218 lines
11 KiB
Markdown
218 lines
11 KiB
Markdown
# File set library
|
||
|
||
This is the internal contributor documentation.
|
||
The user documentation is [in the Nixpkgs manual](https://nixos.org/manual/nixpkgs/unstable/#sec-fileset).
|
||
|
||
## Goals
|
||
|
||
The main goal of the file set library is to be able to select local files that should be added to the Nix store.
|
||
It should have the following properties:
|
||
- Easy:
|
||
The functions should have obvious semantics, be low in number and be composable.
|
||
- Safe:
|
||
Throw early and helpful errors when mistakes are detected.
|
||
- Lazy:
|
||
Only compute values when necessary.
|
||
|
||
Non-goals are:
|
||
- Efficient:
|
||
If the abstraction proves itself worthwhile but too slow, it can be still be optimized further.
|
||
|
||
## Tests
|
||
|
||
Tests are declared in [`tests.sh`](./tests.sh) and can be run using
|
||
```
|
||
./tests.sh
|
||
```
|
||
|
||
## Benchmark
|
||
|
||
A simple benchmark against the HEAD commit can be run using
|
||
```
|
||
./benchmark.sh HEAD
|
||
```
|
||
|
||
This is intended to be run manually and is not checked by CI.
|
||
|
||
## Internal representation
|
||
|
||
The internal representation is versioned in order to allow file sets from different Nixpkgs versions to be composed with each other, see [`internal.nix`](./internal.nix) for the versions and conversions between them.
|
||
This section describes only the current representation, but past versions will have to be supported by the code.
|
||
|
||
### `fileset`
|
||
|
||
An attribute set with these values:
|
||
|
||
- `_type` (constant string `"fileset"`):
|
||
Tag to indicate this value is a file set.
|
||
|
||
- `_internalVersion` (constant `3`, the current version):
|
||
Version of the representation.
|
||
|
||
- `_internalIsEmptyWithoutBase` (bool):
|
||
Whether this file set is the empty file set without a base path.
|
||
If `true`, `_internalBase*` and `_internalTree` are not set.
|
||
This is the only way to represent an empty file set without needing a base path.
|
||
|
||
Such a value can be used as the identity element for `union` and the return value of `unions []` and co.
|
||
|
||
- `_internalBase` (path):
|
||
Any files outside of this path cannot influence the set of files.
|
||
This is always a directory.
|
||
|
||
- `_internalBaseRoot` (path):
|
||
The filesystem root of `_internalBase`, same as `(lib.path.splitRoot _internalBase).root`.
|
||
This is here because this needs to be computed anyway, and this computation shouldn't be duplicated.
|
||
|
||
- `_internalBaseComponents` (list of strings):
|
||
The path components of `_internalBase`, same as `lib.path.subpath.components (lib.path.splitRoot _internalBase).subpath`.
|
||
This is here because this needs to be computed anyway, and this computation shouldn't be duplicated.
|
||
|
||
- `_internalTree` ([filesetTree](#filesettree)):
|
||
A tree representation of all included files under `_internalBase`.
|
||
|
||
- `__noEval` (error):
|
||
An error indicating that directly evaluating file sets is not supported.
|
||
|
||
## `filesetTree`
|
||
|
||
One of the following:
|
||
|
||
- `{ <name> = filesetTree; }`:
|
||
A directory with a nested `filesetTree` value for directory entries.
|
||
Entries not included may either be omitted or set to `null`, as necessary to improve efficiency or laziness.
|
||
|
||
- `"directory"`:
|
||
A directory with all its files included recursively, allowing early cutoff for some operations.
|
||
This specific string is chosen to be compatible with `builtins.readDir` for a simpler implementation.
|
||
|
||
- `"regular"`, `"symlink"`, `"unknown"` or any other non-`"directory"` string:
|
||
A nested file with its file type.
|
||
These specific strings are chosen to be compatible with `builtins.readDir` for a simpler implementation.
|
||
Distinguishing between different file types is not strictly necessary for the functionality this library,
|
||
but it does allow nicer printing of file sets.
|
||
|
||
- `null`:
|
||
A file or directory that is excluded from the tree.
|
||
It may still exist on the file system.
|
||
|
||
## API design decisions
|
||
|
||
This section justifies API design decisions.
|
||
|
||
### Internal structure
|
||
|
||
The representation of the file set data type is internal and can be changed over time.
|
||
|
||
Arguments:
|
||
- (+) The point of this library is to provide high-level functions, users don't need to be concerned with how it's implemented
|
||
- (+) It allows adjustments to the representation, which is especially useful in the early days of the library.
|
||
- (+) It still allows the representation to be stabilized later if necessary and if it has proven itself
|
||
|
||
### Influence tracking
|
||
|
||
File set operations internally track the top-most directory that could influence the exact contents of a file set.
|
||
Specifically, `toSource` requires that the given `fileset` is completely determined by files within the directory specified by the `root` argument.
|
||
For example, even with `dir/file.txt` being the only file in `./.`, `toSource { root = ./dir; fileset = ./.; }` gives an error.
|
||
This is because `fileset` may as well be the result of filtering `./.` in a way that excludes `dir`.
|
||
|
||
Arguments:
|
||
- (+) This gives us the guarantee that adding new files to a project never breaks a file set expression.
|
||
This is also true in a lesser form for removed files:
|
||
only removing files explicitly referenced by paths can break a file set expression.
|
||
- (+) This can be removed later, if we discover it's too restrictive
|
||
- (-) It leads to errors when a sensible result could sometimes be returned, such as in the above example.
|
||
|
||
### Empty file set without a base
|
||
|
||
There is a special representation for an empty file set without a base path.
