fix: drop no-longer-necessary half-disabling of instrumentations (#2444)

When disableInstrumentations support was added in #353 it only
half-disabled these instrumentations, briefly mentioning "continuation
patches". I believe this was about user-land callback queue handling
that should no longer be necessary after run-context and #2430 work.

In #2498 it was discussed and decided that if listed in
disableInstrumentations the agent should not touch the module at all.

Refs: #2498

CHANGELOG.asciidoc

@@ -34,6 +34,13 @@ Notes:
 [float]
 ===== Breaking changes
 
+* Change the `redis` and `mysql` instrumentations to not patch at all if
+  they are listed in <<disable-instrumentations, `disableInstrumentations`>>*.
+  This means that an application that uses one of these packages *and* lists
+  that package in `disableInstrumentations` could see changes in the async
+  run-context of callbacks.  See {issues}2498[#2498] and the
+  <<release-notes-3.26.0,v3.26.0 release notes>> which has a similar change.
+
 [float]
 ===== Features
 

lib/instrumentation/modules/mysql.js

@@ -10,6 +10,9 @@ var symbols = require('../../symbols')
 var { getDBDestination } = require('../context')
 
 module.exports = function (mysql, agent, { version, enabled }) {
+  if (!enabled) {
+    return mysql
+  }
   if (!semver.satisfies(version, '^2.0.0')) {
     agent.logger.debug('mysql version %s not supported - aborting...', version)
     return mysql
@@ -21,8 +24,6 @@ module.exports = function (mysql, agent, { version, enabled }) {
   agent.logger.debug('shimming mysql.createPoolCluster')
   shimmer.wrap(mysql, 'createPoolCluster', wrapCreatePoolCluster)
 
-  if (!enabled) return mysql
-
   agent.logger.debug('shimming mysql.createConnection')
   shimmer.wrap(mysql, 'createConnection', wrapCreateConnection)
 
@@ -75,7 +76,7 @@ module.exports = function (mysql, agent, { version, enabled }) {
 
       if (typeof cb === 'function') {
         arguments[0] = agent._instrumentation.bindFunction(function wrapedCallback (err, connection) { // eslint-disable-line handle-callback-err
-          if (connection && enabled) wrapQueryable(connection, 'getConnection() > connection', agent)
+          if (connection) wrapQueryable(connection, 'getConnection() > connection', agent)
           return cb.apply(this, arguments)
         })
       }

lib/instrumentation/modules/redis.js

@@ -12,6 +12,9 @@ const TYPE = 'cache'
 const SUBTYPE = 'redis'
 
 module.exports = function (redis, agent, { version, enabled }) {
+  if (!enabled) {
+    return redis
+  }
   if (!semver.satisfies(version, '>=2.0.0 <4.0.0')) {
     agent.logger.debug('redis version %s not supported - aborting...', version)
     return redis
@@ -70,7 +73,7 @@ module.exports = function (redis, agent, { version, enabled }) {
 
       const command = commandObj.command
       agent.logger.debug({ command: command }, 'intercepted call to RedisClient.prototype.internal_send_command')
-      const span = enabled && ins.createSpan(command.toUpperCase(), TYPE, SUBTYPE)
+      const span = ins.createSpan(command.toUpperCase(), TYPE, SUBTYPE)
       if (!span) {
         return original.apply(this, arguments)
       }
@@ -102,7 +105,7 @@ module.exports = function (redis, agent, { version, enabled }) {
       }
 
       agent.logger.debug({ command: command }, 'intercepted call to RedisClient.prototype.send_command')
-      var span = enabled && ins.createSpan(command.toUpperCase(), TYPE, SUBTYPE)
+      var span = ins.createSpan(command.toUpperCase(), TYPE, SUBTYPE)
       if (!span) {
         return original.apply(this, arguments)
       }