Make new message UUIDs more URL encoding friendly

=, + and / require triplet encoding in URLs. Cleanup the base64
encoding string to use different characters. This only impacts
newly created string, existing strings will continue to require a
much longer encoding when sent in JSON messages from the REST API.

Also strip leading 'A'. Most strings start with a run of 'A' due
to the first few bytes of uuidPrefix always being 0.

Avoid long runs of - in the resulting IDs by mixing the bits using
the uuidPrefix as the salt for the mixer.

Change-Id: I0a7da0aec071329c840447a0914d14118f2f20a4

gerrit-server/src/main/java/com/google/gerrit/server/ChangeUtil.java

@@ -14,6 +14,7 @@
 
 package com.google.gerrit.server;
 
+import com.google.common.base.CharMatcher;
 import com.google.gerrit.common.ChangeHooks;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.reviewdb.client.ChangeMessage;
@@ -35,6 +36,7 @@ import com.google.gerrit.server.patch.PatchSetInfoFactory;
 import com.google.gerrit.server.patch.PatchSetInfoNotAvailableException;
 import com.google.gerrit.server.project.InvalidChangeOperationException;
 import com.google.gerrit.server.project.NoSuchChangeException;
+import com.google.gerrit.server.util.IdGenerator;
 import com.google.gwtorm.server.AtomicUpdate;
 import com.google.gwtorm.server.OrmConcurrencyException;
 import com.google.gwtorm.server.OrmException;
@@ -66,7 +68,6 @@ import java.util.Set;
 import java.util.regex.Matcher;
 
 public class ChangeUtil {
-
   private static final Logger log = LoggerFactory.getLogger(ChangeUtil.class);
 
   private static int uuidPrefix;
@@ -83,7 +84,12 @@ public class ChangeUtil {
   public static String messageUUID(final ReviewDb db) throws OrmException {
     final byte[] raw = new byte[8];
     fill(raw, db);
-    return Base64.encodeBytes(raw);
+
+    // Make the resulting base64 string more URL friendly.
+    return CharMatcher.is('A').trimLeadingFrom(
+           CharMatcher.is('=').trimTrailingFrom(Base64.encodeBytes(raw)))
+        .replace('+', '.')
+        .replace('/', '-');
   }
 
   private static synchronized void fill(byte[] raw, ReviewDb db)
@@ -93,7 +99,7 @@ public class ChangeUtil {
       uuidSeq = Integer.MAX_VALUE;
     }
     NB.encodeInt32(raw, 0, uuidPrefix);
-    NB.encodeInt32(raw, 4, uuidSeq--);
+    NB.encodeInt32(raw, 4, IdGenerator.mix(uuidPrefix, uuidSeq--));
   }
 
   public static void touch(final Change change, ReviewDb db)

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

@@ -43,13 +43,13 @@ public class IdGenerator {
 
   /** Produce the next identifier. */
   public int next() {
-    return mix(gen.getAndIncrement());
+    return mix(salt, gen.getAndIncrement());
   }
 
   private static final int salt = 0x9e3779b9;
 
   /** A very simple bit permutation to mask a simple incrementer. */
-  static int mix(final int in) {
+  public static int mix(final int salt, final int in) {
     short v0 = hi16(in);
     short v1 = lo16(in);
     v0 += ((v1 << 2) + 0 ^ v1) + (salt ^ (v1 >>> 3)) + 1;