panic ui test: Provide comprehensive test for panic after fork

This tests that we can indeed safely panic after fork, both
a raw libc::fork and in a Command pre_exec hook.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Co-authored-by: Mara Bos <m-ou.se@m-ou.se>

library/std/src/sys/unix/process/process_unix/tests.rs

@@ -2,6 +2,9 @@ use crate::os::unix::process::{CommandExt, ExitStatusExt};
 use crate::panic::catch_unwind;
 use crate::process::Command;
 
+// Many of the other aspects of this situation, including heap alloc concurrency
+// safety etc., are tested in src/test/ui/process/process-panic-after-fork.rs
+
 #[test]
 fn exitstatus_display_tests() {
     // In practice this is the same on every Unix.

src/test/ui/panics/abort-on-panic.rs

@@ -23,41 +23,45 @@ extern "Rust" fn panic_in_rust_abi() {
     panic!("TestRust");
 }
 
-fn test() {
-    let _ = panic::catch_unwind(|| { panic_in_ffi(); });
-    // The process should have aborted by now.
+fn should_have_aborted() {
     io::stdout().write(b"This should never be printed.\n");
     let _ = io::stdout().flush();
 }
 
+fn test() {
+    let _ = panic::catch_unwind(|| { panic_in_ffi(); });
+    should_have_aborted();
+}
+
 fn testrust() {
     let _ = panic::catch_unwind(|| { panic_in_rust_abi(); });
-    // The process should have aborted by now.
-    io::stdout().write(b"This should never be printed.\n");
-    let _ = io::stdout().flush();
+    should_have_aborted();
 }
 
 fn main() {
+    let tests: &[(_, fn())] = &[
+        ("test", test),
+        ("testrust", testrust),
+    ];
+
     let args: Vec<String> = env::args().collect();
     if args.len() > 1 {
         // This is inside the self-executed command.
-        match &*args[1] {
-            "test" => return test(),
-            "testrust" => return testrust(),
-            _ => panic!("bad test"),
+        for (a,f) in tests {
+            if &args[1] == a { return f() }
         }
+        panic!("bad test");
     }
 
-    // These end up calling the self-execution branches above.
-    let mut p = Command::new(&args[0])
-                        .stdout(Stdio::piped())
-                        .stdin(Stdio::piped())
-                        .arg("test").spawn().unwrap();
-    assert!(!p.wait().unwrap().success());
+    let execute_self_expecting_abort = |arg| {
+        let mut p = Command::new(&args[0])
+                            .stdout(Stdio::piped())
+                            .stdin(Stdio::piped())
+                            .arg(arg).spawn().unwrap();
+        assert!(!p.wait().unwrap().success());
+    };
 
-    let mut p = Command::new(&args[0])
-                        .stdout(Stdio::piped())
-                        .stdin(Stdio::piped())
-                        .arg("testrust").spawn().unwrap();
-    assert!(!p.wait().unwrap().success());
+    for (a,_f) in tests {
+        execute_self_expecting_abort(a);
+    }
 }

