implement .nowatchman discovery in repo root

Summary:
Implement .nowatchman check as required in the task

If .nowatchman is presented, not spawning watchman

As required, we will check .nowatchman under repo root, only .nowatchman under repo root will count.

Reviewed By: kmancini

Differential Revision: D49429032

fbshipit-source-id: f959f5bbd3e2e08e6bc888000491415829fc14f9

watchman/cmds/watch.cpp

@@ -191,6 +191,17 @@ bool find_project_root(
   return false;
 }
 
+void check_no_watchman(w_string resolved_root) {
+  // check if .nowatchman exists under the resolved path
+  auto no_watchman_path = w_string::pathCat({resolved_root, ".nowatchman"});
+  if (w_path_exists(no_watchman_path.c_str())) {
+    CommandValidationError::throwf(
+        "resolve_projpath: the repository is configured to not enable watchman. "
+        "Found .nowatchman at {}",
+        no_watchman_path);
+  }
+}
+
 // For watch-project, take a root path string and resolve the
 // containing project directory, then update the args to reflect
 // that path.
@@ -235,6 +246,7 @@ static w_string resolve_projpath(
   if (find_project_root(*root_files, resolvedpiece, relpiece)) {
     relpath = relpiece.asWString();
     resolved = resolvedpiece.asWString();
+    check_no_watchman(resolved);
     args[1] = w_string_to_json(resolved);
     return resolved;
   }