feat(nix-compat/nixcpp): init nix.conf config parsing r/8319

This allows parsing files like `/etc/nix/nix.conf` into the `NixConfig`
struct.

Change-Id: I90b25f43c429dd56127500ff5068e83852adee13
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11888
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Brian Olsen <me@griff.name>

3 files changed, 212 insertions, 0 deletions

diff --git a/tvix/nix-compat/src/lib.rs b/tvix/nix-compat/src/lib.rs
index a71ede3eecf0..cc1ee082be38 100644
--- a/tvix/nix-compat/src/lib.rs
+++ b/tvix/nix-compat/src/lib.rs
@@ -3,6 +3,7 @@ pub mod derivation;
 pub mod nar;
 pub mod narinfo;
 pub mod nixbase32;
+pub mod nixcpp;
 pub mod nixhash;
 pub mod path_info;
 pub mod store_path;
diff --git a/tvix/nix-compat/src/nixcpp/conf.rs b/tvix/nix-compat/src/nixcpp/conf.rs
new file mode 100644
index 000000000000..6969e05856ac
--- /dev/null
+++ b/tvix/nix-compat/src/nixcpp/conf.rs
@@ -0,0 +1,202 @@
+use std::{fmt::Display, str::FromStr};
+
+/// Represents configuration as stored in /etc/nix/nix.conf.
+/// This list is not exhaustive, feel free to add more.
+#[derive(Clone, Debug, Default, Eq, PartialEq)]
+pub struct NixConfig<'a> {
+    allowed_users: Option<Vec<&'a str>>,
+    auto_optimise_store: Option<bool>,
+    cores: Option<u64>,
+    max_jobs: Option<u64>,
+    require_sigs: Option<bool>,
+    sandbox: Option<SandboxSetting>,
+    sandbox_fallback: Option<bool>,
+    substituters: Option<Vec<&'a str>>,
+    system_features: Option<Vec<&'a str>>,
+    trusted_public_keys: Option<Vec<crate::narinfo::PubKey>>,
+    trusted_substituters: Option<Vec<&'a str>>,
+    trusted_users: Option<Vec<&'a str>>,
+    extra_platforms: Option<Vec<&'a str>>,
+    extra_sandbox_paths: Option<Vec<&'a str>>,
+    experimental_features: Option<Vec<&'a str>>,
+    builders_use_substitutes: Option<bool>,
+}
+
+impl<'a> NixConfig<'a> {
+    /// Parses configuration from a file like `/etc/nix/nix.conf`, returning
+    /// a [NixConfig] with all values contained in there.
+    /// It does not support parsing multiple config files, merging semantics,
+    /// and also does not understand `include` and `!include` statements.
+    pub fn parse(input: &'a str) -> Result<Self, Error> {
+        let mut out = Self::default();
+
+        for line in input.lines() {
+            // strip comments at the end of the line
+            let line = if let Some((line, _comment)) = line.split_once('#') {
+                line
+            } else {
+                line
+            };
+
+            // skip comments and empty lines
+            if line.trim().is_empty() {
+                continue;
+            }
+
+            let (tag, val) = line
+                .split_once('=')
+                .ok_or_else(|| Error::InvalidLine(line.to_string()))?;
+
+            // trim whitespace
+            let tag = tag.trim();
+            let val = val.trim();
+
+            #[inline]
+            fn parse_val<'a>(this: &mut NixConfig<'a>, tag: &str, val: &'a str) -> Option<()> {
+                match tag {
+                    "allowed-users" => {
+                        this.allowed_users = Some(val.split_whitespace().collect());
+                    }
+                    "auto-optimise-store" => {
+                        this.auto_optimise_store = Some(val.parse::<bool>().ok()?);
+                    }
+                    "cores" => {
+                        this.cores = Some(val.parse().ok()?);
+                    }
+                    "max-jobs" => {
+                        this.max_jobs = Some(val.parse().ok()?);
+                    }
+                    "require-sigs" => {
+                        this.require_sigs = Some(val.parse().ok()?);
+                    }
+                    "sandbox" => this.sandbox = Some(val.parse().ok()?),
+                    "sandbox-fallback" => this.sandbox_fallback = Some(val.parse().ok()?),
+                    "substituters" => this.substituters = Some(val.split_whitespace().collect()),
+                    "system-features" => {
+                        this.system_features = Some(val.split_whitespace().collect())
+                    }
+                    "trusted-public-keys" => {
+                        this.trusted_public_keys = Some(
+                            val.split_whitespace()
+                                .map(crate::narinfo::PubKey::parse)
+                                .collect::<Result<Vec<crate::narinfo::PubKey>, _>>()
+                                .ok()?,
+                        )
+                    }
+                    "trusted-substituters" => {
+                        this.trusted_substituters = Some(val.split_whitespace().collect())
