fix(shell): drain stdout before reading output to avoid "(no output)" on exit 0 by chethanuk · Pull Request #2 · chethanuk/opencode

chethanuk · 2026-07-06T07:30:39Z

Issue for this PR

Closes anomalyco#35511 (staged on fork against dev).

Type of change

Bug fix
New feature
Refactor / code improvement
Documentation

What does this PR do?

The shell tool can return (no output) with exit 0 on the first call, even when the command clearly printed to stdout.

Root cause. The merged stdout+stderr reader runs in a forkScoped fiber, but the main flow only awaits handle.exitCode. When the process exits, the enclosing Effect.scoped block closes and interrupts the reader fiber before it has drained the buffered chunks to EOF, so the assembled list is empty and the tool reports (no output) despite a clean exit.

Fix. Capture the reader fiber and, on the clean-exit branch, Fiber.join it so the merged stream drains to EOF before the scope closes. A backgrounded child can keep the stdout/stderr write end open past the parent's exit, so the join is bounded by the same abort/timeout races to avoid hanging forever. Because the process already exited cleanly on this branch, a cut-short drain is not relabelled as aborted/terminated — that would contradict the clean exit code and print a misleading "terminated after exceeding timeout" message.

Ordering that the fix guarantees:

sequenceDiagram
    participant Tool as ShellTool
    participant Proc as Process
    participant Reader as reader fiber (forkScoped)
    participant Scope as Effect.scoped

    Tool->>Proc: spawn(command)
    Tool->>Reader: forkScoped(Stream.runForEach handle.all)
    Tool->>Tool: raceAll [exitCode, abort, timeout]
    Proc-->>Tool: exitCode resolves (exit.kind = exit)
    Note over Tool,Reader: Fiber.join(reader), bounded by abort/timeout
    Reader-->>Tool: buffered stdout/stderr drained to EOF
    Tool->>Scope: scope closes (reader already finished)
    Note over Tool: output assembled from full list
    Note over Tool,Scope: Before: scope closed on exit, reader interrupted mid-buffer -> "(no output)"

How did you verify your code works?

bun run typecheck (tsgo, packages/opencode) — passes.
bun test test/tool/shell.test.ts — 24 pass / 0 fail. Includes a new regression that runs echo hi 10x and asserts exit 0 with output (and metadata.output) never equal to (no output); this reliably reproduced the race before the fix.

Screenshots / recordings

Not a UI change.

Checklist

I have tested my changes locally
I have not included unrelated changes in this PR

codeant-ai · 2026-07-06T07:30:43Z

CodeAnt AI is reviewing your PR.

codeant-ai · 2026-07-06T07:30:44Z

Thanks for using CodeAnt! 🎉

We're free for open-source projects. if you're enjoying it, help us grow by sharing.

Share on X ·
Reddit ·
LinkedIn

coderabbitai · 2026-07-06T07:30:47Z

Warning

Review limit reached

@chethanuk, you've reached your PR review limit, so we couldn't start this review.

Next review available in: 36 minutes

Enable usage-based reviews in Billing to review now. Otherwise, wait until the next included review is available.
You're only billed for reviews past your plan's rate limits ($0.25/file).

How can I continue?

After more reviews become available, a review can be triggered using the @coderabbitai review command as a PR comment. Alternatively, push new commits to this PR.

To avoid repeated limits, reduce automatic review volume by pausing incremental auto-reviews earlier, using label-based review opt-in, excluding WIP or generated PR titles, or requesting reviews manually when the PR is ready. If your team needs uninterrupted high-volume reviews, an organization admin can enable usage-based reviews.

How do review limits work?

CodeRabbit enforces per-developer PR review limits for each organization. Most developers receive the normal plan review availability.

For paid Pro and Pro+ PR reviews, CodeRabbit uses adaptive limits for sustained high-volume activity. When a developer's recent PR review activity reaches the 95th percentile or higher among CodeRabbit users, additional reviews become available more gradually as earlier reviews age out of the rolling window.

Please refer docs for additional details.

Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 8f170095-5258-462e-8816-bae13e0c3859

📥 Commits

Reviewing files that changed from the base of the PR and between 3a149ba and 13c9c38.

