Show banner after dismissal if message is updated

The banner is currently not displayed to the user if they dismiss it
until the next day even if the message was updated. When the message
content is updated, the intent is usually to show it to everyone, even
those who dismissed the previous message. This is achieved by updating
the REST API to return the 'id' based on the message content, as the
cookie to manage dismissals is named using this 'id'.

Change-Id: I2ccaaa55ac549cb37eed851f8ed40f4d5f345f72

src/main/java/com/googlesource/gerrit/plugins/messageoftheday/GetMessage.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/messageoftheday/GetMessage.java b/src/main/java/com/googlesource/gerrit/plugins/messageoftheday/GetMessage.java
index b90d4a7..d1ec85a 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/messageoftheday/GetMessage.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/messageoftheday/GetMessage.java
@@ -69,8 +69,8 @@
       return null;
     }
 
-    motd.id = cfg.getString(SECTION_MESSAGE, null, KEY_ID);
-    if (Strings.isNullOrEmpty(motd.id)) {
+    String htmlFileId = cfg.getString(SECTION_MESSAGE, null, KEY_ID);
+    if (Strings.isNullOrEmpty(htmlFileId)) {
       log.warn("id not defined, no message will be shown");
       return Response.none();
     }
@@ -95,14 +95,15 @@
     }
 
     try {
-      motd.html = new String(Files.readAllBytes(dataDirPath.resolve(motd.id + ".html")), UTF_8);
+      motd.html = new String(Files.readAllBytes(dataDirPath.resolve(htmlFileId + ".html")), UTF_8);
     } catch (IOException e1) {
       log.warn(
           String.format(
-              "No HTML-file was found for message %s, no message will be shown", motd.id));
+              "No HTML-file was found for message %s, no message will be shown", htmlFileId));
       return Response.none();
     }
 
+    motd.id = Integer.toString(motd.html.hashCode());
     return Response.ok(motd);
   }
 }

src/main/resources/Documentation/about.md

diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
index 26cc63d..46e0ca9 100644
--- a/src/main/resources/Documentation/about.md
+++ b/src/main/resources/Documentation/about.md
@@ -3,6 +3,7 @@
 The plugin makes use of Gerrit's MessageOfTheDay extension point.
 
 The plugin will display the configured message at the top of the screen.
-The user can dismiss the message banner. Upon dismissal, the banner will be
-redisplayed on the web UI the next day at 00:00. (The next day is calculated
-with respect to the client)
+The user can dismiss the message banner. Upon dismissal, the banner will
+be redisplayed on the web UI the next day at 00:00. The next day is
+calculated with respect to the client. Note that, after dismissal, the
+banner will be redisplayed (before the next day) if its message is updated.

src/main/resources/Documentation/rest-api-config.md

diff --git a/src/main/resources/Documentation/rest-api-config.md b/src/main/resources/Documentation/rest-api-config.md
index 84ed041..d13d089 100644
--- a/src/main/resources/Documentation/rest-api-config.md
+++ b/src/main/resources/Documentation/rest-api-config.md
@@ -44,7 +44,7 @@
 
 The `MessageOfTheDayInfo` entity contains information about the message of the day.
 
-* `id`: ID of the message.
+* `id`: ID of the message generated based on the message content.
 * `starts_at`: Date, when the message will be first displayed
 * `expires_at`: Date, after which the message will not be displayed anymore
 * `html`: String, containing the HTML-formatted message