Add empty ctrlc handler on Windows.

When exec_replacing the cargo process, we want the new process to get any signals. This already works fine on Unix.

On Windows, pressing ctrlc kills the cargo process and doesn't pass the signal on to the child process. By adding an empty handler, we allow the child process to handle the signal instead.

src/cargo/util/process_builder.rs

@@ -150,30 +150,23 @@ impl ProcessBuilder {
         }
     }
 
-    /// On unix, executes the process using the unix syscall `execvp`, which will block this
-    /// process, and will only return if there is an error. On windows this is a synonym for
-    /// `exec`.
-    #[cfg(unix)]
-    pub fn exec_replace(&self) -> CargoResult<()> {
-        use std::os::unix::process::CommandExt;
-
-        let mut command = self.build_command();
-        let error = command.exec();
-        Err(::util::CargoError::from(error)
-            .context(process_error(
-                &format!("could not execute process {}", self),
-                None,
-                None,
-            ))
-            .into())
-    }
-
-    /// On unix, executes the process using the unix syscall `execvp`, which will block this
-    /// process, and will only return if there is an error. On windows this is a synonym for
-    /// `exec`.
-    #[cfg(windows)]
+    /// Replaces the current process with the target process.
+    ///
+    /// On Unix, this executes the process using the unix syscall `execvp`, which will block
+    /// this process, and will only return if there is an error.
+    ///
+    /// On Windows this isn't technically possible. Instead we emulate it to the best of our
+    /// ability. One aspect we fix here is that we specify a handler for the ctrl-c handler.
+    /// In doing so (and by effectively ignoring it) we should emulate proxying ctrl-c
+    /// handling to the application at hand, which will either terminate or handle it itself.
+    /// According to microsoft's documentation at:
+    /// https://docs.microsoft.com/en-us/windows/console/ctrl-c-and-ctrl-break-signals
+    /// the ctrl-c signal is sent to all processes attached to a terminal, which should
+    /// include our child process. If the child terminates then we'll reap them in Cargo
+    /// pretty quickly, and if the child handles the signal then we won't terminate
+    /// (and we shouldn't!) until the process itself later exits.
     pub fn exec_replace(&self) -> CargoResult<()> {
-        self.exec()
+        imp::exec_replace(self)
     }
 
     /// Execute the process, returning the stdio output, or an error if non-zero exit status.
@@ -324,3 +317,51 @@ pub fn process<T: AsRef<OsStr>>(cmd: T) -> ProcessBuilder {
         jobserver: None,
     }
 }
+
+#[cfg(unix)]
+mod imp {
+    use CargoResult;
+    use std::os::unix::process::CommandExt;
+    use util::{process_error, ProcessBuilder};
+
+    pub fn exec_replace(process_builder: &ProcessBuilder) -> CargoResult<()> {
+        let mut command = process_builder.build_command();
+        let error = command.exec();
+        Err(::util::CargoError::from(error)
+            .context(process_error(
+                &format!("could not execute process {}", process_builder),
+                None,
+                None,
+            ))
+            .into())
+    }
+}
+
+#[cfg(windows)]
+mod imp {
+    extern crate winapi;
+
+    use CargoResult;
+    use util::{process_error, ProcessBuilder};
+    use self::winapi::shared::minwindef::{BOOL, DWORD, FALSE, TRUE};
+    use self::winapi::um::consoleapi::SetConsoleCtrlHandler;
+
+    unsafe extern "system" fn ctrlc_handler(_: DWORD) -> BOOL {
+        // Do nothing. Let the child process handle it.
+        TRUE
+    }
+
+    pub fn exec_replace(process_builder: &ProcessBuilder) -> CargoResult<()> {
+        unsafe {
+            if SetConsoleCtrlHandler(Some(ctrlc_handler), TRUE) == FALSE {
+                return Err(process_error(
+                    "Could not set Ctrl-C handler.",
+                    None,
+                    None).into());
+            }
+        }
+
+        // Just exec the process as normal.
+        process_builder.exec()
+    }
+}