Make outputHashAlgo accept "nar", stay in sync

Now that we have a few things identifying content address methods by
name, we should be consistent about it.

Move up the `parseHashAlgoOpt` for tidiness too.

Discussed this change for consistency's sake as part of NixOS#8876

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

doc/manual/src/language/advanced-attributes.md

@@ -207,12 +207,17 @@ Derivations can declare some infrequently used optional attributes.
 
         This is the default.
 
-      - `"recursive"`\
-        The hash is computed over the NAR archive dump of the output
+      - `"recursive"` or `"nar"`\
+        The hash is computed over the [NAR archive](@docroot@/glossary.md#gloss-nar) dump of the output
         (i.e., the result of [`nix-store --dump`](@docroot@/command-ref/nix-store/dump.md)). In
         this case, the output can be anything, including a directory
         tree.
 
+    `"recursive"` is the traditional way of indicating this,
+    and is supported since 2005 (virtually the entire history of Nix).
+    `"nar"` is more clear, and consistent with other parts of Nix (such as the CLI),
+    however support for it is only added in Nix version 2.21.
+
   - [`__contentAddressed`]{#adv-attr-__contentAddressed}
     > **Warning**
     > This attribute is part of an [experimental feature](@docroot@/contributing/experimental-features.md).

src/libexpr/primops.cc

@@ -1139,18 +1139,20 @@ drvName, Bindings * attrs, Value & v)
         vomit("processing attribute '%1%'", key);
 
         auto handleHashMode = [&](const std::string_view s) {
-            if (s == "recursive") ingestionMethod = FileIngestionMethod::Recursive;
-            else if (s == "flat") ingestionMethod = FileIngestionMethod::Flat;
-            else if (s == "git") {
-                experimentalFeatureSettings.require(Xp::GitHashing);
-                ingestionMethod = FileIngestionMethod::Git;
-            } else if (s == "text") {
-                experimentalFeatureSettings.require(Xp::DynamicDerivations);
-                ingestionMethod = TextIngestionMethod {};
-            } else
+            if (s == "recursive") {
+                // back compat, new name is "nar"
+                ingestionMethod = FileIngestionMethod::Recursive;
+            } else try {
+                ingestionMethod = ContentAddressMethod::parse(s);
+            } catch (UsageError &) {
                 state.error<EvalError>(
                     "invalid value '%s' for 'outputHashMode' attribute", s
                 ).atPos(v).debugThrow();
+            }
+            if (ingestionMethod == TextIngestionMethod {})
+                experimentalFeatureSettings.require(Xp::DynamicDerivations);
+            if (ingestionMethod == FileIngestionMethod::Git)
+                experimentalFeatureSettings.require(Xp::GitHashing);
         };
 
         auto handleOutputs = [&](const Strings & ss) {

tests/functional/fixed.nix

@@ -57,11 +57,13 @@ rec {
     outputHashMode = "flat";
   };
 
-  # Test for building two derivations in parallel that produce the
+  # Test for building derivations in parallel that produce the
   # same output path because they're fixed-output derivations.
   parallelSame = [
     (f2 "foo" ./fixed.builder2.sh "recursive" "md5" "3670af73070fa14077ad74e0f5ea4e42")
     (f2 "bar" ./fixed.builder2.sh "recursive" "md5" "3670af73070fa14077ad74e0f5ea4e42")
+    (f2 "foo" ./fixed.builder2.sh "nar" "md5" "3670af73070fa14077ad74e0f5ea4e42")
+    (f2 "bar" ./fixed.builder2.sh "nar" "md5" "3670af73070fa14077ad74e0f5ea4e42")
   ];
 
 }