Make escapeSystemdPath implement the correct systemd escaping algorithm

lib/ascii-table.nix

@@ -0,0 +1,96 @@
+{ " "  = 32;
+  "!"  = 33;
+  "\"" = 34;
+  "#"  = 35;
+  "$"  = 36;
+  "%"  = 37;
+  "&"  = 38;
+  "'"  = 39;
+  "("  = 40;
+  ")"  = 41;
+  "*"  = 42;
+  "+"  = 43;
+  ","  = 44;
+  "-"  = 45;
+  "."  = 46;
+  "/"  = 47;
+  "0"  = 48;
+  "1"  = 49;
+  "2"  = 50;
+  "3"  = 51;
+  "4"  = 52;
+  "5"  = 53;
+  "6"  = 54;
+  "7"  = 55;
+  "8"  = 56;
+  "9"  = 57;
+  ":"  = 58;
+  ";"  = 59;
+  "<"  = 60;
+  "="  = 61;
+  ">"  = 62;
+  "?"  = 63;
+  "@"  = 64;
+  "A"  = 65;
+  "B"  = 66;
+  "C"  = 67;
+  "D"  = 68;
+  "E"  = 69;
+  "F"  = 70;
+  "G"  = 71;
+  "H"  = 72;
+  "I"  = 73;
+  "J"  = 74;
+  "K"  = 75;
+  "L"  = 76;
+  "M"  = 77;
+  "N"  = 78;
+  "O"  = 79;
+  "P"  = 80;
+  "Q"  = 81;
+  "R"  = 82;
+  "S"  = 83;
+  "T"  = 84;
+  "U"  = 85;
+  "V"  = 86;
+  "W"  = 87;
+  "X"  = 88;
+  "Y"  = 89;
+  "Z"  = 90;
+  "["  = 91;
+  "\\" = 92;
+  "]"  = 93;
+  "^"  = 94;
+  "_"  = 95;
+  "`"  = 96;
+  "a"  = 97;
+  "b"  = 98;
+  "c"  = 99;
+  "d"  = 100;
+  "e"  = 101;
+  "f"  = 102;
+  "g"  = 103;
+  "h"  = 104;
+  "i"  = 105;
+  "j"  = 106;
+  "k"  = 107;
+  "l"  = 108;
+  "m"  = 109;
+  "n"  = 110;
+  "o"  = 111;
+  "p"  = 112;
+  "q"  = 113;
+  "r"  = 114;
+  "s"  = 115;
+  "t"  = 116;
+  "u"  = 117;
+  "v"  = 118;
+  "w"  = 119;
+  "x"  = 120;
+  "y"  = 121;
+  "z"  = 122;
+  "{"  = 123;
+  "|"  = 124;
+  "}"  = 125;
+  "~"  = 126;
+}

lib/strings.nix

@@ -185,6 +185,16 @@ rec {
   */
   makeBinPath = makeSearchPathOutput "bin" "bin";
 
+  /* Normalize path, removing extranous /s
+
+     Type: normalizePath :: string -> string
+
+     Example:
+       normalizePath "/a//b///c/"
+       => "/a/b/c/"
+  */
+  normalizePath = s: (builtins.foldl' (x: y: if y == "/" && hasSuffix "/" x then x else x+y) "" (splitString "" s));
+
   /* Depending on the boolean `cond', return either the given string
      or the empty string. Useful to concatenate against a bigger string.
 
@@ -294,6 +304,21 @@ rec {
       map f (stringToCharacters s)
     );
 
+  /* Convert char to ascii value, must be in printable range
+
+     Type: charToInt :: string -> int
+
+     Example:
+       charToInt "A"
+       => 65
+       charToInt "("
+       => 40
+
+  */
+  charToInt = let
+    table = import ./ascii-table.nix;
+  in c: builtins.getAttr c table;
+
   /* Escape occurrence of the elements of `list` in `string` by
      prefixing it with a backslash.
 
@@ -305,6 +330,19 @@ rec {
   */
   escape = list: replaceChars list (map (c: "\\${c}") list);
 
+  /* Escape occurence of the element of `list` in `string` by
+     converting to its ASCII value and prefixing it with \\x.
+     Only works for printable ascii characters.
+
+     Type: escapeC = [string] -> string -> string
+
+     Example:
+       escapeC [" "] "foo bar"
+       => "foo\\x20bar"
+
+  */
+  escapeC = list: replaceChars list (map (c: "\\x${ toLower (lib.toHexString (charToInt c))}") list);
+
   /* Quote string to be used safely within the Bourne shell.
 
      Type: escapeShellArg :: string -> string

lib/tests/misc.nix

@@ -312,6 +312,21 @@ runTests {
     expected = true;
   };
 
+  testNormalizePath = {
+    expr = strings.normalizePath "//a/b//c////d/";
+    expected = "/a/b/c/d/";
+  };
+
+  testCharToInt = {
+    expr = strings.charToInt "A";
+    expected = 65;
+  };
+
+  testEscapeC = {
+    expr = strings.escapeC [ " " ] "Hello World";
+    expected = "Hello\\x20World";
+  };
+
 # LISTS
 
   testFilter = {

nixos/lib/utils.nix

@@ -39,11 +39,19 @@ rec {
     || hasPrefix a'.mountPoint b'.mountPoint
     || any (hasPrefix a'.mountPoint) b'.depends;
 
-  # Escape a path according to the systemd rules, e.g. /dev/xyzzy
-  # becomes dev-xyzzy.  FIXME: slow.
-  escapeSystemdPath = s:
-   replaceChars ["/" "-" " "] ["-" "\\x2d" "\\x20"]
-   (removePrefix "/" s);
+  # Escape a path according to the systemd rules. FIXME: slow
+  # The rules are described in systemd.unit(5) as follows:
+  # The escaping algorithm operates as follows: given a string, any "/" character is replaced by "-", and all other characters which are not ASCII alphanumerics, ":", "_" or "." are replaced by C-style "\x2d" escapes. In addition, "." is replaced with such a C-style escape when it would appear as the first character in the escaped string.
+  # When the input qualifies as absolute file system path, this algorithm is extended slightly: the path to the root directory "/" is encoded as single dash "-". In addition, any leading, trailing or duplicate "/" characters are removed from the string before transformation. Example: /foo//bar/baz/ becomes "foo-bar-baz".
+  escapeSystemdPath = s: let
+    replacePrefix = p: r: s: (if (hasPrefix p s) then r + (removePrefix p s) else s);
+    trim = s: removeSuffix "/" (removePrefix "/" s);
+    normalizedPath = strings.normalizePath s;
+  in
+    replaceChars ["/"] ["-"]
+    (replacePrefix "." (strings.escapeC ["."] ".")
+    (strings.escapeC (stringToCharacters " !\"#$%&'()*+,;<=>=@[\\]^`{|}~-")
+    (if normalizedPath == "/" then normalizedPath else trim normalizedPath)));
 
   # Quotes an argument for use in Exec* service lines.
   # systemd accepts "-quoted strings with escape sequences, toJSON produces