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# Release 2.21.0 (2024-03-11)

- Fix a fixed-output derivation sandbox escape (CVE-2024-27297)

  Cooperating Nix derivations could send file descriptors to files in the Nix store to each other via Unix domain sockets in the abstract namespace. This allowed one derivation to modify the output of the other derivation, after Nix has registered the path as "valid" and immutable in the Nix database. In particular, this allowed the output of fixed-output derivations to be modified from their expected content.

  This isn't the case any more.

- CLI options `--arg-from-file` and `--arg-from-stdin` #10122

  The new CLI option `--arg-from-file` name path passes the contents of file path as a string value via the function argument name to a Nix expression. Similarly, the new option `--arg-from-stdin` name reads the contents of the string from standard input.

- Concise error printing in `nix repl` #9928

  Previously, if an element of a list or attribute set threw an error while evaluating, `nix repl` would print the entire error (including source location information) inline. This output was clumsy and difficult to parse:

      nix-repl> { err = builtins.throw "uh oh!"; }
      { err = «error:
             … while calling the 'throw' builtin
               at «string»:1:9:
                  1| { err = builtins.throw "uh oh!"; }
                   |         ^

             error: uh oh!»; }

  Now, only the error message is displayed, making the output much more readable.

      nix-repl> { err = builtins.throw "uh oh!"; }
      { err = «error: uh oh!»; }

  However, if the whole expression being evaluated throws an error, source locations and (if applicable) a stack trace are printed, just like you'd expect:

      nix-repl> builtins.throw "uh oh!"
      error:
             … while calling the 'throw' builtin
               at «string»:1:1:
                  1| builtins.throw "uh oh!"
                   | ^

             error: uh oh!

- `--debugger` can now access bindings from `let` expressions #8827 #9918

  Breakpoints and errors in the bindings of a `let` expression can now access those bindings in the debugger. Previously, only the body of `let` expressions could access those bindings.

- Enter the `--debugger` when `builtins.trace` is called if `debugger-on-trace` is set #9914

  If the `debugger-on-trace` option is set and `--debugger` is given, `builtins.trace` calls will behave similarly to `builtins.break` and will enter the debug REPL. This is useful for determining where warnings are being emitted from.

- Debugger prints source position information #9913

  The `--debugger` now prints source location information, instead of the pointers of source location information. Before:

      nix-repl> :bt
      0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
      0x600001522598

  After:

      0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
      /nix/store/hg65h51xnp74ikahns9hyf3py5mlbbqq-source/overrides/default.nix:132:27

         131|
         132|       bootstrappingBase = pkgs.${self.python.pythonAttr}.pythonForBuild.pkgs;
            |                           ^
         133|     in

- The `--debugger` will start more reliably in `let` expressions and function calls #6649 #9917

  Previously, if you attempted to evaluate this file with the debugger:

      let
        a = builtins.trace "before inner break" (
          builtins.break "hello"
        );
        b = builtins.trace "before outer break" (
          builtins.break a
        );
      in
        b

  Nix would correctly enter the debugger at `builtins.break a`, but if you asked it to `:continue`, it would skip over the `builtins.break "hello"` expression entirely.

  Now, Nix will correctly enter the debugger at both breakpoints.

- Nested debuggers are no longer supported #9920

  Previously, evaluating an expression that throws an error in the debugger would enter a second, nested debugger:

      nix-repl> builtins.throw "what"
      error: what

      Starting REPL to allow you to inspect the current state of the evaluator.

      Welcome to Nix 2.18.1. Type :? for help.

      nix-repl>

  Now, it just prints the error message like `nix repl`:

      nix-repl> builtins.throw "what"
      error:
             … while calling the 'throw' builtin
               at «string»:1:1:
                  1| builtins.throw "what"
                   | ^

             error: what

- Consistent order of function arguments in printed expressions #9874

  Function arguments are now printed in lexicographic order rather than the internal, creation-time based symbol order.

