etc: allow replacing files with known content

This enables replacing existing system files like /etc/bashrc by default
while keeping the safer behaviour for other files like /etc/passwd, etc.
that could potentially cause major problems for the system when replaced.

modules/lib/write-text.nix

@@ -44,6 +44,12 @@ in
         Path of the source file.
       '';
     };
+
+    knownSha256Hashes = mkOption {
+      internal = true;
+      type = types.listOf types.str;
+      default = [];
+    };
   };
 
   config = {

modules/system/etc.nix

@@ -44,6 +44,9 @@ in
       # Set up the statically computed bits of /etc.
       echo "setting up /etc..." >&2
 
+      declare -A etcSha256Hashes
+      ${concatMapStringsSep "\n" (attr: "etcSha256Hashes['/etc/${attr.target}']='${concatStringsSep " " attr.knownSha256Hashes}'") etc}
+
       ln -sfn "$(readlink -f $systemConfig/etc)" /etc/static
 
       for f in $(find /etc/static/* -type l); do
@@ -53,9 +56,24 @@ in
           mkdir -p "$d"
         fi
         if [ -e "$l" ]; then
-          if [ "$(readlink $l)" != "$f" ]; then
+          if [ "$(readlink "$l")" != "$f" ]; then
             if ! grep -q /etc/static "$l"; then
-              echo "warning: not linking environment.etc.\"''${l#/etc/}\" because $l exists, skipping..." >&2
+              o=''$(shasum -a256 "$l")
+              o=''${o%% *}
+              for h in ''${etcSha256Hashes["$l"]}; do
+                if [ "$o" = "$h" ]; then
+                  mv "$l" "$l.orig"
+                  ln -s "$f" "$l"
+                  break
+                else
+                  h=
+                fi
+              done
+
+              if [ -z "$h" ]; then
+                echo "error: not linking environment.etc.\"''${l#/etc/}\" because $l already exists, skipping..." >&2
+                echo "existing file has unknown content $o, move and activate again to apply" >&2
+              fi
             fi
           fi
         else
@@ -66,7 +84,7 @@ in
       for l in $(find /etc/* -type l 2> /dev/null); do
         f="$(echo $l | sed 's,/etc/,/etc/static/,')"
         f=/etc/static/''${l#/etc/}
-        if [ "$(readlink $l)" = "$f" -a ! -e "$(readlink -f $l)" ]; then
+        if [ "$(readlink "$l")" = "$f" -a ! -e "$(readlink -f "$l")" ]; then
           rm "$l"
         fi
       done