Add documentation for NAR spec in kaitai

* Add a new flake check
* Add unit tests
* Add Kaitai spec
* Updated documentation

Author: fzakaria

doc/manual/source/SUMMARY.md.in

@@ -125,7 +125,7 @@
     - [Deriving Path](protocols/json/deriving-path.md)
   - [Serving Tarball Flakes](protocols/tarball-fetcher.md)
   - [Store Path Specification](protocols/store-path.md)
-  - [Nix Archive (NAR) Format](protocols/nix-archive.md)
+  - [Nix Archive (NAR) Format](protocols/nix-archive/index.md)
   - [Derivation "ATerm" file format](protocols/derivation-aterm.md)
 - [C API](c-api.md)
 - [Glossary](glossary.md)

doc/manual/source/protocols/nix-archive.md renamed to doc/manual/source/protocols/nix-archive/index.md

@@ -41,3 +41,15 @@ The `str` function / parameterized rule is defined as follows:
 - `int(n)` = the 64-bit little endian representation of the number `n`
 
 - `pad(s)` = the byte sequence `s`, padded with 0s to a multiple of 8 byte
+
+## Kaitai Struct Specification
+
+The Nix Archive (NAR) format is also formally described using [Kaitai Struct](https://kaitai.io/), an Interface Description Language (IDL) for defining binary data structures.
+
+> Kaitai Struct provides a language-agnostic, machine-readable specification that can be compiled into parsers for various programming languages (e.g., C++, Python, Java, Rust).
+
+```yaml
+{{#include nar.ksy}}
+```
+
+The source of the spec can be found [here](https://github.com/nixos/nix/blob/master/src/nix-manual/source/protocols/nix-archive/nar.ksy). Contributions and improvements to the spec are welcomed.

doc/manual/source/protocols/nix-archive/nar.ksy

@@ -0,0 +1,169 @@
+meta:
+  id: nix_nar
+  title: Nix Archive (NAR)
+  file-extension: nar
+  endian: le
+doc: |
+    Nix Archive (NAR) format. A simple, reproducible binary archive
+    format used by the Nix package manager to serialize file system objects.
+doc-ref: 'https://nixos.org/manual/nix/stable/command-ref/nix-store.html#nar-format'
+
+seq:
+  - id: magic
+    type: padded_str
+    doc: "Magic string, must be 'nix-archive-1'."
+    valid:
+      expr: _.body == 'nix-archive-1'
+  - id: root_node
+    type: node
+    doc: "The root of the archive, which is always a single node."
+
+types:
+  padded_str:
+    doc: |
+      A string, prefixed with its length (u8le) and
+      padded with null bytes to the next 8-byte boundary.
+    seq:
+      - id: len_str
+        type: u8
+      - id: body
+        type: str
+        size: len_str
+        encoding: 'ascii'
+      - id: padding
+        size: (8 - (len_str % 8)) % 8
+
+  node:
+    doc: "A single filesystem node (file, directory, or symlink)."
+    seq:
+      - id: open_paren
+        type: padded_str
+        doc: "Must be '(', a token starting the node definition."
+        valid:
+          expr: _.body == '('
+      - id: type_key
+        type: padded_str
+        doc: "Must be 'type'."
+        valid:
+          expr: _.body == 'type'
+      - id: type_val
+        type: padded_str
+        doc: "The type of the node: 'regular', 'directory', or 'symlink'."
+      - id: body
+        type:
+          switch-on: type_val.body
+          cases:
+            "'directory'": type_directory
+            "'regular'": type_regular
+            "'symlink'": type_symlink
+      - id: close_paren
+        type: padded_str
+        valid:
+          expr: _.body == ')'
+        if: "type_val.body != 'directory'"
+        doc: "Must be ')', a token ending the node definition."
+
+  type_directory:
+    doc: "A directory node, containing a list of entries. Entries must be ordered by their names."
+    seq:
+      - id: entries
+        type: dir_entry
+        repeat: until
+        repeat-until: _.kind.body == ')'
+    types:
+      dir_entry:
+        doc: "A single entry within a directory, or a terminator."
+        seq:
+          - id: kind
+            type: padded_str
+            valid:
+              expr: _.body == 'entry' or _.body == ')'
+            doc: "Must be 'entry' (for a child node) or '' (for terminator)."
+          - id: open_paren
+            type: padded_str
+            valid:
+              expr: _.body == '('
+            if: 'kind.body == "entry"'
+          - id: name_key
+            type: padded_str
+            valid:
+              expr: _.body == 'name'
+            if: 'kind.body == "entry"'
+          - id: name
+            type: padded_str
+            if: 'kind.body == "entry"'
+          - id: node_key
+            type: padded_str
+            valid:
+              expr: _.body == 'node'
+            if: 'kind.body == "entry"'
+          - id: node
+            type: node
+            if: 'kind.body == "entry"'
+            doc: "The child node, present only if kind is 'entry'."
+          - id: close_paren
+            type: padded_str
+            valid:
+              expr: _.body == ')'
+            if: 'kind.body == "entry"'
+        instances:
+          is_terminator:
+            value: kind.body == ')'
+
+  type_regular:
+    doc: "A regular file node."
+    seq:
+      # Read attributes (like 'executable') until we hit 'contents'
+      - id: attributes
+        type: reg_attribute
+        repeat: until
+        repeat-until: _.key.body == "contents"
+      # After the 'contents' token, read the file data
+      - id: file_data
+        type: file_content
+    instances:
+      is_executable:
+        value: 'attributes[0].key.body == "executable"'
+        doc: "True if the file has the 'executable' attribute."
+    types:
+      reg_attribute:
+        doc: "An attribute of the file, e.g., 'executable' or 'contents'."
+        seq:
+          - id: key
+            type: padded_str
+            doc: "Attribute key, e.g., 'executable' or 'contents'."
+            valid:
+              expr: _.body == 'executable' or _.body == 'contents'
+          - id: value
+            type: padded_str
+            if: 'key.body == "executable"'
+            valid:
+              expr: _.body == ''
+            doc: "Must be '' if key is 'executable'."
+      file_content:
+        doc: "The raw data of the file, prefixed by length."
+        seq:
+          - id: len_contents
+            type: u8
+          # # This relies on the property of instances that they are lazily evaluated and cached.
+          - size: 0
+            if: nar_offset < 0
+          - id: contents
+            size: len_contents
+          - id: padding
+            size: (8 - (len_contents % 8)) % 8
+        instances:
+          nar_offset:
+            value: _io.pos
+
+  type_symlink:
+    doc: "A symbolic link node."
+    seq:
+      - id: target_key
+        type: padded_str
+        doc: "Must be 'target'."
+        valid:
+          expr: _.body == 'target'
+      - id: target_val
+        type: padded_str
+        doc: "The destination path of the symlink."

