Fix: REPL completion adds trailing backslash after keyword completion.

The REPL completer implemented by `rlcompleter.py` adds a trailing whitespace to keywords (except `finally` and `try`).
This makes sense as nothing else can follow those keywords, however, the current GraalPython completer implementation
via `JLineConsoleHandler` uses a DefaultParser for parsing completion candidates which escapes whitespace characters.
Therefore a trailing whitespace got replaced by '\ ' leading to a wrong completion. As a fix we overwrite the
`isDelimiterChar` method from the DefaultParser to never count the last character of a character sequence as a delimiter.
This works because we otherwise only use this check to split a line into it's words and in this case the last
character of the line also cannot be a delimiter.

Author: phoeinx

graalpython/com.oracle.graal.python.shell/src/com/oracle/graal/python/shell/JLineConsoleHandler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2017, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * The Universal Permissive License (UPL), Version 1.0
@@ -67,6 +67,7 @@
 import org.graalvm.shadowed.org.jline.reader.Macro;
 import org.graalvm.shadowed.org.jline.reader.ParsedLine;
 import org.graalvm.shadowed.org.jline.reader.UserInterruptException;
+import org.graalvm.shadowed.org.jline.reader.impl.DefaultParser;
 import org.graalvm.shadowed.org.jline.terminal.Terminal;
 import org.graalvm.shadowed.org.jline.terminal.TerminalBuilder;
 
@@ -113,6 +114,20 @@ public void complete(LineReader r, ParsedLine pl, List<Candidate> candidates) {
                 }
             });
         }
+
+        builder.parser(new DefaultParser() {
+            @Override
+            public boolean isDelimiterChar(CharSequence buffer, int pos) {
+                // Never count a last character of a char sequence as delimiter. The REPL completer implemented
+                // by `rlcompleter.py` adds a trailing whitespace to keywords, e.g. 'raise '. The default DefaultParser
+                // implementation always escaped this whitespace leading to wrong completions like 'raise\ '.
+                if (pos == buffer.length() - 1) {
+                    return false;
+                }
+                return Character.isWhitespace(buffer.charAt(pos));
+            }
+        });
+
         reader = builder.build();
         reader.option(LineReader.Option.DISABLE_EVENT_EXPANSION, true);
         reader.option(LineReader.Option.INSERT_TAB, true);