Refactor XMLFileResourceProvider to support read-only configuration d…

…irectories

Enhance `XMLFileResourceProvider` to allow GeoWebCache to start with a
read-only configuration directory and `geowebcache.xml` file. Previously, the provider
assumed that the configuration directory was always writable, which caused startup
failures when running in environments with restricted file permissions.

- **Support read-only configuration directories**:
  - `hasInput()` now correctly returns `true` if the configuration file
     exists or if a template is available and the directory is writable.
  - `hasOutput()` ensures that the configuration file can be written before
     allowing output operations.
  - Improved error handling when checking directory and file writability.

- **Refactored file existence checks**:
  - Consolidated logic for checking configuration file existence and writability
    into `findConfigFile()`.
  - Moved the directory writability check from `findConfigFile()` to
    `findOrCreateConfFile()` to prevent premature failures.

- **Improved logging and exception handling**:
  - `hasInput()` and `hasOutput()` now log warnings when encountering errors
     instead of failing silently.
  - `findOrCreateConfFile()` now explicitly throws an exception when it cannot
     create a new configuration file due to permission issues.

- GeoWebCache can now start successfully with a pre-existing, read-only configuration directory.
- Configurations stored in a read-only filesystem (e.g., mounted from a
  read-only volume) can still be used without requiring write access.
- The provider only attempts to create a configuration file when explicitly
  required, eliminating side effects from query methods `hasInput()` and
  `hasOutput()`.

This change improves the resilience of GeoWebCache in constrained environments
while maintaining backward compatibility for writable configurations.

geowebcache/core/src/main/java/org/geowebcache/config/XMLFileResourceProvider.java

@@ -33,6 +33,7 @@
 import org.geowebcache.util.ApplicationContextProvider;
 import org.geowebcache.util.GWCVars;
 import org.springframework.context.ApplicationContext;
+import org.springframework.lang.NonNull;
 import org.springframework.web.context.WebApplicationContext;
 
 /** Default implementation of ConfigurationResourceProvider that uses the file system. */
@@ -49,9 +50,11 @@ public class XMLFileResourceProvider implements ConfigurationResourceProvider {
     private final WebApplicationContext context;
 
     /** Location of the configuration file */
+    @NonNull
     private final File configDirectory;
 
     /** Name of the configuration file */
+    @NonNull
     private final String configFileName;
 
     private String templateLocation;
@@ -135,11 +138,27 @@ public XMLFileResourceProvider(
         this(configFileName, appCtx, getConfigDirVar(appCtx), storageDirFinder);
     }
 
+    /**
+     * {@inheritDoc}
+     *
+     * <p>If the configuration file doesn't exist and {@link #hasInput() == true}, the file will be first created from
+     * the {@link #setTemplate(String) template}
+     *
+     * @throws IOException if the file can't be created or copied from the template
+     */
     @Override
     public InputStream in() throws IOException {
         return new FileInputStream(findOrCreateConfFile());
     }
 
+    /**
+     * {@inheritDoc}
+     *
+     * <p>If the configuration file doesn't exist and {@link #hasOutput() == true}, the file will be first created from
+     * the {@link #setTemplate(String) template}
+     *
+     * @throws IOException if the file can't be created or copied from the template
+     */
     @Override
     public OutputStream out() throws IOException {
         return new FileOutputStream(findOrCreateConfFile());
@@ -165,22 +184,13 @@ public void setTemplate(final String templateLocation) {
     }
 
     private File findConfigFile() throws IOException {
-        if (null == configDirectory) {
-            throw new IllegalStateException();
-        }
 
         if (!configDirectory.exists() && !configDirectory.mkdirs()) {
             throw new IOException("TileLayerConfiguration directory does not exist and cannot be created: '"
                     + configDirectory.getAbsolutePath()
                     + "'");
         }
-        if (!configDirectory.canWrite()) {
-            throw new IOException(
-                    "TileLayerConfiguration directory is not writable: '" + configDirectory.getAbsolutePath() + "'");
-        }
-
-        File xmlFile = new File(configDirectory, configFileName);
-        return xmlFile;
+        return new File(configDirectory, configFileName);
     }
 
     @Override
@@ -200,6 +210,11 @@ private File findOrCreateConfFile() throws IOException {
         if (xmlFile.exists()) {
             log.config("Found configuration file in " + configDirectory.getAbsolutePath());
         } else if (templateLocation != null) {
+            if (!configDirectory.canWrite()) {
+                throw new IOException("TileLayerConfiguration directory is not writable: '"
+                        + configDirectory.getAbsolutePath() + "'");
+            }
+
             log.warning("Found no configuration file in config directory, will create one at '"
                     + xmlFile.getAbsolutePath()
                     + "' from template "
@@ -249,17 +264,40 @@ private void backUpConfig(final File xmlFile) throws IOException {
         log.fine("Config backup done");
     }
 
+    /**
+     * Determines if the config file exists and is readable, or doesn't exist but can be created.
+     *
+     * <p>Calling this method has no side effects. The target file either exists, or can be created throught the
+     * {@link #setTemplate(String) template}, if a template has been set. In such case, it'll be created by either
+     * {@link #in()} or {@link #out()}.
+     */
     @Override
     public boolean hasInput() {
         try {
-            return findOrCreateConfFile().exists();
+            File file = findConfigFile();
+            return file.exists() || (templateLocation != null && configDirectory.canWrite());
         } catch (IOException e) {
+            log.log(Level.WARNING, "Error obtaining config file", e);
             return false;
         }
     }
 
+    /**
+     * Determines if the configuration can be {@link #out() written} to the {@link #getLocation() output file}.
+     *
+     * <p>Calling this method has no side effects. The target file may or may not exist. The target directory must be
+     * writable, and so must the target file in case it does exist.
+     *
+     * @return {@code true} if the {@link #getLocation() configuration file} can be written to.
+     */
     @Override
     public boolean hasOutput() {
-        return true;
+        try {
+            File file = findConfigFile();
+            return configDirectory.canWrite() && (!file.exists() || file.canWrite());
+        } catch (IOException e) {
+            log.log(Level.WARNING, "Error obtaining config file", e);
+            return false;
+        }
     }
 }