in-memory database not threadsafe #4459

[joel-rieke]

Please see below for the concurrency issue.

This exception happened intermittently while performing a combinations of SELECT/INSERT/UPDATES on a single table with many rows...

Looking at the code however, it is clear to me why this happened.

I tried fixing this by applying a table level lock around the partition access, but this seems heavy handed. I think there could be a more elegant approach to solving this.

**This was due to a classic insert, but many simultaneous inserts: Something like: INSERT INTO tableA (columnP, columnQ, columnR, columnS) VALUES (?, ?, ?, ?)

tablename and column names and counts changed obviously.**

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"concurrent map read and map write" Stack: 2 0x000000000117e905 in github.com/dolthub/go-mysql-server/memory.partitionssort.Less at /go/pkg/mod/github.com/trimble-oss/go-mysql-server@v0.12.0-1.4/memory/table.go:1523 3 0x000000000118fc3c in github.com/dolthub/go-mysql-server/memory.(partitionssort).Less at :1 4 0x00000000004d4318 in sort.partialInsertionSort at /usr/local/go/src/sort/zsortinterface.go:202 5 0x00000000004d3b29 in sort.pdqsort at /usr/local/go/src/sort/zsortinterface.go:101 6 0x00000000004d070e in sort.Sort at /usr/local/go/src/sort/sort.go:48 7 0x000000000117e23c in github.com/dolthub/go-mysql-server/memory.(Table).sortRows at /go/pkg/mod/github.com/trimble-oss/go-mysql-server@v0.12.0-1.4/memory/table.go:1502 8 0x00000000011859d1 in github.com/dolthub/go-mysql-server/memory.(pkTableEditAccumulator).ApplyEdits at /go/pkg/mod/github.com/trimble-oss/go-mysql-server@v0.12.0-1.4/memory/table_editor.go:460 9 0x00000000011819e2 in github.com/dolthub/go-mysql-server/memory.(tableEditor).StatementComplete at /go/pkg/mod/github.com/trimble-oss/go-mysql-server@v0.12.0-1.4/memory/table_editor.go:101 10 0x0000000001181493 in github.com/dolthub/go-mysql-server/memory.(tableEditor).Close at /go/pkg/mod/github.com/trimble-oss/go-mysql-server@v0.12.0-1.4/memory/table_editor.go:76 11 0x00000000013cf20b in github.com/dolthub/go-mysql-server/sql/plan.(insertIter).Close at /go/pkg/mod/github.com/trimble-oss/go-mysql-server@v0.12.0-1.4/sql/plan/insert.go:508 12 0x000000000143d28a in github.com/dolthub/go-mysql-server/sql/plan.checkpointingTableEditorIter.Close at /go/pkg/mod/github.com/trimble-oss/go-mysql-server@v0.12.0-1.4/sql/plan/table_editor.go:118 13 0x000000000148e99d in github.com/dolthub/go-mysql-server/sql/plan.(checkpointingTableEditorIter).Close at :1 14 0x0000000001409f4e in github.com/dolthub/go-mysql-server/sql/plan.(accumulatorIter).Next.func2 at /go/pkg/mod/github.com/trimble-oss/go-mysql-server@v0.12.0-1.4/sql/plan/row_update_accumulator.go:322 16 0x0000000001409bc6 in github.com/dolthub/go-mysql-server/sql/plan.(accumulatorIter).Next at /go/pkg/mod/github.com/trimble-oss/go-mysql-server@v0.12.0-1.4/sql/plan/row_update_accumulator.go:360 17 0x00000000014437c8 in github.com/dolthub/go-mysql-server/sql/plan.transactionCommittingIter.Next at /go/pkg/mod/github.com/trimble-oss/go-mysql-server@v0.12.0-1.4/sql/plan/transaction_committing_iter.go:106 18 0x000000000148fa0d in github.com/dolthub/go-mysql-server/sql/plan.(transactionCommittingIter).Next at :1 19 0x00000000013f3502 in github.com/dolthub/go-mysql-server/sql/plan.(*trackedRowIter).Next at /go/pkg/mod/github.com/trimble-oss/go-mysql-server@v0.12.0-1.4/sql/plan/process.go:381

[zachmu]

Yes, unfortunately the in-memory database is not completely threadsafe right now. We have this on our roadmap to address, but it's a bit of work to accomplish.

[joel-rieke]

RWLock might be a good option in table.go layer.

[joel-rieke]

SELECT FOR UPDATE would also be nice if anyone is in this layer implementing.

[kom0055]

concurrent map writes it also happened in a very high frequency in v0.17.0 github.com/dolthub/go-mysql-server@v0.17.0/memory/table_editor.go:424