# Protect OpenResty stock decisions across workers with a shared lock

A Lua module variable is per worker, while ngx.shared provides shared state. A get-then-set sequence still needs a lock when the operation consists of several steps. Check current availability under that lock and release it even if the operation raises.

Save this requirement in Perch and check it after each agent edit. The rule gives your agent a named check to run, a finding to repair, and the same command to recheck.

## Implement the OpenResty boundary

Save this illustrative integration module as `lua/reserve.lua`.

```lua
local resty_lock = require "resty.lock"
local stock = ngx.shared.stock

local function reserve(sku, quantity)
    if type(quantity) ~= "number" or quantity <= 0 or quantity % 1 ~= 0 then
        return nil, "invalid quantity"
    end
    local lock, err = resty_lock:new("inventory_locks", {timeout=0.2, exptime=2})
    if not lock then return nil, err end
    local elapsed, lock_err = lock:lock("stock:" .. sku)
    if not elapsed then return nil, lock_err end
    local ok, result, operation_err = pcall(function()
        local available = stock:get(sku) or 0
        if available < quantity then return nil, "insufficient stock" end
        local saved, save_err = stock:set(sku, available - quantity)
        if not saved then return nil, save_err end
        return true
    end)
    lock:unlock()
    if not ok then return nil, result end
    return result, operation_err
end

return reserve
```

The order, inventory, or resource-lifetime scenario is adapted from Perch’s public [order-service source at commit 4af67af](https://github.com/lakeday-org/perch/blob/4af67af46d76ac99398d61c7ad0a45441760d4dc/test/fixtures/order-service/inventory.py). This OpenResty implementation is an illustrative adaptation.

## Integrate with your application

Declare lua_shared_dict stock and lua_shared_dict inventory_locks in nginx.conf and install lua-resty-lock. Resolve and authorize the tenant before constructing a tenant-qualified SKU key. Never perform yielding network work inside this short pcall-protected section; expiration must exceed the maximum critical-section duration. Shared dictionary state is not durable inventory and can be evicted: use a transactional database for real stock and treat this as a coordination example.



## Exercise the boundary and failure paths

| Scenario | Expected outcome |
| --- | --- |
| Two workers reserve final unit | One succeeds under lock |
| Insufficient stock | No decrement |
| Shared-dict set fails | Error returned, lock released |
| Operation throws | pcall catches, unlock still runs |

Add regression tests for these cases in your existing OpenResty test project. Use separate connections or deliberately reordered completion where the case involves concurrency; a sequential happy-path test does not exercise that failure. Run the test command after creating the matching test file or suite:

```sh
resty tests/reserve_spec.lua
```

The scenarios above specify tests to implement. A completed Perch source check below is separate from running these application integration tests.

## Introduce the specific agent regression

In a throwaway copy, replace this source span:

```lua
if available < quantity then return nil, "insufficient stock" end
```

with:

```lua
// Remove the source span shown above.
```

Run the relevant negative test and the Perch command on that broken copy. Restore the corrected source before committing the application. The supplied mutation reproduces the exact regression for the rule check.

## Preserve the requirement as a Perch rule

Save this in `.perch/rules/perch-guide-openresty-shared-dictionary-lock.yaml` at the repository root, keeping your existing `perch.yaml`:

```yaml
- name: shared-stock-availability
  where: "lua/reserve.lua"
  ensure: >-
    Inside the acquired inventory lock, reserve must reject available < quantity before changing the shared dictionary.
```

Install and authenticate the CLI once, then check the current working file:

```sh
npm install -g @lakeday/perch
perch login
perch check lua/reserve.lua --rules shared-stock-availability --json
```

Use Node 22 or newer. The framework can use another language; Perch’s CLI remains a Node command. A gated finding exits 3. Exits 1 or 2 mean the check did not complete and must be reported as a blocker.

## Inspect the captured rule check

On 2026-10-14, Perch using jev-latest checked the exact fixed and broken source provided here. The capture records SHA-256 hashes of both sources and the requirement.

| Source control | Exit | Outcome |
| --- | --- | --- |
| Broken | 3 | Rule finding |
| Corrected | 0 | No rule finding |

The rule distinguished these controls in this run. [Raw Perch capture](/shared-dictionary-lock/capture.json) · [Exact sources and mutation](/shared-dictionary-lock/source.json).

## Install the rule pack and configure your agent

[Download this rule pack ZIP](/shared-dictionary-lock/rule-pack.zip) · [Download the YAML rules](/shared-dictionary-lock/rules.yaml) · [Agent task](/shared-dictionary-lock/agent-task.txt). The pack contains the rule, its example paths and the repair/recheck instructions. Review the `where` selector before using it in an existing project. Keep your root `perch.yaml`; install this rule as a separate file under `.perch/rules/`.

1. Install Perch using Node 22 or newer and sign in once:

```sh
npm install -g @lakeday/perch
perch login
```

2. Download the pack, inspect it, and extract it from your repository root. The extraction preserves any existing files:

```sh
curl -fL 'https://reporules.fyi/openresty/shared-dictionary-lock/rule-pack.zip' -o 'perch-guide-openresty-shared-dictionary-lock.zip'
unzip -n 'perch-guide-openresty-shared-dictionary-lock.zip'
```

The installed rule is `.perch/rules/perch-guide-openresty-shared-dictionary-lock.yaml`. Its selector matches `lua/reserve.lua` in this example; adapt the selector to your application file before checking an existing project.

3. Configure the coding agent you use. Run one of these commands:

Claude Code:

```sh
perch setup claude-code
```

Codex:

```sh
perch setup codex
```

Cursor:

```sh
perch setup cursor
```

Pi:

```sh
perch setup pi
```

4. Ask the agent to edit the implementation and check the saved requirement:

> Use the installed Perch skill while editing lua/reserve.lua. Preserve shared-stock-availability. Run perch check lua/reserve.lua --rules shared-stock-availability --json after the edit. Read each finding and the affected code, fix the implementation, and run the identical check again after the fix. Keep the rule and regression tests intact. Run the application's relevant regression tests and report the commands and actual results. Exit 3 means a rule finding; exit 1 or 2 means the check did not complete. Report an unavailable or incomplete check as blocked. Do not claim the application tests ran if only the Perch check ran.

5. After repairing a finding, run the same check again and inspect the result:

```sh
perch check lua/reserve.lua --rules shared-stock-availability --json
```

Commit the downloaded rule and the installed agent configuration with your application. The rule file adds no guarantee of correctness; keep the regression tests from this guide.

With the base reference fetched, use `perch scan --since origin/main --filter type=lint` in CI.

## Framework references and verification scope

- [github.com reference](https://github.com/openresty/lua-resty-lock)

The dated capture records the source check shown above. Implement the adapters, schema and regression tests described in this guide in your application. Unchanged requests reuse cached answers; fresh model judgments can vary. Model analysis sends relevant source to the configured service.