Support empty heredoc and fix catastrophic backtracking issue

hcl2/hcl2.lark

@@ -50,8 +50,8 @@ tuple : "[" (new_line_or_comment? expression (new_line_or_comment? "," new_line_
 object : "{" new_line_or_comment? (object_elem (new_line_and_or_comma object_elem )* new_line_and_or_comma?)? "}"
 object_elem : (identifier | expression) ("=" | ":") expression
 
-heredoc_template : /<<(?P<heredoc>[a-zA-Z][a-zA-Z0-9._-]+)\n(?:.|\n)+?\n+\s*(?P=heredoc)/
-heredoc_template_trim : /<<-(?P<heredoc_trim>[a-zA-Z][a-zA-Z0-9._-]+)\n(?:.|\n)+?\n+\s*(?P=heredoc_trim)/
+heredoc_template : /<<(?P<heredoc>[a-zA-Z][a-zA-Z0-9._-]+)\n(?:.|\n)*?(?P=heredoc)/
+heredoc_template_trim : /<<-(?P<heredoc_trim>[a-zA-Z][a-zA-Z0-9._-]+)\n(?:.|\n)*?(?P=heredoc_trim)/
 
 function_call : identifier "(" new_line_or_comment? arguments? new_line_or_comment? ")"
 arguments : (expression (new_line_or_comment? "," new_line_or_comment?  expression)* ("," | "...")? new_line_or_comment?)

hcl2/transformer.py

@@ -6,10 +6,8 @@
 
 from lark.visitors import Transformer, Discard, _DiscardType
 
-HEREDOC_PATTERN = re.compile(r"<<([a-zA-Z][a-zA-Z0-9._-]+)\n((.|\n)*?)\n\s*\1", re.S)
-HEREDOC_TRIM_PATTERN = re.compile(
-    r"<<-([a-zA-Z][a-zA-Z0-9._-]+)\n((.|\n)*?)\n\s*\1", re.S
-)
+HEREDOC_PATTERN = re.compile(r"<<([a-zA-Z][a-zA-Z0-9._-]+)\n([\s\S]*)\1", re.S)
+HEREDOC_TRIM_PATTERN = re.compile(r"<<-([a-zA-Z][a-zA-Z0-9._-]+)\n([\s\S]*)\1", re.S)
 
 Attribute = namedtuple("Attribute", ("key", "value"))
 
@@ -191,7 +189,9 @@ def heredoc_template(self, args: List) -> str:
         match = HEREDOC_PATTERN.match(str(args[0]))
         if not match:
             raise RuntimeError(f"Invalid Heredoc token: {args[0]}")
-        return f'"{match.group(2)}"'
+
+        trim_chars = "\n\t "
+        return f'"{match.group(2).rstrip(trim_chars)}"'
 
     def heredoc_template_trim(self, args: List) -> str:
         # See https://github.com/hashicorp/hcl2/blob/master/hcl/hclsyntax/spec.md#template-expressions
@@ -202,7 +202,8 @@ def heredoc_template_trim(self, args: List) -> str:
         if not match:
             raise RuntimeError(f"Invalid Heredoc token: {args[0]}")
 
-        text = match.group(2)
+        trim_chars = "\n\t "
+        text = match.group(2).rstrip(trim_chars)
         lines = text.split("\n")
 
         # calculate the min number of leading spaces in each line

test/helpers/terraform-config-json/empty-heredoc.json

@@ -0,0 +1 @@
+{"bar": ""}

test/helpers/terraform-config/empty-heredoc.hcl2

@@ -0,0 +1,2 @@
+bar = <<EOF
+EOF