feat(vm): capture the payload's output into a host directory

The console is one stream, kernel and guest mixed, so a consumer that
needs the payload's stdout and stderr apart - rhema's sandbox seam
reports them as separate fields - could not get it. With capture set,
the run adds the directory as an internal rw share (guest path
/.ecrou-io, reserved) and the init wraps the payload in a redirection
group pointing at it: the streams land host-side as DIR/stdout and
DIR/stderr, written live, owned by the host user, and there when run()
returns. The console stays on stdout for diagnostics; the CLI exposes
the flag as --capture.

crates/ecrou/src/guest.rs

@@ -18,6 +18,23 @@ pub struct GuestShare {
 /// scratch directory.
 pub const EXIT_FILE: &str = "/.ecrou-exit";
 
+/// The guest path the capture share mounts at, when the run captures
+/// the payload's output. Reserved: no caller share may take it.
+pub const CAPTURE_DIR: &str = "/.ecrou-io";
+/// The payload's stdout, host-side under the capture directory.
+pub const CAPTURE_STDOUT: &str = "/.ecrou-io/stdout";
+/// The payload's stderr, host-side under the capture directory.
+pub const CAPTURE_STDERR: &str = "/.ecrou-io/stderr";
+
+/// Where the init points the payload's stdout and stderr when the run
+/// captures them: guest paths under [`CAPTURE_DIR`], which the caller
+/// reads host-side as files in its capture directory.
+#[derive(Debug, Clone, PartialEq)]
+pub struct GuestCapture {
+    pub stdout: String,
+    pub stderr: String,
+}
+
 /// Shell-quote a string for safe interpolation into the init script.
 /// Single quotes wrap everything; an embedded single quote closes,
 /// escapes, and reopens.
@@ -33,6 +50,7 @@ pub fn init_script(
     payload: &[String],
     workdir: &str,
     hostname: &str,
+    capture: Option<&GuestCapture>,
 ) -> String {
     let mut script = String::from(
         "#!/bin/sh\n\
@@ -75,10 +93,27 @@ pub fn init_script(
         sh_quote(if workdir.is_empty() { "/" } else { workdir })
     ));
 
+    // Captured runs wrap the payload in a redirection group: the
+    // streams cross the capture share as files, and the group's exit
+    // status - the payload's - is what `code=$?` reads below. The
+    // console stays free for kernel and init diagnostics.
+    if capture.is_some() {
+        script.push_str("{ ");
+    }
     for arg in payload {
         script.push_str(&sh_quote(arg));
         script.push(' ');
     }
+    if let Some(cap) = capture {
+        script.push_str(
+            format!(
+                "; }} > {} 2> {} ",
+                sh_quote(&cap.stdout),
+                sh_quote(&cap.stderr)
+            )
+            .as_str(),
+        );
+    }
     script.push_str("\n\ncode=$?\n");
     script.push_str(&format!("echo \"$code\" > {}\n", sh_quote(EXIT_FILE)));
     // When this script exits, the guest init blob reaps it, reports the
@@ -116,7 +151,7 @@ mod tests {
                 read_only: true,
             },
         ];
-        let script = init_script(&shares, &["true".to_string()], "/", "ecrou-vm-x");
+        let script = init_script(&shares, &["true".to_string()], "/", "ecrou-vm-x", None);
         assert!(script.contains("mount_share 'ws0' '/workspace' 'rw'"));
         assert!(script.contains("mount_share 'sk1' '/mnt/skills' 'ro'"));
     }
@@ -128,7 +163,7 @@ mod tests {
             "-c".to_string(),
             "echo $HOME; rm -rf '/'".to_string(),
         ];
-        let script = init_script(&[], &payload, "/", "ecrou-vm-x");
+        let script = init_script(&[], &payload, "/", "ecrou-vm-x", None);
         assert!(script.contains(r"'/bin/bash' '-c' 'echo $HOME; rm -rf '\''/'\''' "));
         // The payload line must appear after the cd and before the code
         // capture, so its exit code is what gets written.
@@ -140,7 +175,7 @@ mod tests {
 
     #[test]
     fn writes_exit_code_then_exits() {
-        let script = init_script(&[], &["true".to_string()], "/work", "ecrou-vm-x");
+        let script = init_script(&[], &["true".to_string()], "/work", "ecrou-vm-x", None);
         assert!(script.contains(&format!("echo \"$code\" > '{}'", EXIT_FILE)));
         // The file write must precede the exit: it is the only
         // diagnostic when a boot dies without the init running.
@@ -156,14 +191,45 @@ mod tests {
 
     #[test]
     fn hostname_lands_in_proc_and_etc() {
-        let script = init_script(&[], &["true".to_string()], "/", "ecrou-vm-x");
+        let script = init_script(&[], &["true".to_string()], "/", "ecrou-vm-x", None);
         assert!(script.contains("echo 'ecrou-vm-x' > /proc/sys/kernel/hostname"));
         assert!(script.contains("echo 'ecrou-vm-x' > /etc/hostname"));
     }
 
