test(e2e): compare two RUSTFS_S3_STACK servers over every S3 operation (#8435)

Add dual_stack_differential to e2e_test. It starts two rustfs processes
from the same binary, stack A on RUSTFS_S3_STACK=legacy and stack B on
RUSTFS_E2E_STACK_B (default legacy until rustfs/backlog#2734 T2.5),
sends both the same aws-sdk-s3 script covering the 73 methods of
impl S3 for FS (a normal request and a 4xx negative each), and compares
status, error code, an allowlist of response headers and a few typed
body fields. Error message text is never compared. Each run writes
<target>/dual-stack/<run>.json with compared_ops and the (op, field,
a, b) differences; any difference fails the test.

differential_detects_an_injected_difference starts B with
RUSTFS_REGION=eu-west-1 and requires GetBucketLocation to be reported.
The comparator is pure and unit-tested without a server.

Wire the module into the e2e-smoke filter and update the e2e-smoke and
e2e-full selection digests and the e2e_test test-count row.

Refs rustfs/backlog#2740

.config/e2e-full-selection.txt

@@ -1,2 +1,2 @@
-sha256-darwin=98dff0961c6eb21d6128bfd2d4a6100a624ddf1fb7db05dba360a5d3d0a310c2
-sha256-linux=890f5925b87e1bf3710c356253f3b2276d901e156df9ccc04ec9315374de6d22
+sha256-darwin=b9438fd1b82558f1fcd7e5a19dc0ceb0b34b5357d4ce99b7033de3ef019333ab
+sha256-linux=d4622517c13283247a022f49962c2cdb69e5ed0e13874fb55e0ab34fb923c044

.config/e2e-smoke-selection.txt

@@ -1 +1 @@
-sha256=ba8f392b617cf9f2338142233a417e43927b28096635e628233ce8a60cf49d10
+sha256=58df0d240cd74b916ce8f094ed1d8ad3e136f2f9bac157a155192a8786dae65c

.config/nextest.toml

@@ -579,6 +579,14 @@ test-group = 'ecstore-serial-flaky'
 # the PR-lane e2e coverage for scanner usage snapshots consumed by quota and
 # admin surfaces. It uses the same single-node, random-port, isolated-temp-dir
 # fixture as the existing smoke modules.
+#
+# Dual-stack differential (rustfs/backlog#2740): `dual_stack_differential::`
+# starts two single-node servers from the same binary, one per
+# RUSTFS_S3_STACK value, on random ports with isolated temp dirs, and runs one
+# SDK script covering every operation of `impl S3 for FS` against both. Its
+# comparator unit tests need no server. It meets the admission criteria
+# unchanged; the module clause admits a new differential test without another
+# filter edit.
 [profile.e2e-smoke]
 default-filter = """
     package(e2e_test) & (
@@ -586,6 +594,7 @@ default-filter = """
         | test(/^replication_extension_test::(test_replication_check_succeeds_with_remote_target|test_replication_check_rejects_target_without_object_lock|test_set_remote_target_rejects_unversioned_source_bucket|test_replication_check_rejects_unversioned_source_bucket|test_replication_check_rejects_missing_replication_config|test_replication_check_rejects_invalid_bucket|test_set_remote_target_rejects_same_bucket_on_same_deployment|test_set_remote_target_rejects_unversioned_target_bucket|test_set_remote_target_update_requires_arn|test_set_remote_target_update_rejects_missing_target|test_set_remote_target_rejects_invalid_target_url|test_set_remote_target_rejects_self_signed_https_target_without_skip_tls_verify|test_set_remote_target_rejects_private_ca_https_target_without_ca_cert_pem|test_list_remote_targets_rejects_empty_bucket|test_list_remote_targets_rejects_invalid_bucket|test_remove_remote_target_rejects_missing_target|test_remove_remote_target_rejects_missing_arn|test_remove_remote_target_rejects_invalid_bucket|test_remove_remote_target_rejects_target_used_by_replication|test_delete_bucket_replication_removes_remote_target)$/)
         | test(/^reliant::lifecycle::/)
         | test(/^reliant::tiering::/)
+        | test(/^dual_stack_differential::/)
         | test(/^upgrade_compatibility_test::upgrade_write_readiness_tests::/)
         | test(/^on_demand_migration::(get_basic_test::(get_miss_pulls_inline_and_serves_locally_afterwards|head_miss_answers_from_the_source_without_persisting)|interaction_test::test_odm_admin_config_is_redacted_and_status_counts_match_the_source)$/)
     )

