Cache server_version_info and guard against re-entrant reads#622
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hashhar wants to merge 1 commit into
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Cache server_version_info and guard against re-entrant reads#622hashhar wants to merge 1 commit into
hashhar wants to merge 1 commit into
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The SQLAlchemy dialect exposed server_version_info as an un-cached property whose getter runs "SELECT version()" through the connection. Because the query executes via the same SQLAlchemy machinery any code that reads server_version_info while a query is in flight re-enters the getter and issues another query, recursing forever. aws-xray-sdk is an example of this pattern. Cache the resolved value on the dialect instance and seed the cache with None before issuing the query. A re-entrant read that arrives while the version query is in flight now returns immediately from the seeded cache instead of recursing and subsequent reads are served from the cache without additional HTTP calls.
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Description
The SQLAlchemy dialect exposed server_version_info as an un-cached property whose getter runs "SELECT version()" through the connection. Because the query executes via the same SQLAlchemy machinery any code that reads server_version_info while a query is in flight re-enters the getter and issues another query, recursing forever.
aws-xray-sdk is an example of this pattern.
Cache the resolved value on the dialect instance and seed the cache with None before issuing the query. A re-entrant read that arrives while the version query is in flight now returns immediately from the seeded cache instead of recursing and subsequent reads are served from the cache without additional HTTP calls.
Non-technical explanation
Cache server_version_info and guard against re-entrant reads. Fixes #559.
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