📒 Files selected for processing (2)

packages/opencode/src/tool/shell.ts
packages/opencode/test/tool/shell.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

Create PR with unit tests
Commit unit tests in branch fix/issue-35511-shell-no-output-race

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

_{Comment @coderabbitai help to get the list of available commands.}

gemini-code-assist

Code Review

This pull request addresses issue anomalyco#35511 by capturing the reader fiber and joining it on a clean exit to ensure that the merged stdout/stderr stream drains to EOF before the scope closes, preventing lost buffered output. A regression test is also added to verify this behavior. The reviewer pointed out a potential issue where the tool could hang indefinitely if a background process keeps the stdout/stderr streams open, since Fiber.join(reader) is called outside of the race block. They suggested racing the join with abort and timeout effects to prevent this hang.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.

greptile-apps · 2026-07-06T07:33:22Z

Greptile Summary

This PR fixes the (no output) regression on exit-0 shell commands by capturing the forkScoped reader fiber and joining it before the Effect.scoped block closes, ensuring buffered stdout/stderr chunks are fully drained. A three-way race (join vs abort vs fresh timeout) guards the drain phase against backgrounded children holding the pipe open indefinitely.

shell.ts: Captures the reader fiber, adds a post-exit drain step with a Fiber.join raced against abort and a fresh timeout; sets aborted/expired flags if the drain is cut short.
shell.test.ts: Adds a 10-iteration regression loop asserting both result.output and result.metadata.output contain the echoed text, catching the original race reliably.

Confidence Score: 4/5

The core drain fix is sound, but setting expired = true when only the post-exit drain times out causes a false "shell tool terminated command" message for a process that exited cleanly — worth addressing before merging.

The drain-timeout path sets the same expired flag that the kill-on-timeout path uses, so the tool emits a misleading "terminated command after exceeding timeout" message when a backgrounded child holds the pipe open past the drain window. The process exited cleanly and was never killed, but the metadata tells the caller otherwise.

packages/opencode/src/tool/shell.ts — specifically the if (drained === "timeout") expired = true line inside the drain block.

Important Files Changed

Filename	Overview
packages/opencode/src/tool/shell.ts	Captures the `forkScoped` reader fiber and joins it on clean exit to drain buffered output; introduces a three-way race (join vs abort vs fresh timeout) for the drain phase, but reuses the `expired` flag when the drain times out, which causes a misleading "terminated command" metadata message for a process that exited cleanly.
packages/opencode/test/tool/shell.test.ts	Adds a 10-iteration regression test for anomalyco#35511 that asserts both `result.output` and `result.metadata.output` contain the expected text and are not the sentinel "(no output)" string; addresses the field-consistency concern raised in the previous review thread.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant Scope as Effect.scoped
    participant Spawner as spawner.spawn
    participant Reader as forkScoped reader fiber
    participant Race1 as raceAll (exit / abort / timeout)
    participant Race2 as drain race (join / abort / timeout)

    Scope->>Spawner: spawn process
    Scope->>Reader: fork Stream.runForEach (stdout+stderr)
    Scope->>Race1: race exitCode vs abort vs timeout

    alt "exit.kind === exit"
        Race1-->>Scope: process exited cleanly
        Scope->>Race2: Fiber.join(reader) race abort race fresh-timeout
        alt reader drains to EOF
            Race2-->>Scope: "drained = exit"
        else abort fires during drain
            Race2-->>Scope: "drained = abort → aborted = true"
        else fresh timeout fires during drain
            Race2-->>Scope: "drained = timeout → expired = true (misleading)"
        end
    else "exit.kind === abort"
        Race1-->>Scope: abort signal
        Scope->>Spawner: kill forceKillAfter 3s
    else "exit.kind === timeout"
        Race1-->>Scope: timeout elapsed
        Scope->>Spawner: kill forceKillAfter 3s
    end

    Scope->>Scope: assemble output from list
    Note over Scope: if expired append terminated message

%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
    participant Scope as Effect.scoped
    participant Spawner as spawner.spawn
    participant Reader as forkScoped reader fiber
    participant Race1 as raceAll (exit / abort / timeout)
    participant Race2 as drain race (join / abort / timeout)

    Scope->>Spawner: spawn process
    Scope->>Reader: fork Stream.runForEach (stdout+stderr)
    Scope->>Race1: race exitCode vs abort vs timeout

    alt "exit.kind === exit"
        Race1-->>Scope: process exited cleanly
        Scope->>Race2: Fiber.join(reader) race abort race fresh-timeout
        alt reader drains to EOF
            Race2-->>Scope: "drained = exit"
        else abort fires during drain
            Race2-->>Scope: "drained = abort → aborted = true"
        else fresh timeout fires during drain
            Race2-->>Scope: "drained = timeout → expired = true (misleading)"
        end
    else "exit.kind === abort"
        Race1-->>Scope: abort signal
        Scope->>Spawner: kill forceKillAfter 3s
    else "exit.kind === timeout"
        Race1-->>Scope: timeout elapsed
        Scope->>Spawner: kill forceKillAfter 3s
    end

    Scope->>Scope: assemble output from list
    Note over Scope: if expired append terminated message

_{Reviews (2): Last reviewed commit: "fix(shell): drain stdout before reading ..." | Re-trigger Greptile}

codeant-ai · 2026-07-06T07:34:18Z

CodeAnt AI finished reviewing your PR.

