BACKSPIN_MODE=record clobbers cross-command snapshot patterns

[rsanheim]

Summary

When using cross-command snapshot patterns (test A records command X, test B verifies command Y against the same snapshot), BACKSPIN_MODE=record breaks the pattern because both calls are forced to record mode. The second call overwrites the first's snapshot instead of verifying against it.

Cross-command pattern

# Test A: record rspec output
Backspin.run(rspec_command, name: "comparison_snapshot", filter: normalize)

# Test B: verify plur output matches
Backspin.run(plur_command, name: "comparison_snapshot", filter: normalize)

In auto mode, test A creates the file (record), test B finds it (verify). With BACKSPIN_MODE=record, both calls record, and test B overwrites test A's snapshot with plur output.

Observed behavior

• The snapshot ends up containing the second command's output instead of the first
• result.verified? returns nil in record mode, which surprises tests that assert on it (e.g. expect(result.verified?).to be(true))

Mode precedence is correct

The precedence (mode: kwarg > env var > auto-detection) works as documented. The issue is that cross-command patterns rely on auto-detection, and the global override doesn't know about the record-then-verify sequence within a single test.

Possible improvements

• Warn on overwrite: Log a warning when BACKSPIN_MODE=record overwrites a snapshot that was recorded earlier in the same process/run
• verified? in record mode: Consider returning true (recording succeeded) instead of nil so callers don't need to check result.mode before checking result.verified?
• Document the limitation: Note that cross-command patterns using the same snapshot name are incompatible with BACKSPIN_MODE=record

Context

Discovered while upgrading plur from backspin v0.10 to v0.12 and using BACKSPIN_MODE=record to re-record all snapshots at format v4.1.