doc/manual/source/store/file-system-object/content-address.md

@@ -46,7 +46,7 @@ be many different serialisations.
 For these reasons, Nix has its very own archive format—the Nix Archive (NAR) format,
 which is carefully designed to avoid the problems described above.
 
-The exact specification of the Nix Archive format is in [specified here](../../protocols/nix-archive.md).
+The exact specification of the Nix Archive format is in [specified here](../../protocols/nix-archive/index.md).
 
 ## Content addressing File System Objects beyond a single serialisation pass
 

flake.nix

@@ -417,6 +417,10 @@
                 supportsCross = false;
               };
 
+              "nix-kaitai-struct-checks" = {
+                supportsCross = false;
+              };
+
               "nix-perl-bindings" = {
                 supportsCross = false;
               };

meson.build

@@ -61,3 +61,4 @@ if get_option('unit-tests')
 endif
 subproject('nix-functional-tests')
 subproject('json-schema-checks')
+subproject('kaitai-struct-checks')

packaging/components.nix

@@ -443,6 +443,11 @@ in
   */
   nix-json-schema-checks = callPackage ../src/json-schema-checks/package.nix { };
 
+  /**
+    Kaitai struct schema validation checks
+  */
+  nix-kaitai-struct-checks = callPackage ../src/kaitai-struct-checks/package.nix { };
+
   nix-perl-bindings = callPackage ../src/perl/package.nix { };
 
   /**

packaging/dev-shell.nix

@@ -109,6 +109,7 @@ pkgs.nixComponents2.nix-util.overrideAttrs (
           ++ pkgs.nixComponents2.nix-external-api-docs.nativeBuildInputs
           ++ pkgs.nixComponents2.nix-functional-tests.externalNativeBuildInputs
           ++ pkgs.nixComponents2.nix-json-schema-checks.externalNativeBuildInputs
+          ++ pkgs.nixComponents2.nix-kaitai-struct-checks.externalNativeBuildInputs
           ++ lib.optional (
             !buildCanExecuteHost
             # Hack around https://github.com/nixos/nixpkgs/commit/bf7ad8cfbfa102a90463433e2c5027573b462479
@@ -148,6 +149,7 @@ pkgs.nixComponents2.nix-util.overrideAttrs (
     ++ pkgs.nixComponents2.nix-expr.externalPropagatedBuildInputs
     ++ pkgs.nixComponents2.nix-cmd.buildInputs
     ++ lib.optionals havePerl pkgs.nixComponents2.nix-perl-bindings.externalBuildInputs
-    ++ lib.optional havePerl pkgs.perl;
+    ++ lib.optional havePerl pkgs.perl
+    ++ pkgs.nixComponents2.nix-kaitai-struct-checks.externalBuildInputs;
   }
 )

packaging/hydra.nix

@@ -63,6 +63,7 @@ let
         "nix-cli"
         "nix-functional-tests"
         "nix-json-schema-checks"
+        "nix-kaitai-struct-checks"
       ]
       ++ lib.optionals enableBindings [
         "nix-perl-bindings"

src/kaitai-struct-checks/.version

@@ -0,0 +1 @@
+../../.version