src: add 'dynamic' process.release.lts property

This makes the process.release.lts property configurable by a constant.

This ref is the original PR to v6.x.
Refs: nodejs#3212

 Conflicts:
        doc/api/process.md

PR-URL: nodejs#16656
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>

doc/api/process.md

@@ -1444,7 +1444,11 @@ tarball.
   compiling Node.js native add-ons. _This property is only present on Windows
   builds of Node.js and will be missing on all other platforms._
 * `lts` {string} a string label identifying the [LTS][] label for this release.
-  If the Node.js release is not an LTS release, this will be `undefined`.
+  This property only exists for LTS releases and is `undefined` for all other
+  release types, including _Current_ releases.  Currently the valid values are:
+  - `'Argon'` for the v4.x LTS line beginning with v4.2.0.
+  - `'Boron'` for the v6.x LTS line beginning with v6.9.0.
+  - `'Carbon'` for the v8.x LTS line beginning with v8.9.1.
 
 For example:
 

src/node.cc

@@ -3496,6 +3496,11 @@ void SetupProcessObject(Environment* env,
   READONLY_PROPERTY(release, "name",
                     OneByteString(env->isolate(), NODE_RELEASE));
 
+#if NODE_VERSION_IS_LTS
+  READONLY_PROPERTY(release, "lts",
+                    OneByteString(env->isolate(), NODE_VERSION_LTS_CODENAME));
+#endif
+
 // if this is a release build and no explicit base has been set
 // substitute the standard release download URL
 #ifndef NODE_RELEASE_URLBASE

test/parallel/test-process-release.js

@@ -0,0 +1,16 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const versionParts = process.versions.node.split('.');
+
+assert.equal(process.release.name, 'node');
+
+// it's expected that future LTS release lines will have additional
+// branches in here
+if (versionParts[0] === '4' && versionParts[1] >= 2) {
+  assert.equal(process.release.lts, 'Argon');
+} else if (versionParts[0] === '6' && versionParts[1] >= 9) {
+  assert.equal(process.release.lts, 'Boron');
+} else {
+  assert.strictEqual(process.release.lts, undefined);
+}