feat(macos): support agent environment hints

refs https://github.com/ogulcancelik/herdr/discussions/679

3 files changed, 105 insertions, 42 deletions

diff --git a/src/platform/linux.rs b/src/platform/linux.rs
index e700a7ff..48f1ef1c 100644
--- a/src/platform/linux.rs
+++ b/src/platform/linux.rs
@@ -280,18 +280,7 @@ pub fn process_agent_hint(pid: u32) -> Option<crate::detect::Agent> {
         return None;
     }
     let environ = std::fs::read(format!("/proc/{pid}/environ")).ok()?;
-    parse_agent_env_hint(&environ)
-}
-
-fn parse_agent_env_hint(environ: &[u8]) -> Option<crate::detect::Agent> {
-    for record in environ.split(|&byte| byte == 0) {
-        let Some(value) = record.strip_prefix(b"HERDR_AGENT=") else {
-            continue;
-        };
-        let value = std::str::from_utf8(value).ok()?;
-        return crate::detect::parse_agent_label(value);
-    }
-    None
+    super::parse_agent_env_hint(&environ)
 }
 
 pub fn session_processes(child_pid: u32) -> Vec<u32> {
@@ -809,24 +798,6 @@ mod tests {
     }
 
     #[test]
-    fn parse_agent_env_hint_accepts_known_agents() {
-        assert_eq!(
-            parse_agent_env_hint(b"PATH=/bin\0HERDR_AGENT=claude\0TERM=xterm\0"),
-            Some(crate::detect::Agent::Claude)
-        );
-        assert_eq!(
-            parse_agent_env_hint(b"HERDR_AGENT=codex"),
-            Some(crate::detect::Agent::Codex)
-        );
-    }
-
-    #[test]
-    fn parse_agent_env_hint_ignores_missing_or_unknown_agents() {
-        assert_eq!(parse_agent_env_hint(b"PATH=/bin\0TERM=xterm\0"), None);
-        assert_eq!(parse_agent_env_hint(b"HERDR_AGENT=not-an-agent\0"), None);
-    }
-
-    #[test]
     fn clipboard_commands_prefer_wayland_when_available() {
         let _guard = env_lock().lock().unwrap();
         unsafe {
diff --git a/src/platform/macos.rs b/src/platform/macos.rs
index 70eea7ec..f35fd1eb 100644
--- a/src/platform/macos.rs
+++ b/src/platform/macos.rs
@@ -842,6 +842,33 @@ fn process_argv(pid: u32) -> Option<Vec<String>> {
     procargs2_argv(&buf)
 }
 
+/// Read a Herdr agent identity hint from a process environment.
+pub fn process_agent_hint(pid: u32) -> Option<crate::detect::Agent> {
+    if pid == 0 {
+        return None;
+    }
+    let buf = kern_procargs2(pid)?;
+    super::parse_agent_env_hint(procargs2_env(&buf)?)
+}
+
+fn procargs2_argv_start(rest: &[u8]) -> Option<usize> {
+    let exec_end = rest.iter().position(|&byte| byte == 0)?;
+    let mut pos = exec_end;
+    while pos < rest.len() && rest[pos] == 0 {
+        pos += 1;
+    }
+    (pos < rest.len()).then_some(pos)
+}
+
+fn skip_nul_strings(bytes: &[u8], start: usize, count: usize) -> Option<usize> {
+    let mut current = start;
+    for _ in 0..count {
+        let end = bytes.get(current..)?.iter().position(|&byte| byte == 0)?;
+        current = current.checked_add(end)?.checked_add(1)?;
+    }
+    Some(current)
+}
+
 fn procargs2_argv(buf: &[u8]) -> Option<Vec<String>> {
     if buf.len() < 4 {
         return None;
@@ -852,19 +879,10 @@ fn procargs2_argv(buf: &[u8]) -> Option<Vec<String>> {
         return None;
     }
 
-    // Layout: [argc: i32] [exec_path\0] [padding\0...] [argv[0]\0] [argv[1]\0] ... [env\0] ...
+    // Layout: [argc: i32] [exec_path\0] [padding\0...] [argv[0]\0] ... [env\0] ...
     let rest = &buf[4..];
-    let exec_end = rest.iter().position(|&b| b == 0)?;
-    let mut pos = exec_end;
-    while pos < rest.len() && rest[pos] == 0 {
-        pos += 1;
-    }
-    if pos >= rest.len() {
-        return None;
-    }
-
+    let mut current = procargs2_argv_start(rest)?;
     let mut argv = Vec::with_capacity(argc as usize);
-    let mut current = pos;
     for _ in 0..argc {
         if current >= rest.len() {
             return None;
@@ -884,6 +902,22 @@ fn procargs2_argv(buf: &[u8]) -> Option<Vec<String>> {
     Some(argv)
 }
 