.config/test-count-baseline.txt

@@ -9,7 +9,7 @@
 # the PR that adds tests; lowering one is a reviewed statement that tests were
 # intentionally removed. Regenerate with:
 #   scripts/check_test_count_ratchet.sh --update
-e2e_test	870	18
+e2e_test	890	18
 rustfs	5490	62
 rustfs-audit	70	0
 rustfs-checksums	26	0

crates/e2e_test/src/dual_stack_differential/compare.rs

@@ -0,0 +1,512 @@
+// Copyright 2024 RustFS Team
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! What one stack answered, and how two answers are compared.
+//!
+//! An [`Observation`] keeps only what a client acts on: the HTTP status, the S3 error code, the
+//! allowlisted response headers and a few typed fields decoded from the response. The error
+//! message is kept for diagnostics and never compared: message wording is judged by the wire
+//! corpus, not here. The comparator is pure so it can be tested without a server.
+
+use serde::Serialize;
+use std::collections::{BTreeMap, BTreeSet};
+use std::path::{Path, PathBuf};
+
+/// Response headers compared exactly (after [`normalized_header_value`]).
+const HEADER_ALLOWLIST: [&str; 7] = [
+    "content-type",
+    "x-amz-version-id",
+    "etag",
+    "x-amz-delete-marker",
+    "content-range",
+    "accept-ranges",
+    "x-amz-request-charged",
+];
+
+/// Every response header with this prefix is compared as well.
+const HEADER_ALLOWLIST_PREFIX: &str = "x-amz-server-side-encryption";
+
+/// Placeholder for a field one side did not send.
+const ABSENT: &str = "<absent>";
+
+/// The lowercase header name if the comparator looks at this header.
+pub(crate) fn compared_header(name: &str) -> Option<String> {
+    let name = name.to_ascii_lowercase();
+    (HEADER_ALLOWLIST.contains(&name.as_str()) || name.starts_with(HEADER_ALLOWLIST_PREFIX)).then_some(name)
+}
+
+/// A version ID is minted per process, so its value never matches across two servers; only
+/// whether it is the literal `null` (an unversioned write) carries meaning.
+fn normalized_header_value(name: &str, value: &str) -> String {
+    if name == "x-amz-version-id" && value != "null" {
+        "<version-id>".to_owned()
+    } else {
+        value.to_owned()
+    }
+}
+
+/// One request's outcome as a client sees it.
+#[derive(Clone, Debug, Serialize)]
+pub(crate) struct Observation {
+    /// The S3 operation, named as in the `s3s::S3` trait (`GetObject`).
+    pub op: &'static str,
+    /// Which request of the script this was (`normal`, `negative`, or a setup label).
+    pub case: &'static str,
+    /// HTTP status; 0 when no response arrived.
+    pub status: u16,
+    /// The S3 error code, or the SDK's error kind when there was no service error.
+    pub error_code: Option<String>,
+    /// Allowlisted headers only, lowercase names, normalized values.
+    pub headers: BTreeMap<String, String>,
+    /// Typed values decoded from the response body, never free text.
+    pub fields: BTreeMap<String, String>,
+    /// Diagnostic only; never compared.
+    #[serde(skip)]
+    pub message: Option<String>,
+}
+
+impl Observation {
+    /// Builds an observation, keeping only the allowlisted headers.
+    pub(crate) fn new<'h>(
+        op: &'static str,
+        case: &'static str,
+        status: u16,
+        error_code: Option<String>,
+        headers: impl IntoIterator<Item = (&'h str, &'h str)>,
+    ) -> Self {
+        let headers = headers
+            .into_iter()
+            .filter_map(|(name, value)| compared_header(name).map(|name| (name, value)))
+            .map(|(name, value)| {
+                let value = normalized_header_value(&name, value);
+                (name, value)
+            })
+            .collect();
