bitsets are misaligned on 32-bit architectures #527
Description
Testing on my trusty x86-32 machine:
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x804a9eb]
goroutine 21 [running]:
runtime/internal/atomic.Load64(0x980c394, 0x2800, 0x84e4060)
/usr/local/go/src/runtime/internal/atomic/asm_386.s:194 +0xb
github.com/oragono/oragono/irc/utils.BitsetGet(0x980c394, 0x1, 0x1, 0xc, 0x980c2d0)
/home/shivaram/go/src/github.com/oragono/oragono/irc/utils/bitset.go:16 +0x37
github.com/oragono/oragono/irc/caps.(*Set).Has(...)
/home/shivaram/go/src/github.com/oragono/oragono/irc/caps/set.go:63
github.com/oragono/oragono/irc.(*Session).SetMaxlenRest(0x980c2d0)
/home/shivaram/go/src/github.com/oragono/oragono/irc/client.go:137 +0x46
github.com/oragono/oragono/irc.(*Server).RunClient(0x98b6000, 0x85de3a0, 0x99b0200, 0x9860000)
/home/shivaram/go/src/github.com/oragono/oragono/irc/client.go:223 +0x4cf
created by github.com/oragono/oragono/irc.(*Server).createListener.func1
/home/shivaram/go/src/github.com/oragono/oragono/irc/server.go:316 +0x10a
sync.atomic guarantees that "the first word in a variable or in an allocated struct, array, or slice can be relied upon to be 64-bit aligned." I misunderstood what "allocated" means; it does not include a [1]uint64 that is a struct field. This article was helpful: https://go101.org/article/memory-layout.html
We got lucky with 1.0.0: the three bitsets we use (client caps, client modes, and channel modes) happened to be 64-bit aligned.
Anyway, this should be an easy fix, we just have to use []uint32 in place of []uint64 for the bitsets. I'll do a round of benchmarking to see if there's any impact on performance on amd64.