Add "which" functionality behind feature gate

This patch adds a "which" feature, which uses the "which" crate to check
whether the program to call actually exists.

The functionality is feature-gated to preserve backwards compatibility.

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

Author: matthiasbeyer

Cargo.toml

@@ -19,5 +19,11 @@ regex = "1"
 tempfile = "3"
 thiserror = "1.0.34"
 
+which = { version = "4.4", optional = true }
+
 [badges]
 maintenance = { status = "passively-maintained" }
+
+[features]
+default = []
+which = ["dep:which"]

src/error.rs

@@ -39,4 +39,8 @@ pub enum Error {
 
     #[error("The provided program arguments cannot be parsed")]
     BadProgramArguments,
+
+    #[cfg(feature = "which")]
+    #[error(transparent)]
+    Which(#[from] which::Error),
 }

src/session.rs

@@ -232,6 +232,10 @@ pub fn spawn(program: &str, timeout_ms: Option<u64>) -> Result<PtySession, Error
 
 /// See `spawn`
 pub fn spawn_command(command: Command, timeout_ms: Option<u64>) -> Result<PtySession, Error> {
+    #[cfg(feature = "which")]
+    {
+        let _ = which::which(command.get_program())?;
+    }
     let mut process = PtyProcess::new(command)?;
     process.set_kill_timeout(timeout_ms);