feat: improve stdio handling with async writers (#160)

## Summary

Extends the existing "stdin suggestion" system to a full "stdio" system.
Previously, only stdin was conditionally inherited vs null. Now, all
three stdio FDs (stdin, stdout, stderr) are conditionally inherited vs
piped, and the reporter provides `Box<dyn AsyncWrite + Unpin>` streams
for receiving child process output.

## Changes

### New types and API
- **`StdioSuggestion`** replaces `StdinSuggestion` — two modes: `Piped`
(stdin=null, stdout/stderr piped into AsyncWrite streams) and
`Inherited` (all three inherited from parent)
- **`StdioConfig`** returned by `LeafExecutionReporter::start()` —
contains the suggestion plus `stdout_writer` and `stderr_writer` async
streams
- Removed `stdin_suggestion()` and `output(kind, content)` from
`LeafExecutionReporter` trait
- Removed `OutputKind` from `event.rs` (only `spawn.rs` keeps its own
`OutputKind`)

### Reporter changes
- Removed the `W` generic from all reporters (`PlainReporter`,
`LabeledReporterBuilder`, `LabeledGraphReporter`, `LabeledLeafReporter`,
`SharedReporterState`)
- Display output (command lines, summaries, errors) written directly to
`std::io::stdout()` using sync writes
- `tokio::io::stdout()` / `tokio::io::stderr()` provided as the
`AsyncWrite` streams in `StdioConfig`
- `PlainReporter` returns `tokio::io::sink()` writers when
`silent_if_cache_hit && is_cache_hit`

### Execution flow
- `spawn_with_tracking()` — stdin is always `Stdio::null()`, takes `&mut
dyn AsyncWrite` for stdout/stderr instead of an `on_output` callback
- New `spawn_inherited()` — inherits all three stdio FDs, no fspy
tracking, just `cmd.spawn()` + `wait()`
- Decision matrix: inherited mode only when suggestion is `Inherited`
AND `cache_metadata.is_none()` (caching fully disabled)
- Cache replay writes directly to `StdioConfig` writers
- In-process execution writes to `stdout_writer` from `StdioConfig`

### Test updates
- Updated 8 stdio suggestion unit tests to use `start()` + assert
`StdioConfig.suggestion`
- Updated `task-list` E2E "vp run in script" test: inner `vp run` now
sees a terminal (inherited stdout) and shows interactive selector — test
uses `expect-milestone` + Enter to select and run a task

Author: branchseer

crates/vite_task/Cargo.toml

@@ -31,7 +31,7 @@ serde = { workspace = true, features = ["derive", "rc"] }
 serde_json = { workspace = true }
 smallvec.workspace = true
 thiserror = { workspace = true }
-tokio = { workspace = true, features = ["rt-multi-thread", "io-std", "macros", "sync"] }
+tokio = { workspace = true, features = ["rt-multi-thread", "io-std", "io-util", "macros", "sync"] }
 tracing = { workspace = true }
 twox-hash = { workspace = true }
 vite_glob = { workspace = true }

crates/vite_task/src/session/event.rs

@@ -43,12 +43,6 @@ pub enum ExecutionError {
     PostRunFingerprint(#[source] anyhow::Error),
 }
 
-#[derive(Debug)]
-pub enum OutputKind {
-    Stdout,
-    Stderr,
-}
-
 #[derive(Debug)]
 pub enum CacheDisabledReason {
     InProcessExecution,

crates/vite_task/src/session/execute/mod.rs

@@ -4,22 +4,25 @@ pub mod spawn;
 use std::{process::Stdio, sync::Arc};
 
 use futures_util::FutureExt;
+use tokio::io::AsyncWriteExt as _;
 use vite_path::AbsolutePath;
-use vite_task_plan::{ExecutionGraph, ExecutionItemKind, LeafExecutionKind, SpawnExecution};
+use vite_task_plan::{
+    ExecutionGraph, ExecutionItemKind, LeafExecutionKind, SpawnCommand, SpawnExecution,
+};
 
