Ensure termination of syslog writer (#857)

Closing #856 

This particular code has been struck by lightning a couple of times now
and is pretty hard to reason about using formal reasoning (e.g. a loop
invariant like `self.cursor < LIMIT` would seen to have to hold, but it
doesn't really). This PR doesn't improve that situation (i.e. the formal
reasoning), but it does ensure termination.

As a minor side-fix, the code could also panic on line 54 if we hit the
middle of a UTF8 character in exactly the wrong position, that is fixed
as well.

I'll work on an implementation of this writer in a more declarative
style.

src/log/syslog.rs

@@ -51,14 +51,20 @@ impl Write for SysLogWriter {
             if self.cursor + message.len() > LIMIT {
                 // floor_char_boundary is currently unstable
                 let mut truncate_boundary = LIMIT - self.cursor;
-                while !message.is_char_boundary(truncate_boundary) {
+                while truncate_boundary > 0 && !message.is_char_boundary(truncate_boundary) {
                     truncate_boundary -= 1;
                 }
 
+                // don't overzealously truncate log messages
                 truncate_boundary = message[..truncate_boundary]
                     .rfind(|c: char| c.is_ascii_whitespace())
                     .unwrap_or(truncate_boundary);
 
+                if truncate_boundary == 0 {
+                    // we failed to find a "nice" cut off point, abruptly cut off the msg
+                    truncate_boundary = LIMIT - self.cursor;
+                }
+
                 let left = &message[..truncate_boundary];
                 let right = &message[truncate_boundary..];
 

test-framework/e2e-tests/src/lib.rs

@@ -1,6 +1,7 @@
 #![cfg(test)]
 
 mod pty;
+mod regression;
 mod su;
 
 type Error = Box<dyn std::error::Error>;

test-framework/e2e-tests/src/regression.rs

@@ -0,0 +1,17 @@
+use sudo_test::{Command, Env, TextFile};
+
+use crate::Result;
+
+#[test]
+fn syslog_writer_should_not_hang() -> Result<()> {
+    let env = Env(TextFile("ALL ALL=(ALL:ALL) NOPASSWD: ALL").chmod("644")).build()?;
+
+    let stdout = Command::new("sudo")
+        .args(["env", "CC=clang-18", "CXX=clang++-18", "FOO=\"........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................\"", "whoami"])
+        .output(&env)?
+        .stdout()?;
+
+    assert_eq!(stdout, "root");
+
+    Ok(())
+}