Summary

• Detect READONLY errors embedded in Lua script error messages on read-only replicas
• Check for the exact substring "-READONLY You can't write against a read only replica" to avoid false positives
• Add comprehensive unit tests for both direct and wrapped error detection

Supersedes #2770 — incorporates reviewer feedback from @ndyakov (stricter matching) and @ofekshenawa (unit tests).

Problem

When a Lua script containing a write command executes against a read-only replica in a Redis Cluster, the error may contain -READONLY You can't write against a read only replica embedded within a larger error message rather than starting with READONLY :

ERR Error running script (call to f_<sha>): @user_script:<line>: -READONLY You can't write against a read only replica.

The existing code only checks strings.HasPrefix(s, "READONLY "), missing this embedded format. This causes:

• shouldRetry() to not retry the command
• isReadOnlyError() to not detect it, preventing connection pool cleanup via isBadConn()
• LazyReload() to not trigger cluster state refresh

Changes

Addressing PR #2770 Reviewer Feedback

@ndyakov's concern: The original PR used strings.Contains(s, "-READONLY") which is too broad — could match keys like "something-READONLY".

This fix: Checks for the exact substring "-READONLY You can't write against a read only replica" — specific enough to only match actual READONLY errors without any false positives.

Integration Test Report (Redis 7.2.7 Cluster)

Tested against a local 6-node Redis Cluster (3 masters + 3 replicas):

======================================================================
REDIS CLUSTER INTEGRATION TEST REPORT
======================================================================
Redis version: 7.2.7
Replica: 127.0.0.1:7104 -> Master: 127.0.0.1:7101 (slots: 5461-10922)
Test key: "testkey-12591" (slot 5462)

Test 1: EVAL with write command on replica (Lua READONLY)
  Error:          "READONLY You can't write against a read only replica.
                   script: d8f2fad9f8e86a53d2a6ebd960b33c4972cacc37,
                   on @user_script:1."
  IsReadOnlyError: true
  ShouldRetry:     true
  Result:          PASS

Test 2: Script.Run with write command on replica
  Error:          "READONLY You can't write against a read only replica.
                   script: d8f2fad9f8e86a53d2a6ebd960b33c4972cacc37,
                   on @user_script:1."
  IsReadOnlyError: true
  ShouldRetry:     true
  Result:          PASS

Test 3: Multi-command Lua script on replica
  Error:          "READONLY You can't write against a read only replica.
                   script: 02c9e874aad609b773e015a3025d0e43fb7b732b,
                   on @user_script:3."
  IsReadOnlyError: true
  ShouldRetry:     true
  Result:          PASS

Test 4: Simulated old-format Lua READONLY
  Error:          "ERR Error running script (call to f_abc123):
                   @user_script:1: -READONLY You can't write against
                   a read only replica."
  IsReadOnlyError: true
  ShouldRetry:     true
  Result:          PASS

======================================================================
ALL TESTS PASSED
======================================================================

Test plan

• Unit tests for IsReadOnlyError with Lua script error format (proto package)
• Unit tests for ShouldRetry with Lua script READONLY error
• Unit tests for wrapped Lua script READONLY error detection
• False positive tests (non-script errors, script errors without READONLY)
• Integration test against local Redis 7.2.7 Cluster
• All existing tests pass (go test ./internal/proto/ and go test -run Test)

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