test_fork_core/

fork.rs

//-
// Copyright 2018 Jason Lingle
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use std::env;
use std::io;
use std::io::BufRead;
use std::io::Read;
use std::panic;
use std::process;
use std::process::Child;
use std::process::ExitCode;
use std::process::Stdio;
use std::process::Termination;

use crate::cmdline;
use crate::error::*;

const OCCURS_ENV: &str = "TEST_FORK_OCCURS";
const OCCURS_TERM_LENGTH: usize = 17; /* ':' plus 16 hexits */


/// Simulate a process fork.
///
/// Since this is not a true process fork, the calling code must be structured
/// to ensure that the child process, upon starting from the same entry point,
/// also reaches this same `fork()` call. Recursive forks are supported; the
/// child branch is taken from all child processes of the fork even if it is
/// not directly the child of a particular branch. However, encountering the
/// same fork point more than once in a single execution sequence of a child
/// process is not (e.g., putting this call in a recursive function) and
/// results in unspecified behaviour.
///
/// `test_name` must exactly match the full path of the test function being
/// run.
///
/// `fork_id` is a unique identifier identifying this particular fork location.
/// This *must* be stable across processes of the same executable; pointers are
/// not suitable stable, and string constants may not be suitably unique. The
/// [`fork_id!()`] macro is the recommended way to supply this
/// parameter.
///
/// If this is the parent process, `in_parent` is invoked, and the return value
/// becomes the return value from this function. The callback is passed a
/// handle to the file which receives the child's output. If is the callee's
/// responsibility to wait for the child to exit. If this is the child process,
/// `in_child` is invoked, and when the callback returns, the child process
/// exits.
///
/// If `in_parent` returns or panics before the child process has terminated,
/// the child process is killed.
///
/// If `in_child` panics, the child process exits with a failure code
/// immediately rather than let the panic propagate out of the `fork()` call.
///
/// `process_modifier` is invoked on the `std::process::Command` immediately
/// before spawning the new process. The callee may modify the process
/// parameters if desired, but should not do anything that would modify or
/// remove any environment variables beginning with `RUSTY_FORK_`.
///
/// ## Panics
///
/// Panics if the environment indicates that there are already at least 16
/// levels of fork nesting.
///
/// Panics if `std::env::current_exe()` fails determine the path to the current
/// executable.
///
/// Panics if any argument to the current process is not valid UTF-8.
pub fn fork<MODIFIER, PARENT, CHILD, R, T>(
    test_name: &str,
    fork_id: &str,
    process_modifier: MODIFIER,
    in_parent: PARENT,
    in_child: CHILD,
) -> Result<R>
where
    MODIFIER: FnOnce(&mut process::Command),
    PARENT: FnOnce(&mut Child) -> R,
    T: Termination,
    CHILD: FnOnce() -> T,
{
    // Erase the generics so we don't instantiate the actual implementation for
    // every single test
    let mut return_value = None;
    let mut process_modifier = Some(process_modifier);
    let mut in_parent = Some(in_parent);
    let mut in_child = Some(in_child);

    fork_impl(
        test_name,
        fork_id,
        &mut |cmd| process_modifier.take().unwrap()(cmd),
        &mut |child| return_value = Some(in_parent.take().unwrap()(child)),
        &mut || in_child.take().unwrap()(),
    )
    .map(|_| return_value.unwrap())
}

fn fork_impl<T: Termination>(
    test_name: &str,
    fork_id: &str,
    process_modifier: &mut dyn FnMut(&mut process::Command),
    in_parent: &mut dyn FnMut(&mut Child),
    in_child: &mut dyn FnMut() -> T,
) -> Result<()> {
    let mut occurs = env::var(OCCURS_ENV).unwrap_or_else(|_| String::new());
    if occurs.contains(fork_id) {
        match panic::catch_unwind(panic::AssertUnwindSafe(in_child)) {
            Ok(test_result) => {
                let rc = if test_result.report() == ExitCode::SUCCESS {
                    0
                } else {
                    70
                };
                process::exit(rc)
            }
            // Assume that the default panic handler already printed something
            //
            // We don't use process::abort() since it produces core dumps on
            // some systems and isn't something more special than a normal
            // panic.
            Err(_) => process::exit(70 /* EX_SOFTWARE */),
        }
    } else {
        // Prevent misconfiguration creating a fork bomb
        if occurs.len() > 16 * OCCURS_TERM_LENGTH {
            panic!("test-fork: Not forking due to >=16 levels of recursion");
        }

        struct KillOnDrop(Child);
        impl Drop for KillOnDrop {
            fn drop(&mut self) {
                // Kill the child if it hasn't exited yet
                let _ = self.0.kill();

                // Copy the child's output to our own
                // Awkwardly, `print!()` and `println!()` are our only gateway
                // to putting things in the captured output. Generally test
                // output really is text, so work on that assumption and read
                // line-by-line, converting lossily into UTF-8 so we can
                // println!() it.

