Make it possible to start multiple transactions (possibly in different attachments) using the same initial transaction snapshot [CORE6018]

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Submitted by: @asfernandes

With this feature it's possible to create parallel (via different attachments) processes reading consistent data from a database.

For example, a backup process may create multiple threads paralleling read data from the database.

Also a web service may dispatch distributed sub services paralleling doing some processing.

That is accomplished creating a transaction with SET TRANSACTION SNAPSHOT [ AT NUMBER <snapshot number> ] or isc_tpb_at_snapshot_number.

The <snapshot number> from the first transaction may be obtained with RDB$GET_CONTEXT('SYSTEM', 'SNAPSHOT_NUMBER') or transaction info call with fb_info_tra_snapshot_number.

Commits: cfbcbed

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Modified by: @asfernandes

assignee: Adriano dos Santos Fernandes [ asfernandes ]

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Modified by: @asfernandes

status: Open [ 1 ] => Resolved [ 5 ]

resolution: Fixed [ 1 ]

Fix Version: 4.0 Beta 2 [ 10888 ]

[firebird-automations]

Modified by: @pavel-zotov

status: Resolved [ 5 ] => Resolved [ 5 ]

QA Status: No test => Done successfully

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Modified by: @pavel-zotov

status: Resolved [ 5 ] => Closed [ 6 ]

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Commented by: @dyemanov

Adriano, please add the description to README.set_transaction.txt

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Commented by: @asfernandes

Dmitry, it's documented in doc/README.transaction_at_snapshot.md

README.set_transaction.txt is also outdated in relation to doc/README.read_consistency.md.

Will everything be in README.set_transaction.txt or is these different docs sufficient?

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Commented by: @dyemanov

Sorry, I've missed doc/README.transaction_at_snapshot.md -- this is perfectly enough for me. Thanks.