+        Self {
+            op,
+            case,
+            status,
+            error_code,
+            headers,
+            fields: BTreeMap::new(),
+            message: None,
+        }
+    }
+
+    pub(crate) fn with_field(mut self, name: &str, value: impl Into<String>) -> Self {
+        self.fields.insert(name.to_owned(), value.into());
+        self
+    }
+
+    pub(crate) fn is_success(&self) -> bool {
+        (200..300).contains(&self.status)
+    }
+}
+
+/// One field on which the two stacks answered differently.
+#[derive(Clone, Debug, PartialEq, Eq, Serialize)]
+pub(crate) struct Difference {
+    pub op: String,
+    pub case: String,
+    pub field: String,
+    pub a: String,
+    pub b: String,
+}
+
+/// Every difference between two transcripts of the same script, matched by `(op, case)`.
+pub(crate) fn compare(a: &[Observation], b: &[Observation]) -> Vec<Difference> {
+    let mut differences = Vec::new();
+    let (index_a, duplicates_a) = index(a);
+    let (index_b, duplicates_b) = index(b);
+    for (op, case) in duplicates_a.union(&duplicates_b) {
+        let count = |side: &[Observation]| {
+            side.iter()
+                .filter(|observation| (observation.op, observation.case) == (*op, *case))
+                .count()
+                .to_string()
+        };
+        differences.push(difference(op, case, "duplicate_case", count(a), count(b)));
+    }
+    let keys: BTreeSet<_> = index_a.keys().chain(index_b.keys()).copied().collect();
+    for key @ (op, case) in keys {
+        match (index_a.get(&key), index_b.get(&key)) {
+            (Some(a), Some(b)) => compare_one(a, b, &mut differences),
+            (a, b) => {
+                let presence = |side: Option<&&Observation>| if side.is_some() { "present" } else { ABSENT }.to_owned();
+                differences.push(difference(op, case, "observation", presence(a), presence(b)));
+            }
+        }
+    }
+    differences
+}
+
+type Key = (&'static str, &'static str);
+
+fn index(side: &[Observation]) -> (BTreeMap<Key, &Observation>, BTreeSet<Key>) {
+    let mut index = BTreeMap::new();
+    let mut duplicates = BTreeSet::new();
+    for observation in side {
+        let key = (observation.op, observation.case);
+        if index.insert(key, observation).is_some() {
+            duplicates.insert(key);
+        }
+    }
+    (index, duplicates)
+}
+
+fn difference(op: &str, case: &str, field: &str, a: String, b: String) -> Difference {
+    Difference {
+        op: op.to_owned(),
+        case: case.to_owned(),
+        field: field.to_owned(),
+        a,
+        b,
+    }
+}
+
+fn compare_one(a: &Observation, b: &Observation, differences: &mut Vec<Difference>) {
+    let mut field = |name: String, value_a: Option<&str>, value_b: Option<&str>| {
+        if value_a != value_b {
+            let show = |value: Option<&str>| value.unwrap_or(ABSENT).to_owned();
+            differences.push(difference(a.op, a.case, &name, show(value_a), show(value_b)));
+        }
+    };
+    field(
+        "status".to_owned(),
+        Some(a.status.to_string().as_str()),
+        Some(b.status.to_string().as_str()),
+    );
+    field("error_code".to_owned(), a.error_code.as_deref(), b.error_code.as_deref());
+    let headers: BTreeSet<_> = a.headers.keys().chain(b.headers.keys()).collect();
+    for name in headers {
+        field(
+            format!("header.{name}"),
+            a.headers.get(name).map(String::as_str),
+            b.headers.get(name).map(String::as_str),
+        );
+    }
+    let fields: BTreeSet<_> = a.fields.keys().chain(b.fields.keys()).collect();
+    for name in fields {
+        field(