                fn drain(read: &mut dyn Read, stderr: bool) {
                    let mut buf = Vec::new();
                    let mut br = io::BufReader::new(read);
                    loop {
                        // We can't use read_line() or lines() since they break if
                        // there's any non-UTF-8 output at all. \n occurs at the
                        // end of the line endings on all major platforms, so we
                        // can just use that as a delimiter.
                        if br.read_until(b'\n', &mut buf).is_err() {
                            break;
                        }
                        if buf.is_empty() {
                            break;
                        }

                        // not println!() because we already have a line ending
                        // from above.
                        let s = String::from_utf8_lossy(&buf);
                        if stderr {
                            eprint!("{s}");
                        } else {
                            print!("{s}");
                        }
                        buf.clear();
                    }
                }

                if let Some(stdout) = self.0.stdout.as_mut() {
                    let () = drain(stdout, false);
                }

                if let Some(stderr) = self.0.stderr.as_mut() {
                    let () = drain(stderr, true);
                }
            }
        }

        occurs.push_str(fork_id);
        let mut command =
            process::Command::new(env::current_exe().expect("current_exe() failed, cannot fork"));
        command
            .args(cmdline::strip_cmdline(env::args())?)
            .args(cmdline::RUN_TEST_ARGS)
            .arg(test_name)
            .env(OCCURS_ENV, &occurs)
            .stdin(Stdio::null())
            .stdout(Stdio::piped())
            .stderr(Stdio::piped());
        process_modifier(&mut command);

        let mut child = command.spawn().map(KillOnDrop)?;
        let () = in_parent(&mut child.0);

        Ok(())
    }
}


#[cfg(test)]
mod test {
    use super::*;

    use std::io::Read;
    use std::thread;

    use crate::fork_id;


    fn sleep(ms: u64) {
        thread::sleep(::std::time::Duration::from_millis(ms));
    }

    fn capturing_output(cmd: &mut process::Command) {
        cmd.stdout(Stdio::piped()).stderr(Stdio::inherit());
    }

    fn inherit_output(cmd: &mut process::Command) {
        cmd.stdout(Stdio::inherit()).stderr(Stdio::inherit());
    }

    fn wait_for_child_output(child: &mut Child) -> String {
        let mut output = String::new();
        child
            .stdout
            .as_mut()
            .unwrap()
            .read_to_string(&mut output)
            .unwrap();
        assert!(child.wait().unwrap().success());
        output
    }

    fn wait_for_child(child: &mut Child) {
        assert!(child.wait().unwrap().success());
    }

    #[test]
    fn fork_basically_works() {
        let status = fork(
            "fork::test::fork_basically_works",
            fork_id!(),
            |_| (),
            |child| child.wait().unwrap(),
            || println!("hello from child"),
        )
        .unwrap();
        assert!(status.success());
    }

    #[test]
    fn child_output_captured_and_repeated() {
        let output = fork(
            "fork::test::child_output_captured_and_repeated",
            fork_id!(),
            capturing_output,
            wait_for_child_output,
            || {
                fork(
                    "fork::test::child_output_captured_and_repeated",
                    fork_id!(),
                    |_| (),
                    wait_for_child,
                    || println!("hello from child"),
                )
                .unwrap()
            },
        )
        .unwrap();
        assert!(output.contains("hello from child"));
    }

    #[test]
    fn child_killed_if_parent_exits_first() {
        let output = fork(
            "fork::test::child_killed_if_parent_exits_first",
            fork_id!(),
            capturing_output,
            wait_for_child_output,
            || {
                fork(
                    "fork::test::child_killed_if_parent_exits_first",
                    fork_id!(),
                    inherit_output,
                    |_| (),
                    || {
                        sleep(100);
                        println!("hello from child");
                    },
                )
                .unwrap()
            },
        )
        .unwrap();

        sleep(200);
        assert!(
            !output.contains("hello from child"),
            "Had unexpected output:\n{}",
            output
        );
    }

    #[test]
    fn child_killed_if_parent_panics_first() {
        let output = fork(
            "fork::test::child_killed_if_parent_panics_first",
            fork_id!(),
            capturing_output,
            wait_for_child_output,
            || {
                assert!(panic::catch_unwind(panic::AssertUnwindSafe(|| fork(
                    "fork::test::child_killed_if_parent_panics_first",
                    fork_id!(),
                    inherit_output,
                    |_| panic!("testing a panic, nothing to see here"),
                    || {
                        sleep(100);
                        println!("hello from child");
                    }
                )
                .unwrap()))
                .is_err());
            },
        )
        .unwrap();

        sleep(200);
        assert!(
            !output.contains("hello from child"),
            "Had unexpected output:\n{}",
            output
        );
    }

    #[test]
    fn child_aborted_if_panics() {
        let status = fork::<_, _, _, _, ()>(
            "fork::test::child_aborted_if_panics",
            fork_id!(),
            |_| (),
            |child| child.wait().unwrap(),
            || panic!("testing a panic, nothing to see here"),
        )
        .unwrap();
        assert_eq!(70, status.code().unwrap());
    }
}