feat(detector): implement FNM detection with mocked tests#1
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Replace the FNM stub with a real implementation of the detection
surface of the Manager interface:
- Detect() — cheap PATH probe via platform.LookupManagerBinary
- Version() — wraps `fnm --version`, parses leading "fnm " token
- ListInstalled()— wraps `fnm list`, skips the "system" line and the
"*" default marker, returns a non-nil sorted-asc
[]semver.Version
Mutation methods (Install, Uninstall, Use, SetDefault,
GlobalNpmPrefix) now return the new ErrFNMNotImplemented sentinel
instead of a silent zero value, so callers can use errors.Is to
distinguish "not yet implemented" from "succeeded".
Tests use a package-level runShell var (signature-compatible with
platform.RunShell) that is swapped per-test via withStubShell. The
test-stub pattern intentionally matches what was already established
in detector_test.go for future manager files (nvm, volta, asdf,
mise, n, nodenv, nvm-windows) to follow.
Verified against the local /opt/homebrew/bin/fnm 1.39.0: detected,
version "1.39.0" parsed, installed versions [24.15.0 25.9.0]
returned sorted asc with the "system" line correctly excluded.
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This was referenced Jun 29, 2026
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Summary
Replaces the FNM stub in
internal/detector/fnm.gowith a real implementation of the detection surface of theManagerinterface. Mutation methods (Install,Uninstall,Use,SetDefault,GlobalNpmPrefix) now return a newErrFNMNotImplementedsentinel rather than a silent zero value.What's implemented
Detect()platform.LookupManagerBinary("fnm") != ""— zero subprocessesVersion()fnm --version, parses leading"fnm "tokenListInstalled()fnm list, strips"* "default marker, drops"system"line, returns sorted-asc[]semver.VersionErrFNMNotImplementedsentinelWhat's tested
internal/detector/fnm_test.go— 21 tests covering:parseFNMVersion: standard output, bare version, trailing whitespace, emptyparseFNMInstalled: happy path, unparseable lines skipped, empty input, system-onlyFNM.Version(): success, RunShell error wrapping, parse error on blankFNM.ListInstalled(): success, RunShell error wrappingFNM.MutationMethods: each returnsErrFNMNotImplemented(errors.Is-verified)FNM.Detect()does not invokerunShell(cheap-probe contract)withStubShellhelperLive verification
Against the local
/opt/homebrew/bin/fnm 1.39.0:The
"* system"line infnm listoutput is correctly excluded.Test-stub pattern (for future managers)
fnm.godeclares a package-levelrunShell = platform.RunShellvar. Tests usewithStubShell(t, ...)to swap it for the duration of the test (auto-restored viat.Cleanup). Future manager implementations (nvm, volta, asdf, mise, n, nodenv, nvm-windows) should follow the same pattern for consistency.Out of scope (tracked separately)
Install,Uninstall,Use,SetDefault,GlobalNpmPrefix) — will land in their own commit when the upgrade command (Phase 4) needs to call themTest plan
go build ./...go vet ./...go test -race ./internal/detector/...(21/21 pass)