+            format!("body.{name}"),
+            a.fields.get(name).map(String::as_str),
+            b.fields.get(name).map(String::as_str),
+        );
+    }
+}
+
+/// Operations observed on both sides.
+pub(crate) fn compared_ops(a: &[Observation], b: &[Observation]) -> BTreeSet<&'static str> {
+    let ops_b: BTreeSet<_> = b.iter().map(|observation| observation.op).collect();
+    a.iter()
+        .map(|observation| observation.op)
+        .filter(|op| ops_b.contains(op))
+        .collect()
+}
+
+/// `(op, case)` pairs observed on both sides.
+pub(crate) fn compared_cases(a: &[Observation], b: &[Observation]) -> usize {
+    let cases_b: BTreeSet<_> = b.iter().map(|observation| (observation.op, observation.case)).collect();
+    a.iter()
+        .map(|observation| (observation.op, observation.case))
+        .collect::<BTreeSet<_>>()
+        .intersection(&cases_b)
+        .count()
+}
+
+/// The JSON report written for every run.
+#[derive(Debug, Serialize)]
+pub(crate) struct Report {
+    pub schema: u32,
+    pub run: String,
+    pub stack_a: String,
+    pub stack_b: String,
+    pub region_a: Option<String>,
+    pub region_b: Option<String>,
+    pub compared_ops: usize,
+    pub compared_cases: usize,
+    pub differences: Vec<Difference>,
+}
+
+impl Report {
+    /// Writes the report as `<directory>/<run>.json` and returns its path.
+    pub(crate) fn write_to(&self, directory: &Path) -> std::io::Result<PathBuf> {
+        std::fs::create_dir_all(directory)?;
+        let path = directory.join(format!("{}.json", self.run));
+        let json = serde_json::to_vec_pretty(self).map_err(std::io::Error::other)?;
+        std::fs::write(&path, json)?;
+        Ok(path)
+    }
+}
+
+/// One line per difference, for a failing assertion.
+pub(crate) fn render(differences: &[Difference]) -> String {
+    differences
+        .iter()
+        .map(|d| format!("({}, {}, {}, a={:?}, b={:?})", d.op, d.case, d.field, d.a, d.b))
+        .collect::<Vec<_>>()
+        .join("\n")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn ok(op: &'static str) -> Observation {
+        Observation::new(op, "normal", 200, None, [("content-type", "application/xml")])
+    }
+
+    fn fields(differences: &[Difference]) -> Vec<&str> {
+        differences.iter().map(|d| d.field.as_str()).collect()
+    }
+
+    #[test]
+    fn identical_transcripts_have_no_difference() {
+        let a = vec![
+            ok("HeadBucket"),
+            ok("GetBucketLocation").with_field("location_constraint", "us-east-1"),
+        ];
+        let b = a.clone();
+        assert_eq!(compare(&a, &b), Vec::new());
+        assert_eq!(compared_ops(&a, &b).len(), 2);
+    }
+
+    #[test]
+    fn a_status_difference_is_reported() {
+        let a = vec![ok("PutBucketAcl")];
+        let b = vec![Observation::new(
+            "PutBucketAcl",
+            "normal",
+            403,
+            None,
+            [("content-type", "application/xml")],
+        )];
+        assert_eq!(
+            compare(&a, &b),
+            vec![Difference {
+                op: "PutBucketAcl".into(),
+                case: "normal".into(),
+                field: "status".into(),
+                a: "200".into(),
+                b: "403".into(),
+            }]
+        );
+    }
+
+    #[test]
+    fn an_error_code_difference_is_reported() {
+        let a = vec![Observation::new("GetObject", "negative", 404, Some("NoSuchKey".into()), [])];
+        let b = vec![Observation::new(
+            "GetObject",
+            "negative",
+            404,
+            Some("NoSuchBucket".into()),
+            [],
+        )];
+        let differences = compare(&a, &b);
