basalt-26.3.2-rc-20260908: sql/importer: TestCSVImportMarksFilesFullyProcessed failed

sql/importer.TestCSVImportMarksFilesFullyProcessed [failed](https://mesolite.cluster.engflow.com/invocations/default/3e6250f1-ec23-4556-b888-ca3350756d3b?testReportRun=1&testReportShard=3&testReportAttempt=1#targets-Ly9wa2cvc3FsL2ltcG9ydGVyOmltcG9ydGVyX3Rlc3Q=) on basalt-26.3.2-rc-20260908 @ [807558e5e81666af257c811e925e4cae570abc28](https://github.com/cockroachdb/cockroach/commits/807558e5e81666af257c811e925e4cae570abc28):

```
external/org_golang_x_sync/errgroup/errgroup.go:93 +0x86

Previous read at 0x00c01d8bc518 by goroutine 8899:
github.com/cockroachdb/cockroach/pkg/kv/bulk.(*kvBuf).Len()
pkg/kv/bulk/kv_buf.go:177 +0x2e
github.com/cockroachdb/cockroach/pkg/kv/bulk.(*BufferingAdder).IsEmpty()
pkg/kv/bulk/buffering_adder.go:271 +0xe
github.com/cockroachdb/cockroach/pkg/sql/bulksst.(*BulkAdderSink).IsEmpty()
:1 +0x3a
github.com/cockroachdb/cockroach/pkg/sql/importer.(*importProgressTracker).flushedRowsForSink()
pkg/sql/importer/import_processor.go:679 +0x4e8
github.com/cockroachdb/cockroach/pkg/sql/importer.(*importProgressTracker).formatProgress()
pkg/sql/importer/import_processor.go:639 +0x556
github.com/cockroachdb/cockroach/pkg/sql/importer.ingestKvs.func1()
pkg/sql/importer/import_processor.go:349 +0x8f
github.com/cockroachdb/cockroach/pkg/sql/importer.ingestKvs.func3()
pkg/sql/importer/import_processor.go:453 +0x20a
github.com/cockroachdb/cockroach/pkg/sql/importer.ingestKvs.Group.GoCtx.func5()
pkg/util/ctxgroup/ctxgroup.go:200 +0x115
golang.org/x/sync/errgroup.(*Group).Go.func1()
external/org_golang_x_sync/errgroup/errgroup.go:93 +0x86

Goroutine 8900 (running) created at:
golang.org/x/sync/errgroup.(*Group).Go()
external/org_golang_x_sync/errgroup/errgroup.go:78 +0x11c
github.com/cockroachdb/cockroach/pkg/util/ctxgroup.Group.GoCtx()
pkg/util/ctxgroup/ctxgroup.go:191 +0x1809
github.com/cockroachdb/cockroach/pkg/sql/importer.ingestKvs()
pkg/sql/importer/import_processor.go:460 +0x1457
github.com/cockroachdb/cockroach/pkg/sql/importer.runImport.func2()
pkg/sql/importer/read_import_base.go:127 +0xdb
github.com/cockroachdb/cockroach/pkg/sql/importer.runImport.Group.GoCtx.func5()
pkg/util/ctxgroup/ctxgroup.go:200 +0x115
golang.org/x/sync/errgroup.(*Group).Go.func1()
external/org_golang_x_sync/errgroup/errgroup.go:93 +0x86

Goroutine 8899 (running) created at:
golang.org/x/sync/errgroup.(*Group).Go()
external/org_golang_x_sync/errgroup/errgroup.go:78 +0x11c
github.com/cockroachdb/cockroach/pkg/util/ctxgroup.Group.GoCtx()
pkg/util/ctxgroup/ctxgroup.go:191 +0x1456
github.com/cockroachdb/cockroach/pkg/sql/importer.ingestKvs()
pkg/sql/importer/import_processor.go:442 +0x1285
github.com/cockroachdb/cockroach/pkg/sql/importer.runImport.func2()
pkg/sql/importer/read_import_base.go:127 +0xdb
github.com/cockroachdb/cockroach/pkg/sql/importer.runImport.Group.GoCtx.func5()
pkg/util/ctxgroup/ctxgroup.go:200 +0x115
golang.org/x/sync/errgroup.(*Group).Go.func1()
external/org_golang_x_sync/errgroup/errgroup.go:93 +0x86
==================
```

Parameters:
- attempt=1
- race=true
- run=1
- shard=3
Help

See also: [How To Investigate a Go Test Failure \(internal\)](https://cockroachlabs.atlassian.net/l/c/HgfXfJgM)

Same failure on other branches

- #174927 basalt-26.3: sql/importer: TestCSVImportMarksFilesFullyProcessed failed [C-test-failure O-robot T-sql-foundations branch-basalt-26.3 release-blocker]

/cc @cockroachlabs/sql-foundations

[This test on roachdash](https://roachdash.crdb.dev/?filter=status:open%20t:.*TestCSVImportMarksFilesFullyProcessed.*&sort=title+created&display=lastcommented+project) | [Improve this report!](https://github.com/cockroachdb/cockroach/tree/master/pkg/cmd/bazci/githubpost/issues)

Start by reproducing sql/importer.TestCSVImportMarksFilesFullyProcessed with race=true, using the linked test report and commit 807558e5e81666af257c811e925e4cae570abc28. Read pkg/sql/importer/import_processor.go around lines 349, 439-460, 639, and 679, then inspect the reported kv/bulk reads in kv_buf.go and buffering_adder.go. Done means the test no longer reports a race and files are marked fully processed.

Written by the indexing model from the issue text.