From f78b159b08e50c794527a8ea8cabaa1b2a2f04fc Mon Sep 17 00:00:00 2001
From: closer-claudio <claudio@farcloser.world>
Date: Mon, 9 Nov 2026 21:41:06 -0800
Subject: [PATCH] tarcmd tests: the passthrough filter is written once, before
 any child

TestCompressProgram wrote its filter script and ran it through tar's
-I in the same test, in parallel with tests that run zstdmt, unzstd
and the system tar. A child forked by one of those between the
script's open and close inherits the write descriptor until its exec,
and running the script in that window fails with "text file busy"
(golang/go#22315); the verify lane met it on ubuntu-24.04-arm and
passed on a rerun. TestMain now writes the script before any test
runs, with no child alive, and the test runs it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Signed-off-by: closer-claudio <claudio@farcloser.world>
---
 internal/tarcmd/run_test.go | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/internal/tarcmd/run_test.go b/internal/tarcmd/run_test.go
index 8b5e74b..98f774c 100644
--- a/internal/tarcmd/run_test.go
+++ b/internal/tarcmd/run_test.go
@@ -9,6 +9,7 @@ import (
 	"archive/tar"
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"io/fs"
 	"os"
@@ -26,6 +27,35 @@ import (
 	"github.com/forkcloser/compress/internal/tarcmd"
 )
 
+// passthroughFilter is the one executable the tests run, and TestMain
+// writes it before any test runs: a script written while another test's
+// child is between fork and exec is inherited by that child, write
+// descriptor and all, until the exec, and running it in that window fails
+// with "text file busy" (golang/go#22315; the verify lane met it with
+// zstdmt and the system tar in flight). Written once with no child alive,
+// it is run freely by the parallel tests.
+var passthroughFilter string //nolint:gochecknoglobals // set once in TestMain, read by the tests
+
+func TestMain(m *testing.M) {
+	dir, err := os.MkdirTemp("", "tarcmd-filter-*")
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	passthroughFilter = filepath.Join(dir, "passthrough")
+
+	script := "#!/bin/sh\nif [ \"$1\" = -d ]; then shift; fi\nexec cat\n"
+	if err := os.WriteFile(passthroughFilter, []byte(script), 0o755); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	m.Run()
+
+	_ = os.RemoveAll(dir)
+}
+
 // run invokes the command with args and stdin, returning its status and the
 // two output streams.
 func run(t *testing.T, stdin []byte, args ...string) (status int, stdout, stderr string) {
@@ -1019,8 +1049,7 @@ func TestCompressProgram(t *testing.T) {
 		t.Skip("the filter is a shell script")
 	}
 
-	filter := filepath.Join(t.TempDir(), "passthrough")
-	writeFile(t, filter, "#!/bin/sh\nif [ \"$1\" = -d ]; then shift; fi\nexec cat\n", 0o755)
+	filter := passthroughFilter
 
 	src := tree(t)
 	archive := filepath.Join(t.TempDir(), "through.tar")