+        assert_eq!(fields(&differences), ["error_code"]);
+        assert_eq!((differences[0].a.as_str(), differences[0].b.as_str()), ("NoSuchKey", "NoSuchBucket"));
+    }
+
+    #[test]
+    fn an_error_code_on_one_side_only_is_reported() {
+        let a = vec![Observation::new("HeadBucket", "negative", 404, Some("NotFound".into()), [])];
+        let b = vec![Observation::new("HeadBucket", "negative", 404, None, [])];
+        let differences = compare(&a, &b);
+        assert_eq!(fields(&differences), ["error_code"]);
+        assert_eq!(differences[0].b, ABSENT);
+    }
+
+    #[test]
+    fn an_allowlisted_header_difference_is_reported() {
+        let a = vec![Observation::new("HeadObject", "normal", 200, None, [("ETag", "\"aa\"")])];
+        let b = vec![Observation::new("HeadObject", "normal", 200, None, [("etag", "\"bb\"")])];
+        assert_eq!(fields(&compare(&a, &b)), ["header.etag"]);
+    }
+
+    #[test]
+    fn a_header_sent_by_one_side_only_is_reported() {
+        let a = vec![Observation::new(
+            "DeleteObject",
+            "normal",
+            204,
+            None,
+            [("x-amz-delete-marker", "true")],
+        )];
+        let b = vec![Observation::new("DeleteObject", "normal", 204, None, [])];
+        let differences = compare(&a, &b);
+        assert_eq!(fields(&differences), ["header.x-amz-delete-marker"]);
+        assert_eq!((differences[0].a.as_str(), differences[0].b.as_str()), ("true", ABSENT));
+    }
+
+    #[test]
+    fn every_server_side_encryption_header_is_compared() {
+        let header = "x-amz-server-side-encryption-customer-key-md5";
+        let a = vec![Observation::new("PutObject", "sse-c", 200, None, [(header, "one")])];
+        let b = vec![Observation::new("PutObject", "sse-c", 200, None, [(header, "two")])];
+        assert_eq!(fields(&compare(&a, &b)), [format!("header.{header}")]);
+    }
+
+    #[test]
+    fn headers_outside_the_allowlist_are_not_compared() {
+        let a = vec![Observation::new(
+            "ListBuckets",
+            "normal",
+            200,
+            None,
+            [
+                ("date", "Mon"),
+                ("x-amz-request-id", "1"),
+                ("server", "a"),
+                ("content-length", "10"),
+            ],
+        )];
+        let b = vec![Observation::new(
+            "ListBuckets",
+            "normal",
+            200,
+            None,
+            [
+                ("date", "Tue"),
+                ("x-amz-request-id", "2"),
+                ("server", "b"),
+                ("content-length", "11"),
+            ],
+        )];
+        assert_eq!(compare(&a, &b), Vec::new());
+        assert!(a[0].headers.is_empty(), "non-allowlisted headers were kept: {:?}", a[0].headers);
+    }
+
+    #[test]
+    fn distinct_version_ids_are_not_a_difference() {
+        let a = vec![Observation::new(
+            "PutObject",
+            "normal",
+            200,
+            None,
+            [("x-amz-version-id", "0b1c")],
+        )];
+        let b = vec![Observation::new(
+            "PutObject",
+            "normal",
+            200,
+            None,
+            [("x-amz-version-id", "9f8e")],
+        )];
+        assert_eq!(compare(&a, &b), Vec::new());
+    }
+
+    #[test]
+    fn a_null_version_id_against_a_real_one_is_reported() {
+        let a = vec![Observation::new(
+            "PutObject",
+            "normal",
+            200,
+            None,
+            [("x-amz-version-id", "null")],
+        )];
+        let b = vec![Observation::new(
+            "PutObject",
+            "normal",
+            200,
+            None,
