Optimize /etc/hosts writes

client/src/main.rs

@@ -280,18 +280,20 @@ fn update_hosts_file(
     hosts_path: PathBuf,
     peers: &[Peer],
 ) -> Result<(), WrappedIoError> {
-    log::info!("updating {} with the latest peers.", "/etc/hosts".yellow());
-
     let mut hosts_builder = HostsBuilder::new(format!("innernet {interface}"));
     for peer in peers {
         hosts_builder.add_hostname(
             peer.contents.ip,
             &format!("{}.{}.wg", peer.contents.name, interface),
         );
     }
-    if let Err(e) = hosts_builder.write_to(&hosts_path).with_path(hosts_path) {
-        log::warn!("failed to update hosts ({})", e);
-    }
+    match hosts_builder.write_to(&hosts_path).with_path(hosts_path) {
+        Ok(has_written) if has_written => {
+            log::info!("updated {} with the latest peers.", "/etc/hosts".yellow())
+        },
+        Ok(_) => {},
+        Err(e) => log::warn!("failed to update hosts ({})", e),
+    };
 
     Ok(())
 }

hostsfile/src/lib.rs

@@ -1,5 +1,5 @@
 use std::{
-    collections::HashMap,
+    collections::BTreeMap,
     fmt,
     fs::OpenOptions,
     io::{self, BufRead, BufReader, ErrorKind, Write},
@@ -81,7 +81,7 @@ impl std::error::Error for Error {
 /// ```
 pub struct HostsBuilder {
     tag: String,
-    hostname_map: HashMap<IpAddr, Vec<String>>,
+    hostname_map: BTreeMap<IpAddr, Vec<String>>,
 }
 
 impl HostsBuilder {
@@ -90,7 +90,7 @@ impl HostsBuilder {
     pub fn new<S: Into<String>>(tag: S) -> Self {
         Self {
             tag: tag.into(),
-            hostname_map: HashMap::new(),
+            hostname_map: BTreeMap::new(),
         }
     }
 
@@ -116,7 +116,8 @@ impl HostsBuilder {
 
     /// Inserts a new section to the system's default hosts file.  If there is a section with the
     /// same tag name already, it will be replaced with the new list instead.
-    pub fn write(&self) -> io::Result<()> {
+    /// Returns true if the hosts file has changed.
+    pub fn write(&self) -> io::Result<bool> {
         self.write_to(Self::default_path()?)
     }
 
@@ -178,7 +179,9 @@ impl HostsBuilder {
     ///
     /// On Windows, the format of one hostname per line will be used, all other systems will use
     /// the same format as Unix and Unix-like systems (i.e. allow multiple hostnames per line).
-    pub fn write_to<P: AsRef<Path>>(&self, hosts_path: P) -> io::Result<()> {
+    ///
+    /// Returns true if the hosts file has changed.
+    pub fn write_to<P: AsRef<Path>>(&self, hosts_path: P) -> io::Result<bool> {
         let hosts_path = hosts_path.as_ref();
         if hosts_path.is_dir() {
             // TODO(jake): use io::ErrorKind::IsADirectory when it's stable.
@@ -206,9 +209,33 @@ impl HostsBuilder {
         let begin = lines.iter().position(|line| line.trim() == begin_marker);
         let end = lines.iter().position(|line| line.trim() == end_marker);
 
+        let mut lines_to_insert = vec![];
+        if !self.hostname_map.is_empty() {
+            lines_to_insert.push(begin_marker);
+            for (ip, hostnames) in &self.hostname_map {
+                if cfg!(windows) {
+                    // windows only allows one hostname per line
+                    for hostname in hostnames {
+                        lines_to_insert.push(format!("{ip} {hostname}"));
+                    }
+                } else {
+                    // assume the same format as Unix
+                    lines_to_insert.push(format!("{} {}", ip, hostnames.join(" ")));
+                }
+            }
+            lines_to_insert.push(end_marker);
+        }
+
         let insert = match (begin, end) {
             (Some(begin), Some(end)) => {
-                lines.drain(begin..end + 1);
+                let old_section: Vec<String> = lines.drain(begin..end + 1).collect();
+
+                let sections_are_eq = old_section == lines_to_insert;
+
+                if sections_are_eq {
+                    return Ok(false);
+                }
+
                 begin
             },
             (None, None) => {
@@ -233,21 +260,12 @@ impl HostsBuilder {
         for line in &lines[..insert] {
             writeln!(s, "{line}")?;
         }
-        if !self.hostname_map.is_empty() {
-            writeln!(s, "{begin_marker}")?;
-            for (ip, hostnames) in &self.hostname_map {
-                if cfg!(windows) {
-                    // windows only allows one hostname per line
-                    for hostname in hostnames {
-                        writeln!(s, "{ip} {hostname}")?;
-                    }
-                } else {
-                    // assume the same format as Unix
-                    writeln!(s, "{} {}", ip, hostnames.join(" "))?;
-                }
-            }
-            writeln!(s, "{end_marker}")?;
+
+        // Append hostnames_map section
+        for line in lines_to_insert {
+            writeln!(s, "{line}")?;
         }
+
         for line in &lines[insert..] {
             writeln!(s, "{line}")?;
         }
@@ -260,8 +278,9 @@ impl HostsBuilder {
             _ => {
                 log::debug!("wrote hosts file with the write-and-swap strategy");
             },
-        }
-        Ok(())
+        };
+
+        Ok(true)
     }
 
     fn write_and_swap(temp_path: &Path, hosts_path: &Path, contents: &[u8]) -> io::Result<()> {
@@ -314,7 +333,8 @@ mod tests {
         temp_file.write_all(b"preexisting\ncontent").unwrap();
         let mut builder = HostsBuilder::new("foo");
         builder.add_hostname([1, 1, 1, 1].into(), "whatever");
-        builder.write_to(&temp_path).unwrap();
+        assert!(builder.write_to(&temp_path).unwrap());
+        assert!(!builder.write_to(&temp_path).unwrap());
 
         let contents = std::fs::read_to_string(&temp_path).unwrap();
         println!("contents: {contents}");