Use atomic.Uint64 for MemoryBackend upload counting on i386

[Copilot]

The internal/ctlog tests use a MemoryBackend upload counter that is updated atomically during sequencing. On i386, the previous raw uint64 field could be misaligned inside the struct, triggering panic: unaligned 64-bit atomic operation.

• Switch the counter to a typed atomic

  • Replace MemoryBackend.uploads uint64 with atomic.Uint64
  • Use the typed API for increments in Upload

• Update test reads to use atomic loads

  • Adjust TestSequenceUploadCount to read the counter with .Load() before computing deltas
  • Preserve existing test behavior while avoiding direct access to the atomic value

• Net effect

  • Keeps the change scoped to the test backend
  • Makes the upload counter safe on 32-bit architectures without changing sequencing logic

type MemoryBackend struct {
    // ...
    uploads atomic.Uint64
}

func (b *MemoryBackend) Upload(...) error {
    b.uploads.Add(1)
    // ...
}

new := tl.Config.Backend.(*MemoryBackend).uploads.Load()

Original prompt

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This section details on the original issue you should resolve

<issue_title>i386: FAIL: TestSequenceOneLeaf (0.03s) panic: unaligned 64-bit atomic operation [recovered, repanicked]</issue_title>
<issue_description>Hi! I'm packaging this for Debian, and builds on 32-bit architecture i386 fails like this:

--- FAIL: TestSequenceOneLeaf (0.03s)
panic: unaligned 64-bit atomic operation [recovered, repanicked]
goroutine 21 [running]:
testing.tRunner.func1.2({0x8aa5940, 0x8c35da0})
	/usr/lib/go-1.26/src/testing/testing.go:1974 +0x29b
testing.tRunner.func1()
	/usr/lib/go-1.26/src/testing/testing.go:1977 +0x409
panic({0x8aa5940, 0x8c35da0})
	/usr/lib/go-1.26/src/runtime/panic.go:860 +0x10b
internal/runtime/atomic.panicUnaligned()
	/usr/lib/go-1.26/src/internal/runtime/atomic/unaligned.go:8 +0x2d
internal/runtime/atomic.Xadd64(0xd0400dc, 0x1)
	/usr/lib/go-1.26/src/internal/runtime/atomic/atomic_386.s:127 +0x11
filippo.io/sunlight/internal/ctlog_test.(*MemoryBackend).Upload(0xd0400c0, {0x8c3df84, 0xd13b740}, {0x8be22a9, 0xa}, {0xd0dc000, 0x24f, 0x280}, 0x93b32e8)
	/build/package/package/_build/src/filippo.io/sunlight/internal/ctlog/testlog_test.go:455 +0x58
filippo.io/sunlight/internal/ctlog.CreateLog({0x8c3df84, 0xd13b740}, 0xd0aa0c0)
	/build/package/package/_build/src/filippo.io/sunlight/internal/ctlog/ctlog.go:151 +0x799
filippo.io/sunlight/internal/ctlog_test.NewEmptyTestLog({0x8c41efc, 0xd108b48})
	/build/package/package/_build/src/filippo.io/sunlight/internal/ctlog/testlog_test.go:68 +0x814
filippo.io/sunlight/internal/ctlog_test.TestSequenceOneLeaf(0xd108b48)
	/build/package/package/_build/src/filippo.io/sunlight/internal/ctlog/ctlog_test.go:38 +0x2f
testing.tRunner(0xd108b48, 0x8c30e24)
	/usr/lib/go-1.26/src/testing/testing.go:2036 +0x114
created by testing.(*T).Run in goroutine 1
	/usr/lib/go-1.26/src/testing/testing.go:2101 +0x511
FAIL	filippo.io/sunlight/internal/ctlog	0.047s

https://salsa.debian.org/jas/golang-filippo-sunlight/-/jobs/9252969

Thanks,
Simon</issue_description>

<agent_instructions>Fix this by replacing the uint64 uploads field with a sync/atomic.Uint64.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes i386: FAIL: TestSequenceOneLeaf (0.03s) panic: unaligned 64-bit atomic operation [recovered, repanicked] #59

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