Agent-mode Go rollback and remove after go mod tidy drop the replace but not restore the module's go.sum lines, so the next default go build fails with "missing go.sum entry" while rollback exits 0 with no warning #549

[agent] Found by the scheduled Go modules bug-hunt routine (ledger #317).

Summary

In agent mode, apply points the patched module at .socket/go-patches/<module>@<version>/ with a directory replace. Go doesn't need go.sum lines for a directory replace, so go mod tidy removes the original module's h1: lines from go.sum. docs/ecosystems.md says "the wiring survives go mod tidy", so running tidy after apply is a supported workflow.

rollback (and remove <purl>) then drops the replace and the copy but leaves go.sum as tidy left it. The result is a go.mod that requires the module with no go.sum entry. The next go build with default flags (-mod=readonly) fails with missing go.sum entry for module providing package …. Rollback still reports status: success with warnings: [].

Impact

After a routine socket-patch rollback or remove, every CI build and every fresh go build/go test is red until someone runs go mod tidy or go get by hand. Nothing in the output says this is needed. Other paths already handle this:

• The hosted leg re-resolves and restores the original module's go.sum lines.
• The hosted→vendored takeover warns that go mod tidy will be needed after vendor --revert (vendor/golang.rs ~L364).

The agent leg does neither.

Repro (hermetic, Linux, go 1.24.7)

SP=/path/to/target/release/socket-patch
W=$(mktemp -d); M=example.com/upstream; V=v1.0.0
mkdir -p $W/stage/$M@$V $W/proxy/$M/@v $W/c/.socket/blobs
printf 'module %s\n\ngo 1.21\n' $M > $W/stage/$M@$V/go.mod
printf 'package upstream\n\nfunc Greeting() string { return "PRISTINE" }\n' > $W/stage/$M@$V/lib.go
printf 'package upstream\n\nfunc Greeting() string { return "PATCHED" }\n' > $W/patched.go
echo "{\"Version\":\"$V\"}" > $W/proxy/$M/@v/$V.info; cp $W/stage/$M@$V/go.mod $W/proxy/$M/@v/$V.mod
(cd $W/stage && zip -qrD $W/proxy/$M/@v/$V.zip $M@$V)
export GOMODCACHE=$W/modcache GOPROXY=file://$W/proxy GOSUMDB=off GOTOOLCHAIN=local
cd $W/c
printf 'module example.com/consumer\n\ngo 1.21\n\nrequire %s %s\n' $M $V > go.mod
printf 'package main\n\nimport (\n\t"fmt"\n\t"%s"\n)\n\nfunc main() { fmt.Println("OUT:", upstream.Greeting()) }\n' $M > main.go
go mod download $M@$V && go mod tidy            # go.sum: 2 lines
gsha(){ python3 -c "import hashlib,sys;d=open(sys.argv[1],'rb').read();print(hashlib.sha256(b'blob %d\0'%len(d)+d).hexdigest())" "$1"; }
B=$(gsha $W/stage/$M@$V/lib.go); A=$(gsha $W/patched.go); cp $W/patched.go .socket/blobs/$A
cat > .socket/manifest.json <<J
{"patches":{"pkg:golang/$M@$V":{"uuid":"4d5e6f70-8192-4a1b-8c2d-0123456789ab","exportedAt":"t","files":{"lib.go":{"beforeHash":"$B","afterHash":"$A"}},"vulnerabilities":{},"description":"","license":"","tier":""}},"setup":{"manual":["golang"]}}
J
$SP apply                 # exit 0, replace written
go mod tidy               # supported per docs; go.sum is now empty
go build -o /dev/null .   # OK (PATCHED)
$SP rollback              # exit 0, "Rolled back packages: pkg:golang/example.com/upstream@v1.0.0"
go build -o /dev/null .   # exit 1: main.go:5:2: missing go.sum entry for module providing package example.com/upstream

$SP remove pkg:golang/example.com/upstream@v1.0.0 instead of rollback gives the same result. Without the go mod tidy step, both pass (go.sum keeps its 2 lines).

Expected vs actual

• Expected: CLI_CONTRACT.md "Rollback command contract (v5.0)" says a bare rollback "restores the SYSTEM to unpatched". docs/ecosystems.md "Go: directory replaces and go.sum" says the wiring "survives go mod tidy". So after apply → tidy → rollback, the project should build exactly as it did before apply. That means either restoring the module's h1: and /go.mod h1: lines (as the hosted leg does) or, at minimum, a warning plus a non-silent outcome telling the user to run go mod tidy.
• Actual: go.mod's require is left with no go.sum lines. Rollback/remove exit 0 with warnings: [] (--json), and the next default go build fails.

Matrix (Linux; each cell run 2× on go 1.24.7, 1× on the others)

Vendored mode (vendor --revert after tidy) is untested here: it needs the mock patch service. The same drop-without-go.sum path looks likely.

Tested on main 61cfb9b (CLI 4.0.0). There have been no Go code changes since 4.0.0, so this isn't a regression in the 4.x line.

Suspect code

• crates/socket-patch-core/src/patch/redirect/golang_local.rs:316 (remove_go_redirect) only calls go_mod_edit::drop_replace_entry (crates/socket-patch-core/src/vendor/go_mod_edit.rs:193) and removes the copy. go.sum is never reconciled and no warning is raised. Compare the hosted-takeover warning at crates/socket-patch-core/src/vendor/golang.rs:364-377.