x/tools/gopls/internal/analysis: analyzer for slice aliasing mistakes

[adonovan]

In a discussion about slice aliasing bugs, @randall77 suggested an analyzer that detects "aliasing of two slice variables, where the capacity of the slice is unknown (due to append starting from nil, say), and then an append on each of the aliased slices". We should prototype a simple analyzer to detect this pattern and see what turns up.

[nitinmewar]

@adonovan I’d like to prototype this analyzer. I’ll start with a minimal version that detects simple aliasing patterns with appends. Will share a WIP soon!

[nitinmewar]

Hey @adonovan I've now published the initial prototype of the analyzer here:
👉 github.com/nitinmewar/alias-analyzer

The analyzer detects cases where:

• A slice with unknown capacity (e.g., from make([]T, 0) or []T{}) is aliased
• And both the original and the alias are independently appended to

Would love feedback on:
• Is this worth upstreaming into x/tools/go/analysis/passes?
• What next steps should I take to improve/align it with Go vet standards?

Thanks!

[nitinmewar]

Hello @adonovan @randall77,

Let me know how would you want to proceed with it

[adonovan]

Thanks, I'm looking forward to reviewing it. Monday was a holiday and we have a number of urgent tasks for the upcoming gopls release, but I will take a look at it this week.

[adonovan]

I ran your analyzer across a sample of 15K modules from the module mirror corpus, and got the following set of 155 diagnostics (alias.txt), of which a random 20 are shown below. I haven't inspected them yet for precision.

https://go-mod-viewer.appspot.com/code.vegaprotocol.io/vega@v0.79.0/core/processor/abci.go#L1471: append to alias 'appHash' of unknown-capacity slice 'snapHash' may cause memory divergence
https://go-mod-viewer.appspot.com/github.com/admpub/mysql-schema-sync@v0.2.6/sqlite/compare.go#L153: append to alias 'alters' of unknown-capacity slice 'alterIndexLines' may cause memory divergence
https://go-mod-viewer.appspot.com/cuelang.org/go@v0.13.0/tools/trim/trimv3.go#L594: append to alias 'worklist' of unknown-capacity slice 'branches' may cause memory divergence
https://go-mod-viewer.appspot.com/github.com/admpub/mysql-schema-sync@v0.2.6/sqlite/compare.go#L163: append to alias 'alters' of unknown-capacity slice 'alterIndexLines' may cause memory divergence
https://go-mod-viewer.appspot.com/github.com/GoogleCloudPlatform/testgrid@v0.0.174/pkg/updater/updater.go#L673: append to alias 'cols' of unknown-capacity slice 'oldCols' may cause memory divergence
https://go-mod-viewer.appspot.com/github.com/alibaba/ilogtail/pkg@v0.0.0-20250526110833-c53b480d046c/helper/profile/pyroscope/jfr/jfr.go#L112: append to alias 'res' of unknown-capacity slice 'content' may cause memory divergence
https://go-mod-viewer.appspot.com/github.com/admpub/mysql-schema-sync@v0.2.6/sqlite/compare.go#L170: append to alias 'alters' of unknown-capacity slice 'alterIndexLines' may cause memory divergence
https://go-mod-viewer.appspot.com/code.vegaprotocol.io/vega@v0.79.0/core/processor/abci.go#L1464: append to alias 'appHash' of unknown-capacity slice 'snapHash' may cause memory divergence
https://go-mod-viewer.appspot.com/github.com/Velocidex/yaml/v2@v2.2.8/parserc.go#L439: append to alias 'tag' of unknown-capacity slice 'tag_suffix' may cause memory divergence
https://go-mod-viewer.appspot.com/github.com/C-Sto/goWMIExec@v0.0.1-deva.0.20210704154847-b8ebd6464a06/pkg/wmiexec/wmiexec.go#L731: append to alias 'rqUUID' of unknown-capacity slice 'ipid2' may cause memory divergence
https://go-mod-viewer.appspot.com/github.com/Hyperledger-TWGC/ccs-gm@v1.0.0/x509/verify.go#L335: append to alias 'chains' of unknown-capacity slice 'candidateChains' may cause memory divergence
https://go-mod-viewer.appspot.com/github.com/admpub/mysql-schema-sync@v0.2.6/sqlite/compare.go#L179: append to alias 'alters' of unknown-capacity slice 'alterIndexLines' may cause memory divergence
https://go-mod-viewer.appspot.com/github.com/Hyperledger-TWGC/tjfoc-gm@v1.4.0/x509/verify.go#L338: append to alias 'chains' of unknown-capacity slice 'candidateChains' may cause memory divergence
https://go-mod-viewer.appspot.com/github.com/GoogleCloudPlatform/testgrid@v0.0.174/pkg/updater/updater.go#L708: append to alias 'cols' of unknown-capacity slice 'oldCols' may cause memory divergence
https://go-mod-viewer.appspot.com/code.vegaprotocol.io/vega@v0.79.0/core/processor/abci.go#L1461: append to alias 'appHash' of unknown-capacity slice 'snapHash' may cause memory divergence
https://go-mod-viewer.appspot.com/decred.org/dcrdex@v1.0.3/client/asset/eth/deploy.go#L171: append to alias 'txData' of unknown-capacity slice 'bytecode' may cause memory divergence
https://go-mod-viewer.appspot.com/code.vegaprotocol.io/vega@v0.79.0/core/processor/abci.go#L1462: append to alias 'appHash' of unknown-capacity slice 'snapHash' may cause memory divergence
https://go-mod-viewer.appspot.com/cuelang.org/go@v0.13.0/tools/trim/trimv3.go#L590: append to alias 'worklist' of unknown-capacity slice 'branches' may cause memory divergence
https://go-mod-viewer.appspot.com/code.vegaprotocol.io/vega@v0.79.0/core/processor/abci.go#L1463: append to alias 'appHash' of unknown-capacity slice 'snapHash' may cause memory divergence
https://go-mod-viewer.appspot.com/github.com/GoogleCloudPlatform/testgrid@v0.0.174/pkg/updater/updater.go#L696: append to alias 'cols' of unknown-capacity slice 'oldCols' may cause memory divergence

[nitinmewar]

Thanks for running it at scale!
I’ve reviewed the results and many of them are valid risks for memory divergence.
I’d love feedback on:
• How to improve precision and reduce false positives
• Whether this could be useful as a vet check (or a golang.org/x/tools check)

[adonovan]

Can you quantify what fraction are true vs false positives? We usually aim for a false positive rate of around 5% or less.