agent: add tests for is_signal_handled function

Add test coverage for is_signal_handled function in rpc.rs. Includes
refactors to make the function testable and handle additional cases.

Fixes #3939

Signed-off-by: Braden Rayhorn <bradenrayhorn@fastmail.com>

src/agent/src/rpc.rs

@@ -407,7 +407,8 @@ impl AgentService {
             // For container initProcess, if it hasn't installed handler for "SIGTERM" signal,
             // it will ignore the "SIGTERM" signal sent to it, thus send it "SIGKILL" signal
             // instead of "SIGTERM" to terminate it.
-            if p.init && sig == libc::SIGTERM && !is_signal_handled(p.pid, sig as u32) {
+            let proc_status_file = format!("/proc/{}/status", p.pid);
+            if p.init && sig == libc::SIGTERM && !is_signal_handled(&proc_status_file, sig as u32) {
                 sig = libc::SIGKILL;
             }
             p.signal(sig)?;
@@ -1635,21 +1636,33 @@ fn append_guest_hooks(s: &Sandbox, oci: &mut Spec) -> Result<()> {
     Ok(())
 }
 
-// Check is the container process installed the
+// Check if the container process installed the
 // handler for specific signal.
-fn is_signal_handled(pid: pid_t, signum: u32) -> bool {
-    let sig_mask: u64 = 1u64 << (signum - 1);
-    let file_name = format!("/proc/{}/status", pid);
+fn is_signal_handled(proc_status_file: &str, signum: u32) -> bool {
+    let shift_count: u64 = if signum == 0 {
+        // signum 0 is used to check for process liveness.
+        // Since that signal is not part of the mask in the file, we only need
+        // to know if the file (and therefore) process exists to handle
+        // that signal.
+        return fs::metadata(proc_status_file).is_ok();
+    } else if signum > 64 {
+        // Ensure invalid signum won't break bit shift logic
+        warn!(sl!(), "received invalid signum {}", signum);
+        return false;
+    } else {
+        (signum - 1).into()
+    };
 
     // Open the file in read-only mode (ignoring errors).
-    let file = match File::open(&file_name) {
+    let file = match File::open(proc_status_file) {
         Ok(f) => f,
         Err(_) => {
-            warn!(sl!(), "failed to open file {}\n", file_name);
+            warn!(sl!(), "failed to open file {}", proc_status_file);
             return false;
         }
     };
 
+    let sig_mask: u64 = 1 << shift_count;
     let reader = BufReader::new(file);
 
     // Read the file line by line using the lines() iterator from std::io::BufRead.
@@ -1657,21 +1670,21 @@ fn is_signal_handled(pid: pid_t, signum: u32) -> bool {
         let line = match line {
             Ok(l) => l,
             Err(_) => {
-                warn!(sl!(), "failed to read file {}\n", file_name);
+                warn!(sl!(), "failed to read file {}", proc_status_file);
                 return false;
             }
         };
         if line.starts_with("SigCgt:") {
             let mask_vec: Vec<&str> = line.split(':').collect();
             if mask_vec.len() != 2 {
-                warn!(sl!(), "parse the SigCgt field failed\n");
+                warn!(sl!(), "parse the SigCgt field failed");
                 return false;
             }
             let sig_cgt_str = mask_vec[1];
             let sig_cgt_mask = match u64::from_str_radix(sig_cgt_str, 16) {
                 Ok(h) => h,
                 Err(_) => {
-                    warn!(sl!(), "failed to parse the str {} to hex\n", sig_cgt_str);
+                    warn!(sl!(), "failed to parse the str {} to hex", sig_cgt_str);
                     return false;
                 }
             };
@@ -2305,6 +2318,102 @@ mod tests {
         }
     }
 
+    #[tokio::test]
+    async fn test_is_signal_handled() {
+        #[derive(Debug)]
+        struct TestData<'a> {
+            status_file_data: Option<&'a str>,
+            signum: u32,
+            result: bool,
+        }
+
+        let tests = &[
+            TestData {
+                status_file_data: Some(
+                    r#"
+SigBlk:0000000000010000
+SigCgt:0000000000000001
+OtherField:other
+                "#,
+                ),
+                signum: 1,
+                result: true,
+            },
+            TestData {
+                status_file_data: Some("SigCgt:000000004b813efb"),
+                signum: 4,
+                result: true,
+            },
+            TestData {
+                status_file_data: Some("SigCgt:000000004b813efb"),
+                signum: 3,
+                result: false,
+            },
+            TestData {
+                status_file_data: Some("SigCgt:000000004b813efb"),
+                signum: 65,
+                result: false,
+            },
+            TestData {
+                status_file_data: Some("SigCgt:000000004b813efb"),
+                signum: 0,
+                result: true,
+            },
+            TestData {
+                status_file_data: Some("SigCgt:ZZZZZZZZ"),
+                signum: 1,
+                result: false,
+            },
+            TestData {
+                status_file_data: Some("SigCgt:-1"),
+                signum: 1,
+                result: false,
+            },
+            TestData {
+                status_file_data: Some("SigCgt"),
+                signum: 1,
+                result: false,
+            },
+            TestData {
+                status_file_data: Some("any data"),
+                signum: 0,
+                result: true,
+            },
+            TestData {
+                status_file_data: Some("SigBlk:0000000000000001"),
+                signum: 1,
+                result: false,
+            },
+            TestData {
+                status_file_data: None,
+                signum: 1,
+                result: false,
+            },
+            TestData {
+                status_file_data: None,
+                signum: 0,
+                result: false,
+            },
+        ];
+
+        for (i, d) in tests.iter().enumerate() {
+            let msg = format!("test[{}]: {:?}", i, d);
+
+            let dir = tempdir().expect("failed to make tempdir");
+            let proc_status_file_path = dir.path().join("status");
+
+            if let Some(file_data) = d.status_file_data {
+                fs::write(&proc_status_file_path, file_data).unwrap();
+            }
+
+            let result = is_signal_handled(proc_status_file_path.to_str().unwrap(), d.signum);
+
+            let msg = format!("{}, result: {:?}", msg, result);
+
+            assert_eq!(d.result, result, "{}", msg);
+        }
+    }
+
     #[tokio::test]
     async fn test_verify_cid() {
         #[derive(Debug)]