download on demand (#2631)

* draft a download-api

* basic implementation

* allow partial downloads for protected chats

* use a separate column for download_state

* force a minimal timeout for delete_server_after in combination with partial messages

* add a warning if a possible download may expire by delete_server_after

* test load_imap_deletion_msgid()

* add a test for a partial download

* improve documentation and visibility

* let get_download_limit() return Result<Option>

* rusty getters

* apply MIN_DELETE_SERVER_AFTER to shown availability time

* move stub-creation to download.rs, use stock-strings, nicer logging

* make clippy happy (cargo clippy --tests)

* refine tests and comments

* fix typo

* remove superfluous closure in ffi

* respect partial_download for immediately scheduled DeleteMsgOnImap jobs

src/mimeparser.rs

@@ -136,6 +136,18 @@ const MIME_AC_SETUP_FILE: &str = "application/autocrypt-setup";
 
 impl MimeMessage {
     pub async fn from_bytes(context: &Context, body: &[u8]) -> Result<Self> {
+        MimeMessage::from_bytes_with_partial(context, body, None).await
+    }
+
+    /// Parse a mime message.
+    ///
+    /// If `partial` is set, it contains the full message size in bytes
+    /// and `body` contains the header only.
+    pub async fn from_bytes_with_partial(
+        context: &Context,
+        body: &[u8],
+        partial: Option<u32>,
+    ) -> Result<Self> {
         let mail = mailparse::parse_mail(body)?;
 
         let message_time = mail
@@ -274,7 +286,18 @@ impl MimeMessage {
             is_mime_modified: false,
             decoded_data: Vec::new(),
         };
-        parser.parse_mime_recursive(context, &mail, false).await?;
+
+        match partial {
+            Some(org_bytes) => {
+                parser
+                    .create_stub_from_partial_download(context, org_bytes)
+                    .await?;
+            }
+            None => {
+                parser.parse_mime_recursive(context, &mail, false).await?;
+            }
+        };
+
         parser.maybe_remove_bad_parts();
         parser.maybe_remove_inline_mailinglist_footer();
         parser.heuristically_parse_ndn(context).await;
@@ -1443,9 +1466,9 @@ pub struct Part {
     pub msg_raw: Option<String>,
     pub bytes: usize,
     pub param: Params,
-    org_filename: Option<String>,
+    pub(crate) org_filename: Option<String>,
     pub error: Option<String>,
-    dehtml_failed: bool,
+    pub(crate) dehtml_failed: bool,
 
     /// the part is a child or a descendant of multipart/related.
     /// typically, these are images that are referenced from text/html part
@@ -1453,7 +1476,7 @@ pub struct Part {
     ///
     /// note that multipart/related may contain further multipart nestings
     /// and all of them needs to be marked with `is_related`.
-    is_related: bool,
+    pub(crate) is_related: bool,
 }
 
 /// return mimetype and viewtype for a parsed mail