Extract a type for MergeOp's submissionId

This stronger type allows easier reuse from other classes,
particularly ReceiveCommits, without duplicating the logic for
computing a unique string; it also cuts down on the use of Strings in
long method signatures.

Change-Id: Ida22c44b926f02f3e002a8736783a72a7e0118e6

gerrit-server/src/main/java/com/google/gerrit/server/util/RequestId.java

@@ -0,0 +1,65 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// 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 com.google.gerrit.server.util;
+
+import com.google.common.hash.Hasher;
+import com.google.common.hash.Hashing;
+import com.google.gerrit.common.TimeUtil;
+import com.google.gerrit.reviewdb.client.Change;
+import com.google.gerrit.reviewdb.client.Project;
+
+import java.net.InetAddress;
+import java.net.UnknownHostException;
+
+/** Unique identifier for an end-user request, used in logs and similar. */
+public class RequestId {
+  private static final String MACHINE_ID;
+  static {
+    String id;
+    try {
+      id = InetAddress.getLocalHost().getHostAddress();
+    } catch (UnknownHostException e) {
+      id = "unknown";
+    }
+    MACHINE_ID = id;
+  }
+
+  public static RequestId forChange(Change c) {
+    return new RequestId(c.getId().toString());
+  }
+
+  public static RequestId forProject(Project.NameKey p) {
+    return new RequestId(p.toString());
+  }
+
+  private final String str;
+
+  private RequestId(String resourceId) {
+    Hasher h = Hashing.sha1().newHasher();
+    h.putLong(Thread.currentThread().getId())
+        .putUnencodedChars(MACHINE_ID);
+    str = "[" + resourceId + "-" + TimeUtil.nowTs().getTime() +
+        "-" + h.hash().toString().substring(0, 8) + "]";
+  }
+
+  @Override
+  public String toString() {
+    return str;
+  }
+
+  public String toStringForStorage() {
+    return str.substring(1, str.length() - 1);
+  }
+}