src/test/ui/process/process-panic-after-fork.rs

@@ -0,0 +1,150 @@
+// run-pass
+// no-prefer-dynamic
+// ignore-wasm32-bare no libc
+// ignore-windows
+// ignore-sgx no libc
+// ignore-emscripten no processes
+// ignore-sgx no processes
+
+#![feature(bench_black_box)]
+#![feature(rustc_private)]
+#![feature(never_type)]
+#![feature(panic_always_abort)]
+
+extern crate libc;
+
+use std::alloc::{GlobalAlloc, Layout};
+use std::fmt;
+use std::panic::{self, panic_any};
+use std::os::unix::process::{CommandExt, ExitStatusExt};
+use std::process::{self, Command, ExitStatus};
+use std::sync::atomic::{AtomicU32, Ordering};
+
+use libc::c_int;
+
+/// This stunt allocator allows us to spot heap allocations in the child.
+struct PidChecking<A> {
+    parent: A,
+    require_pid: AtomicU32,
+}
+
+#[global_allocator]
+static ALLOCATOR: PidChecking<std::alloc::System> = PidChecking {
+    parent: std::alloc::System,
+    require_pid: AtomicU32::new(0),
+};
+
+impl<A> PidChecking<A> {
+    fn engage(&self) {
+        let parent_pid = process::id();
+        eprintln!("engaging allocator trap, parent pid={}", parent_pid);
+        self.require_pid.store(parent_pid, Ordering::Release);
+    }
+    fn check(&self) {
+        let require_pid = self.require_pid.load(Ordering::Acquire);
+        if require_pid != 0 {
+            let actual_pid = process::id();
+            if require_pid != actual_pid {
+                unsafe {
+                    libc::raise(libc::SIGTRAP);
+                }
+            }
+        }
+    }
+}
+
+unsafe impl<A:GlobalAlloc> GlobalAlloc for PidChecking<A> {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        self.check();
+        self.parent.alloc(layout)
+    }
+
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        self.check();
+        self.parent.dealloc(ptr, layout)
+    }
+
+    unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
+        self.check();
+        self.parent.alloc_zeroed(layout)
+    }
+
+    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
+        self.check();
+        self.parent.realloc(ptr, layout, new_size)
+    }
+}
+
+fn expect_aborted(status: ExitStatus) {
+    dbg!(status);
+    let signal = status.signal().expect("expected child process to die of signal");
+    assert!(signal == libc::SIGABRT || signal == libc::SIGILL);
+}
+
+fn main() {
+    ALLOCATOR.engage();
+
+    fn run(do_panic: &dyn Fn()) -> ExitStatus {
+        let child = unsafe { libc::fork() };
+        assert!(child >= 0);
+        if child == 0 {
+            panic::always_abort();
+            do_panic();
+            process::exit(0);
+        }
+        let mut status: c_int = 0;
+        let got = unsafe { libc::waitpid(child, &mut status, 0) };
+        assert_eq!(got, child);
+        let status = ExitStatus::from_raw(status.into());
+        status
+    }
+
+    fn one(do_panic: &dyn Fn()) {
+        let status = run(do_panic);
+        expect_aborted(status);
+    }
+
+    one(&|| panic!());
+    one(&|| panic!("some message"));
+    one(&|| panic!("message with argument: {}", 42));
+
+    #[derive(Debug)]
+    struct Wotsit { }
+    one(&|| panic_any(Wotsit { }));
+
+    let mut c = Command::new("echo");
+    unsafe {
+        c.pre_exec(|| panic!("{}", "crash now!"));
+    }
+    let st = c.status().expect("failed to get command status");
+    expect_aborted(st);
+
+    struct DisplayWithHeap;
+    impl fmt::Display for DisplayWithHeap {
+        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
+            let s = vec![0; 100];
+            let s = std::hint::black_box(s);
+            write!(f, "{:?}", s)
+        }
+    }
+
+    // Some panics in the stdlib that we want not to allocate, as
+    // otherwise these facilities become impossible to use in the
+    // child after fork, which is really quite awkward.
+
+    one(&|| { None::<DisplayWithHeap>.unwrap(); });
+    one(&|| { None::<DisplayWithHeap>.expect("unwrapped a none"); });
+    one(&|| { std::str::from_utf8(b"\xff").unwrap(); });
+    one(&|| {
+        let x = [0, 1, 2, 3];
+        let y = x[std::hint::black_box(4)];
+        let _z = std::hint::black_box(y);
+    });
+
+    // Finally, check that our stunt allocator can actually catch an allocation after fork.
+    // ie, that our test is effective.
+
+    let status = run(&|| panic!("allocating to display... {}", DisplayWithHeap));
+    dbg!(status);
+    assert_eq!(status.signal(), Some(libc::SIGTRAP));
+}