Make mount error messages a bit more consistent

 - Move most error message constants to a new MountHelpers class.
 - Be a little more consistent in when we throw "No such file" vs "Not a
   file/directory" messages.

projects/common/src/main/java/dan200/computercraft/shared/computer/core/ResourceMount.java

@@ -21,6 +21,8 @@
 import java.util.HashMap;
 import java.util.Map;
 
+import static dan200.computercraft.core.filesystem.MountHelpers.NO_SUCH_FILE;
+
 /**
  * A mount backed by Minecraft's {@link ResourceManager}.
  *

projects/core/src/main/java/dan200/computercraft/core/filesystem/AbstractInMemoryMount.java

@@ -17,14 +17,14 @@
 import java.util.Map;
 import java.util.function.Function;
 
+import static dan200.computercraft.core.filesystem.MountHelpers.*;
+
 /**
  * An abstract mount which stores its file tree in memory.
  *
  * @param <T> The type of file.
  */
 public abstract class AbstractInMemoryMount<T extends AbstractInMemoryMount.FileEntry<T>> implements Mount {
-    protected static final String NO_SUCH_FILE = "No such file";
-
     @Nullable
     protected T root;
 
@@ -57,7 +57,8 @@ public final boolean isDirectory(String path) {
     @Override
     public final void list(String path, List<String> contents) throws IOException {
         var file = get(path);
-        if (file == null || file.children == null) throw new FileOperationException(path, "Not a directory");
+        if (file == null) throw new FileOperationException(path, NO_SUCH_FILE);
+        if (file.children == null) throw new FileOperationException(path, NOT_A_DIRECTORY);
 
         contents.addAll(file.children.keySet());
     }
@@ -82,7 +83,8 @@ public final long getSize(String path) throws IOException {
     @Override
     public final SeekableByteChannel openForRead(String path) throws IOException {
         var file = get(path);
-        if (file == null || file.isDirectory()) throw new FileOperationException(path, NO_SUCH_FILE);
+        if (file == null) throw new FileOperationException(path, NO_SUCH_FILE);
+        if (file.isDirectory()) throw new FileOperationException(path, NOT_A_FILE);
         return openForRead(path, file);
     }
 

projects/core/src/main/java/dan200/computercraft/core/filesystem/ArchiveMount.java

@@ -21,8 +21,6 @@
  * @param <T> The type of file.
  */
 public abstract class ArchiveMount<T extends ArchiveMount.FileEntry<T>> extends AbstractInMemoryMount<T> {
-    protected static final String NO_SUCH_FILE = "No such file";
-
     /**
      * Limit the entire cache to 64MiB.
      */

projects/core/src/main/java/dan200/computercraft/core/filesystem/FileMount.java

@@ -17,6 +17,9 @@
 import java.util.List;
 import java.util.Set;
 
+import static dan200.computercraft.core.filesystem.MountHelpers.NOT_A_FILE;
+import static dan200.computercraft.core.filesystem.MountHelpers.NO_SUCH_FILE;
+
 /**
  * A {@link Mount} implementation which provides read-only access to a directory.
  */
@@ -84,7 +87,9 @@ public BasicFileAttributes getAttributes(String path) throws FileOperationExcept
     @Override
     public SeekableByteChannel openForRead(String path) throws FileOperationException {
         var file = resolvePath(path);
-        if (!Files.isRegularFile(file)) throw new FileOperationException(path, "No such file");
+        if (!Files.isRegularFile(file)) {
+            throw new FileOperationException(path, Files.exists(file) ? NOT_A_FILE : NO_SUCH_FILE);
+        }
 
         try {
             return Files.newByteChannel(file, READ_OPTIONS);
@@ -103,7 +108,7 @@ public SeekableByteChannel openForRead(String path) throws FileOperationExceptio
     protected FileOperationException remapException(String fallbackPath, IOException exn) {
         return exn instanceof FileSystemException fsExn
             ? remapException(fallbackPath, fsExn)
-            : new FileOperationException(fallbackPath, exn.getMessage() == null ? "Access denied" : exn.getMessage());
+            : new FileOperationException(fallbackPath, exn.getMessage() == null ? MountHelpers.ACCESS_DENIED : exn.getMessage());
     }
 
