for that issue.

My email address is already in the AUTHORS file as I apparently have contributed before :)

[1] https://ci.debian.net/data/autopkgtest/testing/armhf/g/golang-github-gocql-gocql/29281221/log.gz
[2] https://ci.debian.net/data/autopkgtest/testing/i386/g/golang-github-gocql-gocql/29271590/log.gz
[3] https://pkg.go.dev/sync/atomic#pkg-note-BUG
[4] golang/go#11891 (comment)

[martin-sucha]

Thank you for the report!

Could you please rebase the commit and add the information from the pull request description to the commit message? The description is nicely written, it would be great to also include it in the commit message so that it's easier to find in IDEs etcetera.

[satta]

Thanks for the quick reply. Just added the requested information.

[martin-sucha]

Thanks! I've released this in 1.3.1 🎉