tools: add tool to check for N-API modules

Adds tools/check-napi.js which uses `nm -a` on UNIX and
`dumpbin /imports` on Windows to check whether a given `.node` file
is an N-API module or not. Intentionally ignores files named
`nothing.node` because they are node-addon-api build artefacts.

Sets the target type for `nothing` (which gets built when a built-in
N-API is found to be present) to `'static_library'` so as to avoid the
creation of `nothing.node` files which incorrectly end up showing up in
the output of `check-napi.js` as non-N-API modules.

tools/check-napi.js

@@ -0,0 +1,96 @@
+// Descend into a directory structure and, for each file matching *.node, output
+// based on the imports found in the file whether it's an N-API module or not.
+
+const fs = require('fs');
+const path = require('path');
+const child_process = require('child_process');
+
+// Read the output of the command, break it into lines, and use the reducer to
+// decide whether the file is an N-API module or not.
+function checkFile(file, command, arguments, reducer) {
+  const child = child_process.spawn(command, arguments, {
+    stdio: ['inherit', 'pipe', 'inherit']
+  });
+  let leftover = '';
+  let isNapi = undefined;
+  child.stdout.on('data', (chunk) => {
+    if (isNapi === undefined) {
+      chunk = leftover + chunk.toString();
+      const haveLeftover = !!chunk.match(/[\r\n]+$/);
+      chunk = chunk.split(/[\r\n]+/);
+      leftover = chunk.pop();
+      isNapi = chunk.reduce(reducer, isNapi);
+    }
+  });
+  child.on('exit', (code, signal) => {
+    if ((code === null && signal !== null) || (code !== 0)) {
+      console.log(
+        command + ' exited with code: '  + code + ' and signal: ' + signal);
+    } else {
+      console.log(
+          '\033[' + (isNapi ? '42' : '41') + 'm' +
+          (isNapi ? '    N-API' : 'Not N-API') +
+          '\033[0m: ' + file);
+    }
+  });
+}
+
+// Use nm -a to list symbols.
+function checkFileUNIX(file) {
+  checkFile(file, 'nm', ['-a', file], (soFar, line) => {
+      if (soFar === undefined) {
+        line = line.match(/([0-9a-f]*)? ([a-zA-Z]) (.*$)/);
+        line.shift();
+        if (line[1] === 'U') {
+          if (line[2].match(/^napi/)) {
+            soFar = true;
+          }
+        }
+      }
+      return soFar;
+  });
+}
+
+// Use dumpbin /imports to list symbols.
+function checkFileWin32(file) {
+  checkFile(file, 'dumpbin', ['/imports', file], (soFar, line) => {
+      if (soFar === undefined) {
+        line = line.match(/([0-9a-f]*)? +([a-zA-Z0-9]) (.*$)/);
+        if (line && line[line.length - 1].match(/^napi/)) {
+          soFar = true;
+        }
+      }
+      return soFar;
+  });
+}
+
+// Descend into a directory structure and pass each file ending in '.node' to
+// one of the above checks, depending on the OS.
+function recurse(top) {
+  fs.readdir(top, (error, items) => {
+    items.forEach((item) => {
+      item = path.join(top, item);
+      fs.stat(item, ((item) => (error, stats) => {
+        if (!error) {
+          if (stats.isDirectory()) {
+            recurse(item);
+          } else if (item.match(/[.]node$/) &&
+              // Explicitly ignore files called 'nothing.node' because they are
+              // artefacts of node-addon-api having identified a version of
+              // Node.js that ships with a correct implementation of N-API.
+              path.basename(item) !== 'nothing.node') {
+            process.platform === 'win32' ?
+                checkFileWin32(item) :
+                checkFileUNIX(item);
+          }
+        } else {
+          throw ("error about " + item + ": " + error);
+        }
+      })(item));
+    });
+  });
+}
+
+// Start with the directory given on the command line or the current directory
+// if nothing was given.
+recurse(process.argv.length > 3 ? process.argv[2] : '.');