Expand skipped headers for sigv4 canonical request signing to include x-amzn-trace-id and authorization headers. (#2815)

## Motivation and Context
When customers add x-ray headers to requests, the SigV4 signer should
exclude them, or the generated canonical signature will not match the
remote service's, since many services being called are written with
non-rust SDKs that automatically exclude these common headers.

The Rust SDK should exclude a similar set of headers to the other
popular AWS SDKs. While this is not uniform across the SDKs, a minimal
set should be excluded and others should be considered to be excluded in
future PRs.

## Description

* Expands the set of headers excluded from canonical request calculation
to include "x-amzn-trace-id" and "authorization" (since authorization
will be added as a part of this process).

## Testing

* Added headers to exclusion test & validated with `cargo test`
* `./gradlew :aws:sdk:test`  

## Checklist
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

Co-authored-by: Sam Bartlett <sbartl@amazon.com>
Co-authored-by: Zelda Hessler <zhessler@amazon.com>

Author: 3 people

CHANGELOG.next.toml

@@ -11,6 +11,12 @@
 # meta = { "breaking" = false, "tada" = false, "bug" = false, "target" = "client | server | all"}
 # author = "rcoh"
 
+ [[aws-sdk-rust]]
+ message = "Automatically exclude X-Ray trace ID headers and authorization headers from SigV4 canonical request calculations."
+ references = ["smithy-rs#2815"]
+ meta = { "breaking" = false, "tada" = false, "bug" = true }
+ author = "relevantsam"
+
  [[aws-sdk-rust]]
  message = "Add accessors to Builders"
  references = ["smithy-rs#2791"]

aws/rust-runtime/aws-sigv4/src/http_request/canonical_request.rs

@@ -774,14 +774,46 @@ mod tests {
         assert_eq!(creq.values.signed_headers().as_str(), "host;x-amz-date");
     }
 
-    // It should exclude user-agent and x-amz-user-agent headers from presigning
+    // It should exclude authorization, user-agent, x-amzn-trace-id headers from presigning
+    #[test]
+    fn non_presigning_header_exclusion() {
+        let request = http::Request::builder()
+            .uri("https://some-endpoint.some-region.amazonaws.com")
+            .header("authorization", "test-authorization")
+            .header("content-type", "application/xml")
+            .header("content-length", "0")
+            .header("user-agent", "test-user-agent")
+            .header("x-amzn-trace-id", "test-trace-id")
+            .header("x-amz-user-agent", "test-user-agent")
+            .body("")
+            .unwrap();
+        let request = SignableRequest::from(&request);
+
+        let settings = SigningSettings {
+            signature_location: SignatureLocation::Headers,
+            ..Default::default()
+        };
+
+        let signing_params = signing_params(settings);
+        let canonical = CanonicalRequest::from(&request, &signing_params).unwrap();
+
+        let values = canonical.values.as_headers().unwrap();
+        assert_eq!(
+            "content-length;content-type;host;x-amz-date;x-amz-user-agent",
+            values.signed_headers.as_str()
+        );
+    }
+
+    // It should exclude authorization, user-agent, x-amz-user-agent, x-amzn-trace-id headers from presigning
     #[test]
     fn presigning_header_exclusion() {
         let request = http::Request::builder()
             .uri("https://some-endpoint.some-region.amazonaws.com")
+            .header("authorization", "test-authorization")
             .header("content-type", "application/xml")
             .header("content-length", "0")
             .header("user-agent", "test-user-agent")
+            .header("x-amzn-trace-id", "test-trace-id")
             .header("x-amz-user-agent", "test-user-agent")
             .body("")
             .unwrap();

aws/rust-runtime/aws-sigv4/src/http_request/settings.rs

@@ -3,12 +3,14 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-use http::header::{HeaderName, USER_AGENT};
+use http::header::{HeaderName, AUTHORIZATION, USER_AGENT};
 use std::time::Duration;
 
 /// HTTP signing parameters
 pub type SigningParams<'a> = crate::SigningParams<'a, SigningSettings>;
 
+const HEADER_NAME_X_RAY_TRACE_ID: &str = "x-amzn-trace-id";
+
 /// HTTP-specific signing settings
 #[derive(Debug, PartialEq)]
 #[non_exhaustive]
@@ -97,15 +99,30 @@ pub enum SessionTokenMode {
 
 impl Default for SigningSettings {
     fn default() -> Self {
-        // The user agent header should not be signed because it may be altered by proxies
-        const EXCLUDED_HEADERS: [HeaderName; 1] = [USER_AGENT];
-
+        // Headers that are potentially altered by proxies or as a part of standard service operations.
+        // Reference:
+        // Go SDK: <https://github.com/aws/aws-sdk-go/blob/v1.44.289/aws/signer/v4/v4.go#L92>
+        // Java SDK: <https://github.com/aws/aws-sdk-java-v2/blob/master/core/auth/src/main/java/software/amazon/awssdk/auth/signer/internal/AbstractAws4Signer.java#L70>
+        // JS SDK: <https://github.com/aws/aws-sdk-js/blob/master/lib/signers/v4.js#L191>
+        // There is no single source of truth for these available, so this uses the minimum common set of the excluded options.
+        // Instantiate this every time, because SigningSettings takes a Vec (which cannot be const);
+        let excluded_headers = Some(
+            [
+                // This header is calculated as part of the signing process, so if it's present, discard it
+                AUTHORIZATION,
+                // Changes when sent by proxy
+                USER_AGENT,
+                // Changes based on the request from the client
+                HeaderName::from_static(HEADER_NAME_X_RAY_TRACE_ID),
+            ]
+            .to_vec(),
+        );
         Self {
             percent_encoding_mode: PercentEncodingMode::Double,
             payload_checksum_kind: PayloadChecksumKind::NoHeader,
             signature_location: SignatureLocation::Headers,
             expires_in: None,
-            excluded_headers: Some(EXCLUDED_HEADERS.to_vec()),
+            excluded_headers,
             uri_path_normalization_mode: UriPathNormalizationMode::Enabled,
             session_token_mode: SessionTokenMode::Include,
         }