     /**
@@ -115,7 +120,7 @@ protected FileOperationException remapException(String fallbackPath, IOException
      * @return The wrapped exception.
      */
     protected FileOperationException remapException(String fallbackPath, FileSystemException exn) {
-        var reason = getReason(exn);
+        var reason = MountHelpers.getReason(exn);
 
         var failedFile = exn.getFile();
         if (failedFile == null) return new FileOperationException(fallbackPath, reason);
@@ -125,20 +130,4 @@ protected FileOperationException remapException(String fallbackPath, FileSystemE
             ? new FileOperationException(Joiner.on('/').join(root.relativize(failedPath)), reason)
             : new FileOperationException(fallbackPath, reason);
     }
-
-    /**
-     * Get the user-friendly reason for a {@link FileSystemException}.
-     *
-     * @param exn The exception that occurred.
-     * @return The friendly reason for this exception.
-     */
-    protected String getReason(FileSystemException exn) {
-        if (exn instanceof FileAlreadyExistsException) return "File exists";
-        if (exn instanceof NoSuchFileException) return "No such file";
-        if (exn instanceof NotDirectoryException) return "Not a directory";
-        if (exn instanceof AccessDeniedException) return "Access denied";
-
-        var reason = exn.getReason();
-        return reason != null ? reason.trim() : "Operation failed";
-    }
 }

projects/core/src/main/java/dan200/computercraft/core/filesystem/FileSystem.java

@@ -24,6 +24,8 @@
 import java.util.function.Function;
 import java.util.regex.Pattern;
 
+import static dan200.computercraft.core.filesystem.MountHelpers.*;
+
 public class FileSystem {
     /**
      * Maximum depth that {@link #copyRecursive(String, MountWrapper, String, MountWrapper, int)} will descend into.
@@ -206,9 +208,9 @@ public synchronized void move(String sourcePath, String destPath) throws FileSys
         sourcePath = sanitizePath(sourcePath);
         destPath = sanitizePath(destPath);
 
-        if (isReadOnly(sourcePath) || isReadOnly(destPath)) throw new FileSystemException("Access denied");
-        if (!exists(sourcePath)) throw new FileSystemException("No such file");
-        if (exists(destPath)) throw new FileSystemException("File exists");
+        if (isReadOnly(sourcePath) || isReadOnly(destPath)) throw new FileSystemException(ACCESS_DENIED);
+        if (!exists(sourcePath)) throw new FileSystemException(NO_SUCH_FILE);
+        if (exists(destPath)) throw new FileSystemException(FILE_EXISTS);
         if (contains(sourcePath, destPath)) throw new FileSystemException("Can't move a directory inside itself");
 
         var mount = getMount(sourcePath);
@@ -223,15 +225,9 @@ public synchronized void move(String sourcePath, String destPath) throws FileSys
     public synchronized void copy(String sourcePath, String destPath) throws FileSystemException {
         sourcePath = sanitizePath(sourcePath);
         destPath = sanitizePath(destPath);
-        if (isReadOnly(destPath)) {
-            throw new FileSystemException("/" + destPath + ": Access denied");
-        }
-        if (!exists(sourcePath)) {
-            throw new FileSystemException("/" + sourcePath + ": No such file");
-        }
-        if (exists(destPath)) {
-            throw new FileSystemException("/" + destPath + ": File exists");
-        }
+        if (isReadOnly(destPath)) throw new FileSystemException("/" + destPath + ": " + ACCESS_DENIED);
+        if (!exists(sourcePath)) throw new FileSystemException("/" + sourcePath + ": " + NO_SUCH_FILE);
+        if (exists(destPath)) throw new FileSystemException("/" + destPath + ": " + FILE_EXISTS);
         if (contains(sourcePath, destPath)) {
             throw new FileSystemException("/" + sourcePath + ": Can't copy a directory inside itself");
         }
@@ -264,7 +260,7 @@ private synchronized void copyRecursive(String sourcePath, MountWrapper sourceMo
                 // Copy bytes as fast as we can
                 ByteStreams.copy(source, destination);
             } catch (AccessDeniedException e) {
-                throw new FileSystemException("Access denied");
+                throw new FileSystemException(ACCESS_DENIED);
             } catch (IOException e) {
                 throw FileSystemException.of(e);
             }

projects/core/src/main/java/dan200/computercraft/core/filesystem/FileSystemException.java

