feat: new feature flag default-engine-rustls (#572)

We had a request to remove our (implicit) dependency on native-tls. The
`default-engine` feature flag pulls in `reqwest` with default features
which requires native-tls. This PR introduces a new feature flag:
`default-engine-rustls` which instead specifies `default-features =
false` for `reqwest` enables the same features which are used in
`object_store`: `reqwest = { version = "0.12", default-features = false,
features = ["rustls-tls-native-roots", "http2"], optional = true }
`

### Details
This PR actually introduces two new feature flags: and 'internal'
feature flag `default-engine-base` and the new `default-engine-rustls`.
The former doesn't work on its own, we throw a compile error if you
attempt to use the 'base' feature without either `default-engine` or
`default-engine-rustls`:

```
cargo b -p delta_kernel --features default-engine-base
   Compiling delta_kernel v0.6.0 (/Users/zach.schuermann/dev/delta-kernel-rs2/kernel)
error: The default-engine-base feature flag is not meant to be used directly. Please use either default-engine or default-engine-rustls.
   --> kernel/src/lib.rs:470:1
    |
470 | / compile_error!(
471 | |     "The default-engine-base feature flag is not meant to be used directly. \
472 | |     Please use either default-engine or default-engine-rustls."
473 | | );
    | |_^

```

Lastly, a new crate `feature-tests` in the workspace was created to
ensure that the features work as expected.

### This PR affects the following public APIs
New `default-engine-rustls` feature flag. (New private
`_default-engine-base` feature flag)


## How was this change tested?
New `feature-tests` crate and added to CI.

## Future work
1. this flag isn't exposed in the FFI (just normal `default-engine` is)
2. need to clean up feature flags in general (lots of overlap, etc.)
3. need to add a PresignedUrl test (does not yet exist)

.github/workflows/build.yml

@@ -107,6 +107,10 @@ jobs:
         run: cargo clippy --benches --tests --all-features -- -D warnings
       - name: lint without default features
         run: cargo clippy --no-default-features -- -D warnings
+      - name: check kernel builds with default-engine
+        run: cargo build -p feature_tests --features default-engine
+      - name: check kernel builds with default-engine-rustls
+        run: cargo build -p feature_tests --features default-engine-rustls
   test:
     runs-on: ${{ matrix.os }}
     strategy:

Cargo.toml

@@ -6,6 +6,7 @@ members = [
     "kernel",
     "kernel/examples/*",
     "test-utils",
+    "feature-tests",
 ]
 # Only check / build main crates by default (check all with `--workspace`)
 default-members = ["acceptance", "kernel"]

feature-tests/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "feature_tests"
+edition.workspace = true
+homepage.workspace = true
+keywords.workspace = true
+license.workspace = true
+repository.workspace = true
+readme.workspace = true
+version.workspace = true
+
+[dependencies]
+delta_kernel = { path = "../kernel" }
+
+[features]
+default-engine = [ "delta_kernel/default-engine" ]
+default-engine-rustls = [ "delta_kernel/default-engine-rustls" ]

feature-tests/src/lib.rs

@@ -0,0 +1,12 @@
+/// This is a compilation test to ensure that the default-engine feature flags are working
+/// correctly. Run (from workspace root) with:
+/// 1. `cargo b -p feature_tests --features default-engine-rustls`
+/// 2. `cargo b -p feature_tests --features default-engine`
+/// These run in our build CI.
+pub fn test_default_engine_feature_flags() {
+    #[cfg(any(feature = "default-engine", feature = "default-engine-rustls"))]
+    {
+        #[allow(unused_imports)]
+        use delta_kernel::engine::default::DefaultEngine;
+    }
+}

kernel/Cargo.toml

@@ -54,7 +54,7 @@ hdfs-native-object-store = { workspace = true, optional = true }
 # Used in default and sync engine
 parquet = { workspace = true, optional = true }
 # Used for fetching direct urls (like pre-signed urls)
-reqwest = { version = "0.12.7", optional = true }
+reqwest = { version = "0.12.8", default-features = false, optional = true }
 strum = { version = "0.26", features = ["derive"] }
 
