wal: separate read interface from Manager

[jbowens]

This commit separates the interface for reading WAL files from the Manager interface. This helps unify some code on the read path, but it also makes it easier to support changing configuration (eg, changing secondary location or disabling failover).

[cockroach-teamcity]

This change is 

[sumeerbhola]

Reviewed 10 of 10 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @jbowens)

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wal/failover_manager.go line 447 at r1 (raw file):

// List implements Manager.
func (wm *failoverManager) List() (Logs, error) {
	return Scan(wm.opts.Primary, wm.opts.Secondary)

I suppose since we don't have anything like StandaloneManager.mu.queue we have to go to the filesystem. And this may change once we implement stats etc.?

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wal/reader.go line 44 at r1 (raw file):

}

// NumSegments returns the number of consitutent physical log files that make up

nit: constituent

[jbowens]

TFTR!

Reviewable status: 9 of 10 files reviewed, 1 unresolved discussion (waiting on @sumeerbhola)

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wal/failover_manager.go line 447 at r1 (raw file):

Previously, sumeerbhola wrote…

I suppose since we don't have anything like StandaloneManager.mu.queue we have to go to the filesystem. And this may change once we implement stats etc.?

Yeah, I think it might—there's also the ambiguity as to whether the files that we know we've attempted to create actually do exist. I haven't thought through that stats plumbing.

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wal/reader.go line 44 at r1 (raw file):

Previously, sumeerbhola wrote…

nit: constituent

Done.