 use self::{
     fingerprint::PostRunFingerprint,
-    spawn::{OutputKind as SpawnOutputKind, spawn_with_tracking},
+    spawn::{SpawnResult, spawn_with_tracking},
 };
 use super::{
     cache::{CommandCacheValue, ExecutionCache},
     event::{
         CacheDisabledReason, CacheErrorKind, CacheNotUpdatedReason, CacheStatus, CacheUpdateStatus,
-        ExecutionError, OutputKind,
+        ExecutionError,
     },
     reporter::{
         ExitStatus, GraphExecutionReporter, GraphExecutionReporterBuilder, LeafExecutionPath,
-        LeafExecutionReporter, StdinSuggestion,
+        LeafExecutionReporter, StdioSuggestion,
     },
 };
 use crate::{Session, session::execute::spawn::SpawnTrackResult};
@@ -116,10 +119,13 @@ impl ExecutionContext<'_> {
         match leaf_execution_kind {
             LeafExecutionKind::InProcess(in_process_execution) => {
                 // In-process (built-in) commands: caching is disabled, execute synchronously
-                leaf_reporter.start(CacheStatus::Disabled(CacheDisabledReason::InProcessExecution));
+                let mut stdio_config = leaf_reporter
+                    .start(CacheStatus::Disabled(CacheDisabledReason::InProcessExecution));
 
                 let execution_output = in_process_execution.execute();
-                leaf_reporter.output(OutputKind::Stdout, execution_output.stdout.into());
+                // Write output to the stdout writer from StdioConfig
+                let _ = stdio_config.stdout_writer.write_all(&execution_output.stdout).await;
+                let _ = stdio_config.stdout_writer.flush().await;
 
                 leaf_reporter.finish(
                     None,
@@ -147,9 +153,10 @@ impl ExecutionContext<'_> {
 ///
 /// The full lifecycle is:
 /// 1. Cache lookup (determines cache status)
-/// 2. `leaf_reporter.start(cache_status)`
-/// 3. If cache hit: replay cached outputs → finish
-/// 4. If cache miss/disabled: spawn process → stream output → update cache → finish
+/// 2. `leaf_reporter.start(cache_status)` → `StdioConfig`
+/// 3. If cache hit: replay cached outputs via `StdioConfig` writers → finish
+/// 4. If `Inherited` suggestion AND caching disabled: `spawn_inherited()` → finish
+/// 5. Else (piped): `spawn_with_tracking()` with writers → cache update → finish
 ///
 /// Errors (cache lookup failure, spawn failure, cache update failure) are reported
 /// through `leaf_reporter.finish()` and do not abort the caller.
@@ -197,20 +204,20 @@ pub async fn execute_spawn(
         (CacheStatus::Disabled(CacheDisabledReason::NoCacheMetadata), None)
     };
 
-    // 2. Report execution start with the determined cache status
-    leaf_reporter.start(cache_status);
+    // 2. Report execution start with the determined cache status.
+    //    Returns StdioConfig with the reporter's suggestion and async writers.
+    let mut stdio_config = leaf_reporter.start(cache_status);
 
-    // 3. If cache hit, replay outputs and finish early.
+    // 3. If cache hit, replay outputs via the StdioConfig writers and finish early.
     //    No need to actually execute the command — just replay what was cached.
     if let Some(cached) = cached_value {
         for output in cached.std_outputs.iter() {
-            leaf_reporter.output(
-                match output.kind {
-                    SpawnOutputKind::StdOut => OutputKind::Stdout,
-                    SpawnOutputKind::StdErr => OutputKind::Stderr,
-                },
-                output.content.clone().into(),
-            );
+            let writer: &mut (dyn tokio::io::AsyncWrite + Unpin) = match output.kind {
+                spawn::OutputKind::StdOut => &mut stdio_config.stdout_writer,
+                spawn::OutputKind::StdErr => &mut stdio_config.stderr_writer,
+            };
+            let _ = writer.write_all(&output.content).await;
+            let _ = writer.flush().await;
         }
         leaf_reporter.finish(
             None,
@@ -220,41 +227,53 @@ pub async fn execute_spawn(
         return SpawnOutcome::CacheHit;
     }
 