|
||
This is used for return values that should be empty but when there's no base path that would makes sense.
|
||
|
||
Arguments:
|
||
- Alternative: This could also be represented using `_internalBase = /.` and `_internalTree = null`.
|
||
- (+) Removes the need for a special representation.
|
||
- (-) Due to [influence tracking](#influence-tracking),
|
||
`union empty ./.` would have `/.` as the base path,
|
||
which would then prevent `toSource { root = ./.; fileset = union empty ./.; }` from working,
|
||
which is not as one would expect.
|
||
- (-) With the assumption that there can be multiple filesystem roots (as established with the [path library](../path/README.md)),
|
||
this would have to cause an error with `union empty pathWithAnotherFilesystemRoot`,
|
||
which is not as one would expect.
|
||
- Alternative: Do not have such a value and error when it would be needed as a return value
|
||
- (+) Removes the need for a special representation.
|
||
- (-) Leaves us with no identity element for `union` and no reasonable return value for `unions []`.
|
||
From a set theory perspective, which has a well-known notion of empty sets, this is unintuitive.
|
||
|
||
### Empty directories
|
||
|
||
File sets can only represent a _set_ of local files, directories on their own are not representable.
|
||
|
||
Arguments:
|
||
- (+) There does not seem to be a sensible set of combinators when directories can be represented on their own.
|
||
Here's some possibilities:
|
||
- `./.` represents the files in `./.` _and_ the directory itself including its subdirectories, meaning that even if there's no files, the entire structure of `./.` is preserved
|
||
|
||
In that case, what should `fileFilter (file: false) ./.` return?
|
||
It could return the entire directory structure unchanged, but with all files removed, which would not be what one would expect.
|
||
|
||
Trying to have a filter function that also supports directories will lead to the question of:
|
||
What should the behavior be if `./foo` itself is excluded but all of its contents are included?
|
||
It leads to having to define when directories are recursed into, but then we're effectively back at how the `builtins.path`-based filters work.
|
||
|
||
- `./.` represents all files in `./.` _and_ the directory itself, but not its subdirectories, meaning that at least `./.` will be preserved even if it's empty.
|
||
|
||
In that case, `intersect ./. ./foo` should only include files and no directories themselves, since `./.` includes only `./.` as a directory, and same for `./foo`, so there's no overlap in directories.
|
||
But intuitively this operation should result in the same as `./foo` – everything else is just confusing.
|
||
- (+) This matches how Git only supports files, so developers should already be used to it.
|
||
- (-) Empty directories (even if they contain nested directories) are neither representable nor preserved when coercing from paths.
|
||
- (+) It is very rare that empty directories are necessary.
|
||
- (+) We can implement a workaround, allowing `toSource` to take an extra argument for ensuring certain extra directories exist in the result.
|
||
- (-) It slows down store imports, since the evaluator needs to traverse the entire tree to remove any empty directories
|
||
- (+) This can still be optimized by introducing more Nix builtins if necessary
|
||
|
||
### String paths
|
||
|
||
File sets do not support Nix store paths in strings such as `"/nix/store/...-source"`.
|
||
|
||
Arguments:
|
||
- (+) Such paths are usually produced by derivations, which means `toSource` would either:
|
||
- Require IFD if `builtins.path` is used as the underlying primitive
|
||
- Require importing the entire `root` into the store such that derivations can be used to do the filtering
|
||
- (+) The convenient path coercion like `union ./foo ./bar` wouldn't work for absolute paths, requiring more verbose alternate interfaces:
|
||
- `let root = "/nix/store/...-source"; in union "${root}/foo" "${root}/bar"`
|
||
|
||
Verbose and dangerous because if `root` was a path, the entire path would get imported into the store.
|
||
|
||
- `toSource { root = "/nix/store/...-source"; fileset = union "./foo" "./bar"; }`
|
||
|
||
Does not allow debug printing intermediate file set contents, since we don't know the paths contents before having a `root`.
|
||
|
||
- `let fs = lib.fileset.withRoot "/nix/store/...-source"; in fs.union "./foo" "./bar"`
|
||
|
||
Makes library functions impure since they depend on the contextual root path, questionable composability.
|
||
|
||
- (+) The point of the file set abstraction is to specify which files should get imported into the store.
|
||
|
||
This use case makes little sense for files that are already in the store.
|
||
This should be a separate abstraction as e.g. `pkgs.drvLayout` instead, which could have a similar interface but be specific to derivations.
|
||
Additional capabilities could be supported that can't be done at evaluation time, such as renaming files, creating new directories, setting executable bits, etc.
|
||
|
||
### Single files
|
||
|
||
File sets cannot add single files to the store, they can only import files under directories.
|
||
|
||
Arguments:
|
||
- (+) There's no point in using this library for a single file, since you can't do anything other than add it to the store or not.
|
||
And it would be unclear how the library should behave if the one file wouldn't be added to the store:
|
||
`toSource { root = ./file.nix; fileset = <empty>; }` has no reasonable result because returing an empty store path wouldn't match the file type, and there's no way to have an empty file store path, whatever that would mean.
|
||
|
||
## To update in the future
|
||
|
||
Here's a list of places in the library that need to be updated in the future:
|
||
- > The file set library is currently somewhat limited but is being expanded to include more functions over time.
|
||
|
||
in [the manual](../../doc/functions/fileset.section.md)
|
||
- Once a tracing function exists, `__noEval` in [internal.nix](./internal.nix) should mention it
|
||
- If/Once a function to convert `lib.sources` values into file sets exists, the `_coerce` and `toSource` functions should be updated to mention that function in the error when such a value is passed
|
||
- If/Once a function exists that can optionally include a path depending on whether it exists, the error message for the path not existing in `_coerce` should mention the new function
|