[__line] Add support for '--state replace'

It is currently counter-intuitive that something like:

    # File '/thing' contents
    #SomeSetting WrongValue

    # Manifest
    __line '/thing' \
           --line 'SomeSeting GoodValue' \
           --regex '^(#[[:space:]]*)?SomeSetting[[:space:]]'

Produces:

    # Resulting '/thing' contents
    #SomeSetting WrongValue

This makes sense given the implementation, but it masks a very common use-case.

Changing the default behaviour for such a base type is not really an option, so
instead we add a `replace` as a valid value for `--state`, which would result
in:

    # Resulting '/thing' contents with: --state replace
    SomeSetting GoodValue

For compatibility, if the regex is missing, `--state replace` behaves just as
`--state present`.

cdist/conf/type/__line/explorer/state

@@ -53,8 +53,10 @@ function _find(_text, _pattern) {
 BEGIN {
    getline anchor < (ENVIRON["__object"] "/parameter/" position)
    getline pattern < (ENVIRON["__object"] "/parameter/" needle)
+   getline line < (ENVIRON["__object"] "/parameter/line")
 
    found_line = 0
+   correct_line = 0
    correct_pos = (position != "after" && position != "before")
 }
 {
@@ -63,15 +65,18 @@ BEGIN {
          getline
          if (_find($0, pattern)) {
             found_line++
+	    if (index($0, line) == 1) { correct_line++ }
             correct_pos = 1
             exit 0
          }
       } else if (_find($0, pattern)) {
          found_line++
+         if (index($0, line) == 1) { correct_line++ }
       }
    } else if (position == "before") {
       if (_find($0, pattern)) {
          found_line++
+         if (index($0, line) == 1) { correct_line++ }
          getline
          if (match($0, anchor)) {
             correct_pos = 1
@@ -81,13 +86,18 @@ BEGIN {
    } else {
       if (_find($0, pattern)) {
          found_line++
+         if (index($0, line) == 1) { correct_line++ }
          exit 0
       }
    }
 }
 END {
    if (found_line && correct_pos) {
-      print "present"
+      if (correct_line) {
+        print "present"
+      } else {
+        print "matching"
+      }
    } else if (found_line) {
       print "wrongposition"
    } else {

cdist/conf/type/__line/gencode-remote

@@ -38,7 +38,11 @@ if [ -z "$state_is" ]; then
    exit 1
 fi
 
-if [ "$state_should" = "$state_is" ]; then
+if [ "$state_should" = "$state_is" ] || \
+	{ [ "$state_should" = "present" ] && [ "$state_is" = "matching" ] ;} || \
+	{ [ "$state_should" = "replace" ] && [ "$state_is" = "present" ] ;} ; then
+   # If state matches already, or 'present' is used and regex matches
+   # or 'replace' is used and the exact line is present, then there is
    # nothing to do
    exit 0
 fi
@@ -61,8 +65,8 @@ fi
 add=0
 remove=0
 case "$state_should" in
-   present)
-      if [ "$state_is" = "wrongposition" ]; then
+   present|replace)
+      if [ "$state_is" = "wrongposition" ] || [ "$state_is" = "matching" ]; then
          echo updated >> "$__messages_out"
          remove=1
       else

cdist/conf/type/__line/man.rst

@@ -31,7 +31,7 @@ file
 line
     Specifies the line which should be absent or present.
 
-    Must be present, if state is 'present'.
+    Must be present, if state is 'present' or 'replace'.
     Ignored if regex is given and state is 'absent'.
 
 regex
@@ -41,10 +41,13 @@ regex
     If state is 'absent', ensure all lines matching the regular expression
     are absent.
 
+    If state is 'replace', ensure all lines matching the regular expression
+    are exactly 'line'.
+
     The regular expression is interpreted by awk's match function.
 
 state
-    'present' or 'absent', defaults to 'present'
+    'present', 'absent' or 'replace', defaults to 'present'.
 
 onchange
     The code to run if line is added, removed or updated.
@@ -99,6 +102,12 @@ EXAMPLES
         --line '-session required pam_exec.so debug log=/tmp/classify.log /usr/local/libexec/classify' \
         --after '^session[[:space:]]+include[[:space:]]+password-auth-ac$'
 
+    # Uncomment as needed and set a value in a configuration file.
+    __line /etc/example.conf \
+        --line 'SomeSetting SomeValue' \
+        --regex '^(#[[:space:]]*)?SomeSetting[[:space:]]' \
+        --state replace
+
 
 SEE ALSO
 --------