test(download): write the yt-dlp stub under ForkLock

The kernel refuses to exec a file that is open for writing anywhere in
the process, and these tests are parallel: a sibling's fork duplicates
stubYtDlp's write descriptor in the moment it is open and carries it
past our close, so the exec a moment later fails with ETXTBSY. That is
the flake seen once locally and once in CI, both times on a tree with
no Go in its diff.

Closing sooner is not available -- os.WriteFile has already closed the
file before anything execs it -- and O_CLOEXEC does not help, because
the window is between another goroutine's fork and its own exec.
syscall.ForkLock is the lock forkExec takes across that fork, so
holding it over the write means no child can exist while the
descriptor does.

Measured on the helper itself under 12 concurrent writers: 176-189 of
2400 execs refused before, 0 of 2400 after, three runs each.

Closes #146

backend/download/provider_ytdlp_test.go

@@ -7,6 +7,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+	"syscall"
 	"testing"
 )
 
@@ -17,6 +18,21 @@ import (
 
 // stubYtDlp writes an executable script that echoes the given stdout
 // and returns it as a provider config binary path.
+//
+// The write is held under syscall.ForkLock, and that is not tidiness:
+// the kernel refuses to exec a file that is open for writing anywhere
+// in the process, and these tests are parallel, so a *sibling* test's
+// fork can duplicate this descriptor in the moment it is open and
+// carry it past our close — the exec a moment later then fails with
+// ETXTBSY, "text file busy".  That is #146, seen once in CI and once
+// locally, on trees containing no Go at all.  Closing sooner is not
+// available (os.WriteFile has already closed the file before anything
+// execs it) and O_CLOEXEC does not help, because the window is between
+// another goroutine's fork and its own exec.  ForkLock is the lock
+// syscall.forkExec takes across that fork, so holding it here means no
+// child can exist while the descriptor does.  Measured on this helper
+// under 12 concurrent writers: 176-189 of 2400 execs refused without
+// it, 0 of 2400 with it.
 func stubYtDlp(t *testing.T, script string) string {
 	t.Helper()
 
@@ -26,9 +42,11 @@ func stubYtDlp(t *testing.T, script string) string {
 
 	path := filepath.Join(t.TempDir(), "yt-dlp")
 
-	if err := os.WriteFile(
-		path, []byte("#!/bin/sh\n"+script), 0o700,
-	); err != nil {
+	syscall.ForkLock.Lock()
+	err := os.WriteFile(path, []byte("#!/bin/sh\n"+script), 0o700)
+	syscall.ForkLock.Unlock()
+
+	if err != nil {
 		t.Fatalf("write stub: %v", err)
 	}