terms: skip injected code for off actions

[1fanwang]

What problem does this PR solve?

Setting a failpoint to off still runs the injected code. This can inject a failure even after the caller has switched the action off.

Fixes #86.

What is changed and how it works?

The off action returns the existing non-triggering error instead of a successful evaluation. Generated injection guards then skip that evaluation. Counted off terms can still advance to a later action.

Check List

• Unit tests cover off actions, term chains, exhausted counts and enabled controls.
• The README states the evaluation result for an off action.
• The manual check below uses the real source rewriter and executes its output.

Testing Done

The same rewritten program prints term=off injected=true before the fix and term=off injected=false afterward. The enabled control remains true.

I ran the probe from an unchanged checkout of 55ac33a and from this branch.

Save this program as /tmp/failpoint-off-probe-XyxNHL/main.go:

package main

import (
	"fmt"

	"github.com/pingcap/failpoint"
)

func main() {
	for _, term := range []string{"return", "off"} {
		if err := failpoint.Enable("main/off-repro", term); err != nil {
			panic(err)
		}
		injected := false
		failpoint.Inject("off-repro", func() {
			injected = true
		})
		fmt.Printf("term=%s injected=%t\n", term, injected)
	}
}

Rewrite it once from the baseline checkout:

GOMAXPROCS=2 go run ./failpoint-ctl enable /tmp/failpoint-off-probe-XyxNHL

Scenario	Command, run from each checkout	Observed result
Before	GOMAXPROCS=2 go run /tmp/failpoint-off-probe-XyxNHL/*.go	Both actions execute the injection.
After	GOMAXPROCS=2 go run /tmp/failpoint-off-probe-XyxNHL/*.go	Only the return action executes it.

Raw logs

Before:

term=return injected=true
term=off injected=true

After:

term=return injected=true
term=off injected=false

The separate rewriter test TestRewriteInjectCall/case-0 fails on both the unchanged baseline and this branch: its expected output still includes an evaluation guard that the rewriter no longer emits. This change leaves that pre-existing failure untouched.

[pingcap-cla-assistant]

All committers have signed the CLA.