Improve error messages when server misconfigured (#1520)

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Bug fixes and new features should include tests.
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## Motivation

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problem
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use case that feature fulfills.
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Relates to #1435

This PR is trying out a way to improve the error message shown when
something is misconfigured.

Currently running the server binary (`cargo run` in the `svix-server/`
folder) panics when some needed config is missing.

e.g. `listen_address` not set
```
thread 'main' panicked at svix-server/src/cfg.rs:550:10:
Error loading configuration: Error { tag: Tag::Default, profile: Some(Profile(Uncased { string: "default" })), metadata: None, path: [], kind: MissingField("listen_address"), prev: None }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

e.g. `jwt_secret` not set - in this case the error message doesn't
indicate the missing field due to an issue with it coming from an
inlined struct inside `ConfigurationInner` -
<SergioBenitez/Figment#80>
```
thread 'main' panicked at svix-server/src/cfg.rs:550:10:
Error loading configuration: Error { tag: Tag::Default, profile: Some(Profile(Uncased { string: "default" })), metadata: None, path: [], kind: Message("data did not match any variant of untagged enum JwtSigningConfig"), prev: None }
```

## Solution

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the code change.
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If `cfg::load()` errors out, terminate with exit code 1 and print the
formatted error to stderr. I tried switching to using `anyhow::Result`
for `main()` and `cfg::load()` to make propagating errors with human
friendly context easier.

```
Error: failed to extract configuration

Caused by:
    missing field `listen_address`
```

Check upfront that `jwt_secret` is set and override the error message to
be more helpful due to
<SergioBenitez/Figment#80>

```
Error: failed to extract configuration

Caused by:
    missing field `jwt_secret`
```

server/svix-server/Cargo.toml

@@ -73,12 +73,12 @@ omniqueue = { git = "https://github.com/svix/omniqueue-rs", rev = "75e5a9510ad33
 # Switch to hyper-http-proxy when upgrading hyper to 1.0.
 hyper-proxy = { version = "=0.9.1", default-features = false, features = ["openssl-tls"] }
 hex = "0.4.3"
+anyhow = "1.0.56"
 
 [target.'cfg(not(target_env = "msvc"))'.dependencies]
 tikv-jemallocator = { version = "0.5", optional = true }
 
 [dev-dependencies]
-anyhow = "1.0.56"
 assert_matches = "1.5.0"
 # NOTE: Purposely not the latest version such as not to mess up the `hyper` fork patch
 axum-server = { version = "0.5", features = ["tls-openssl"] }

server/svix-server/src/cfg.rs

@@ -3,6 +3,7 @@
 
 use std::{borrow::Cow, collections::HashMap, fmt, net::SocketAddr, sync::Arc, time::Duration};
 
+use anyhow::{bail, Context};
 use figment::{
     providers::{Env, Format, Toml},
     Figment,
@@ -533,7 +534,20 @@ impl From<SentinelConfig> for omniqueue::backends::redis::SentinelConfig {
     }
 }
 
-pub fn load() -> Result<Arc<ConfigurationInner>> {
+/// Try to extract a [`ConfigurationInner`] from the provided [`Figment`]. Any error message should
+/// indicate the missing required field(s).
+fn try_extract(figment: Figment) -> anyhow::Result<ConfigurationInner> {
+    // Explicitly override error if `jwt_secret` is not set, as the default error does not mention
+    // the field name due it coming from an inlined field `ConfigurationInner::jwt_signing_config`
+    // See: <https://github.com/SergioBenitez/Figment/issues/80>
+    if !figment.contains("jwt_secret") {
+        bail!("missing field `jwt_secret`");
+    }
+
+    Ok(figment.extract()?)
+}
+
+pub fn load() -> anyhow::Result<Arc<ConfigurationInner>> {
     if let Ok(db_url) = std::env::var("DATABASE_URL") {
         // If we have DATABASE_URL set, we should potentially use it.
         const DB_DSN: &str = "SVIX_DB_DSN";
@@ -542,14 +556,16 @@ pub fn load() -> Result<Arc<ConfigurationInner>> {
         }
     }
 
-    let config: ConfigurationInner = Figment::new()
+    let merged = Figment::new()
         .merge(Toml::string(DEFAULTS))
         .merge(Toml::file("config.toml"))
-        .merge(Env::prefixed("SVIX_"))
-        .extract()
-        .expect("Error loading configuration");
+        .merge(Env::prefixed("SVIX_"));
+
+    let config = try_extract(merged).context("failed to extract configuration")?;
 
-    config.validate().expect("Error validating configuration");
+    config
+        .validate()
+        .context("failed to validate configuration")?;
     Ok(Arc::from(config))
 }
 
@@ -562,7 +578,7 @@ mod tests {
         Figment,
     };
 
-    use super::{load, CacheBackend, CacheType, QueueBackend, QueueType};
+    use super::{load, try_extract, CacheBackend, CacheType, QueueBackend, QueueType};
     use crate::core::security::{JWTAlgorithm, JwtSigningConfig};
 
     #[test]
@@ -582,6 +598,16 @@ mod tests {
         assert_eq!(cfg.cache_backend(), CacheBackend::Redis("test_b"));
     }
 
+    #[test]
+    fn test_try_extract_missing_jwt_secret() {
+        let defaults = Figment::new();
+
+        let actual = try_extract(defaults);
+
+        let err = actual.unwrap_err();
+        assert_eq!(err.to_string(), "missing field `jwt_secret`");
+    }
+
     #[test]
     fn test_jwt_signing_fallback() {
         let raw_config = r#"

server/svix-server/src/main.rs

@@ -4,6 +4,7 @@
 #![warn(clippy::all)]
 #![forbid(unsafe_code)]
 
+use anyhow::bail;
 use clap::{Parser, Subcommand};
 use dotenvy::dotenv;
 use svix_server::{
@@ -108,11 +109,11 @@ fn org_id_parser(s: &str) -> Result<OrganizationId, String> {
 }
 
 #[tokio::main]
-async fn main() {
+async fn main() -> anyhow::Result<()> {
     dotenv().ok();
 
     let args = Args::parse();
-    let cfg = cfg::load().expect("Error loading configuration");
+    let cfg = cfg::load()?;
 
     let (tracing_subscriber, _guard) = setup_tracing(&cfg, /* for_test = */ false);
     tracing_subscriber.init();
@@ -136,8 +137,7 @@ async fn main() {
         }
 
         if let Err(e) = futures::future::try_join_all(wait_for).await {
-            tracing::error!("{e}");
-            return;
+            bail!(e);
         }
     }
 
@@ -201,4 +201,5 @@ async fn main() {
     };
 
     opentelemetry::global::shutdown_tracer_provider();
+    Ok(())
 }