Merge pull request #8817 from iFreilicht/flake-update-lock-overhaul

Overhaul `nix flake update` and `nix flake lock` UX

(cherry picked from commit 12a0ae73dbb37becefa5a442eb4532ff0de9ce65)
Change-Id: Iff3b4f4235ebb1948ec612036b39ab29e4ca22b2

3 files changed, 99 insertions, 46 deletions

diff --git a/src/nix/flake-lock.md b/src/nix/flake-lock.md
index 2af0ad81e..6d10258e3 100644
--- a/src/nix/flake-lock.md
+++ b/src/nix/flake-lock.md
@@ -2,37 +2,39 @@ R""(
 
 # Examples
 
-* Update the `nixpkgs` and `nix` inputs of the flake in the current
-  directory:
+* Create the lock file for the flake in the current directory:
 
   ```console
-  # nix flake lock --update-input nixpkgs --update-input nix
-  * Updated 'nix': 'github:NixOS/nix/9fab14adbc3810d5cc1f88672fde1eee4358405c' -> 'github:NixOS/nix/8927cba62f5afb33b01016d5c4f7f8b7d0adde3c'
-  * Updated 'nixpkgs': 'github:NixOS/nixpkgs/3d2d8f281a27d466fa54b469b5993f7dde198375' -> 'github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293'
+  # nix flake lock
+  warning: creating lock file '/home/myself/repos/testflake/flake.lock':
+  • Added input 'nix':
+      'github:NixOS/nix/9fab14adbc3810d5cc1f88672fde1eee4358405c' (2023-06-28)
+  • Added input 'nixpkgs':
+      'github:NixOS/nixpkgs/3d2d8f281a27d466fa54b469b5993f7dde198375' (2023-06-30)
   ```
 
-# Description
-
-This command updates the lock file of a flake (`flake.lock`) so that
-it contains a lock for every flake input specified in
-`flake.nix`. Existing lock file entries are not updated unless
-required by a flag such as `--update-input`.
+* Add missing inputs to the lock file for a flake in a different directory:
 
-Note that every command that operates on a flake will also update the
-lock file if needed, and supports the same flags. Therefore,
+  ```console
+  # nix flake lock ~/repos/another
+  warning: updating lock file '/home/myself/repos/another/flake.lock':
+  • Added input 'nixpkgs':
+      'github:NixOS/nixpkgs/3d2d8f281a27d466fa54b469b5993f7dde198375' (2023-06-30)
+  ```
 
-```console
-# nix flake lock --update-input nixpkgs
-# nix build
-```
+  > **Note**
+  >
+  > When trying to refer to a flake in a subdirectory, write `./another`
+  > instead of `another`.
+  > Otherwise Nix will try to look up the flake in the registry.
 
-is equivalent to:
+# Description
 
-```console
-# nix build --update-input nixpkgs
-```
+This command adds inputs to the lock file of a flake (`flake.lock`)
+so that it contains a lock for every flake input specified in
+`flake.nix`. Existing lock file entries are not updated.
 
-Thus, this command is only useful if you want to update the lock file
-separately from any other action such as building.
+If you want to update existing lock entries, use
+[`nix flake update`](@docroot@/command-ref/new-cli/nix3-flake-update.md)
 
 )""
diff --git a/src/nix/flake-update.md b/src/nix/flake-update.md
index 8c6042d94..63df3b12a 100644
--- a/src/nix/flake-update.md
+++ b/src/nix/flake-update.md
@@ -2,33 +2,57 @@ R""(
 
 # Examples
 
-* Recreate the lock file (i.e. update all inputs) and commit the new
-  lock file:
+* Update all inputs (i.e. recreate the lock file from scratch):
 
   ```console
-  # nix flake update --commit-lock-file
-  * Updated 'nix': 'github:NixOS/nix/9fab14adbc3810d5cc1f88672fde1eee4358405c' -> 'github:NixOS/nix/8927cba62f5afb33b01016d5c4f7f8b7d0adde3c'
-  * Updated 'nixpkgs': 'github:NixOS/nixpkgs/3d2d8f281a27d466fa54b469b5993f7dde198375' -> 'github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293'
-  …
-  warning: committed new revision '158bcbd9d6cc08ab859c0810186c1beebc982aad'
+  # nix flake update
+  warning: updating lock file '/home/myself/repos/testflake/flake.lock':
+  • Updated input 'nix':
+      'github:NixOS/nix/9fab14adbc3810d5cc1f88672fde1eee4358405c' (2023-06-28)
+    → 'github:NixOS/nix/8927cba62f5afb33b01016d5c4f7f8b7d0adde3c' (2023-07-11)
+  • Updated input 'nixpkgs':
+      'github:NixOS/nixpkgs/3d2d8f281a27d466fa54b469b5993f7dde198375' (2023-06-30)
+    → 'github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293' (2023-07-05)
   ```
 