- Fix duplicate attribute error positions for `inherit` #9874

  When an `inherit` caused a duplicate attribute error the position of the error was not reported correctly, placing the error with the inherit itself or at the start of the bindings block instead of the offending attribute name.

- `inherit (x) ...` evaluates `x` only once #9847

  `inherit (x) a b ...` now evaluates the expression `x` only once for all inherited attributes rather than once for each inherited attribute. This does not usually have a measurable impact, but side-effects (such as `builtins.trace`) would be duplicated and expensive expressions (such as derivations) could cause a measurable slowdown.

- Store paths are allowed to start with `.` #912 #9091 #9095 #9120 #9121 #9122 #9130 #9219 #9224 #9867

  Leading periods were allowed by accident in Nix 2.4. The Nix team has considered this to be a bug, but this behavior has since been relied on by users, leading to unnecessary difficulties. From now on, leading periods are supported. The names `.` and `..` are disallowed, as well as those starting with `.-` or `..-`.

  Nix versions that denied leading periods are documented in the issue.

- `nix repl` pretty-prints values #9931

  `nix repl` will now pretty-print values:

      {
        attrs = {
          a = {
            b = {
              c = { };
            };
          };
        };
        list = [ 1 ];
        list' = [
          1
          2
          3
        ];
      }

- Introduction of `--regex` and `--all` in `nix profile remove` and `nix profile upgrade` #10166

  Previously the command-line arguments for `nix profile remove` and `nix profile upgrade` matched the package entries using regular expression. For instance:

      nix profile remove '.*vim.*'

  This would remove all packages that contain `vim` in their name.

  In most cases, only singular package names were used to remove and upgrade packages. Mixing this with regular expressions sometimes lead to unintended behavior. For instance, `python3.1` could match `python311`.

  To avoid unintended behavior, the arguments are now only matching exact names.

  Matching using regular expressions is still possible by using the new `--regex` flag:

      nix profile remove --regex '.*vim.*'

  One of the most useful cases for using regular expressions was to upgrade all packages. This was previously accomplished by:

      nix profile upgrade '.*'

  With the introduction of the `--all` flag, this now becomes more straightforward:

      nix profile upgrade --all

- Visual clutter in `--debugger` is reduced #9919

  Before:

      info: breakpoint reached

      Starting REPL to allow you to inspect the current state of the evaluator.

      Welcome to Nix 2.20.0pre20231222_dirty. Type :? for help.

      nix-repl> :continue
      error: uh oh

      Starting REPL to allow you to inspect the current state of the evaluator.

      Welcome to Nix 2.20.0pre20231222_dirty. Type :? for help.

      nix-repl>

  After:

      info: breakpoint reached

      Nix 2.20.0pre20231222_dirty debugger
      Type :? for help.
      nix-repl> :continue
      error: uh oh

      nix-repl>

- Cycle detection in `nix repl` is simpler and more reliable #8672 #9926

  The cycle detection in `nix repl`, `nix eval`, `builtins.trace`, and everywhere else values are printed is now simpler and matches the cycle detection in `nix-instantiate --eval` output.

  Before:

      nix eval --expr 'let self = { inherit self; }; in self'
      { self = { self = «repeated»; }; }

  After:

      { self = «repeated»; }

- In the debugger, `while evaluating the attribute` errors now include position information #9915

  Before:

      0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
      0x600001522598

  After:

      0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
      /nix/store/hg65h51xnp74ikahns9hyf3py5mlbbqq-source/overrides/default.nix:132:27

         131|
         132|       bootstrappingBase = pkgs.${self.python.pythonAttr}.pythonForBuild.pkgs;
            |                           ^
         133|     in

- Stack size is increased on macOS #9860

  Previously, Nix would set the stack size to 64MiB on Linux, but would leave the stack size set to the default (approximately 8KiB) on macOS. Now, the stack size is correctly set to 64MiB on macOS as well, which should reduce stack overflow segfaults in deeply-recursive Nix expressions.