Blocking co::Object::commit

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Needed by @tribal-tec for codash: Implement a way to configure per master or per slave object how many versions might be queued up before a commit blocks.

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Issue 1: Per master or per slave?

Resolved: per slave seems to give more flexibility and cleaner

Per master:

• Owner decides
• Master can request ack with stream
• Potentially simpler impl

Per slave:

• different receivers can have different queuing strategies
• It's the slave's memory being used

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Issue 2: Runtime-changeable?

Resolved: no.

For simplicity and memory footprint will go with a overridable method to configure max queue size. If the return value changes at runtime the behavior is undefined for now.

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API

virtual uint64_t co::Object::getMaxVersions() const { return std::numeric_limits< uint64_t >::max(); }

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Issue 3: Also for slave object commits?

Not for now. Requires a completely different implementation, and is not needed yet.

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Implementation

• Master CM has per-slave 'maxVersion', defaults to uint64_t::max
• Slave CM queries getMaxVersions() after each sync
  • sends version_ack with synced+maxVersions to master
  • master updates per-slave maxVersion to received value, updated global maxVersion monitor
• Master commit monitors maxVersion.waitGE( current+1 ) before committing