feat(tvix/glue): support builtin:fetchurl

nixpkgs calls <nix/fetchurl.nix> during nixpkgs bootstrap.

This produces a fake derivation with system = builtin
and builder = builtin:fetchurl, and needs to download files from the
internet.

At the end of the Derivation construction, if we have such a derivation,
also synthesize a `Fetch` struct, which we add to the known fetch paths.

This will then cause these fetches to be picked up like all other
fetches in TvixStoreIO.

Change-Id: I72cbca4f85da106b25eda97693a6a6e59911cd57
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10975
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/glue/src/fetchurl.rs

@@ -0,0 +1,82 @@
+//! This contains the code translating from a `builtin:derivation` [Derivation]
+//! to a [Fetch].
+use crate::fetchers::Fetch;
+use nix_compat::{derivation::Derivation, nixhash::CAHash};
+use tracing::instrument;
+use url::Url;
+
+/// Takes a derivation produced by a call to `builtin:fetchurl` and returns the
+/// synthesized [Fetch] for it, as well as the name.
+#[instrument]
+pub(crate) fn fetchurl_derivation_to_fetch(drv: &Derivation) -> Result<(String, Fetch), Error> {
+    if drv.builder != "builtin:fetchurl" {
+        return Err(Error::BuilderInvalid);
+    }
+    if !drv.arguments.is_empty() {
+        return Err(Error::ArgumentsInvalud);
+    }
+    if drv.system != "builtin" {
+        return Err(Error::SystemInvalid);
+    }
+
+    // ensure this is a fixed-output derivation
+    if drv.outputs.len() != 1 {
+        return Err(Error::NoFOD);
+    }
+    let out_output = &drv.outputs.get("out").ok_or(Error::NoFOD)?;
+    let ca_hash = out_output.ca_hash.clone().ok_or(Error::NoFOD)?;
+
+    let name: String = drv
+        .environment
+        .get("name")
+        .ok_or(Error::NameMissing)?
+        .to_owned()
+        .try_into()
+        .map_err(|_| Error::NameInvalid)?;
+
+    let url: Url = std::str::from_utf8(drv.environment.get("url").ok_or(Error::URLMissing)?)
+        .map_err(|_| Error::URLInvalid)?
+        .parse()
+        .map_err(|_| Error::URLInvalid)?;
+
+    match ca_hash {
+        CAHash::Flat(hash) => {
+            return Ok((
+                name,
+                Fetch::URL {
+                    url,
+                    exp_hash: Some(hash),
+                },
+            ))
+        }
+        CAHash::Nar(hash) => {
+            if drv.environment.get("executable").map(|v| v.as_slice()) == Some(b"1") {
+                Ok((name, Fetch::Executable { url, hash }))
+            } else {
+                Ok((name, Fetch::NAR { url, hash }))
+            }
+        }
+        // you can't construct derivations containing this
+        CAHash::Text(_) => panic!("Tvix bug: got CaHash::Text in drv"),
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub(crate) enum Error {
+    #[error("Invalid builder")]
+    BuilderInvalid,
+    #[error("invalid arguments")]
+    ArgumentsInvalud,
+    #[error("Invalid system")]
+    SystemInvalid,
+    #[error("Derivation is not fixed-output")]
+    NoFOD,
+    #[error("Missing URL")]
+    URLMissing,
+    #[error("Invalid URL")]
+    URLInvalid,
+    #[error("Missing Name")]
+    NameMissing,
+    #[error("Name invalid")]
+    NameInvalid,
+}