process_matcher: Fix off-by-one in signal mask

Apparently the signal masks in /proc/status are off-by-one, ie. bit 0
represents signal 1. In original pgrep signal 0 results in matching
signal 64 so replicate this behaviour also.

Author: dezgeg

src/uu/pgrep/src/process_matcher.rs

@@ -140,10 +140,17 @@ pub fn find_matching_pids(settings: &Settings) -> Vec<ProcessInformation> {
     let mut pids = collect_matched_pids(settings);
     #[cfg(unix)]
     if settings.require_handler {
+        // Bits in SigCgt are off by one (ie. bit 0 represents signal 1, etc.)
+        let mask_to_test = if settings.signal == 0 {
+            // In original pgrep, testing for signal 0 seems to return results for signal 64 instead.
+            1 << (64 - 1)
+        } else {
+            1 << (settings.signal - 1)
+        };
         pids.retain(|pid| {
             let mask =
                 u64::from_str_radix(pid.clone().status().get("SigCgt").unwrap(), 16).unwrap();
-            mask & (1 << settings.signal) != 0
+            mask & mask_to_test != 0
         });
     }
     if pids.is_empty() {

tests/by-util/test_pgrep.rs

@@ -369,6 +369,17 @@ fn test_invalid_signal() {
         .stderr_contains("Unknown signal 'foo'");
 }
 
+#[test]
+#[cfg(target_os = "linux")]
+fn test_signal_that_never_matches() {
+    new_ucmd!()
+        .arg("--require-handler")
+        .arg("--signal=KILL")
+        .arg(".*")
+        .fails()
+        .no_output();
+}
+
 #[test]
 #[cfg(target_os = "linux")]
 fn test_does_not_match_pid() {