+fn procargs2_env(buf: &[u8]) -> Option<&[u8]> {
+    if buf.len() < 4 {
+        return None;
+    }
+
+    let argc = i32::from_ne_bytes([buf[0], buf[1], buf[2], buf[3]]);
+    if argc < 1 {
+        return None;
+    }
+
+    let rest = &buf[4..];
+    let argv_start = procargs2_argv_start(rest)?;
+    let env_start = skip_nul_strings(rest, argv_start, argc as usize)?;
+    rest.get(env_start..)
+}
+
 /// Get the current working directory of a process.
 ///
 /// Uses `proc_pidinfo(PROC_PIDVNODEPATHINFO)` to read `pvi_cdir.vip_path`.
@@ -1068,6 +1102,33 @@ mod tests {
     }
 
     #[test]
+    fn procargs2_env_reads_agent_hint_after_argv() {
+        let buf = build_procargs2(
+            "/opt/homebrew/bin/nono",
+            &["nono", "run", "HERDR_AGENT=codex", "--", "claude"],
+            &["PATH=/usr/bin", "HERDR_AGENT=claude", "TERM=xterm-256color"],
+        );
+
+        let env = procargs2_env(&buf).expect("expected env block");
+        assert_eq!(
+            crate::platform::parse_agent_env_hint(env),
+            Some(crate::detect::Agent::Claude)
+        );
+    }
+
+    #[test]
+    fn procargs2_env_does_not_treat_argv_as_environment() {
+        let buf = build_procargs2(
+            "/opt/homebrew/bin/nono",
+            &["nono", "run", "HERDR_AGENT=claude"],
+            &["PATH=/usr/bin"],
+        );
+
+        let env = procargs2_env(&buf).expect("expected env block");
+        assert_eq!(crate::platform::parse_agent_env_hint(env), None);
+    }
+
+    #[test]
     fn terminal_bundle_identifier_maps_known_terminal_env() {
         assert_eq!(
             terminal_bundle_identifier_from_env(Some("ghostty"), None, false, false),
diff --git a/src/platform/mod.rs b/src/platform/mod.rs
index e396966b..3583df5e 100644
--- a/src/platform/mod.rs
+++ b/src/platform/mod.rs
@@ -148,11 +148,22 @@ mod fallback;
 #[cfg(not(any(target_os = "linux", target_os = "macos", target_os = "windows")))]
 pub use fallback::*;
 
-#[cfg(not(target_os = "linux"))]
+#[cfg(not(any(target_os = "linux", target_os = "macos")))]
 pub fn process_agent_hint(_pid: u32) -> Option<crate::detect::Agent> {
     None
 }
 
+#[cfg(any(target_os = "linux", target_os = "macos"))]
+pub(crate) fn parse_agent_env_hint(environ: &[u8]) -> Option<crate::detect::Agent> {
+    for record in environ.split(|&byte| byte == 0) {
+        let Some(value) = record.strip_prefix(b"HERDR_AGENT=") else {
+            continue;
+        };
+        return crate::detect::parse_agent_label(std::str::from_utf8(value).ok()?);
+    }
+    None
+}
+
 #[cfg(not(target_os = "macos"))]
 #[derive(Debug)]
 pub(crate) struct InputSourceRestore;
@@ -229,6 +240,26 @@ mod tests {
         );
     }
 
+    #[cfg(any(target_os = "linux", target_os = "macos"))]
+    #[test]
+    fn parse_agent_env_hint_accepts_known_agents() {
+        assert_eq!(
+            parse_agent_env_hint(b"PATH=/bin\0HERDR_AGENT=claude\0TERM=xterm\0"),
+            Some(crate::detect::Agent::Claude)
+        );
+        assert_eq!(
+            parse_agent_env_hint(b"HERDR_AGENT=codex"),
+            Some(crate::detect::Agent::Codex)
+        );
+    }
+
+    #[cfg(any(target_os = "linux", target_os = "macos"))]
+    #[test]
+    fn parse_agent_env_hint_ignores_missing_or_unknown_agents() {
+        assert_eq!(parse_agent_env_hint(b"PATH=/bin\0TERM=xterm\0"), None);
+        assert_eq!(parse_agent_env_hint(b"HERDR_AGENT=not-an-agent\0"), None);
+    }
+
     #[test]
     fn read_limited_reader_returns_complete_data_under_limit() {
         let input = std::io::Cursor::new(b"image".to_vec());