… on exit 0 The output reader was forked with forkScoped but only handle.exitCode was awaited, so the scope could close and interrupt the reader before buffered stdout/stderr drained to EOF, yielding "(no output)" on a clean exit 0 (anomalyco#35511). Capture the reader fiber and, on a clean exit, join it so the merged stream drains before the scope closes. Bound the best-effort drain with the abort/timeout races so a backgrounded child holding the pipe open cannot hang the tool; a clean exit is never relabelled aborted/terminated if the drain is cut short.

greptile-apps

chethanuk has reached the 50-credit limit for trial accounts. To continue receiving code reviews, upgrade your plan.

codeant-ai Bot added the size:S This PR changes 10-29 lines, ignoring generated files label Jul 6, 2026

gemini-code-assist Bot reviewed Jul 6, 2026

View reviewed changes

Comment thread packages/opencode/src/tool/shell.ts Resolved

greptile-apps Bot reviewed Jul 6, 2026

View reviewed changes

Comment thread packages/opencode/src/tool/shell.ts Resolved

Comment thread packages/opencode/test/tool/shell.test.ts Resolved

codeant-ai Bot reviewed Jul 6, 2026

View reviewed changes

Comment thread packages/opencode/src/tool/shell.ts Resolved

chethanuk force-pushed the fix/issue-35511-shell-no-output-race branch from d779212 to 895e46d Compare July 6, 2026 07:46

chethanuk force-pushed the fix/issue-35511-shell-no-output-race branch from 895e46d to 13c9c38 Compare July 6, 2026 11:13

greptile-apps Bot reviewed Jul 6, 2026

View reviewed changes

Navigation Menu

Repository navigation

fix(shell): drain stdout before reading output to avoid "(no output)" on exit 0 - #2

fix(shell): drain stdout before reading output to avoid "(no output)" on exit 0#2
chethanuk wants to merge 1 commit into
devfrom
fix/issue-35511-shell-no-output-race

chethanuk commented Jul 6, 2026 •

edited

Loading

Uh oh!

codeant-ai Bot commented Jul 6, 2026

Uh oh!

codeant-ai Bot commented Jul 6, 2026

Uh oh!

coderabbitai Bot commented Jul 6, 2026 •

edited

Loading

Review limit reached

Uh oh!

gemini-code-assist Bot left a comment

Uh oh!

Uh oh!

greptile-apps Bot commented Jul 6, 2026 •

edited

Loading

Sequence Diagram

Uh oh!

Uh oh!

Uh oh!

Uh oh!

codeant-ai Bot commented Jul 6, 2026

Uh oh!

greptile-apps Bot left a comment

Uh oh!

Reviewers

Uh oh!

Assignees

Labels

Projects

Milestone

Development

Uh oh!

1 participant

Conversation

chethanuk commented Jul 6, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Issue for this PR

Type of change

What does this PR do?

How did you verify your code works?

Screenshots / recordings

Checklist

Uh oh!

codeant-ai Bot commented Jul 6, 2026

Uh oh!

codeant-ai Bot commented Jul 6, 2026

Thanks for using CodeAnt! 🎉

Uh oh!

coderabbitai Bot commented Jul 6, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Review limit reached

Uh oh!

gemini-code-assist Bot left a comment

Choose a reason for hiding this comment

Code Review

Uh oh!

Uh oh!

greptile-apps Bot commented Jul 6, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Greptile Summary

Confidence Score: 4/5

Important Files Changed

Sequence Diagram

Uh oh!

Uh oh!

Uh oh!

Uh oh!

codeant-ai Bot commented Jul 6, 2026

Uh oh!

greptile-apps Bot left a comment

Choose a reason for hiding this comment

Uh oh!

Reviewers

Uh oh!

Assignees

Labels

Projects

Milestone

Development

Uh oh!

1 participant

chethanuk commented Jul 6, 2026 •

edited

Loading

coderabbitai Bot commented Jul 6, 2026 •

edited

Loading

greptile-apps Bot commented Jul 6, 2026 •

edited

Loading