ZFS: Request credentials only for selected pools

This change introduces more fine-grained requestEncryptionCredentials.
While previously when requestEncryptionCredentials = true, the
credentials for all imported pools and all datasets in these imported
pools were requested, it is now possible to select exactly the pools and
datasets for which credentials should be requested.

It is still possible to set requestEncryptionCredentials = true, which
continues to act as a wildcard for all pools and datasets, so the change
is backwards compatible.

nixos/modules/tasks/filesystems/zfs.nix

@@ -191,13 +191,14 @@ in
       };
 
       requestEncryptionCredentials = mkOption {
-        type = types.bool;
+        type = types.either types.bool (types.listOf types.str);
         default = true;
+        example = [ "tank" "data" ];
         description = ''
-          Request encryption keys or passwords for all encrypted datasets on import.
-          For root pools the encryption key can be supplied via both an
-          interactive prompt (keylocation=prompt) and from a file
-          (keylocation=file://).
+          If true on import encryption keys or passwords for all encrypted datasets
+          are requested. To only decrypt selected datasets supply a list of dataset
+          names instead. For root pools the encryption key can be supplied via both
+          an interactive prompt (keylocation=prompt) and from a file (keylocation=file://).
         '';
       };
 
@@ -419,9 +420,13 @@ in
               fi
               poolImported "${pool}" || poolImport "${pool}"  # Try one last time, e.g. to import a degraded pool.
             fi
-            ${lib.optionalString cfgZfs.requestEncryptionCredentials ''
-              zfs load-key -a
-            ''}
+            ${if isBool cfgZfs.requestEncryptionCredentials
+              then optionalString cfgZfs.requestEncryptionCredentials ''
+                zfs load-key -a
+              ''
+              else concatMapStrings (fs: ''
+                zfs load-key ${fs}
+              '') cfgZfs.requestEncryptionCredentials}
         '') rootPools));
       };
 
@@ -517,9 +522,16 @@ in
               done
               poolImported "${pool}" || poolImport "${pool}"  # Try one last time, e.g. to import a degraded pool.
               if poolImported "${pool}"; then
-                ${optionalString cfgZfs.requestEncryptionCredentials ''
+                ${optionalString (if isBool cfgZfs.requestEncryptionCredentials
+                                  then cfgZfs.requestEncryptionCredentials
+                                  else cfgZfs.requestEncryptionCredentials != []) ''
                   ${packages.zfsUser}/sbin/zfs list -rHo name,keylocation ${pool} | while IFS=$'\t' read ds kl; do
-                    (case "$kl" in
+                    (${optionalString (!isBool cfgZfs.requestEncryptionCredentials) ''
+                         if ! echo '${concatStringsSep "\n" cfgZfs.requestEncryptionCredentials}' | grep -qFx "$ds"; then
+                           continue
+                         fi
+                       ''}
+                    case "$kl" in
                       none )
                         ;;
                       prompt )