bug: Pipeline stdin not forwarded to user-defined functions

[chaliy]

Description

When a user-defined function is invoked as part of a pipeline, the stdin from the pipe is not forwarded to the function body. The function executes with no stdin, so commands like tr, cat, read inside the function receive nothing and produce no output.

Reproduction

to_upper() { tr '[:lower:]' '[:upper:]'; }
echo "hello world" | to_upper
# Expected: HELLO WORLD
# Actual: (empty)

Also fails with source:

# /lib/utils.sh
to_upper() { tr '[:lower:]' '[:upper:]'; }

source /lib/utils.sh
echo "hello world" | to_upper
# Expected: HELLO WORLD
# Actual: (empty)

Root Cause

In crates/bashkit/src/interpreter/mod.rs around line 2756-2778, when execute_dispatched_command handles a user-defined function, the stdin parameter is available but never set on self.pipeline_stdin before calling execute_command(&func_def.body):

if let Some(func_def) = self.functions.get(name).cloned() {
    // ...
    let mut result = self.execute_command(&func_def.body).await?; // stdin NOT passed
    // ...
}

Compare to how compound commands in pipelines correctly handle this at lines 2269-2271:

let prev_pipeline_stdin = self.pipeline_stdin.take();
self.pipeline_stdin = stdin_data.take();
// ... execute ...
self.pipeline_stdin = prev_pipeline_stdin;

Suggested Fix

Set self.pipeline_stdin = stdin; before executing the function body, then restore it after. Same pattern used for compound commands and subshells (e.g., line 3412-3419).

Impact

High — All 4 eval models (Opus, Sonnet, Haiku, GPT) hit this bug. Breaks the script_function_lib eval task entirely. Any function that reads stdin via pipe will silently produce no output.

Found In

Eval run 2026-02-25, task script_function_lib — failed across all models.