Add PrintableString utility

Strings defined by third parties may contain control characters. Provide
a wrapper such that these are replaced when displayed. Useful in node
aliases and offer fields.

lightning/src/routing/gossip.rs

@@ -31,6 +31,7 @@ use crate::util::ser::{Readable, ReadableArgs, Writeable, Writer, MaybeReadable}
 use crate::util::logger::{Logger, Level};
 use crate::util::events::{Event, EventHandler, MessageSendEvent, MessageSendEventsProvider};
 use crate::util::scid_utils::{block_from_scid, scid_from_parts, MAX_SCID_BLOCK};
+use crate::util::string::PrintableString;
 
 use crate::io;
 use crate::io_extras::{copy, sink};
@@ -1022,23 +1023,17 @@ pub struct NodeAlias(pub [u8; 32]);
 
 impl fmt::Display for NodeAlias {
 	fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
-		let control_symbol = core::char::REPLACEMENT_CHARACTER;
 		let first_null = self.0.iter().position(|b| *b == 0).unwrap_or(self.0.len());
 		let bytes = self.0.split_at(first_null).0;
 		match core::str::from_utf8(bytes) {
-			Ok(alias) => {
-				for c in alias.chars() {
-					let mut bytes = [0u8; 4];
-					let c = if !c.is_control() { c } else { control_symbol };
-					f.write_str(c.encode_utf8(&mut bytes))?;
-				}
-			},
+			Ok(alias) => PrintableString(alias).fmt(f)?,
 			Err(_) => {
+				use core::fmt::Write;
 				for c in bytes.iter().map(|b| *b as char) {
 					// Display printable ASCII characters
-					let mut bytes = [0u8; 4];
+					let control_symbol = core::char::REPLACEMENT_CHARACTER;
 					let c = if c >= '\x20' && c <= '\x7e' { c } else { control_symbol };
-					f.write_str(c.encode_utf8(&mut bytes))?;
+					f.write_char(c)?;
 				}
 			},
 		};

lightning/src/util/mod.rs

@@ -21,6 +21,7 @@ pub mod ser;
 pub mod message_signing;
 pub mod invoice;
 pub mod persist;
+pub mod string;
 pub mod wakers;
 
 pub(crate) mod atomic_counter;

lightning/src/util/string.rs

@@ -0,0 +1,41 @@
+// This file is Copyright its original authors, visible in version control
+// history.
+//
+// This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
+// or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
+// You may not use this file except in accordance with one or both of these
+// licenses.
+
+//! Utilities for strings.
+
+use core::fmt;
+
+/// A string that displays only printable characters, replacing control characters with
+/// [`core::char::REPLACEMENT_CHARACTER`].
+pub struct PrintableString<'a>(pub &'a str);
+
+impl<'a> fmt::Display for PrintableString<'a> {
+	fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+		use core::fmt::Write;
+		for c in self.0.chars() {
+			let c = if c.is_control() { core::char::REPLACEMENT_CHARACTER } else { c };
+			f.write_char(c)?;
+		}
+
+		Ok(())
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::PrintableString;
+
+	#[test]
+	fn displays_printable_string() {
+		assert_eq!(
+			format!("{}", PrintableString("I \u{1F496} LDK!\t\u{26A1}")),
+			"I \u{1F496} LDK!\u{FFFD}\u{26A1}",
+		);
+	}
+}