Add 'nix registry' manpages

This also documents the registry format and matching/unification
semantics (though not quite correctly).

src/nix/registry-add.md

@@ -0,0 +1,33 @@
+R""(
+
+# Examples
+
+* Set the `nixpkgs` flake identifier to a specific branch of Nixpkgs:
+
+  ```console
+  # nix registry add nixpkgs github:NixOS/nixpkgs/nixos-20.03
+  ```
+
+* Pin `nixpkgs` to a specific revision:
+
+  ```console
+  # nix registry add nixpkgs github:NixOS/nixpkgs/925b70cd964ceaedee26fde9b19cc4c4f081196a
+  ```
+
+* Add an entry that redirects a specific branch of `nixpkgs` to
+  another fork:
+
+  ```console
+  # nix registry add nixpkgs/nixos-20.03 ~/Dev/nixpkgs
+  ```
+
+# Description
+
+This command adds an entry to the user registry that maps flake
+reference *from-url* to flake reference *to-url*. If an entry for
+*from-url* already exists, it is overwritten.
+
+Entries can be removed using [`nix registry
+remove`](./nix3-registry-remove.md).
+
+)""

src/nix/registry-list.md

@@ -0,0 +1,29 @@
+R""(
+
+# Examples
+
+* Show the contents of all registries:
+
+  ```console
+  # nix registry list
+  user   flake:dwarffs github:edolstra/dwarffs/d181d714fd36eb06f4992a1997cd5601e26db8f5
+  system flake:nixpkgs path:/nix/store/fxl9mrm5xvzam0lxi9ygdmksskx4qq8s-source?lastModified=1605220118&narHash=sha256-Und10ixH1WuW0XHYMxxuHRohKYb45R%2fT8CwZuLd2D2Q=&rev=3090c65041104931adda7625d37fa874b2b5c124
+  global flake:blender-bin github:edolstra/nix-warez?dir=blender
+  global flake:dwarffs github:edolstra/dwarffs
+  …
+  ```
+
+# Description
+
+This command displays the contents of all registries on standard
+output. Each line represents one registry entry in the format *type*
+*from* *to*, where *type* denotes the registry containing the entry:
+
+* `flags`: entries specified on the command line using `--override-flake`.
+* `user`: the user registry.
+* `system`: the system registry.
+* `global`: the global registry.
+
+See the [`nix registry` manual page](./nix3-registry.md) for more details.
+
+)""

src/nix/registry-pin.md

@@ -0,0 +1,38 @@
+R""(
+
+# Examples
+
+* Pin `nixpkgs` to its most recent Git revision:
+
+  ```console
+  # nix registry pin nixpkgs
+  ```
+
+  Afterwards the user registry will have an entry like this:
+
+  ```console
+  nix registry list | grep '^user '
+  user   flake:nixpkgs github:NixOS/nixpkgs/925b70cd964ceaedee26fde9b19cc4c4f081196a
+  ```
+
+  and `nix flake info` will say:
+
+  ```console
+  # nix flake info nixpkgs
+  Resolved URL:  github:NixOS/nixpkgs/925b70cd964ceaedee26fde9b19cc4c4f081196a
+  Locked URL:    github:NixOS/nixpkgs/925b70cd964ceaedee26fde9b19cc4c4f081196a
+  …
+  ```
+
+# Description
+
+This command adds an entry to the user registry that maps flake
+reference *url* to the corresponding *locked* flake reference, that
+is, a flake reference that specifies an exact revision or content
+hash. This ensures that until this registry entry is removed, all uses
+of *url* will resolve to exactly the same flake.
+
+Entries can be removed using [`nix registry
+remove`](./nix3-registry-remove.md).
+
+)""

src/nix/registry-remove.md

@@ -0,0 +1,16 @@
+R""(
+
+# Examples
+
+* Remove the entry `nixpkgs` from the user registry:
+
+  ```console
+  # nix registry remove nixpkgs
+  ```
+
+# Description
+
+This command removes from the user registry any entry for flake
+reference *url*.
+
+)""

