Add a "shell" pseudo-command to nomulus tool

Add the "shell" command which lets you run multiple other command in a single
session, sparing you the initialization costs for all but the first of them.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188712815

java/google/registry/tools/ShellCommand.java

@@ -0,0 +1,93 @@
+// Copyright 2018 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.tools;
+
+import static java.nio.charset.StandardCharsets.US_ASCII;
+
+import com.beust.jcommander.Parameters;
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.collect.ImmutableList;
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.StreamTokenizer;
+import java.io.StringReader;
+
+/**
+ * Implements a tiny shell interpreter for the nomulus tool.
+ *
+ * <p>Parses a very simple command grammar. Tokens are either whitespace delimited words or
+ * double-quoted strings.
+ */
+@Parameters(commandDescription = "Run an interactive shell")
+public class ShellCommand implements Command {
+
+  private final CommandRunner runner;
+  private final BufferedReader lineReader;
+
+  ShellCommand(InputStream in, CommandRunner runner) {
+    this.runner = runner;
+    this.lineReader = new BufferedReader(new InputStreamReader(in, US_ASCII));
+  }
+
+  /** Run the shell until the user presses "Ctrl-D". */
+  @Override
+  public void run() {
+    String line;
+    while ((line = getLine()) != null) {
+      String[] lineArgs = parseCommand(line);
+      try {
+        runner.run(lineArgs);
+      } catch (Exception e) {
+        System.err.println("Got an exception:\n" + e);
+      }
+    }
+  }
+
+  private String getLine() {
+    if (System.console() != null) {
+      System.err.print("nom> ");
+    }
+    try {
+      return lineReader.readLine();
+    } catch (IOException e) {
+      return null;
+    }
+  }
+
+  @VisibleForTesting
+  static String[] parseCommand(String line) {
+    ImmutableList.Builder<String> resultBuilder = new ImmutableList.Builder<>();
+
+    // Create a tokenizer, make everything word characters except quoted strings and unprintable
+    // ascii chars and space (just treat them all as whitespace).
+    StreamTokenizer tokenizer = new StreamTokenizer(new StringReader(line));
+    tokenizer.resetSyntax();
+    tokenizer.whitespaceChars(0, ' ');
+    tokenizer.wordChars('!', '~');
+    tokenizer.quoteChar('"');
+
+    try {
+      while (tokenizer.nextToken() != StreamTokenizer.TT_EOF) {
+        resultBuilder.add(tokenizer.sval);
+      }
+    } catch (IOException e) {
+      throw new RuntimeException(e);
+    }
+
+    return resultBuilder.build().toArray(new String[0]);
+  }
+}