test(e2e): pin GET codec-streaming body/header parity vs legacy duplex (backlog#1183) (#4745)

backlog#1183 tracks flipping the default GET data path from the legacy
tokio::io::duplex double-copy to the zero-duplex codec-streaming fast path.
That flip is gated behind RUSTFS_GET_CODEC_STREAMING_BODY_COMPAT_CONFIRMED /
..._HEADER_COMPAT_CONFIRMED, which need empirical evidence the two paths are
byte- and header-identical.

Add an e2e regression net that runs the same object matrix twice against the
same on-disk EC shards, changing only the codec-streaming env gates: phase A
(default) takes the legacy duplex path, phase B (gates opened) takes codec
streaming. It asserts byte-for-byte (sha256) and header-for-header equality
across inline / small / multi-block (1.5M/3M/5M+) / multipart objects, plus a
ranged GET (which falls back to legacy by gate design). Path confirmation is
not assumed: the legacy path logs "Created duplex pipe ..." per full GET, so
the test counts that marker per phase and asserts the codec phase created zero
duplex pipes, proving the fast path actually ran rather than silently falling
back to the path it is compared against.

To capture the child server's logs for that assertion, add an optional
RustFSTestEnvironment.capture_log_path (default None = inherit stdio,
backward compatible) that redirects the spawned server's stdout+stderr to a file.

Co-authored-by: heihutu <heihutu@gmail.com>

crates/e2e_test/src/common.rs

@@ -28,7 +28,7 @@ use reqwest::Client as HttpClient;
 use std::ffi::OsStr;
 use std::fs as stdfs;
 use std::path::{Path, PathBuf};
-use std::process::{Child, Command};
+use std::process::{Child, Command, Stdio};
 use std::sync::Once;
 use std::time::Duration;
 use tokio::fs;
@@ -293,6 +293,10 @@ pub struct RustFSTestEnvironment {
     pub access_key: String,
     pub secret_key: String,
     pub process: Option<Child>,
+    /// When set, the spawned server's stdout+stderr are redirected (append) to
+    /// this file instead of being inherited by the test process. Lets a test
+    /// grep the child's logs (e.g. to confirm which GET reader path ran).
+    pub capture_log_path: Option<String>,
 }
 
 impl RustFSTestEnvironment {
@@ -313,6 +317,7 @@ impl RustFSTestEnvironment {
             access_key: DEFAULT_ACCESS_KEY.to_string(),
             secret_key: DEFAULT_SECRET_KEY.to_string(),
             process: None,
+            capture_log_path: None,
         })
     }
 
@@ -330,6 +335,7 @@ impl RustFSTestEnvironment {
             access_key: DEFAULT_ACCESS_KEY.to_string(),
             secret_key: DEFAULT_SECRET_KEY.to_string(),
             process: None,
+            capture_log_path: None,
         })
     }
 
@@ -398,6 +404,13 @@ impl RustFSTestEnvironment {
         for (key, value) in extra_env {
             command.env(key, value);
         }
+        // Optionally capture the child's stdout+stderr to a file so the test can
+        // grep server logs (e.g. to confirm which GET reader path was taken).
+        if let Some(log_path) = &self.capture_log_path {
+            let file = stdfs::OpenOptions::new().create(true).append(true).open(log_path)?;
+            let stderr_file = file.try_clone()?;
+            command.stdout(Stdio::from(file)).stderr(Stdio::from(stderr_file));
+        }
         let process = command.args(&args).spawn()?;
 
         self.process = Some(process);