src/nix/registry.cc

@@ -17,6 +17,13 @@ struct CmdRegistryList : StoreCommand
         return "list available Nix flakes";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "registry-list.md"
+          ;
+    }
+
     void run(nix::ref<nix::Store> store) override
     {
         using namespace fetchers;
@@ -47,6 +54,13 @@ struct CmdRegistryAdd : MixEvalArgs, Command
         return "add/replace flake in user flake registry";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "registry-add.md"
+          ;
+    }
+
     CmdRegistryAdd()
     {
         expectArg("from-url", &fromUrl);
@@ -75,6 +89,13 @@ struct CmdRegistryRemove : virtual Args, MixEvalArgs, Command
         return "remove flake from user flake registry";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "registry-remove.md"
+          ;
+    }
+
     CmdRegistryRemove()
     {
         expectArg("url", &url);
@@ -97,6 +118,13 @@ struct CmdRegistryPin : virtual Args, EvalCommand
         return "pin a flake to its current version in user flake registry";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "registry-pin.md"
+          ;
+    }
+
     CmdRegistryPin()
     {
         expectArg("url", &url);
@@ -132,6 +160,13 @@ struct CmdRegistry : virtual NixMultiCommand
         return "manage the flake registry";
     }
 
+    std::string doc() override
+    {
+        return
+          #include "registry.md"
+          ;
+    }
+
     Category category() override { return catSecondary; }
 
     void run() override

src/nix/registry.md

@@ -0,0 +1,98 @@
+R""(
+
+# Description
+
+`nix flake` provides subcommands for managing *flake
+registries*. Flake registries are a convenience feature that allows
+you to refer to flakes using symbolic identifiers such as `nixpkgs`,
+rather than full URLs such as `git://github.com/NixOS/nixpkgs`. You
+can use these identifiers on the command line (e.g. when you do `nix
+run nixpkgs#hello`) or in flake input specifications in `flake.nix`
+files. The latter are automatically resolved to full URLs and recorded
+in the flake's `flake.lock` file.
+
+In addition, the flake registry allows you to redirect arbitrary flake
+references (e.g. `github:NixOS/patchelf`) to another location, such as
+a local fork.
+
+There are multiple registries. These are, in order from lowest to
+highest precedence:
+
+* The global registry, which is a file downloaded from the URL
+  specified by the setting `flake-registry`. It is cached locally and
+  updated automatically when it's older than `tarball-ttl`
+  seconds. The default global registry is kept in [a GitHub
+  repository](https://github.com/NixOS/flake-registry).
+
+* The system registry, which is shared by all users. The default
+  location is `/etc/nix/registry.json`. On NixOS, the system registry
+  can be specified using the NixOS option `nix.registry`.
+
+* The user registry `~/.config/nix/registry.json`. This registry can
+  be modified by commands such as `nix flake pin`.
+
+* Overrides specified on the command line using the option
+  `--override-flake`.
+
+# Registry format
+
+A registry is a JSON file with the following format:
+
+```json
+{
+  "version": 2,
+  [
+    {
+      "from": {
+        "type": "indirect",
+        "id": "nixpkgs"
+      },
+      "to": {
+        "type": "github",
+        "owner": "NixOS",
+        "repo": "nixpkgs"
+      }
+    },
+    ...
+  ]
+}
+```
+
+That is, it contains a list of objects with attributes `from` and
+`to`, both of which contain a flake reference in attribute
+representation. (For example, `{"type": "indirect", "id": "nixpkgs"}`
+is the attribute representation of `nixpkgs`, while `{"type":
+"github", "owner": "NixOS", "repo": "nixpkgs"}` is the attribute
+representation of `github:NixOS/nixpkgs`.)
+
+Given some flake reference *R*, a registry entry is used if its
+`from` flake reference *matches* *R*. *R* is then replaced by the
+*unification* of the `to` flake reference with *R*.
+
+# Matching
+
+The `from` flake reference in a registry entry *matches* some flake
+reference *R* if the attributes in `from` are the same as the
+attributes in `R`. For example:
+
+* `nixpkgs` matches with `nixpkgs`.
+
+* `nixpkgs` matches with `nixpkgs/nixos-20.09`.
+
+* `nixpkgs/nixos-20.09` does not match with `nixpkgs`.
+
+* `nixpkgs` does not match with `git://github.com/NixOS/patchelf`.
+
+# Unification
+
+The `to` flake reference in a registry entry is *unified* with some flake
+reference *R* by taking `to` and applying the `rev` and `ref`
+attributes from *R*, if specified. For example:
+
+* `github:NixOS/nixpkgs` unified with `nixpkgs` produces `github:NixOS/nixpkgs`.
+
+* `github:NixOS/nixpkgs` unified with `nixpkgs/nixos-20.09` produces `github:NixOS/nixpkgs/nixos-20.09`.
+
+* `github:NixOS/nixpkgs/master` unified with `nixpkgs/nixos-20.09` produces `github:NixOS/nixpkgs/nixos-20.09`.
+
+)""