 
@@ -76,7 +76,10 @@ cloud = [
   "hdfs-native-object-store",
 ]
 default = []
-default-engine = [
+
+# this is an 'internal' feature flag which has all the shared bits from default-engine and
+# default-engine-rustls
+default-engine-base = [
   "arrow-conversion",
   "arrow-expression",
   "arrow-array",
@@ -89,12 +92,24 @@ default-engine = [
   "object_store",
   "parquet/async",
   "parquet/object_store",
-  "reqwest",
   "tokio",
   "uuid/v4",
   "uuid/fast-rng",
 ]
 
+# the default-engine use the reqwest crate with default features which uses native-tls. if you want
+# to instead use rustls, use 'default-engine-rustls' which has no native-tls dependency
+default-engine = [
+  "default-engine-base",
+  "reqwest/default",
+]
+
+default-engine-rustls = [
+  "default-engine-base",
+  "reqwest/rustls-tls-native-roots",
+  "reqwest/http2",
+]
+
 developer-visibility = []
 sync-engine = [
   "arrow-cast",

kernel/src/actions/set_transaction.rs

@@ -80,7 +80,7 @@ impl SetTransactionScanner {
     }
 }
 
-#[cfg(all(test, feature = "default-engine"))]
+#[cfg(all(test, feature = "sync-engine"))]
 mod tests {
     use std::path::PathBuf;
 

kernel/src/engine/mod.rs

@@ -7,11 +7,11 @@ pub(crate) mod arrow_conversion;
 
 #[cfg(all(
     feature = "arrow-expression",
-    any(feature = "default-engine", feature = "sync-engine")
+    any(feature = "default-engine-base", feature = "sync-engine")
 ))]
 pub mod arrow_expression;
 
-#[cfg(feature = "default-engine")]
+#[cfg(feature = "default-engine-base")]
 pub mod default;
 
 #[cfg(feature = "sync-engine")]
@@ -25,7 +25,7 @@ macro_rules! declare_modules {
     };
 }
 
-#[cfg(any(feature = "default-engine", feature = "sync-engine"))]
+#[cfg(any(feature = "default-engine-base", feature = "sync-engine"))]
 declare_modules!(
     (pub, arrow_data),
     (pub, parquet_row_group_skipping),

kernel/src/error.rs

@@ -27,7 +27,7 @@ pub enum Error {
     },
 
     /// An error performing operations on arrow data
-    #[cfg(any(feature = "default-engine", feature = "sync-engine"))]
+    #[cfg(any(feature = "default-engine-base", feature = "sync-engine"))]
     #[error(transparent)]
     Arrow(arrow_schema::ArrowError),
 
@@ -75,7 +75,7 @@ pub enum Error {
     #[error("Object store path error: {0}")]
     ObjectStorePath(#[from] object_store::path::Error),
 
-    #[cfg(feature = "default-engine")]
+    #[cfg(any(feature = "default-engine", feature = "default-engine-rustls"))]
     #[error("Reqwest Error: {0}")]
     Reqwest(#[from] reqwest::Error),
 
@@ -303,7 +303,7 @@ from_with_backtrace!(
     (std::io::Error, IOError)
 );
 
-#[cfg(any(feature = "default-engine", feature = "sync-engine"))]
+#[cfg(any(feature = "default-engine-base", feature = "sync-engine"))]
 impl From<arrow_schema::ArrowError> for Error {
     fn from(value: arrow_schema::ArrowError) -> Self {
         Self::Arrow(value).with_backtrace()

kernel/src/lib.rs

@@ -456,3 +456,15 @@ pub trait Engine: AsAny {
     /// Get the connector provided [`ParquetHandler`].
     fn get_parquet_handler(&self) -> Arc<dyn ParquetHandler>;
 }
+
+// we have an 'internal' feature flag: default-engine-base, which is actually just the shared
+// pieces of default-engine and default-engine-rustls. the crate can't compile with _only_
+// default-engine-base, so we give a friendly error here.
+#[cfg(all(
+    feature = "default-engine-base",
+    not(any(feature = "default-engine", feature = "default-engine-rustls",))
+))]
+compile_error!(
+    "The default-engine-base feature flag is not meant to be used directly. \
+    Please use either default-engine or default-engine-rustls."
+);