+    #[test]
+    fn capture_wraps_the_payload_and_points_at_the_reserved_share() {
+        let cap = GuestCapture {
+            stdout: CAPTURE_STDOUT.into(),
+            stderr: CAPTURE_STDERR.into(),
+        };
+        let script = init_script(
+            &[],
+            &["true".to_string()],
+            "/work",
+            "ecrou-vm-x",
+            Some(&cap),
+        );
+        // The payload line is a redirection group: the group's exit
+        // status, which `code=$?` reads, is the payload's.
+        assert!(script.contains("{ 'true' ; } > '/.ecrou-io/stdout' 2> '/.ecrou-io/stderr' "));
+        let payload_at = script.find("{ 'true'").unwrap();
+        let capture_at = script.find("code=$?").unwrap();
+        assert!(payload_at < capture_at);
+    }
+
+    #[test]
+    fn capture_paths_are_quoted_like_any_operand() {
+        let cap = GuestCapture {
+            stdout: "/.ecrou-io/stdout".into(),
+            stderr: "/.ecrou-io/stderr".into(),
+        };
+        let script = init_script(&[], &["echo it's".to_string()], "/", "x", Some(&cap));
+        assert!(script.contains("echo it'\\''s"));
+    }
+
     #[test]
     fn empty_workdir_falls_back_to_root() {
-        let script = init_script(&[], &["true".to_string()], "", "ecrou-vm-x");
+        let script = init_script(&[], &["true".to_string()], "", "ecrou-vm-x", None);
         assert!(script.contains("cd '/'"));
     }
 }

crates/ecrou/src/vm.rs

@@ -45,6 +45,15 @@ pub struct RunOptions {
     /// How much of the engine's own logging reaches stderr
     pub log_level: LogLevel,
     pub no_cache: bool,
+    /// When set, the payload's stdout and stderr are captured into
+    /// files in this host directory (`stdout` and `stderr` under it):
+    /// the run adds the directory as an internal read-write share,
+    /// mounted in the guest at [`crate::guest::CAPTURE_DIR`], and the
+    /// init points the payload's streams at it. The console stays on
+    /// stdout for diagnostics; the files land host-side as the guest
+    /// writes them and are simply there when `run` returns. A boot
+    /// failure leaves them absent.
+    pub capture: Option<std::path::PathBuf>,
 }
 
 /// How much of the engine's own logging reaches stderr. The engine
@@ -75,6 +84,7 @@ impl Default for RunOptions {
             hostname: String::new(),
             log_level: LogLevel::Normal,
             no_cache: false,
+            capture: None,
         }
     }
 }
@@ -105,7 +115,21 @@ pub fn run(options: &RunOptions, progress: &dyn Progress) -> Result<i32> {
         options.hostname.clone()
     };
 
-    let share_pairs = collect_shares(&rootfs, options).map_err(Error::Other)?;
+    let capture_share = options
+        .capture
+        .as_ref()
+        .map(|dir| {
+            std::fs::create_dir_all(dir)
+                .with_context(|| format!("creating the capture directory {}", dir.display()))?;
+            Ok(Share {
+                host: dir.clone(),
+                guest: guest::CAPTURE_DIR.to_string(),
+                read_only: false,
+            })
+        })
+        .transpose()
+        .map_err(Error::Other)?;
+    let share_pairs = collect_shares(&rootfs, options, capture_share).map_err(Error::Other)?;
     let shares = share_pairs
         .iter()
         .map(|(_, gs)| gs.clone())
@@ -117,7 +141,17 @@ pub fn run(options: &RunOptions, progress: &dyn Progress) -> Result<i32> {
     // contract (it writes the control file), the share mounter, and the
     // hostname applier.
     let init_path = rootfs.dir().join("ecrou-init");
-    let script = guest::init_script(&shares, &payload, &options.workdir, &hostname);
+    let capture = options.capture.as_ref().map(|_| guest::GuestCapture {
+        stdout: guest::CAPTURE_STDOUT.to_string(),
+        stderr: guest::CAPTURE_STDERR.to_string(),
+    });
+    let script = guest::init_script(
+        &shares,
+        &payload,
+        &options.workdir,
+        &hostname,
+        capture.as_ref(),
+    );
     std::fs::write(&init_path, script)
         .with_context(|| format!("writing {}", init_path.display()))?;
     set_exec_bit(&init_path)?;
@@ -344,12 +378,17 @@ fn decode_status(status: libc::c_int, exit_file_host: &Path) -> Result<i32> {
 fn collect_shares(
     rootfs: &PreparedRootfs,
     options: &RunOptions,
+    capture: Option<Share>,
 ) -> anyhow::Result<Vec<(Share, GuestShare)>> {
     let mut all = Vec::new();
     if let Some(ws) = &options.workspace {
         all.push(ws.clone());
     }
     all.extend(options.shares.iter().cloned());
+    // The capture share rides last, after every caller share.
+    if let Some(cap) = capture {
+        all.push(cap);
+    }
 
     let mut out = Vec::with_capacity(all.len());
     for (i, share) in all.iter().enumerate() {