execfile

Overview ¶

Package execfile writes a file that is about to be executed: a fake docker or git a test puts on PATH, a hook, a helper script.

os.WriteFile is not safe for that in a program that starts processes from several goroutines, as every parallel test binary does. Between the open and the close of the file being written, another goroutine may fork a child. The child inherits the write descriptor and holds it until its own exec closes it (O_CLOEXEC acts only at exec). If our exec of the new file lands in that gap, Linux refuses it with ETXTBSY ("text file busy"), because the file is still open for writing somewhere. This is Go issue 22315; the standard library has no fix, and it shows up as a rare failure on Linux CI that macOS seldom reproduces.

Write holds syscall.ForkLock for reading while the file is open. The standard library takes it for writing around every fork (os.StartProcess, exec.Cmd). A fork that started earlier cannot inherit a descriptor not yet opened, and a later one cannot start until the file is closed.

It does not help with processes started some other way (cgo, raw clone), and it is no use for a file written by a different process.