+                    }
+                    "trusted-users" => this.trusted_users = Some(val.split_whitespace().collect()),
+                    "extra-platforms" => {
+                        this.extra_platforms = Some(val.split_whitespace().collect())
+                    }
+                    "extra-sandbox-paths" => {
+                        this.extra_sandbox_paths = Some(val.split_whitespace().collect())
+                    }
+                    "experimental-features" => {
+                        this.experimental_features = Some(val.split_whitespace().collect())
+                    }
+                    "builders-use-substitutes" => {
+                        this.builders_use_substitutes = Some(val.parse().ok()?)
+                    }
+                    _ => return None,
+                }
+                Some(())
+            }
+
+            parse_val(&mut out, tag, val)
+                .ok_or_else(|| Error::InvalidValue(tag.to_string(), val.to_string()))?
+        }
+
+        Ok(out)
+    }
+}
+
+#[derive(thiserror::Error, Debug)]
+pub enum Error {
+    #[error("Invalid line: {0}")]
+    InvalidLine(String),
+    #[error("Unrecognized key: {0}")]
+    UnrecognizedKey(String),
+    #[error("Invalid value '{1}' for key '{0}'")]
+    InvalidValue(String, String),
+}
+
+/// Valid values for the Nix 'sandbox' setting
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum SandboxSetting {
+    True,
+    False,
+    Relaxed,
+}
+
+impl Display for SandboxSetting {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            SandboxSetting::True => write!(f, "true"),
+            SandboxSetting::False => write!(f, "false"),
+            SandboxSetting::Relaxed => write!(f, "relaxed"),
+        }
+    }
+}
+
+impl FromStr for SandboxSetting {
+    type Err = &'static str;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(match s {
+            "true" => Self::True,
+            "false" => Self::False,
+            "relaxed" => Self::Relaxed,
+            _ => return Err("invalid value"),
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{narinfo::PubKey, nixcpp::conf::SandboxSetting};
+
+    use super::NixConfig;
+
+    #[test]
+    pub fn test_parse() {
+        let config = NixConfig::parse(include_str!("../../testdata/nix.conf")).expect("must parse");
+
+        assert_eq!(
+            NixConfig {
+                allowed_users: Some(vec!["*"]),
+                auto_optimise_store: Some(false),
+                cores: Some(0),
+                max_jobs: Some(8),
+                require_sigs: Some(true),
+                sandbox: Some(SandboxSetting::True),
+                sandbox_fallback: Some(false),
+                substituters: Some(vec!["https://nix-community.cachix.org", "https://cache.nixos.org/"]),
+                system_features: Some(vec!["nixos-test", "benchmark", "big-parallel", "kvm"]),
+                trusted_public_keys: Some(vec![
+                    PubKey::parse("cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=")
+                        .expect("failed to parse pubkey"),
+                    PubKey::parse("nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs=")
+                        .expect("failed to parse pubkey")
+                ]),
+                trusted_substituters: Some(vec![]),
+                trusted_users: Some(vec!["flokli"]),
+                extra_platforms: Some(vec!["aarch64-linux", "i686-linux"]),
+                extra_sandbox_paths: Some(vec![
+                    "/run/binfmt", "/nix/store/swwyxyqpazzvbwx8bv40z7ih144q841f-qemu-aarch64-binfmt-P-x86_64-unknown-linux-musl"
+                ]),
+                experimental_features: Some(vec!["nix-command"]),
+                builders_use_substitutes: Some(true)
+            },
+            config
+        );
+
+        // parse a config file using some non-space whitespaces, as well as comments right after the lines.
+        // ensure it contains the same data as initially parsed.
+        let other_config = NixConfig::parse(include_str!("../../testdata/other_nix.conf"))
+            .expect("other config must parse");
+
+        assert_eq!(config, other_config);
+    }
+}
diff --git a/tvix/nix-compat/src/nixcpp/mod.rs b/tvix/nix-compat/src/nixcpp/mod.rs
new file mode 100644
index 000000000000..57518de8cc52
--- /dev/null
+++ b/tvix/nix-compat/src/nixcpp/mod.rs
@@ -0,0 +1,9 @@
+//! Contains code parsing some of the Nixcpp config files etc.
+//! left by Nix *on the local disk*.
+//!
+//! This is only for Nix' own state/config.
+//!
+//! More "standardized" protocols, like parts of the Nix HTTP Binary Cache
+//! protocol live elsewhere.
+
+pub mod conf;