@@ -7,6 +7,8 @@
 import java.io.IOException;
 import java.io.Serial;
 
+import static dan200.computercraft.core.filesystem.MountHelpers.ACCESS_DENIED;
+
 public class FileSystemException extends Exception {
     @Serial
     private static final long serialVersionUID = -2500631644868104029L;
@@ -20,6 +22,6 @@ public static FileSystemException of(IOException e) {
     }
 
     public static String getMessage(IOException e) {
-        return e.getMessage() == null ? "Access denied" : e.getMessage();
+        return e.getMessage() == null ? ACCESS_DENIED : e.getMessage();
     }
 }

projects/core/src/main/java/dan200/computercraft/core/filesystem/JarMount.java

@@ -14,11 +14,12 @@
 import java.io.IOException;
 import java.nio.file.attribute.BasicFileAttributes;
 import java.nio.file.attribute.FileTime;
-import java.time.Instant;
 import java.util.HashMap;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipFile;
 
+import static dan200.computercraft.core.filesystem.MountHelpers.NO_SUCH_FILE;
+
 /**
  * A mount which reads zip/jar files.
  */
@@ -95,9 +96,7 @@ void setup(ZipEntry entry) {
         }
     }
 
-    private static final FileTime EPOCH = FileTime.from(Instant.EPOCH);
-
     private static FileTime orEpoch(@Nullable FileTime time) {
-        return time == null ? EPOCH : time;
+        return time == null ? MountHelpers.EPOCH : time;
     }
 }

projects/core/src/main/java/dan200/computercraft/core/filesystem/MemoryMount.java

@@ -22,14 +22,13 @@
 import java.time.Instant;
 import java.util.*;
 
-import static dan200.computercraft.core.filesystem.WritableFileMount.MINIMUM_FILE_SIZE;
+import static dan200.computercraft.core.filesystem.MountHelpers.*;
 
 /**
  * A basic {@link Mount} which stores files and directories in-memory.
  */
 public final class MemoryMount extends AbstractInMemoryMount<MemoryMount.FileEntry> implements WritableMount {
     private static final byte[] EMPTY = new byte[0];
-    private static final FileTime EPOCH = FileTime.from(Instant.EPOCH);
 
     private final long capacity;
 
@@ -80,7 +79,7 @@ protected long getSize(String path, FileEntry file) {
 
     @Override
     protected SeekableByteChannel openForRead(String path, FileEntry file) throws IOException {
-        if (file.contents == null) throw new FileOperationException(path, "File is a directory");
+        if (file.contents == null) throw new FileOperationException(path, NOT_A_FILE);
         return new EntryChannel(file, 0);
     }
 
@@ -119,7 +118,7 @@ public void makeDirectory(String path) throws IOException {
             if (nextEntry == null) {
                 lastEntry.children.put(part, nextEntry = FileEntry.newDir());
             } else if (nextEntry.children == null) {
-                throw new FileOperationException(path, "File exists");
+                throw new FileOperationException(path, FILE_EXISTS);
             }
 
             lastEntry = nextEntry;
@@ -129,7 +128,7 @@ public void makeDirectory(String path) throws IOException {
 
     @Override
     public void delete(String path) throws IOException {
-        if (path.isEmpty()) throw new AccessDeniedException("Access denied");
+        if (path.isEmpty()) throw new AccessDeniedException(ACCESS_DENIED);
         var node = getParentAndName(path);
         if (node != null) node.parent().remove(node.name());
     }
@@ -139,23 +138,23 @@ public void rename(String source, String dest) throws IOException {
         if (dest.startsWith(source)) throw new FileOperationException(source, "Cannot move a directory inside itself");
 
         var sourceParent = getParentAndName(source);
-        if (sourceParent == null || !sourceParent.exists()) throw new FileOperationException(source, "No such file");
+        if (sourceParent == null || !sourceParent.exists()) throw new FileOperationException(source, NO_SUCH_FILE);
 
