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Write specification for text spec input files #79

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122 changes: 122 additions & 0 deletions cep-??.md
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<table>
<tr><td> Title </td><td> Specification of text spec input files </td>
<tr><td> Status </td><td> Draft </td></tr>
<tr><td> Author(s) </td><td> Jaime Rodríguez-Guerra &lt;jaime.rogue@gmail.com&gt;</td></tr>
<tr><td> Created </td><td> June 3, 2024 </td></tr>
<tr><td> Updated </td><td> June 3, 2024 </td></tr>
<tr><td> Discussion </td><td> https://github.com/conda/ceps/pull/79 </td></tr>
<tr><td> Implementation </td><td> NA </td></tr>
</table>

## Abstract

This CEP standardizes the format of `.txt` input files for conda clients.

## Motivation

The motivation of this CEP is merely informative. It describes the details of an existing file format.

## Nomenclature

This type of input file has not received a specific name. Different conda clients vaguely refer to it as [file specification](https://github.com/conda/conda/blob/841d9d57fd96ad27cda4b7c43549104a96f961ce/conda/cli/helpers.py#L90-L91), or [text spec file](https://github.com/mamba-org/mamba/blob/9300a6530cac4f5575e7f8aa4049fbb9c1150909/docs/source/user_guide/micromamba.rst?plain=1#L143).

In this CEP, we will use the term "text spec file".

## Specification

Text spec files use plain text to specify one package requirement per line. Lines starting with `#` are considered comments and are not taken into account. Empty lines or lines consisting only of whitespace are also ignored.

There are two flavors of this input file: explicit and not explicit.

### Explicit input files

If the line `@EXPLICIT` is found, the file is considered explicit. The word `@EXPLICIT` can contain whitespace before and after, but it's not recommended.

In explicit files, each package requirement line MUST specify a single, direct URL (as in RFC 3986) to a conda artifact. File paths are also supported, but they SHOULD be preferrably expressed as `file://` URLs. Relative paths are allowed, and will be processed as relative to the working directory, not the input file parent directory. Each URL can be immediately followed by an anchor tag (`#<hash>`) that encodes the expected MD5 checksum of the downloaded artifact as a lowercase string of 32 hexadecimal characters. More specifically, whitespace-stripped lines SHOULD be parsable by this Python-style regex:

```re
(?:(?P<url_p>.+)(?:[/\\]))?(?P<fn>[^/\\#]+(?:\.tar\.bz2|\.conda))(:?#(?P<md5>[0-9a-f]{32}))?$
```

Leading tildes (`~`) and environment variables MUST replaced with their values at runtime for backwards compatibility (as Python's [`os.path.expanduser`](https://docs.python.org/3/library/os.path.html#os.path.expanduser) and [`os.path.expandvars`](https://docs.python.org/3/library/os.path.html#os.path.expandvars) would do, respectively). That said, users SHOULD NOT use environment variables in their explicit spec files for the sake of reproducibility.

When an explicit input file is processed, the conda client SHOULD NOT invoke a solver. Because of this, the lines SHOULD be sorted topologically; e.g. if a package `A` depends on `B`, then the URL of B should come first.

Some conda clients tend to include a comment line specifying the platform the file was written for. This line is often found right before the `@EXPLICIT` marker, with the following syntax `# platform: {SUBDIR}`, where `{SUBDIR}` is a platform identifier like `linux-64` or `osx-arm64`. This line is not required, but implementors might choose to parse it if found for logging or verification purposes.

### Non-explicit input files

In the absence of an `@EXPLICIT` line, the file is considered regular or not explicit. Each line will encode a `MatchSpec`-compatible string. The solver SHOULD be invoked and, as such, topological sorting is not required.

