From 7b7c52208042cadfc2c1ee11eec5fdb81606fb09 Mon Sep 17 00:00:00 2001
From: James Brechtel <git@jamesbrechtel.com>
Date: Wed, 20 May 2026 15:35:22 -0400
Subject: [PATCH] fix: force lazy ByteString evaluation before waitForProcess
 to avoid deadlock

The lazy I/O in LB.hGetContents returns a thunk immediately without
reading any data. If waitForProcess is called before the ByteString
is forced, the parent blocks on the process while the child blocks
on a full pipe buffer (no one is reading). Fix by computing LB.length
(which forces full traversal) before calling waitForProcess.
---
 src/Roux/Server.hs | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/src/Roux/Server.hs b/src/Roux/Server.hs
index 269646d..c2199c3 100644
--- a/src/Roux/Server.hs
+++ b/src/Roux/Server.hs
@@ -235,7 +235,11 @@ parseFormBody body =
          in toEnum hexVal : go rest
     go (c : rest) = c : go rest
 
--- | Run a subprocess and capture stdout/stderr as raw bytes (no locale decoding).
+{- | Run a subprocess and capture stdout/stderr as raw bytes (no locale decoding).
+Reads output eagerly to avoid the classic lazy-IO deadlock where
+the parent blocks on waitForProcess while the child blocks on a
+full pipe buffer.
+-}
 readProcessBytes :: FilePath -> [String] -> IO (ExitCode, LB.ByteString, LB.ByteString)
 readProcessBytes cmd args = do
     let process =
@@ -244,16 +248,22 @@ readProcessBytes cmd args = do
                 , std_err = CreatePipe
                 }
     result <- createProcess process
-    -- createProcess returns (stdin, stdout, stderr, process).
-    -- stdin is Inherited → Nothing; stdout/stderr are CreatePipe → Just.
     case result of
         (Nothing, Just outH, Just errH, ph) -> do
             hSetBinaryMode outH True
             hSetBinaryMode errH True
+            -- Read stdout and stderr strictly before waiting for the process.
+            -- Using length forces the lazy ByteString, triggering eager reads
+            -- and preventing the pipe buffer from filling up.
+            -- hSetBuffering would also work but requires knowing a buffer size.
             out <- LB.hGetContents outH
             err <- LB.hGetContents errH
-            ec <- waitForProcess ph
-            pure (ec, out, err)
+            -- Force full evaluation before waitForProcess to avoid deadlock.
+            let outLen = LB.length out
+                errLen = LB.length err
+            outLen `seq` errLen `seq` do
+                ec <- waitForProcess ph
+                pure (ec, out, err)
         _ ->
             ioError $
                 userError $