fce01f7d71
`fromHexString` is backed by `builtins.fromTOML`. Per [the TOML
v1.0.0 specification]:
> Arbitrary 64-bit signed integers (from −2^63 to 2^63−1) should be
> accepted and handled losslessly. If an integer cannot be represented
> losslessly, an error must be thrown.
[the TOML v1.0.0 specification]: <https://toml.io/en/v1.0.0#integer>
The saturating behaviour of the toml11 version currently used
by Nix is not lossless, and is therefore a violation of the TOML
specification. We should not be relying on it. This blocks the update
of toml11, as it became stricter about reporting this condition.
This, yes, is arguably an evaluation compatibility break. However,
integer overflow was recently explicitly defined as an error by
both Nix and Lix, as opposed to the C++ undefined behaviour it was
previously implemented as:
* <https://nix.dev/manual/nix/stable/release-notes/rl-2.25>
* <https://docs.lix.systems/manual/lix/stable/release-notes/rl-2.91.html#fixes>
This included changing `builtins.fromJSON` to explicitly
reject overflowing integer literals. I believe that the case for
`builtins.fromTOML` is comparable, and that we are effectively testing
undefined behaviour in TOML and the Nix language here, in the same way
that we would have been if we had tests relying on overflowing integer
arithmetic. I am not aware of any use of this behaviour outside of
these tests; the reverted toml11 bump in Nix did not break the 23.11
evaluation regression test, for example.
C++ undefined behaviour is not involved here, as toml11 used the C++
formatted input functions that are specified to saturate on invalid
values. But it’s still a violation of the TOML specification caused
by insufficient error checking in the old version of the library,
and inconsistent with the handling of overflowing literals in the
rest of Nix.
Let’s fix this so that Nix implementations can correctly flag up
this error and we can unblock the toml11 update.
(cherry picked from commit 449ad44f16)
Nixpkgs lib
This directory contains the implementation, documentation and tests for the Nixpkgs lib library.
Overview
The evaluation entry point for lib is default.nix.
This file evaluates to an attribute set containing two separate kinds of attributes:
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Sub-libraries: Attribute sets grouping together similar functionality. Each sub-library is defined in a separate file usually matching its attribute name.
Example:
lib.listsis a sub-library containing list-related functionality such aslib.lists.takeandlib.lists.imap0. These are defined in the filelists.nix. -
Aliases: Attributes that point to an attribute of the same name in some sub-library.
Example:
lib.takeis an alias forlib.lists.take.
Most files in this directory are definitions of sub-libraries, but there are a few others:
minver.nix: A string of the minimum version of Nix that is required to evaluate Nixpkgs.tests: Tests, see Running testsrelease.nix: A derivation aggregating all testsmisc.nix: Evaluation unit tests for most sub-libraries*.sh: Bash scripts that run tests for specific sub-libraries- All other files in this directory exist to support the tests
systems: Thelib.systemssub-library, structured into a directory instead of a file due to its complexitypath: Thelib.pathsub-library, which includes tests as well as a document describing the design goals oflib.path- All other files in this directory are sub-libraries
Module system
The module system spans multiple sub-libraries:
modules.nix:lib.modulesfor the core functions and anything not relating to option definitionsoptions.nix:lib.optionsfor anything relating to option definitionstypes.nix:lib.typesfor module system types
PR Guidelines
Follow these guidelines for proposing a change to the interface of lib.
Provide a Motivation
Clearly describe why the change is necessary and its use cases.
Make sure that the change benefits the user more than the added mental effort of looking it up and keeping track of its definition. If the same can reasonably be done with the existing interface, consider just updating the documentation with more examples and links. This is also known as the Fairbairn Threshold.
Through this principle we avoid the human cost of duplicated functionality in an overly large library.
Make one PR for each change
Don't have multiple changes in one PR, instead split it up into multiple ones.
This keeps the conversation focused and has a higher chance of getting merged.
Name the interface appropriately
When introducing new names to the interface, such as new function, or new function attributes, make sure to name it appropriately.
Names should be self-explanatory and consistent with the rest of lib.
If there's no obvious best name, include the alternatives you considered.
Write documentation
Update the reference documentation to reflect the change.
Be generous with links to related functionality.
Write tests
Add good test coverage for the change, including:
-
Tests for edge cases, such as empty values or lists.
-
Tests for tricky inputs, such as a string with string context or a path that doesn't exist.
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Test all code paths, such as
if-then-elsebranches and returned attributes. -
If the tests for the sub-library are written in bash, test messages of custom errors, such as
throworabortMsg,At the time this is only not necessary for sub-libraries tested with
tests/misc.nix.
See running tests for more details on the test suites.
Write tidy code
Name variables well, even if they're internal. The code should be as self-explanatory as possible. Be generous with code comments when appropriate.
As a baseline, follow the Nixpkgs code conventions.
Write efficient code
Nix generally does not have free abstractions. Be aware that seemingly straightforward changes can cause more allocations and a decrease in performance. That said, don't optimise prematurely, especially in new code.
Reference documentation
Reference documentation for library functions is written above each function as a multi-line comment. These comments are processed using nixdoc and rendered in the Nixpkgs manual. The nixdoc README describes the comment format.
See doc/README.md for how to build the manual.
Running tests
All library tests can be run by building the derivation in tests/release.nix:
nix-build tests/release.nix
Some commands for quicker iteration over parts of the test suite are also available:
# Run all evaluation unit tests in tests/misc.nix
# if the resulting list is empty, all tests passed
nix-instantiate --eval --strict tests/misc.nix
# Run the module system tests
tests/modules.sh
# Run the lib.sources tests
tests/sources.sh
# Run the lib.filesystem tests
tests/filesystem.sh
# Run the lib.path property tests
path/tests/prop.sh
# Run the lib.fileset tests
fileset/tests.sh
Commit conventions
-
Make sure you read about the commit conventions common to Nixpkgs as a whole.
-
Format the commit messages in the following way:
lib.(section): (init | add additional argument | refactor | etc) (Motivation for change. Additional information.)Examples:
-
lib.getExe': check arguments
-
lib.fileset: Add an additional argument in the design docs
Closes #264537
-