-    // 4. Execute spawn (cache miss or disabled).
-    //    Track file system access if caching is enabled (for future cache updates).
+    // 4. Determine actual stdio mode based on the suggestion AND cache state.
+    //    Inherited stdio is only used when the reporter suggests it AND caching is
+    //    completely disabled (no cache_metadata). If caching is enabled but missed,
+    //    we still need piped mode to capture output for the cache update.
+    let use_inherited =
+        stdio_config.suggestion == StdioSuggestion::Inherited && cache_metadata.is_none();
+
+    if use_inherited {
+        // Inherited mode: all three stdio FDs (stdin, stdout, stderr) are inherited
+        // from the parent process. No fspy tracking, no output capture.
+        // Drop the StdioConfig writers before spawning to avoid holding tokio::io::Stdout
+        // while the child also writes to the same FD.
+        drop(stdio_config);
+
+        match spawn_inherited(&spawn_execution.spawn_command).await {
+            Ok(result) => {
+                leaf_reporter.finish(
+                    Some(result.exit_status),
+                    CacheUpdateStatus::NotUpdated(CacheNotUpdatedReason::CacheDisabled),
+                    None,
+                );
+                return SpawnOutcome::Spawned(result.exit_status);
+            }
+            Err(err) => {
+                leaf_reporter.finish(
+                    None,
+                    CacheUpdateStatus::NotUpdated(CacheNotUpdatedReason::CacheDisabled),
+                    Some(ExecutionError::Spawn(err)),
+                );
+                return SpawnOutcome::Failed;
+            }
+        }
+    }
+
+    // 5. Piped mode: execute spawn with tracking, streaming output to writers.
     let mut track_result_with_cache_metadata =
         cache_metadata.map(|cache_metadata| (SpawnTrackResult::default(), cache_metadata));
 
-    // Determine the child process's stdin mode based on:
-    // - The reporter's suggestion (inherited only when appropriate, e.g., single task)
-    // - Whether caching is disabled (inherited stdin would make output non-deterministic,
-    //   breaking cache semantics)
-    let stdin = if leaf_reporter.stdin_suggestion() == StdinSuggestion::Inherited
-        && cache_metadata.is_none()
-    {
-        Stdio::inherit()
-    } else {
-        Stdio::null()
-    };
-
-    // Execute command with tracking, streaming output in real-time via the reporter
     #[expect(
         clippy::large_futures,
         reason = "spawn_with_tracking manages process I/O and creates a large future"
     )]
     let result = match spawn_with_tracking(
         &spawn_execution.spawn_command,
         cache_base_path,
-        stdin,
-        |kind, content| {
-            leaf_reporter.output(
-                match kind {
-                    SpawnOutputKind::StdOut => OutputKind::Stdout,
-                    SpawnOutputKind::StdErr => OutputKind::Stderr,
-                },
-                content,
-            );
-        },
+        &mut stdio_config.stdout_writer,
+        &mut stdio_config.stderr_writer,
         track_result_with_cache_metadata.as_mut().map(|(track_result, _)| track_result),
     )
     .await
@@ -270,7 +289,7 @@ pub async fn execute_spawn(
         }
     };
 
-    // 5. Update cache if successful and determine cache update status.
+    // 6. Update cache if successful and determine cache update status.
     //    Errors during cache update are terminal (reported through finish).
     let (cache_update_status, cache_error) = if let Some((track_result, cache_metadata)) =
         track_result_with_cache_metadata
@@ -315,14 +334,37 @@ pub async fn execute_spawn(
         (CacheUpdateStatus::NotUpdated(CacheNotUpdatedReason::CacheDisabled), None)
     };
 
-    // 6. Finish the leaf execution with the result and optional cache error.
+    // 7. Finish the leaf execution with the result and optional cache error.
     //    Cache update/fingerprint failures are reported but do not affect the outcome —
     //    the process ran, so we return its actual exit status.
     leaf_reporter.finish(Some(result.exit_status), cache_update_status, cache_error);
 