         var destParent = getParentAndName(dest);
         if (destParent == null) throw new FileOperationException(dest, "Parent directory does not exist");
-        if (destParent.exists()) throw new FileOperationException(dest, "File exists");
+        if (destParent.exists()) throw new FileOperationException(dest, FILE_EXISTS);
 
         destParent.put(sourceParent.parent().remove(sourceParent.name()));
     }
 
     private FileEntry getForWrite(String path) throws FileOperationException {
-        if (path.isEmpty()) throw new FileOperationException(path, "Cannot write to directory");
+        if (path.isEmpty()) throw new FileOperationException(path, CANNOT_WRITE_TO_DIRECTORY);
 
         var parent = getParentAndName(path);
         if (parent == null) throw new FileOperationException(path, "Parent directory does not exist");
 
         var file = parent.get();
-        if (file != null && file.isDirectory()) throw new FileOperationException(path, "Cannot write to directory");
+        if (file != null && file.isDirectory()) throw new FileOperationException(path, CANNOT_WRITE_TO_DIRECTORY);
         if (file == null) parent.put(file = FileEntry.newFile());
 
         return file;

projects/core/src/main/java/dan200/computercraft/core/filesystem/MountHelpers.java

@@ -0,0 +1,88 @@
+// SPDX-FileCopyrightText: 2023 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.core.filesystem;
+
+import dan200.computercraft.api.filesystem.Mount;
+import dan200.computercraft.api.filesystem.WritableMount;
+
+import java.nio.file.FileSystemException;
+import java.nio.file.*;
+import java.nio.file.attribute.BasicFileAttributes;
+import java.nio.file.attribute.FileTime;
+import java.time.Instant;
+import java.util.List;
+
+/**
+ * Useful constants and helper functions for working with mounts.
+ */
+public final class MountHelpers {
+    /**
+     * A {@link FileTime} set to the Unix EPOCH, intended for {@link BasicFileAttributes}'s file times.
+     */
+    public static final FileTime EPOCH = FileTime.from(Instant.EPOCH);
+
+    /**
+     * The minimum size of a file for file {@linkplain WritableMount#getCapacity() capacity calculations}.
+     */
+    public static final long MINIMUM_FILE_SIZE = 500;
+
+    /**
+     * The error message used when the file does not exist.
+     */
+    public static final String NO_SUCH_FILE = "No such file";
+
+    /**
+     * The error message used when trying to use a file as a directory (for instance when
+     * {@linkplain Mount#list(String, List) listing its contents}).
+     */
+    public static final String NOT_A_DIRECTORY = "Not a directory";
+
+    /**
+     * The error message used when trying to use a directory as a file (for instance when
+     * {@linkplain Mount#openForRead(String) opening for reading}).
+     */
+    public static final String NOT_A_FILE = "Not a file";
+
+    /**
+     * The error message used when attempting to modify a read-only file or mount.
+     */
+    public static final String ACCESS_DENIED = "Access denied";
+
+    /**
+     * The error message used when trying to overwrite a file (for instance when
+     * {@linkplain WritableMount#rename(String, String) renaming files} or {@linkplain WritableMount#makeDirectory(String)
+     * creating directories}).
+     */
+    public static final String FILE_EXISTS = "File exists";
+
+    /**
+     * The error message used when trying to {@linkplain WritableMount#openForWrite(String) opening a directory to read}.
+     */
+    public static final String CANNOT_WRITE_TO_DIRECTORY = "Cannot write to directory";
+
+    /**
+     * The error message used when the mount runs out of space.
+     */
+    public static final String OUT_OF_SPACE = "Out of space";
+
+    private MountHelpers() {
+    }
+
+    /**
+     * Get the user-friendly reason for a {@link java.nio.file.FileSystemException}.
+     *
+     * @param exn The exception that occurred.
+     * @return The friendly reason for this exception.
+     */
+    public static String getReason(FileSystemException exn) {
+        if (exn instanceof FileAlreadyExistsException) return FILE_EXISTS;
+        if (exn instanceof NoSuchFileException) return NO_SUCH_FILE;
+        if (exn instanceof NotDirectoryException) return NOT_A_DIRECTORY;
+        if (exn instanceof AccessDeniedException) return ACCESS_DENIED;
+
+        var reason = exn.getReason();
+        return reason != null ? reason.trim() : "Operation failed";
+    }
+}