Use TTY detection to determine if SIGINT forwarding is enabled (#13925)

Use TTY detection to determine when we should forward SIGINT instead of
counting signals, which can lead to various problems where multiple
SIGINTs are sent to a child after the first signal. Counting does not
make sense in interactive situations that do not exit on interrupt,
e.g., the Python REPL.

Closes https://github.com/astral-sh/uv/issues/13919
Closes https://github.com/astral-sh/uv/issues/12108

crates/uv/src/commands/run.rs

@@ -17,12 +17,11 @@ pub(crate) async fn run_to_completion(mut handle: Child) -> anyhow::Result<ExitS
     // its process group, in which case the terminal driver will _not_ send SIGINT to the child
     // process and uv must take ownership of forwarding the signal.
     //
-    // Note this assumes an interactive terminal. If a signal is sent directly to the uv parent
-    // process (e.g., `kill -2 <pid>`), the process group is not involved and a signal is not sent
-    // to the child by default. In this context, uv must forward the signal to the child. We work
-    // around this by forwarding SIGINT if it is received more than once. We could attempt to infer
-    // if the parent is a terminal using TTY detection(?), but there hasn't been sufficient
-    // motivation to explore alternatives yet.
+    // Note the above only applies in an interactive terminal. If a signal is sent directly to the
+    // uv parent process (e.g., `kill -2 <pid>`), the process group is not involved and a signal is
+    // not sent to the child by default. In this context, uv must forward the signal to the child.
+    // uv checks if stdin is a TTY as a heuristic to determine if uv is running in an interactive
+    // terminal. When not in an interactive terminal, uv will forward SIGINT to the child.
     //
     // Use of SIGTERM is also a bit complicated. If a terminal receives a SIGTERM, it just waits for
     // its children to exit — multiple SIGTERMs do not have any effect and the signals are not
@@ -38,6 +37,7 @@ pub(crate) async fn run_to_completion(mut handle: Child) -> anyhow::Result<ExitS
     // to the child process or the process will not be killable.
     #[cfg(unix)]
     let status = {
+        use std::io::{IsTerminal, stdin};
         use std::ops::Deref;
 
         use anyhow::Context;
@@ -94,7 +94,6 @@ pub(crate) async fn run_to_completion(mut handle: Child) -> anyhow::Result<ExitS
 
         let mut sigterm_handle = handle_signal(SignalKind::terminate())?;
         let mut sigint_handle = handle_signal(SignalKind::interrupt())?;
-        let mut sigint_count = 0;
 
         // The following signals are terminal by default, but can have user defined handlers.
         // Forward them to the child process for handling.
@@ -130,6 +129,9 @@ pub(crate) async fn run_to_completion(mut handle: Child) -> anyhow::Result<ExitS
         )))]
         let mut siginfo_handle = PlatformSpecificSignal::Dummy;
 
+        // Use TTY detection as a heuristic
+        let is_interactive = stdin().is_terminal();
+
         // Notably, we do not handle `SIGCHLD` (sent when a child process status changes) and
         // `SIGIO` or `SIGPOLL` (sent when a file descriptor is ready for io) as they don't make
         // sense for this parent / child relationship.
@@ -151,14 +153,11 @@ pub(crate) async fn run_to_completion(mut handle: Child) -> anyhow::Result<ExitS
                     // Check if the child pgid has changed
                     let child_pgid = getpgid(Some(child_pid)).context("Failed to get PID of child process")?;
 
-                    // Increment the number of interrupts seen
-                    sigint_count += 1;
-
                     // If the pgid _differs_ from the parent, the child will not receive a SIGINT
-                    // and we should forward it. If we've received multiple SIGINTs, forward it
-                    // regardless.
-                    if child_pgid == parent_pgid && sigint_count < 2 {
-                        debug!("Received SIGINT, assuming the child received it as part of the process group");
+                    // and we should forward it. If we think we're not in an interactive terminal,
+                    // forward the signal unconditionally.
+                    if child_pgid == parent_pgid && is_interactive {
+                        debug!("Received SIGINT, assuming the child received it from the terminal driver");
                         continue;
                     }