     SpawnOutcome::Spawned(result.exit_status)
 }
 
+/// Spawn a command with all three stdio file descriptors inherited from the parent.
+///
+/// Used when the reporter suggests inherited stdio AND caching is disabled.
+/// All three FDs (stdin, stdout, stderr) are inherited, allowing interactive input
+/// and direct terminal output. No fspy tracking is performed since there's no
+/// cache to update.
+///
+/// The child process will see `is_terminal() == true` for stdout/stderr when the
+/// parent is running in a terminal. This is expected behavior.
+async fn spawn_inherited(spawn_command: &SpawnCommand) -> anyhow::Result<SpawnResult> {
+    let mut cmd = fspy::Command::new(spawn_command.program_path.as_path());
+    cmd.args(spawn_command.args.iter().map(vite_str::Str::as_str));
+    cmd.envs(spawn_command.all_envs.iter());
+    cmd.current_dir(&*spawn_command.cwd);
+    cmd.stdin(Stdio::inherit()).stdout(Stdio::inherit()).stderr(Stdio::inherit());
+
+    let start = std::time::Instant::now();
+    let mut child = cmd.into_tokio_command().spawn()?;
+    let exit_status = child.wait().await?;
+
+    Ok(SpawnResult { exit_status, duration: start.elapsed() })
+}
+
 impl Session<'_> {
     /// Execute an execution graph, reporting events through the provided reporter builder.
     ///

crates/vite_task/src/session/execute/spawn.rs

@@ -7,11 +7,10 @@ use std::{
 };
 
 use bincode::{Decode, Encode};
-use bstr::BString;
 use fspy::AccessMode;
 use rustc_hash::FxHashSet;
 use serde::Serialize;
-use tokio::io::AsyncReadExt as _;
+use tokio::io::{AsyncReadExt as _, AsyncWrite, AsyncWriteExt as _};
 use vite_path::{AbsolutePath, RelativePathBuf};
 use vite_task_plan::SpawnCommand;
 
@@ -57,28 +56,26 @@ pub struct SpawnTrackResult {
     pub path_writes: FxHashSet<RelativePathBuf>,
 }
 
-/// Spawn a command with file system tracking via fspy.
+/// Spawn a command with file system tracking via fspy, using piped stdio.
 ///
 /// Returns the execution result including captured outputs, exit status,
 /// and tracked file accesses.
 ///
-/// - `stdin` controls the child process's stdin (typically `Stdio::null()` or `Stdio::inherit()`).
-/// - `on_output` is called in real-time as stdout/stderr data arrives.
+/// - stdin is always `/dev/null` (piped mode is for non-interactive execution).
+/// - `stdout_writer`/`stderr_writer` receive the child's stdout/stderr output in real-time.
 /// - `track_result` if provided, will be populated with captured outputs and path accesses for caching. If `None`, tracking is disabled.
+#[expect(clippy::future_not_send, reason = "uses !Send dyn AsyncWrite writers internally")]
 #[expect(
     clippy::too_many_lines,
     reason = "spawn logic is inherently sequential and splitting would reduce clarity"
 )]
-pub async fn spawn_with_tracking<F>(
+pub async fn spawn_with_tracking(
     spawn_command: &SpawnCommand,
     workspace_root: &AbsolutePath,
-    stdin: Stdio,
-    mut on_output: F,
+    stdout_writer: &mut (dyn AsyncWrite + Unpin),
+    stderr_writer: &mut (dyn AsyncWrite + Unpin),
     track_result: Option<&mut SpawnTrackResult>,
-) -> anyhow::Result<SpawnResult>
-where
-    F: FnMut(OutputKind, BString),
-{
+) -> anyhow::Result<SpawnResult> {
     /// The tracking state of the spawned process
     enum TrackingState<'a> {
         /// Tacking is enabled, with the tracked child and result reference
@@ -92,7 +89,7 @@ where
     cmd.args(spawn_command.args.iter().map(vite_str::Str::as_str));
     cmd.envs(spawn_command.all_envs.iter());
     cmd.current_dir(&*spawn_command.cwd);
-    cmd.stdin(stdin).stdout(Stdio::piped()).stderr(Stdio::piped());
+    cmd.stdin(Stdio::null()).stdout(Stdio::piped()).stderr(Stdio::piped());
 
     let mut tracking_state = if let Some(track_result) = track_result {
         // track_result is Some. Spawn with tracking enabled
@@ -122,37 +119,49 @@ where
 
     let start = Instant::now();
 