+            [("x-amz-version-id", "9f8e")],
+        )];
+        assert_eq!(fields(&compare(&a, &b)), ["header.x-amz-version-id"]);
+    }
+
+    #[test]
+    fn a_body_field_difference_is_reported() {
+        let a = vec![ok("GetBucketLocation").with_field("location_constraint", "us-east-1")];
+        let b = vec![ok("GetBucketLocation").with_field("location_constraint", "eu-west-1")];
+        let differences = compare(&a, &b);
+        assert_eq!(fields(&differences), ["body.location_constraint"]);
+        assert_eq!((differences[0].a.as_str(), differences[0].b.as_str()), ("us-east-1", "eu-west-1"));
+    }
+
+    #[test]
+    fn a_body_field_on_one_side_only_is_reported() {
+        let a = vec![ok("ListObjectsV2").with_field("keys", "[]")];
+        let b = vec![ok("ListObjectsV2")];
+        assert_eq!(fields(&compare(&a, &b)), ["body.keys"]);
+    }
+
+    #[test]
+    fn error_messages_are_never_compared() {
+        let mut a = Observation::new("GetObject", "negative", 404, Some("NoSuchKey".into()), []);
+        let mut b = a.clone();
+        a.message = Some("The specified key does not exist.".into());
+        b.message = Some("Object not found".into());
+        assert_eq!(compare(&[a], &[b]), Vec::new());
+    }
+
+    #[test]
+    fn a_case_observed_on_one_side_only_is_reported() {
+        let a = vec![ok("HeadBucket"), ok("ListBuckets")];
+        let b = vec![ok("HeadBucket")];
+        let differences = compare(&a, &b);
+        assert_eq!(fields(&differences), ["observation"]);
+        assert_eq!(differences[0].op, "ListBuckets");
+        assert_eq!(compared_ops(&a, &b).into_iter().collect::<Vec<_>>(), ["HeadBucket"]);
+    }
+
+    #[test]
+    fn a_case_recorded_twice_is_reported() {
+        let a = vec![ok("HeadBucket"), ok("HeadBucket")];
+        let b = vec![ok("HeadBucket")];
+        assert_eq!(fields(&compare(&a, &b)), ["duplicate_case"]);
+    }
+
+    #[test]
+    fn compared_ops_counts_operations_not_cases() {
+        let negative = Observation::new("HeadBucket", "negative", 404, Some("NotFound".into()), []);
+        let a = vec![ok("HeadBucket"), negative, ok("ListBuckets")];
+        let b = a.clone();
+        assert_eq!(compared_ops(&a, &b).len(), 2);
+    }
+
+    #[test]
+    fn the_report_lists_each_difference_as_op_field_a_b() {
+        let directory = std::env::temp_dir().join(format!("dual-stack-report-{}", uuid::Uuid::new_v4()));
+        let report = Report {
+            schema: 1,
+            run: "unit".into(),
+            stack_a: "legacy".into(),
+            stack_b: "legacy".into(),
+            region_a: None,
+            region_b: Some("eu-west-1".into()),
+            compared_ops: 1,
+            compared_cases: 1,
+            differences: compare(
+                &[ok("GetBucketLocation").with_field("location_constraint", "us-east-1")],
+                &[ok("GetBucketLocation").with_field("location_constraint", "eu-west-1")],
+            ),
+        };
+        let path = report.write_to(&directory).expect("write the report");
+        let json: serde_json::Value = serde_json::from_slice(&std::fs::read(&path).expect("read the report")).expect("parse");
+        let _ = std::fs::remove_dir_all(&directory);
+        assert_eq!(path.file_name().and_then(|name| name.to_str()), Some("unit.json"));
+        assert_eq!(json["compared_ops"], 1);
+        assert_eq!(
+            json["differences"],
+            serde_json::json!([{
+                "op": "GetBucketLocation",
+                "case": "normal",
+                "field": "body.location_constraint",
+                "a": "us-east-1",
+                "b": "eu-west-1",
+            }])
+        );
+    }
+}

