GUACAMOLE-374: Automatically read properties from files pointed to by "*_FILE" environment variables.

Dockerfile

@@ -97,7 +97,8 @@ RUN useradd --system --create-home --shell /usr/sbin/nologin --uid $UID --gid $G
 USER guacamole
 
 # Environment variable defaults
-ENV GUACAMOLE_HOME=/etc/guacamole
+ENV GUACAMOLE_HOME=/etc/guacamole \
+    ENABLE_FILE_ENVIRONMENT_PROPERTIES=true
 
 # Start Guacamole under Tomcat, listening on 0.0.0.0:8080
 EXPOSE 8080

guacamole/src/main/java/org/apache/guacamole/GuacamoleServletContextListener.java

@@ -95,7 +95,7 @@ public class GuacamoleServletContextListener extends GuiceServletContextListener
 
     /**
      * A property that determines whether environment variables are evaluated
-     * to override properties specified in guacamole.properties.
+     * to supply properties not specified in guacamole.properties.
      */
     private static final BooleanGuacamoleProperty ENABLE_ENVIRONMENT_PROPERTIES =
         new BooleanGuacamoleProperty() {
@@ -105,6 +105,19 @@ public class GuacamoleServletContextListener extends GuiceServletContextListener
             }
         };
 
+    /**
+     * A property that determines whether environment variables of the form
+     * "*_FILE" are evaluated to supply properties not specified in
+     * guacamole.properties nor in environment variables.
+     */
+    private static final BooleanGuacamoleProperty ENABLE_FILE_ENVIRONMENT_PROPERTIES =
+        new BooleanGuacamoleProperty() {
+            @Override
+            public String getName() {
+                return "enable-file-environment-properties";
+            }
+        };
+
     /**
      * The Guacamole server environment.
      */
@@ -172,6 +185,23 @@ public class GuacamoleServletContextListener extends GuiceServletContextListener
             logger.debug("Error reading \"{}\" property from guacamole.properties.", ENABLE_ENVIRONMENT_PROPERTIES.getName(), e);
         }
 
+        // For any values not defined in GUACAMOLE_HOME/guacamole.properties
+        // nor in the system environment, read from files pointed to by
+        // corresponding "*_FILE" variables in the system environment if
+        // "enable-file-environment-properties" is set to "true"
+        try {
+            if (environment.getProperty(ENABLE_FILE_ENVIRONMENT_PROPERTIES, false)) {
+                environment.addGuacamoleProperties(new SystemFileEnvironmentGuacamoleProperties());
+                logger.info("Additional configuration parameters may be read "
+                        + "from files pointed to by \"*_FILE\" environment "
+                        + "variables.");
+            }
+        }
+        catch (GuacamoleException e) {
+            logger.error("Unable to configure support for file environment properties: {}", e.getMessage());
+            logger.debug("Error reading \"{}\" property from guacamole.properties.", ENABLE_FILE_ENVIRONMENT_PROPERTIES.getName(), e);
+        }
+
         // Now that at least the main guacamole.properties source of
         // configuration information is available, initialize the session map
         sessionMap = new HashTokenSessionMap(environment);

guacamole/src/main/java/org/apache/guacamole/SystemFileEnvironmentGuacamoleProperties.java

@@ -0,0 +1,64 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 org.apache.guacamole;
+
+import com.google.common.io.Files;
+import java.io.File;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import org.apache.guacamole.properties.GuacamoleProperties;
+import org.apache.guacamole.token.TokenName;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * GuacamoleProperties implementation which reads all properties from files
+ * whose filenames are stored in environment variables. The name of the
+ * environment variable corresponding to the filename is determined from the
+ * original property using {@link TokenName#canonicalize(java.lang.String)}
+ * with an additional "_FILE" suffix.
+ */
+public class SystemFileEnvironmentGuacamoleProperties implements GuacamoleProperties {
+
+    /**
+     * Logger for this class.
+     */
+    private static final Logger logger = LoggerFactory.getLogger(SystemFileEnvironmentGuacamoleProperties.class);
+
+    @Override
+    public String getProperty(String name) {
+
+        String filename = System.getenv(TokenName.canonicalize(name) + "_FILE");
+        if (filename != null) {
+            try {
+                return Files.asCharSource(new File(filename), StandardCharsets.UTF_8).read();
+            }
+            catch (IOException e) {
+                logger.error("Property \"{}\" could not be read from file \"{}\": {}", name, filename, e.getMessage());
+                logger.debug("Error reading property value from file.", e);
+            }
+        }
+
+        return null;
+
+    }
+
+}
+