-    // Helper closure to process output chunks
-    let mut process_output = |kind: OutputKind, content: Vec<u8>| {
-        // Emit event immediately
-        on_output(kind, content.clone().into());
-
-        // Store outputs for caching
-        if let Some(outputs) = &mut outputs {
-            // Merge consecutive outputs of the same kind for caching
-            if let Some(last) = outputs.last_mut()
-                && last.kind == kind
-            {
-                last.content.extend(&content);
-            } else {
-                outputs.push(StdOutput { kind, content });
-            }
-        }
-    };
-
     // Read from both stdout and stderr concurrently using select!
     loop {
         tokio::select! {
             result = child_stdout.read(&mut stdout_buf), if !stdout_done => {
                 match result? {
                     0 => stdout_done = true,
-                    n => process_output(OutputKind::StdOut, stdout_buf[..n].to_vec()),
+                    n => {
+                        let content = stdout_buf[..n].to_vec();
+                        // Write to the async writer immediately
+                        stdout_writer.write_all(&content).await?;
+                        stdout_writer.flush().await?;
+                        // Store outputs for caching
+                        if let Some(outputs) = &mut outputs {
+                            if let Some(last) = outputs.last_mut()
+                                && last.kind == OutputKind::StdOut
+                            {
+                                last.content.extend(&content);
+                            } else {
+                                outputs.push(StdOutput { kind: OutputKind::StdOut, content });
+                            }
+                        }
+                    }
                 }
             }
             result = child_stderr.read(&mut stderr_buf), if !stderr_done => {
                 match result? {
                     0 => stderr_done = true,
-                    n => process_output(OutputKind::StdErr, stderr_buf[..n].to_vec()),
+                    n => {
+                        let content = stderr_buf[..n].to_vec();
+                        // Write to the async writer immediately
+                        stderr_writer.write_all(&content).await?;
+                        stderr_writer.flush().await?;
+                        // Store outputs for caching
+                        if let Some(outputs) = &mut outputs {
+                            if let Some(last) = outputs.last_mut()
+                                && last.kind == OutputKind::StdErr
+                            {
+                                last.content.extend(&content);
+                            } else {
+                                outputs.push(StdOutput { kind: OutputKind::StdErr, content });
+                            }
+                        }
+                    }
                 }
             }
             else => break,

crates/vite_task/src/session/mod.rs

@@ -246,8 +246,7 @@ impl<'a> Session<'a> {
                         .await
                     }
                     Ok(graph) => {
-                        let builder =
-                            LabeledReporterBuilder::new(std::io::stdout(), self.workspace_path());
+                        let builder = LabeledReporterBuilder::new(self.workspace_path());
                         Ok(self
                             .execute_graph(graph, Box::new(builder))
                             .await
@@ -392,7 +391,7 @@ impl<'a> Session<'a> {
 
         let cwd = Arc::clone(&self.cwd);
         let graph = self.plan_from_cli_run(cwd, run_command).await?;
-        let builder = LabeledReporterBuilder::new(std::io::stdout(), self.workspace_path());
+        let builder = LabeledReporterBuilder::new(self.workspace_path());
         Ok(self.execute_graph(graph, Box::new(builder)).await.err().unwrap_or(ExitStatus::SUCCESS))
     }
 
@@ -468,7 +467,7 @@ impl<'a> Session<'a> {
         let cache = self.cache()?;
 
         // Create a plain (standalone) reporter — no graph awareness, no summary
-        let plain_reporter = reporter::PlainReporter::new(std::io::stdout(), silent_if_cache_hit);
+        let plain_reporter = reporter::PlainReporter::new(silent_if_cache_hit);
 
         // Execute the spawn directly using the free function, bypassing the graph pipeline
         match execute::execute_spawn(