Clarify implementation (or try to)

Author: saethlin

cargo-miri/bin.rs

@@ -249,14 +249,14 @@ fn xargo_check() -> Command {
     Command::new(env::var_os("XARGO_CHECK").unwrap_or_else(|| OsString::from("xargo-check")))
 }
 
-/// Execute the command. If it fails, fail this process with the same exit code.
-/// Otherwise, continue.
+/// Execute the `Command`, where possible by replacing the current process with a new process
+/// described by the `Command`. Then exit this process with the exit code of the new process.
 fn exec(mut cmd: Command) -> ! {
     // On non-Unix imitate POSIX exec as closely as we can
     #[cfg(not(unix))]
     {
         let exit_status = cmd.status().expect("failed to run command");
-        std::process::exit(exit_status.code().unwrap_or(-1));
+        std::process::exit(exit_status.code().unwrap_or(-1))
     }
     // On Unix targets, actually exec
     // If exec returns, process setup has failed. This is the same error condition as the expect in
@@ -269,10 +269,10 @@ fn exec(mut cmd: Command) -> ! {
     }
 }
 
-/// Execute the command and pipe `input` into its stdin.
-/// If it fails, fail this process with the same exit code.
-/// Otherwise, continue.
-fn exec_with_pipe(mut cmd: Command, input: &[u8]) {
+/// Execute the `Command`, where possible by replacing the current process with a new process
+/// described by the `Command`. Then exit this process with the exit code of the new process.
+/// `input` is also piped to the new process's stdin.
+fn exec_with_pipe(mut cmd: Command, input: &[u8]) -> ! {
     #[cfg(unix)]
     {
         use std::os::unix::io::FromRawFd;
@@ -283,25 +283,15 @@ fn exec_with_pipe(mut cmd: Command, input: &[u8]) {
         let res = unsafe { libc::pipe(fds.as_mut_ptr()) };
         assert_eq!(res, 0, "failed to create pipe");
 
-        // We need to set close-on-exec, otherwise our pipe isn't readable after exec
-        // SAFETY: fcntl has no preconditions
-        unsafe {
-            for fd in &fds {
-                let res = libc::fcntl(
-                    *fd,
-                    libc::F_SETFD,
-                    libc::fcntl(*fd, libc::F_GETFD) | libc::FD_CLOEXEC,
-                );
-                assert_eq!(res, 0, "failed to set close-on-exec for pipe");
-            }
-        }
-
-        // SAFETY: Both elements of fds are open file descriptors, because pipe2 returned 0
+        // SAFETY: Both elements of fds are open file descriptors, because pipe returned 0
         let dst = unsafe { Stdio::from_raw_fd(fds[0]) };
         let mut src = unsafe { File::from_raw_fd(fds[1]) };
 
         src.write_all(input).expect("failed to write out test source");
 
+        // Close the writing part of the pipe in the parent process
+        drop(src);
+
         cmd.stdin(dst);
         exec(cmd)
     }
@@ -936,10 +926,11 @@ fn phase_rustc(mut args: impl Iterator<Item = String>, phase: RustcPhase) {
                 cmd.arg("--emit=metadata");
             }
 
+            // Alter the `-o` parameter so that it does not overwrite the JSON file we stored above.
             let mut args = env.args.clone();
             for i in 0..args.len() {
                 if args[i] == "-o" {
-                    args[i + 1] = format!("{}_miri", args[i + 1]);
+                    args[i + 1].push_str("_miri");
                 }
             }