Turn dc_ensure_secret_key_existy into something more rusty

This marks the function safe and returns Result, it also now returns
the ConfiguredAddr since it has to look this up anyway and it makes
testing more easy.  Turns out it reduces some duplicate SQL query in
some callers too.

More test code has been moved from dc_imex to test_utils as it's
more genrally applicable.

src/dc_e2ee.rs

@@ -16,10 +16,12 @@ use mmime::mmapstring::*;
 use mmime::{mailmime_substitute, MAILIMF_NO_ERROR, MAIL_NO_ERROR};
 
 use crate::aheader::*;
+use crate::config::Config;
 use crate::context::Context;
 use crate::dc_mimeparser::*;
 use crate::dc_securejoin::*;
 use crate::dc_tools::*;
+use crate::error::*;
 use crate::key::*;
 use crate::keyring::*;
 use crate::peerstate::*;
@@ -1045,33 +1047,53 @@ pub unsafe fn dc_e2ee_thanks(helper: &mut dc_e2ee_helper_t) {
     helper.cdata_to_free = 0 as *mut libc::c_void;
 }
 
-/* makes sure, the private key exists, needed only for exporting keys and the case no message was sent before */
-// TODO should return bool /rtn
-pub unsafe fn dc_ensure_secret_key_exists(context: &Context) -> libc::c_int {
-    /* normally, the key is generated as soon as the first mail is send
-    (this is to gain some extra-random-seed by the message content and the timespan between program start and message sending) */
-    let mut success: libc::c_int = 0i32;
-
-    let self_addr = context.sql.get_config(context, "configured_addr");
-    if self_addr.is_none() {
-        warn!(
-            context,
-            0, "Cannot ensure secret key if context is not configured.",
-        );
-    } else if load_or_generate_self_public_key(context, self_addr.unwrap(), 0 as *mut mailmime)
-        .is_some()
-    {
-        /*no random text data for seeding available*/
-        success = 1;
+/// Ensures a private key exists for the configured user.
+///
+/// Normally the private key is generated when the first message is
+/// sent (allowing the use of some extra random seed from the message
+/// content) but in a few locations there are no such guarantees,
+/// e.g. when exporting keys, and calling this function ensures a
+/// private key will be present.
+///
+/// If this succeeds you are also guaranteed that the
+/// [Config::ConfiguredAddr] is configured, this address is returned.
+pub fn dc_ensure_secret_key_exists(context: &Context) -> Result<String> {
+    let self_addr = context
+        .get_config(Config::ConfiguredAddr)
+        .ok_or(format_err!(concat!(
+            "Failed to get self address, ",
+            "cannot ensure secret key if not configured."
+        )))?;
+    unsafe {
+        load_or_generate_self_public_key(context, &self_addr, 0 as *mut mailmime)
+            .ok_or(format_err!("Failed to generate private key."))?;
     }
-
-    success
+    Ok(self_addr)
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    use crate::test_utils::*;
+
+    mod ensure_secret_key_exists {
+        use super::*;
+
+        #[test]
+        fn test_prexisting() {
+            let t = dummy_context();
+            let test_addr = configure_alice_keypair(&t.ctx);
+            assert_eq!(dc_ensure_secret_key_exists(&t.ctx).unwrap(), test_addr);
+        }
+
+        #[test]
+        fn test_not_configured() {
+            let t = dummy_context();
+            assert!(dc_ensure_secret_key_exists(&t.ctx).is_err());
+        }
+    }
+
     #[test]
     fn test_mailmime_parse() {
         let plain = b"Chat-Disposition-Notification-To: holger@deltachat.de