client: Expose a Config constructor to support legacy configurations

The `Config::from_cluster_env` constructor is misleadingly named: it
doesn't use the environment, it uses the default cluster configurations.

This change deprecates the `Config::from_cluster_env` constructor in
favor of `Config::load_in_cluster`. An additional constructor,
`Config::load_in_cluster_from_legacy_env`, uses the
`KUBERNETES_SERVICE_HOST` and `KUBERNETES_SERVICE_PORT` environment
variables to match client-go's behavior.

This changes does NOT alter the default inferred configuration in any
way. It simply allows users to opt-in to using the old behavior.

Related to kubernetes/kubernetes#112263
Closes kube-rs#1000

Signed-off-by: Oliver Gould <ver@buoyant.io>

.devcontainer/devcontainer.json

@@ -8,7 +8,7 @@
 
 	// These extensions are loaded for all users by default.
 	"extensions": [
-		"matklad.rust-analyzer",
+		"rust-lang.rust-analyzer",
 		"NathanRidley.autotrim",
 		"samverschueren.final-newline",
 		"tamasfe.even-better-toml",

kube-client/src/config/incluster_config.rs

@@ -16,6 +16,10 @@ pub enum Error {
     #[error("failed to read a certificate bundle: {0}")]
     ReadCertificateBundle(#[source] std::io::Error),
 
+    /// Failed to parse cluster port value
+    #[error("failed to parse cluster port: {0}")]
+    ParseClusterPort(#[source] std::num::ParseIntError),
+
     /// Failed to parse cluster url
     #[error("failed to parse cluster url: {0}")]
     ParseClusterUrl(#[source] http::uri::InvalidUri),
@@ -29,6 +33,45 @@ pub fn kube_dns() -> http::Uri {
     http::Uri::from_static("https://kubernetes.default.svc/")
 }
 
+pub fn try_kube_from_legacy_env_or_dns() -> Result<http::Uri, Error> {
+    // client-go requires that both environment variables are set, so we do too.
+    let host = match std::env::var("KUBERNETES_SERVICE_HOST") {
+        Ok(h) => h,
+        Err(_) => return Ok(kube_dns()),
+    };
+    let port = match std::env::var("KUBERNETES_SERVICE_PORT") {
+        Ok(p) => p.parse::<u16>().map_err(Error::ParseClusterPort)?,
+        Err(_) => return Ok(kube_dns()),
+    };
+
+    // Format a host and, if not using 443, a port.
+    //
+    // Ensure that IPv6 addresses are properly bracketed.
+    let uri = match host.parse::<std::net::IpAddr>() {
+        Ok(ip) => {
+            if port == 443 {
+                if ip.is_ipv6() {
+                    format!("https://[{ip}]")
+                } else {
+                    format!("https://{ip}")
+                }
+            } else {
+                let addr = std::net::SocketAddr::new(ip, port);
+                format!("https://{addr}")
+            }
+        }
+        Err(_) => {
+            if port == 443 {
+                format!("https://{host}")
+            } else {
+                format!("https://{host}:{port}")
+            }
+        }
+    };
+
+    uri.parse().map_err(Error::ParseClusterUrl)
+}
+
 pub fn token_file() -> String {
     SERVICE_TOKENFILE.to_owned()
 }

kube-client/src/config/mod.rs

@@ -195,7 +195,7 @@ impl Config {
                     "no local config found, falling back to local in-cluster config"
                 );
 
-                Self::from_cluster_env().map_err(|in_cluster_err| InferConfigError {
+                Self::load_in_cluster().map_err(|in_cluster_err| InferConfigError {
                     in_cluster: in_cluster_err,
                     kubeconfig: kubeconfig_err,
                 })?
@@ -206,13 +206,42 @@ impl Config {
         Ok(config)
     }
 
-    /// Create configuration from the cluster's environment variables
-    ///
-    /// This follows the standard [API Access from a Pod](https://kubernetes.io/docs/tasks/access-application-cluster/access-cluster/#accessing-the-api-from-a-pod)
-    /// and relies on you having the service account's token mounted,
-    /// as well as having given the service account rbac access to do what you need.
+    /// Replaced by [`Self::load_in_cluster`].
+    #[deprecated(since = "0.75.0", note = "use Config::load_in_cluster")]
     pub fn from_cluster_env() -> Result<Self, InClusterError> {
-        let cluster_url = incluster_config::kube_dns();
+        Self::load_in_cluster()
+    }
+
+    /// Load the default in-cluster config.
+    ///
+    /// This follows the standard [API Access from a Pod][docs] policy of using
+    /// `https://kubernetes.default.svc` to connect to the Kubernetes API
+    /// server. A service account's token must be available in
+    /// `/var/run/secrets/kubernetes.io/serviceaccount/`.
+    ///
+    /// [docs]: https://kubernetes.io/docs/tasks/access-application-cluster/access-cluster/#accessing-the-api-from-a-pod
+    pub fn load_in_cluster() -> Result<Self, InClusterError> {
+        Self::load_in_cluster_with_uri(incluster_config::kube_dns())
+    }
+
+    /// Load an in-cluster config use the legacy `KUBERNETES_SERVICE_HOST` and
+    /// `KUBERNETES_SERVICE_PORT` environment variables.
+    ///
+    /// This matches the behavior of [client-go], but it will fallback to using
+    /// `kubernetes.default.svc` if the environment is not set.
+    ///
+    /// A service account's token must be available in
+    /// `/var/run/secrets/kubernetes.io/serviceaccount/`.
+    ///
+    /// This will be deprecated/removed when client-go's behavior changes.
+    ///
+    /// [client-go]: https://github.com/kubernetes/kubernetes/blob/67bde9a1023d1805e33d698b28aa6fad991dfb39/staging/src/k8s.io/client-go/rest/config.go#L507-L541
+    pub fn load_in_cluster_from_legacy_env() -> Result<Self, InClusterError> {
+        let uri = incluster_config::try_kube_from_legacy_env_or_dns()?;
+        Self::load_in_cluster_with_uri(uri)
+    }
+
+    fn load_in_cluster_with_uri(cluster_url: http::uri::Uri) -> Result<Self, InClusterError> {
         let default_namespace = incluster_config::load_default_ns()?;
         let root_cert = incluster_config::load_cert()?;
 
@@ -378,7 +407,6 @@ pub use file_config::{
     NamedContext, NamedExtension, Preferences,
 };
 
-
 #[cfg(test)]
 mod tests {
     #[cfg(not(feature = "client"))] // want to ensure this works without client features