Reimplement the hasher

We create a sha1 over the path of the file and the Message-ID of the
mail itself.

The file name is not constant with mail files, because flags are encoded
in the filename. The path is also not constant with mail files, because
they can be moved between boxes.

This approach is not yet the best one, but better than before.

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

lib/domain/libimagmail/src/hasher.rs

@@ -22,6 +22,8 @@ use std::path::Path;
 use failure::Fallible as Result;
 
 use libimagentryref::hasher::Hasher;
+use libimagerror::errors::ErrorMsg;
+use libimagentryref::hasher::sha1::Sha1Hasher;
 
 pub struct MailHasher;
 
@@ -30,11 +32,24 @@ impl Hasher for MailHasher {
 
     /// hash the file at path `path`
     ///
-    /// TODO: This is the expensive implementation. We use the message Id as hash, which is
+    /// We create a sha1 over the path of the file (which is NOT safe, because mails can move) and
-    /// convenient and _should_ be safe
+    /// the Message-ID of the mail itself.
     ///
-    /// TODO: Confirm that this approach is right
+    /// # TODO: Fix
+    ///
+    /// The file name is not constant with mail files, because flags are encoded in the filename.
+    /// The path is not constant with mail files, because they can be moved between boxes.
     fn hash<P: AsRef<Path>>(path: P) -> Result<String> {
-        ::util::get_message_id_for_mailfile(path)
+        let mut path_str = path
+            .as_ref()
+            .to_str()
+            .map(String::from)
+            .ok_or_else(|| ErrorMsg::UTF8Error)?;
+
+        let message_id = ::util::get_message_id_for_mailfile(path)?;
+
+        path_str.push_str(&message_id);
+
+        Ok(Sha1Hasher::sha1_hash(&path_str))
     }
 }