From 46bb856578f7ae958bf218cda505ba03462842b3 Mon Sep 17 00:00:00 2001
From: "Nadine Roth (COO)" <coo@ki-knechtschaft.com>
Date: Mon, 24 Aug 2026 10:31:02 +0200
Subject: [PATCH] fix(core): show stdout tail in exec() failure message when
 stderr is empty
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The error tail used `stderr.trim().split('\n').pop() ?? stdout…pop()`,
but pop() on the single-element array [''] returns '' — not undefined —
so the ?? fallback never fired and commands that fail while writing
their diagnostics to stdout (very common for CLIs) produced errors like:

    git … failed (exit 128):

Now the last non-empty stderr line is preferred, with the last
non-empty stdout line as the actual fallback.
---
 packages/core/src/exec.test.ts | 14 ++++++++++++++
 packages/core/src/exec.ts      |  6 +++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/packages/core/src/exec.test.ts b/packages/core/src/exec.test.ts
index e647aaea..049fb9d8 100644
--- a/packages/core/src/exec.test.ts
+++ b/packages/core/src/exec.test.ts
@@ -41,6 +41,20 @@ describe('exec', () => {
       'quoted "value"',
     ]);
   });
+
+  it('falls back to the last stdout line in the failure message when stderr is empty', async () => {
+    await expect(exec('sh', ['-c', 'echo useful-diagnostic-on-stdout; exit 3'], {
+      log: () => {},
+      throwOnNonZero: true,
+    })).rejects.toThrow('failed (exit 3): useful-diagnostic-on-stdout');
+  });
+
+  it('prefers the last non-empty stderr line in the failure message', async () => {
+    await expect(exec('sh', ['-c', 'echo noisy >&2; echo real-error >&2; exit 2'], {
+      log: () => {},
+      throwOnNonZero: true,
+    })).rejects.toThrow('failed (exit 2): real-error');
+  });
 });
 
 describe('ensureCli', () => {
diff --git a/packages/core/src/exec.ts b/packages/core/src/exec.ts
index 4042abd8..2b335a09 100644
--- a/packages/core/src/exec.ts
+++ b/packages/core/src/exec.ts
@@ -72,7 +72,11 @@ export async function exec(cmd: string, args: string[], opts: ExecOptions): Prom
     child.on('close', (exitCode) => {
       const result: ExecResult = { exitCode: exitCode ?? -1, stdout, stderr };
       if (throwOnNonZero && result.exitCode !== 0) {
-        const tail = stderr.trim().split('\n').pop() ?? stdout.trim().split('\n').pop() ?? '';
+        // Prefer the last non-empty stderr line; fall back to stdout. Note
+        // `''.trim().split('\n').pop()` returns '' (not undefined), so a plain
+        // ??-chain never reaches the stdout fallback when stderr is empty.
+        const lastLine = (s: string) => s.trim().split('\n').filter(Boolean).pop() ?? '';
+        const tail = lastLine(stderr) || lastLine(stdout);
         reject(new Error(`${cmd} ${args.join(' ')} failed (exit ${result.exitCode}): ${tail}`));
       } else {
         resolve(result);