From 758f757cb9b60dddf10efd5d37becfec9f4b5196 Mon Sep 17 00:00:00 2001
From: Tommaso Negri <me@tommasongr.com>
Date: Thu, 15 Oct 2026 11:01:01 +0200
Subject: drain command stdout and stderr concurrently

---
 Sources/bar/Models/Runner.swift | 20 ++++++++++++++------
 Sources/bar/bar.docc/command.md |  6 +++---
 2 files changed, 17 insertions(+), 9 deletions(-)

(limited to 'Sources')

diff --git a/Sources/bar/Models/Runner.swift b/Sources/bar/Models/Runner.swift
index 6c90d54..4b6694f 100644
--- a/Sources/bar/Models/Runner.swift
+++ b/Sources/bar/Models/Runner.swift
@@ -19,14 +19,15 @@ enum Runner {
 	///
 	/// - Returns: Parsed stdout on success; ["ERROR"] on launch failure or
 	///   nonzero exit, or [""] if successful output cannot be decoded as UTF-8.
-	///   Captured stderr is forwarded only after a nonzero exit.
-	static func run(_ command: String, env: [String: String] = [:]) -> [String] {
+	///   Captured stderr is forwarded regardless of exit status.
+	static func run(_ command: String, env: [String: String] = [:],
+	                errorOutput: FileHandle = .standardError) -> [String] {
 		// Detached tasks do not run inside AppKit's main event-loop pool.
 		// Drain temporary Foundation objects at the end of each command.
-		autoreleasepool { execute(command, env: env) }
+		autoreleasepool { execute(command, env: env, errorOutput: errorOutput) }
 	}
 
-	private static func execute(_ command: String, env: [String: String]) -> [String] {
+	private static func execute(_ command: String, env: [String: String], errorOutput: FileHandle) -> [String] {
 		let process = Process()
 		process.executableURL = URL(fileURLWithPath: "/bin/sh")
 		process.arguments = ["-c", command]
@@ -53,14 +54,21 @@ enum Runner {
 		}
 		timer.resume()
 
+		// Drain stderr alongside stdout so neither full pipe can stall the child.
+		// Use GCD for this blocking read, not Swift's cooperative executor.
+		let diagnostics = DispatchGroup()
+		DispatchQueue.global(qos: .utility).async(group: diagnostics) {
+			autoreleasepool {
+				write(stderr.fileHandleForReading.readDataToEndOfFile(), to: errorOutput)
+			}
+		}
 		let data = stdout.fileHandleForReading.readDataToEndOfFile()
-		let errData = stderr.fileHandleForReading.readDataToEndOfFile()
 		process.waitUntilExit()
+		diagnostics.wait()
 
 		timer.cancel()
 
 		guard process.terminationStatus == 0 else {
-			write(errData, to: .standardError)
 			return ["ERROR"]
 		}
 
diff --git a/Sources/bar/bar.docc/command.md b/Sources/bar/bar.docc/command.md
index 3196011..c2ad917 100644
--- a/Sources/bar/bar.docc/command.md
+++ b/Sources/bar/bar.docc/command.md
@@ -29,13 +29,13 @@ On the contrary, if you run `bar` from a launch agent, you'll probably not have
 ## Error handling
 
 If the command exits with a non-zero status or cannot be launched, the block displays a single cell containing the text `ERROR`.
-After a non-zero exit, captured stderr is forwarded to the `bar` process stderr.
-Stderr from a successful command is discarded, and launch failures do not forward a diagnostic.
+Captured stderr is forwarded to the `bar` process stderr, including on successful exits.
+Launch failures do not forward a diagnostic.
 Successful stdout that cannot be decoded as UTF-8 produces a blank cell.
 
 After 30 seconds, *bar* requests termination of the shell with SIGTERM.
 This is not a hard deadline: a shell can ignore the signal, and child processes can keep the output pipes open.
-The current runner reads stdout and then stderr before waiting for exit, so commands with enough stderr output can also block while stdout is being read.
+Stdout and stderr are drained concurrently so a full output pipe does not stall the command.
 
 ```shell
 # Example: simulate a command failure
-- 
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