crates/e2e_test/src/dual_stack_differential/mod.rs

@@ -0,0 +1,197 @@
+// Copyright 2024 RustFS Team
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Dual-stack differential: one `rustfs` binary, two processes, one SDK script (rustfs/backlog#2740).
+//!
+//! Stack A always runs `RUSTFS_S3_STACK=legacy`. Stack B runs the value of `RUSTFS_E2E_STACK_B`
+//! (default `legacy` until the gateway stack is assembled), and `RUSTFS_E2E_STACK_B_REGION`, when
+//! set, starts B in another region so that a difference can be injected by hand. Both servers
+//! answer the script in `scenarios`, which covers every operation of `impl S3 for FS`; `compare`
+//! reports each `(op, field, a, b)` on which the answers differ, and every run writes its report
+//! to `<target>/dual-stack/<run>.json`. Two processes, not two embedded servers: bucket metadata
+//! is process-global, so two stacks in one process would read each other's buckets.
+
+mod compare;
+mod scenarios;
+
+use crate::common::{RustFSTestEnvironment, init_logging, workspace_root};
+use compare::{Difference, Observation, Report, compare, compared_cases, compared_ops, render};
+use std::path::PathBuf;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+type BoxError = Box<dyn std::error::Error + Send + Sync>;
+
+const STACK_A: &str = "legacy";
+const ENV_STACK_B: &str = "RUSTFS_E2E_STACK_B";
+const ENV_STACK_B_REGION: &str = "RUSTFS_E2E_STACK_B_REGION";
+/// Becomes `gateway` once rustfs/backlog#2734 T2.5 assembles the gateway stack.
+const DEFAULT_STACK_B: &str = "legacy";
+/// The region the negative control gives stack A explicitly, so an inherited `RUSTFS_REGION`
+/// cannot erase the injected difference.
+const CONTROL_REGION_A: &str = "us-east-1";
+const CONTROL_REGION_B: &str = "eu-west-1";
+/// `normal` cases the legacy stack must answer with a 2xx. The comparison alone cannot tell a
+/// script that exercises real behavior from one in which both stacks fail the same way. Five of
+/// the 73 fail on legacy by design of this script: PutBucketReplication (no registered remote
+/// target), GetBucketReplication (so no configuration), PutBucketNotificationConfiguration (no
+/// notification target configured), RestoreObject (the object is not archived) and
+/// GetObjectTorrent (legacy answers `NoSuchKey`).
+const MIN_NORMAL_SUCCESSES: usize = 68;
+
+struct PairConfig {
+    stack_b: String,
+    region_a: Option<String>,
+    region_b: Option<String>,
+}
+
+impl PairConfig {
+    fn from_env() -> Self {
+        Self {
+            stack_b: std::env::var(ENV_STACK_B).unwrap_or_else(|_| DEFAULT_STACK_B.to_owned()),
+            region_a: None,
+            region_b: std::env::var(ENV_STACK_B_REGION).ok(),
+        }
+    }
+}
+
+/// Starts one server on `stack`, overriding its region only when asked.
+async fn start_stack(stack: &str, region: Option<&str>) -> Result<RustFSTestEnvironment, BoxError> {
+    let mut env = RustFSTestEnvironment::new().await?;
+    let mut vars = vec![("RUSTFS_S3_STACK", stack)];
+    if let Some(region) = region {
+        vars.push(("RUSTFS_REGION", region));
+    }
+    env.start_rustfs_server_without_cleanup_with_env(&vars).await?;
+    Ok(env)
+}
+
+/// Starts stack A and stack B side by side, each with its own port and data directory.
+async fn spawn_pair(config: &PairConfig) -> Result<(RustFSTestEnvironment, RustFSTestEnvironment), BoxError> {
+    tokio::try_join!(
+        start_stack(STACK_A, config.region_a.as_deref()),
+        start_stack(&config.stack_b, config.region_b.as_deref())
+    )
+}
+
+fn report_directory() -> PathBuf {
+    let target = std::env::var_os("CARGO_TARGET_DIR").map_or_else(|| PathBuf::from("target"), PathBuf::from);
+    let target = if target.is_absolute() {
+        target
+    } else {
+        workspace_root().join(target)
+    };
+    target.join("dual-stack")
+}
+
+struct Run {
+    report: Report,
+    path: PathBuf,
+    a: Vec<Observation>,
+}
+
+/// Runs the script against both stacks concurrently and writes the report.
+async fn run_differential(label: &str, config: &PairConfig) -> Run {
