build: expose napi_build_version variable

Expose `napi_build_version` to allow `node-gyp` to make it
available for building native addons.

Fixes: https://github.com/nodejs/node-gyp/issues/1745
Refs: https://github.com/nodejs/abi-stable-node/issues/371
PR-URL: https://github.com/nodejs/node/pull/27835
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

configure.py

@@ -34,6 +34,7 @@ import nodedownload
 # imports in tools/
 sys.path.insert(0, 'tools')
 import getmoduleversion
+import getnapibuildversion
 from gyp_node import run_gyp
 
 # imports in deps/v8/tools/node
@@ -1147,6 +1148,10 @@ def configure_node(o):
   else:
     o['variables']['node_target_type'] = 'executable'
 
+def configure_napi(output):
+  version = getnapibuildversion.get_napi_version()
+  output['variables']['napi_build_version'] = version
+
 def configure_library(lib, output):
   shared_lib = 'shared_' + lib
   output['variables']['node_' + shared_lib] = b(getattr(options, shared_lib))
@@ -1626,6 +1631,7 @@ if (options.dest_os):
 flavor = GetFlavor(flavor_params)
 
 configure_node(output)
+configure_napi(output)
 configure_library('zlib', output)
 configure_library('http_parser', output)
 configure_library('libuv', output)

doc/api/process.md

@@ -662,6 +662,7 @@ An example of the possible output looks like:
   variables:
    {
      host_arch: 'x64',
+     napi_build_version: 4,
      node_install_npm: 'true',
      node_prefix: '',
      node_shared_cares: 'false',

src/js_native_api.h

@@ -12,7 +12,12 @@
 #ifdef NAPI_EXPERIMENTAL
 #define NAPI_VERSION NAPI_VERSION_EXPERIMENTAL
 #else
-// The baseline version for N-API
+// The baseline version for N-API.
+// The NAPI_VERSION controls which version will be used by default when
+// compilling a native addon. If the addon developer specifically wants to use
+// functions available in a new version of N-API that is not yet ported in all
+// LTS versions, they can set NAPI_VERSION knowing that they have specifically
+// depended on that version.
 #define NAPI_VERSION 4
 #endif
 #endif

src/node_version.h

@@ -91,7 +91,8 @@
  */
 #define NODE_MODULE_VERSION 74
 
-// the NAPI_VERSION provided by this version of the runtime
+// The NAPI_VERSION provided by this version of the runtime. This is the version
+// which the Node binary being built supports.
 #define NAPI_VERSION  4
 
 #endif  // SRC_NODE_VERSION_H_

test/parallel/test-process-versions.js

@@ -45,3 +45,6 @@ for (let i = 0; i < expected_keys.length; i++) {
   const descriptor = Object.getOwnPropertyDescriptor(process.versions, key);
   assert.strictEqual(descriptor.writable, false);
 }
+
+assert.strictEqual(process.config.variables.napi_build_version,
+                   process.versions.napi);

tools/getnapibuildversion.py

@@ -0,0 +1,26 @@
+from __future__ import print_function
+import os
+import re
+
+
+def get_napi_version():
+  napi_version_h = os.path.join(
+    os.path.dirname(__file__),
+    '..',
+    'src',
+    'node_version.h')
+
+  f = open(napi_version_h)
+
+  regex = '^#define NAPI_VERSION'
+
+  for line in f:
+    if re.match(regex, line):
+      napi_version = line.split()[2]
+      return napi_version
+
+  raise Exception('Could not find pattern matching %s' % regex)
+
+
+if __name__ == '__main__':
+  print(get_napi_version())