worker: use copy of process.env

Instead of sharing the OS-backed store for all `process.env` instances,
create a copy of `process.env` for every worker that is created.

The copies do not interact. Native-addons do not see modifications to
`process.env` from Worker threads, but child processes started from
Workers do default to the Worker’s copy of `process.env`.

This makes Workers behave like child processes as far as `process.env`
is concerned, and an option corresponding to the `child_process`
module’s `env` option is added to the constructor.

Fixes: https://github.com/nodejs/node/issues/24947

PR-URL: https://github.com/nodejs/node/pull/26544
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

2 files changed, 54 insertions, 14 deletions

diff --git a/doc/api/process.md b/doc/api/process.md
index fa41109662..3b428fb038 100644
--- a/doc/api/process.md
+++ b/doc/api/process.md
@@ -954,6 +954,11 @@ emitMyWarning();
 <!-- YAML
 added: v0.1.27
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/26544
+    description: Worker threads will now use a copy of the parent thread’s
+                 `process.env` by default, configurable through the `env`
+                 option of the `Worker` constructor.
   - version: v10.0.0
     pr-url: https://github.com/nodejs/node/pull/18990
     description: Implicit conversion of variable value to string is deprecated.
@@ -983,8 +988,9 @@ An example of this object looks like:
 ```
 
 It is possible to modify this object, but such modifications will not be
-reflected outside the Node.js process. In other words, the following example
-would not work:
+reflected outside the Node.js process, or (unless explicitly requested)
+to other [`Worker`][] threads.
+In other words, the following example would not work:
 
 ```console
 $ node -e 'process.env.foo = "bar"' && echo $foo
@@ -1027,7 +1033,12 @@ console.log(process.env.test);
 // => 1
 ```
 
-`process.env` is read-only in [`Worker`][] threads.
+Unless explicitly specified when creating a [`Worker`][] instance,
+each [`Worker`][] thread has its own copy of `process.env`, based on its
+parent thread’s `process.env`, or whatever was specified as the `env` option
+to the [`Worker`][] constructor. Changes to `process.env` will not be visible
+across [`Worker`][] threads, and only the main thread can make changes that
+are visible to the operating system or to native add-ons.
 
 ## process.execArgv
 <!-- YAML
diff --git a/doc/api/worker_threads.md b/doc/api/worker_threads.md
index e19181760e..5d81f3f7f5 100644
--- a/doc/api/worker_threads.md
+++ b/doc/api/worker_threads.md
@@ -125,6 +125,25 @@ if (isMainThread) {
 }
 ```
 
+## worker.SHARE_ENV
+<!-- YAML
+added: REPLACEME
+-->
+
+* {symbol}
+
+A special value that can be passed as the `env` option of the [`Worker`][]
+constructor, to indicate that the current thread and the Worker thread should
+share read and write access to the same set of environment variables.
+
+```js
+const { Worker, SHARE_ENV } = require('worker_threads');
+new Worker('process.env.SET_IN_WORKER = "foo"', { eval: true, env: SHARE_ENV })
+  .on('exit', () => {
+    console.log(process.env.SET_IN_WORKER);  // Prints 'foo'.
+  });
+```
+
 ## worker.threadId
 <!-- YAML
 added: v10.5.0
@@ -380,7 +399,11 @@ Notable differences inside a Worker environment are:
   and [`process.abort()`][] is not available.
 - [`process.chdir()`][] and `process` methods that set group or user ids
   are not available.
-- [`process.env`][] is a read-only reference to the environment variables.
+- [`process.env`][] is a copy of the parent thread's environment variables,
+  unless otherwise specified. Changes to one copy will not be visible in other
+  threads, and will not be visible to native add-ons (unless
+  [`worker.SHARE_ENV`][] has been passed as the `env` option to the
+  [`Worker`][] constructor).
 - [`process.title`][] cannot be modified.
 - Signals will not be delivered through [`process.on('...')`][Signals events].
 - Execution may stop at any point as a result of [`worker.terminate()`][]
@@ -439,13 +462,18 @@ if (isMainThread) {
   If `options.eval` is `true`, this is a string containing JavaScript code
   rather than a path.
 * `options` {Object}
+  * `env` {Object} If set, specifies the initial value of `process.env` inside
+    the Worker thread. As a special value, [`worker.SHARE_ENV`][] may be used
+    to specify that the parent thread and the child thread should share their
+    environment variables; in that case, changes to one thread’s `process.env`
+    object will affect the other thread as well. **Default:** `process.env`.
   * `eval` {boolean} If `true`, interpret the first argument to the constructor
     as a script that is executed once the worker is online.
-  * `workerData` {any} Any JavaScript value that will be cloned and made
-    available as [`require('worker_threads').workerData`][]. The cloning will
-    occur as described in the [HTML structured clone algorithm][], and an error
-    will be thrown if the object cannot be cloned (e.g. because it contains
-    `function`s).
+  * `execArgv` {string[]} List of node CLI options passed to the worker.
+    V8 options (such as `--max-old-space-size`) and options that affect the
+    process (such as `--title`) are not supported. If set, this will be provided
+    as [`process.execArgv`][] inside the worker. By default, options will be
+    inherited from the parent thread.
   * `stdin` {boolean} If this is set to `true`, then `worker.stdin` will
     provide a writable stream whose contents will appear as `process.stdin`
     inside the Worker. By default, no data is provided.
@@ -453,11 +481,11 @@ if (isMainThread) {
     not automatically be piped through to `process.stdout` in the parent.
   * `stderr` {boolean} If this is set to `true`, then `worker.stderr` will
     not automatically be piped through to `process.stderr` in the parent.
-  * `execArgv` {string[]} List of node CLI options passed to the worker.
-    V8 options (such as `--max-old-space-size`) and options that affect the
-    process (such as `--title`) are not supported. If set, this will be provided
-    as [`process.execArgv`][] inside the worker. By default, options will be
-    inherited from the parent thread.
+  * `workerData` {any} Any JavaScript value that will be cloned and made
+    available as [`require('worker_threads').workerData`][]. The cloning will
+    occur as described in the [HTML structured clone algorithm][], and an error
+    will be thrown if the object cannot be cloned (e.g. because it contains
+    `function`s).
 
 ### Event: 'error'
 <!-- YAML
@@ -628,6 +656,7 @@ active handle in the event system. If the worker is already `unref()`ed calling
 [`vm`]: vm.html
 [`worker.on('message')`]: #worker_threads_event_message_1
 [`worker.postMessage()`]: #worker_threads_worker_postmessage_value_transferlist
+[`worker.SHARE_ENV`]: #worker_threads_worker_share_env
 [`worker.terminate()`]: #worker_threads_worker_terminate_callback
 [`worker.threadId`]: #worker_threads_worker_threadid_1
 [Addons worker support]: addons.html#addons_worker_support