hostsfile: support Windows (#79)

Windows has some peculiarities for example it only allows one hostname per line
and the file's location depends on an environment variable. Although in most
cases just using C:\Windows\ for %WinDir% would probably work.

Note that editing the hosts file on Windows will require running with elevated
privileges ("Run as Administrator") and in some cases also antivirus may block
access to the file.

hostsfile/src/lib.rs

@@ -70,6 +70,14 @@ impl std::error::Error for Error {
 /// 1.1.1.1 cloudflare-dns apnic-dns
 /// # DO NOT EDIT dns END
 /// ```
+///
+/// On Windows the host file format is slightly different in this case:
+/// ```text
+/// # DO NOT EDIT dns BEGIN
+/// 1.1.1.1 cloudflare-dns
+/// 1.1.1.1 apnic-dns
+/// # DO NOT EDIT dns END
+/// ```
 pub struct HostsBuilder {
     tag: String,
     hostname_map: HashMap<IpAddr, Vec<String>>,
@@ -106,11 +114,19 @@ 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. Only Unix systems are
+    /// same tag name already, it will be replaced with the new list instead.
-    /// supported now.
     pub fn write(&self) -> Result<()> {
         let hosts_file = if cfg!(unix) {
             PathBuf::from("/etc/hosts")
+        } else if cfg!(windows) {
+            PathBuf::from(
+                // according to https://support.microsoft.com/en-us/topic/how-to-reset-the-hosts-file-back-to-the-default-c2a43f9d-e176-c6f3-e4ef-3500277a6dae
+                // the location depends on the environment variable %WinDir%.
+                format!(
+                    "{}\\System32\\Drivers\\Etc\\hosts",
+                    std::env::var("WinDir")?
+                ),
+            )
         } else {
             return Err(Box::new(Error("unsupported operating system.".to_owned())));
         };
@@ -127,6 +143,9 @@ impl HostsBuilder {
 
     /// Inserts a new section to the specified hosts file.  If there is a section with the same tag
     /// name already, it will be replaced with the new list instead.
+    ///
+    /// 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) -> Result<()> {
         let hosts_path = hosts_path.as_ref();
         let begin_marker = format!("# DO NOT EDIT {} BEGIN", &self.tag);
@@ -185,8 +204,16 @@ impl HostsBuilder {
         if !self.hostname_map.is_empty() {
             writeln!(&mut file, "{}", begin_marker)?;
             for (ip, hostnames) in &self.hostname_map {
+                if cfg!(windows) {
+                    // windows only allows one hostname per line
+                    for hostname in hostnames {
+                        writeln!(&mut file, "{} {}", ip, hostname)?;
+                    }
+                } else {
+                    // assume the same format as Unix
                     writeln!(&mut file, "{} {}", ip, hostnames.join(" "))?;
                 }
+            }
             writeln!(&mut file, "{}", end_marker)?;
         }
         for line in &lines[insert..] {