diff options
| -rw-r--r-- | parser/vyos1x_lexer.mll | 72 |
1 files changed, 64 insertions, 8 deletions
diff --git a/parser/vyos1x_lexer.mll b/parser/vyos1x_lexer.mll index f471ec6..1969b91 100644 --- a/parser/vyos1x_lexer.mll +++ b/parser/vyos1x_lexer.mll @@ -4,7 +4,50 @@ open Vyos1x_parser exception Error of string -let vy_in_string = ref false +(* + +The language of the VyOS 1.x config file has multiple ambiguities that +make it not even context free. + +The main issue is lack of explicit statement separators, so if we ignore whitespace, +a parser is left to guess if, for example + +address dhcp # leaf node with a value +disable # valueless leaf node + +is three valueless nodes, a valueless node followed by a node with a value, +or a node with a value followed by a valueless node. + +The only cue is the newline, which means that newlines are sometimes significant, +and sometimes they aren't. + +interfaces { # doesn't matter + ethernet 'eth0' { # doesn't matter + address '192.0.2.1/24' # significant! + disable # significant! + + # empty line -- doesn't matter + hw-id 00:aa:bb:cc:dd:ee # significant! + } # doesn't matter +} + +If there were explicit terminators (like we do in VyConf, or like JunOS does), +the language would be context free. Enter the lexer hack: let's emit newlines only +when they are significant, so that the parser can use them as terminators. + +The informal idea is that a newline is only significant if it follows a leaf node. +So we need rules for finding out if we are inside a leaf node or not. + +These are the formal rules. A newline is significant if and only if +1. Preceding token is an identifier +2. Preceding token is a quoted string + +We set the vy_inside_node flag to true when we enter a leaf node and reset it when +we reach the end of it. + +*) + +let vy_inside_node = ref false } @@ -12,19 +55,19 @@ rule token = parse | [' ' '\t' '\r'] { token lexbuf } | '\n' - { Lexing.new_line lexbuf; if !vy_in_string then (vy_in_string := false; NEWLINE) else token lexbuf } + { Lexing.new_line lexbuf; if !vy_inside_node then (vy_inside_node := false; NEWLINE) else token lexbuf } | '"' - { vy_in_string := true; read_string (Buffer.create 16) lexbuf } + { vy_inside_node := true; read_string (Buffer.create 16) lexbuf } | ''' - { vy_in_string := true; read_single_quoted_string (Buffer.create 16) lexbuf } + { vy_inside_node := true; read_single_quoted_string (Buffer.create 16) lexbuf } | "/*" - { vy_in_string := false; read_comment (Buffer.create 16) lexbuf } + { vy_inside_node := false; read_comment (Buffer.create 16) lexbuf } | '{' - { vy_in_string := false; LEFT_BRACE } + { vy_inside_node := false; LEFT_BRACE } | '}' - { vy_in_string := false; RIGHT_BRACE } + { vy_inside_node := false; RIGHT_BRACE } | [^ ' ' '\t' '\n' '\r' '{' '}' '[' ']' ';' '#' '"' ''' ]+ as s - { vy_in_string := true; IDENTIFIER s} + { vy_inside_node := true; IDENTIFIER s} | eof { EOF } | _ @@ -78,3 +121,16 @@ and read_comment buf = { Buffer.add_string buf (Lexing.lexeme lexbuf); read_comment buf lexbuf } + +(* + +If you are curious how the original parsers handled the issue: they did not. +The CStore parser cheated by reading data from command definitions to resolve +the ambiguities, which made it impossible to use in standalone config +manipulation programs like migration scripts. + +The XorpConfigParser could not tell tag nodes' name and tag from +a leaf node with a value, which made it impossible to manipulate +tag nodes or change values properly. + +*) |