What if Watchman CLI is missing... (#2643)

Summary:
Here is an updated version of the `GraphQLWatchmanClient.isAvailable`. (as a fix for #2617)

Now, it's also checking if the `watchman` CLI is available.

Test plan (Tested with `relay-examples:relay-publish-test`  (relayjs/relay-examples#89)

First of all, I've removed watchman from my machine.
Then,
  - Build the compiler
  - Install updated dependencies relay-examples (yarn)
  - Run the compiler (yarn build)

Compiled without errors.
Pull Request resolved: #2643

Reviewed By: kassens

Differential Revision: D14240611

Pulled By: alunyov

fbshipit-source-id: 551a9338158debe7db8748676a6dccf0dfd479b0

packages/relay-compiler/bin/RelayCompilerMain.js

@@ -185,7 +185,6 @@ Ensure that one such file exists in ${srcDir} or its parents.
     verbose: options.verbose,
     quiet: options.quiet,
   });
-
   const useWatchman = options.watchman && (await WatchmanClient.isAvailable());
 
   const schema = getSchema(schemaPath);
@@ -270,7 +269,7 @@ Ensure that one such file exists in ${srcDir} or its parents.
     // TODO: allow passing in a flag or detect?
     sourceControl: null,
   });
-  if (!options.validate && !options.watch && options.watchman) {
+  if (!options.validate && !options.watch && useWatchman) {
     // eslint-disable-next-line no-console
     console.log('HINT: pass --watch to keep watching for changes.');
   }

packages/relay-compiler/core/GraphQLWatchmanClient.js

@@ -10,6 +10,7 @@
  */
 'use strict';
 
+const childProcess = require('child_process');
 const watchman = require('fb-watchman');
 
 const MAX_ATTEMPT_LIMIT = 5;
@@ -24,21 +25,18 @@ class GraphQLWatchmanClient {
 
   static isAvailable(): Promise<boolean> {
     return new Promise(resolve => {
-      const client = new GraphQLWatchmanClient(MAX_ATTEMPT_LIMIT);
-      client.on('error', () => {
+      // This command not only will verify that watchman CLI is available
+      // More than that `watchman version` is a command that runs on the server.
+      // And it can tell us that watchman is up and running
+      // Also `watchman version` check ``relative_root`` capability
+      // under the covers
+      const proc = childProcess.spawn('watchman', ['version']);
+      proc.on('error', () => {
         resolve(false);
-        client.end();
       });
-      client.hasCapability('relative_root').then(
-        hasRelativeRoot => {
-          resolve(hasRelativeRoot);
-          client.end();
-        },
-        () => {
-          resolve(false);
-          client.end();
-        },
-      );
+      proc.on('close', code => {
+        resolve(code === 0);
+      });
     });
   }