improve comments

src/certificates.rs

@@ -18,7 +18,8 @@ use {
 /// A struct that holds all loaded certificates and the respective domain
 /// names.
 pub(crate) struct CertStore {
-    // use a Vec of pairs instead of a HashMap because order matters
+    /// Stores the certificates and the domains they apply to, sorted by domain
+    /// names, longest matches first
     certs: Vec<(String, CertifiedKey)>,
 }
 
@@ -133,7 +134,8 @@ impl CertStore {
         // load all certificates from directories
         let mut certs = vec![];
 
-        // try to load fallback certificate and key
+        // Try to load fallback certificate and key directly from the top level
+        // certificate directory. It will be loaded as the `.` domain.
         match load_domain(certs_dir, ".".to_string()) {
             Err(CertLoadError::EmptyDomain(_)) => { /* there are no fallback keys */ }
             Err(CertLoadError::NoReadCertDir) => unreachable!(),
@@ -153,8 +155,9 @@ impl CertStore {
             Err(CertLoadError::MissingCert(_)) => {
                 return Err(CertLoadError::MissingCert("fallback".to_string()))
             }
-            // if there are files, just push them because there is no domain
-            // name to check against
+            // For the fallback keys there is no domain name to verify them
+            // against, so we can skip that step and only have to do it for the
+            // other keys below.
             Ok(key) => certs.push((String::new(), key)),
         }
 
@@ -165,6 +168,8 @@ impl CertStore {
             .filter(|x| x.path().is_dir())
         {
             let path = file.path();
+
+            // the filename should be the domain name
             let filename = path
                 .file_name()
                 .and_then(OsStr::to_str)
@@ -184,14 +189,17 @@ impl CertStore {
         }
 
         certs.sort_unstable_by(|(a, _), (b, _)| {
-            // try to match as many as possible. If one is a substring of the other,
-            // the `zip` will make them look equal and make the length decide.
+            // Try to match as many domain segments as possible. If one is a
+            // substring of the other, the `zip` will only compare the smaller
+            // length of either a or b and the for loop will not decide.
             for (a_part, b_part) in a.split('.').rev().zip(b.split('.').rev()) {
                 if a_part != b_part {
+                    // What we sort by here is not really important, but `str`
+                    // already implements Ord, making it easier for us.
                     return a_part.cmp(b_part);
                 }
             }
-            // longer domains first
+            // Sort longer domains first.
             a.len().cmp(&b.len()).reverse()
         });
 
@@ -203,13 +211,17 @@ impl ResolvesServerCert for CertStore {
     fn resolve(&self, client_hello: rustls::ClientHello<'_>) -> Option<CertifiedKey> {
         if let Some(name) = client_hello.server_name() {
             let name: &str = name.into();
+            // The certificate list is sorted so the longest match will always
+            // appear first. We have to find the first that is either this
+            // domain or a parent domain of the current one.
             self.certs
                 .iter()
                 .find(|(s, _)| name.ends_with(s))
+                // only the key is interesting
                 .map(|(_, k)| k)
                 .cloned()
         } else {
-            // This kind of resolver requires SNI
+            // This kind of resolver requires SNI.
             None
         }
     }