+    let (server_a, server_b) = spawn_pair(config).await.expect("start stack A and stack B");
+    let (a, b) = tokio::join!(Box::pin(scenarios::run(&server_a)), Box::pin(scenarios::run(&server_b)));
+    let millis = SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .map_or(0, |elapsed| elapsed.as_millis());
+    let report = Report {
+        schema: 1,
+        run: format!("{label}-{millis}-{}", std::process::id()),
+        stack_a: STACK_A.to_owned(),
+        stack_b: config.stack_b.clone(),
+        region_a: config.region_a.clone(),
+        region_b: config.region_b.clone(),
+        compared_ops: compared_ops(&a, &b).len(),
+        compared_cases: compared_cases(&a, &b),
+        differences: compare(&a, &b),
+    };
+    let path = report.write_to(&report_directory()).expect("write the dual-stack report");
+    Run { report, path, a }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn both_stacks_answer_every_operation_identically() {
+        init_logging();
+        let config = PairConfig::from_env();
+        let run = run_differential("self-comparison", &config).await;
+        let report = &run.report;
+
+        let mut observed: Vec<_> = run.a.iter().map(|observation| observation.op).collect();
+        observed.sort_unstable();
+        observed.dedup();
+        assert_eq!(
+            observed,
+            scenarios::OPERATIONS,
+            "the script must drive exactly the operations of impl S3 for FS"
+        );
+        assert_eq!(report.compared_ops, scenarios::OPERATIONS.len(), "report: {}", run.path.display());
+        assert!(
+            report.differences.is_empty(),
+            "stack {} and stack {} differ on {} of {} cases (report: {}):\n{}",
+            report.stack_a,
+            report.stack_b,
+            report.differences.len(),
+            report.compared_cases,
+            run.path.display(),
+            render(&report.differences)
+        );
+
+        let not_4xx: Vec<_> = run
+            .a
+            .iter()
+            .filter(|observation| observation.case == "negative" && !(400..500).contains(&observation.status))
+            .collect();
+        assert!(not_4xx.is_empty(), "negative cases must fail with a 4xx on {STACK_A}: {not_4xx:#?}");
+        let successes = run
+            .a
+            .iter()
+            .filter(|observation| observation.case == "normal" && observation.is_success())
+            .count();
+        assert!(
+            successes >= MIN_NORMAL_SUCCESSES,
+            "only {successes} normal cases succeeded on {STACK_A}: {:#?}",
+            run.a
+        );
+    }
+
+    #[tokio::test]
+    async fn differential_detects_an_injected_difference() {
+        init_logging();
+        let config = PairConfig {
+            stack_b: STACK_A.to_owned(),
+            region_a: Some(CONTROL_REGION_A.to_owned()),
+            region_b: Some(CONTROL_REGION_B.to_owned()),
+        };
+        let run = run_differential("injected-region", &config).await;
+        let expected = Difference {
+            op: "GetBucketLocation".to_owned(),
+            case: "normal".to_owned(),
+            field: "body.location_constraint".to_owned(),
+            a: CONTROL_REGION_A.to_owned(),
+            b: CONTROL_REGION_B.to_owned(),
+        };
+        assert!(
+            run.report.differences.contains(&expected),
+            "B started in {CONTROL_REGION_B} but GetBucketLocation was not reported (report: {}):\n{}",
+            run.path.display(),
+            render(&run.report.differences)
+        );
+    }
+}

crates/e2e_test/src/lib.rs

@@ -31,6 +31,12 @@ pub mod chaos;
 #[cfg(test)]
 pub mod fake_s3_target;
 
+// Dual-stack differential (rustfs/backlog#2740): one binary, two processes on
+// different RUSTFS_S3_STACK values, the same SDK script for every S3 operation,
+// compared on status, error code, allowlisted headers and typed body fields.
+#[cfg(test)]
+mod dual_stack_differential;
+
 // On-demand migration (backlog#2147): shared two-server environment, admin
 // wrappers, and the harness self-test (backlog#2151). Behavior scenarios are
 // added by later ODM tasks.