Remove absolute file path from FS errors

When dealing with invalid paths (for instance, ones which are too long
or malformed), Java may throw a FileSystemException. This contains the
absolute path (i.e. C:/Users/Moi/.../.minecraft/...), which is printed
to the user within CC - obviously not ideal!

We simply catch this exception within the MountWrapper and map it back
to the local path. The disadvantage of doing it here is that we can't
map the path in the exception back to the computer - we'd need to catch
it in FileMount for that - so we just assume it referrs to the original
path instead.

Doing it in FileMount ends up being a little uglier, as we already do
all the exception wrangling in FileWrapper, so this'll do for now.

Fixes #495

src/main/java/dan200/computercraft/core/filesystem/MountWrapper.java

@@ -10,6 +10,7 @@ import dan200.computercraft.api.filesystem.IMount;
 import dan200.computercraft.api.filesystem.IWritableMount;
 
 import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import java.io.IOException;
 import java.nio.channels.ReadableByteChannel;
 import java.nio.channels.WritableByteChannel;
@@ -85,7 +86,7 @@ class MountWrapper
         }
         catch( IOException e )
         {
-            throw new FileSystemException( e.getMessage() );
+            throw localExceptionOf( path, e );
         }
     }
 
@@ -98,7 +99,7 @@ class MountWrapper
         }
         catch( IOException e )
         {
-            throw localExceptionOf( e );
+            throw localExceptionOf( path, e );
         }
     }
 
@@ -116,7 +117,7 @@ class MountWrapper
         }
         catch( IOException e )
         {
-            throw localExceptionOf( e );
+            throw localExceptionOf( path, e );
         }
     }
 
@@ -130,7 +131,7 @@ class MountWrapper
         }
         catch( IOException e )
         {
-            throw localExceptionOf( e );
+            throw localExceptionOf( path, e );
         }
     }
 
@@ -145,7 +146,7 @@ class MountWrapper
         }
         catch( IOException e )
         {
-            throw localExceptionOf( e );
+            throw localExceptionOf( path, e );
         }
     }
 
@@ -165,7 +166,7 @@ class MountWrapper
         }
         catch( IOException e )
         {
-            throw localExceptionOf( e );
+            throw localExceptionOf( path, e );
         }
     }
 
@@ -187,7 +188,7 @@ class MountWrapper
         }
         catch( IOException e )
         {
-            throw localExceptionOf( e );
+            throw localExceptionOf( path, e );
         }
     }
 
@@ -195,9 +196,9 @@ class MountWrapper
     {
         if( writableMount == null ) throw exceptionOf( path, "Access denied" );
 
+        path = toLocal( path );
         try
         {
-            path = toLocal( path );
             if( mount.exists( path ) )
             {
                 writableMount.delete( path );
@@ -209,7 +210,7 @@ class MountWrapper
         }
         catch( IOException e )
         {
-            throw localExceptionOf( e );
+            throw localExceptionOf( path, e );
         }
     }
 
@@ -243,7 +244,7 @@ class MountWrapper
         }
         catch( IOException e )
         {
-            throw localExceptionOf( e );
+            throw localExceptionOf( path, e );
         }
     }
 
@@ -281,7 +282,7 @@ class MountWrapper
         }
         catch( IOException e )
         {
-            throw localExceptionOf( e );
+            throw localExceptionOf( path, e );
         }
     }
 
@@ -290,7 +291,7 @@ class MountWrapper
         return FileSystem.toLocal( path, location );
     }
 
-    private FileSystemException localExceptionOf( IOException e )
+    private FileSystemException localExceptionOf( @Nullable String localPath, @Nonnull IOException e )
     {
         if( !location.isEmpty() && e instanceof FileOperationException )
         {
@@ -298,6 +299,14 @@ class MountWrapper
             if( ex.getFilename() != null ) return localExceptionOf( ex.getFilename(), ex.getMessage() );
         }
 
+        if( e instanceof java.nio.file.FileSystemException )
+        {
+            // This error will contain the absolute path, leaking information about where MC is installed. We drop that,
+            // just taking the reason. We assume that the error refers to the input path.
+            String message = ((java.nio.file.FileSystemException) e).getReason().trim();
+            return localPath == null ? new FileSystemException( message ) : localExceptionOf( localPath, message );
+        }
+
         return new FileSystemException( e.getMessage() );
     }
 

src/test/resources/test-rom/spec/apis/fs_spec.lua

@@ -99,6 +99,15 @@ describe("The fs library", function()
                 handle.close()
                 expect.error(handle.close):eq("attempt to use a closed file")
             end)
+
+            it("fails gracefully when opening 'CON' on Windows", function()
+                local ok, err = fs.open("test-files/con", "w")
+                if ok then fs.delete("test-files/con") return end
+
+                -- On my Windows/Java version the message appears to be "Incorrect function.". It may not be
+                -- consistent though, and honestly doesn't matter too much.
+                expect(err):str_match("^/test%-files/con: .*")
+            end)
         end)
 
         describe("writing in binary mode", function()