feat(tvix/glue): Implement builtins.fetchurl r/7678

Implement the fetchurl builtin, and lay the groundwork for implementing
the fetchTarball builtin (which works very similarly, and is implemented
using almost the same code in C++ nix).

An overview of how this works:

1. First, we check if the store path that *would* result from the
   download already exists in the store - if it does, we just return
   that
2. If we need to download the URL, TvixStoreIO has an `http_client:
   reqwest::Client` field now which we use to make the request
3. As we're downloading the blob, we hash the data incrementally into a
   SHA256 hasher
4. We compare the hash against the expected hash (if any) and bail out
   if it doesn't match
5. Finally, we put the blob in the store and return the store path

Since the logic is very similar, this commit also implements a *chunk*
of `fetchTarball` (though the actual implementation will likely include
a refactor to some of the code reuse here).

The main thing that's missing here is caching of downloaded blobs when
fetchurl is called without a hash - I've opened b/381 to track the TODO
there.

Adding the `SSL_CERT_FILE` here is necessary to teach reqwest how to
load it during tests - see 1c16dee20 (feat(tvix/store): use reqwests'
rustls-native-roots feature, 2024-03-03) for  more info.

Change-Id: I83c4abbc7c0c3bfe92461917e23d6d3430fbf137
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11017
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: aspen <root@gws.fyi>

1 files changed, 3 insertions, 38 deletions

diff --git a/tvix/glue/src/builtins/derivation.rs b/tvix/glue/src/builtins/derivation.rs
index 71249f1c7722..4ea9611a946c 100644
--- a/tvix/glue/src/builtins/derivation.rs
+++ b/tvix/glue/src/builtins/derivation.rs
@@ -10,8 +10,7 @@ use std::rc::Rc;
 use tvix_eval::builtin_macros::builtins;
 use tvix_eval::generators::{self, emit_warning_kind, GenCo};
 use tvix_eval::{
-    AddContext, CatchableErrorKind, CoercionKind, ErrorKind, NixAttrs, NixContext,
-    NixContextElement, Value, WarningKind,
+    AddContext, ErrorKind, NixAttrs, NixContext, NixContextElement, Value, WarningKind,
 };
 
 // Constants used for strangely named fields in derivation inputs.
@@ -144,6 +143,8 @@ fn handle_fixed_output(
 pub(crate) mod derivation_builtins {
     use std::collections::BTreeMap;
 
+    use crate::builtins::utils::{select_string, strong_importing_coerce_to_string};
+
     use super::*;
     use bstr::ByteSlice;
     use nix_compat::store_path::hash_placeholder;
@@ -197,27 +198,6 @@ pub(crate) mod derivation_builtins {
         drv.outputs.insert("out".to_string(), Default::default());
         let mut input_context = NixContext::new();
 
-        #[inline]
-        async fn strong_importing_coerce_to_string(
-            co: &GenCo,
-            val: Value,
-        ) -> Result<NixString, CatchableErrorKind> {
-            let val = generators::request_force(co, val).await;
-            match generators::request_string_coerce(
-                co,
-                val,
-                CoercionKind {
-                    strong: true,
-                    import_paths: true,
-                },
-            )
-            .await
-            {
-                Err(cek) => Err(cek),
-                Ok(val_str) => Ok(val_str),
-            }
-        }
-
         /// Inserts a key and value into the drv.environment BTreeMap, and fails if the
         /// key did already exist before.
         fn insert_env(
@@ -385,21 +365,6 @@ pub(crate) mod derivation_builtins {
 
         // Configure fixed-output derivations if required.
         {
-            async fn select_string(
-                co: &GenCo,
-                attrs: &NixAttrs,
-                key: &str,
-            ) -> Result<Result<Option<String>, CatchableErrorKind>, ErrorKind> {
-                if let Some(attr) = attrs.select(key) {
-                    match strong_importing_coerce_to_string(co, attr.clone()).await {
-                        Err(cek) => return Ok(Err(cek)),
-                        Ok(str) => return Ok(Ok(Some(str.to_str()?.to_owned()))),
-                    }
-                }
-
-                Ok(Ok(None))
-            }
-
             let output_hash = match select_string(&co, &input, "outputHash")
                 .await
                 .context("evaluating the `outputHash` parameter")?