-# Description
+* Update only a single input:
+
+  ```console
+  # nix flake update nixpkgs
+  warning: updating lock file '/home/myself/repos/testflake/flake.lock':
+  • Updated input 'nixpkgs':
+      'github:NixOS/nixpkgs/3d2d8f281a27d466fa54b469b5993f7dde198375' (2023-06-30)
+    → 'github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293' (2023-07-05)
+  ```
+
+* Update only a single input of a flake in a different directory:
 
-This command recreates the lock file of a flake (`flake.lock`), thus
-updating the lock for every unlocked input (like `nixpkgs`) to its
-current version. This is equivalent to passing `--recreate-lock-file`
-to any command that operates on a flake. That is,
+  ```console
+  # nix flake update nixpkgs --flake ~/repos/another
+  warning: updating lock file '/home/myself/repos/another/flake.lock':
+  • Updated input 'nixpkgs':
+      'github:NixOS/nixpkgs/3d2d8f281a27d466fa54b469b5993f7dde198375' (2023-06-30)
+    → 'github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293' (2023-07-05)
+  ```
+
+  > **Note**
+  >
+  > When trying to refer to a flake in a subdirectory, write `./another`
+  > instead of `another`.
+  > Otherwise Nix will try to look up the flake in the registry.
+
+# Description
 
-```console
-# nix flake update
-# nix build
-```
+This command updates the inputs in a lock file (`flake.lock`).
+**By default, all inputs are updated**. If the lock file doesn't exist
+yet, it will be created. If inputs are not in the lock file yet, they will be added.
 
-is equivalent to:
+Unlike other `nix flake` commands, `nix flake update` takes a list of names of inputs
+to update as its positional arguments and operates on the flake in the current directory.
+You can pass a different flake-url with `--flake` to override that default.
 
-```console
-# nix build --recreate-lock-file
-```
+The related command [`nix flake lock`](@docroot@/command-ref/new-cli/nix3-flake-lock.md)
+also creates lock files and adds missing inputs, but is safer as it
+will never update inputs already in the lock file.
 
 )""
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index dde4b43bd..7b5d8096a 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -24,8 +24,10 @@ using namespace nix;
 using namespace nix::flake;
 using json = nlohmann::json;
 
+struct CmdFlakeUpdate;
 class FlakeCommand : virtual Args, public MixFlakeOptions
 {
+protected:
     std::string flakeUrl = ".";
 
 public:
@@ -63,6 +65,8 @@ public:
 
 struct CmdFlakeUpdate : FlakeCommand
 {
+public:
+
     std::string description() override
     {
         return "update flake lock file";
@@ -70,9 +74,31 @@ struct CmdFlakeUpdate : FlakeCommand
 
     CmdFlakeUpdate()
     {
+        expectedArgs.clear();
+        addFlag({
+            .longName="flake",
+            .description="The flake to operate on. Default is the current directory.",
+            .labels={"flake-url"},
+            .handler={&flakeUrl},
+            .completer = {[&](AddCompletions & completions, size_t, std::string_view prefix) {
+                completeFlakeRef(completions, getStore(), prefix);
+            }}
+        });
+        expectArgs({
+            .label="inputs",
+            .optional=true,
+            .handler={[&](std::string inputToUpdate){
+                auto inputPath = flake::parseInputPath(inputToUpdate);
+                if (lockFlags.inputUpdates.contains(inputPath))
+                    warn("Input '%s' was specified multiple times. You may have done this by accident.");
+                lockFlags.inputUpdates.insert(inputPath);
+            }},
+            .completer = {[&](AddCompletions & completions, size_t, std::string_view prefix) {
+                completeFlakeInputPath(completions, getEvalState(), getFlakeRefsForCompletion(), prefix);
+            }}
+        });
+
         /* Remove flags that don't make sense. */
-        removeFlag("recreate-lock-file");
-        removeFlag("update-input");
         removeFlag("no-update-lock-file");
         removeFlag("no-write-lock-file");
     }
@@ -87,8 +113,9 @@ struct CmdFlakeUpdate : FlakeCommand
     void run(nix::ref<nix::Store> store) override
     {
         settings.tarballTtl = 0;
+        auto updateAll = lockFlags.inputUpdates.empty();
 
-        lockFlags.recreateLockFile = true;
+        lockFlags.recreateLockFile = updateAll;
         lockFlags.writeLockFile = true;
         lockFlags.applyNixConfig = true;