## Examples

An example of an explicit TXT input file:

```
# This line is a comment. The one below only consists of whitespace and will also be ignored.

# This file may be used to create an environment using:
# $ conda create --name <env> --file <this file>
# platform: osx-arm64
@EXPLICIT
https://conda.anaconda.org/conda-forge/osx-arm64/bzip2-1.0.8-h93a5062_5.conda#1bbc659ca658bfd49a481b5ef7a0f40f
https://conda.anaconda.org/conda-forge/osx-arm64/ca-certificates-2024.2.2-hf0a4a13_0.conda#fb416a1795f18dcc5a038bc2dc54edf9
https://conda.anaconda.org/conda-forge/osx-arm64/libexpat-2.6.2-hebf3989_0.conda#e3cde7cfa87f82f7cb13d482d5e0ad09
https://conda.anaconda.org/conda-forge/osx-arm64/libffi-3.4.2-h3422bc3_5.tar.bz2#086914b672be056eb70fd4285b6783b6
https://conda.anaconda.org/conda-forge/osx-arm64/libzlib-1.2.13-h53f4e23_5.conda#1a47f5236db2e06a320ffa0392f81bd8
https://conda.anaconda.org/conda-forge/osx-arm64/ncurses-6.5-hb89a1cb_0.conda#b13ad5724ac9ae98b6b4fd87e4500ba4
https://conda.anaconda.org/conda-forge/noarch/tzdata-2024a-h0c530f3_0.conda#161081fc7cec0bfda0d86d7cb595f8d8
https://conda.anaconda.org/conda-forge/osx-arm64/xz-5.2.6-h57fd34a_0.tar.bz2#39c6b54e94014701dd157f4f576ed211
https://conda.anaconda.org/conda-forge/osx-arm64/libsqlite-3.45.3-h091b4b1_0.conda#c8c1186c7f3351f6ffddb97b1f54fc58
https://conda.anaconda.org/conda-forge/osx-arm64/openssl-3.3.0-hfb2fe0b_2.conda#c9602073e34599f40b8c4ce9e19cabf6
https://conda.anaconda.org/conda-forge/osx-arm64/readline-8.2-h92ec313_1.conda#8cbb776a2f641b943d413b3e19df71f4
https://conda.anaconda.org/conda-forge/osx-arm64/tk-8.6.13-h5083fa2_1.conda#b50a57ba89c32b62428b71a875291c9b
https://conda.anaconda.org/conda-forge/osx-arm64/python-3.12.3-h4a7b5fc_0_cpython.conda#8643ab37bece6ae8f112464068d9df9c
https://conda.anaconda.org/conda-forge/noarch/setuptools-69.5.1-pyhd8ed1ab_0.conda#7462280d81f639363e6e63c81276bd9e
# Note how MD5 hashes are optional
https://conda.anaconda.org/conda-forge/noarch/wheel-0.43.0-pyhd8ed1ab_1.conda
https://conda.anaconda.org/conda-forge/noarch/pip-24.0-pyhd8ed1ab_0.conda
```

A regular input file:

```
# This file may be used to create an environment using:
# $ conda create --name <env> --file <this file>
# platform: osx-arm64
bzip2=1.0.8=h93a5062_5
ca-certificates=2024.2.2=hf0a4a13_0
certifi=2024.2.2=pypi_0
charset-normalizer=3.3.2=pypi_0
idna=3.7=pypi_0
joblib=1.4.2=pypi_0
libexpat=2.6.2=hebf3989_0
libffi=3.4.2=h3422bc3_5
libsqlite=3.45.3=h091b4b1_0
libzlib=1.2.13=h53f4e23_5
ncurses=6.5=hb89a1cb_0
numpy=1.26.4=pypi_0
openssl=3.3.0=hfb2fe0b_2
pip=24.0=pyhd8ed1ab_0
python=3.12.3=h4a7b5fc_0_cpython
readline=8.2=h92ec313_1
requests=2.32.1=pypi_0
tzdata=2024a=h0c530f3_0
urllib3=2.2.1=pypi_0
wheel=0.43.0=pyhd8ed1ab_1
xz=5.2.6=h57fd34a_0
# Any of these syntaxes is also recognized as a match specification
scikit-learn
scipy=1.13.1
setuptools>=69.5.1
tk[build=h5083fa2_1]
```

## Reference

- [Explicit URL regex](https://github.com/conda/conda/blob/841d9d57fd96ad27cda4b7c43549104a96f961ce/conda/misc.py#L50-L54)
- [`explicit()` implementation in `conda`](https://github.com/conda/conda/blob/841d9d57fd96ad27cda4b7c43549104a96f961ce/conda/misc.py#L57-L145)

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For what it's worth this is the implementation in rattler: https://github.com/mamba-org/rattler/blob/main/crates/rattler_conda_types/src/explicit_environment_spec.rs

Although I don't think it follows the specs from this CEP to the letter.


## Copyright

All CEPs are explicitly [CC0 1.0 Universal](https://creativecommons.org/publicdomain/zero/1.0/).