Add Sq::get_certification_key.

- Add `Sq::get_certification_key`, which is a wrapper around
    `Sq::get_certification_keys`, for the case where only one
    certificate is looked up.

  - Update users.

src/commands/network.rs

@@ -241,12 +241,7 @@ pub fn certify_downloads<'store, 'rstore>(sq: &mut Sq<'store, 'rstore>,
     let ca = || -> Result<_> {
         let ca = ca.to_cert()?;
 
-        let keys = sq.get_certification_keys(&[ca], None)?;
-        assert!(
-            keys.len() == 1,
-            "Expect exactly one result from get_certification_keys()"
-        );
-        Ok(keys.into_iter().next().unwrap().0)
+        Ok(sq.get_certification_key(ca, None)?.0)
     };
     let mut ca_signer = match ca() {
         Ok(signer) => signer,

src/commands/pki/certify.rs

@@ -119,12 +119,7 @@ pub fn certify(sq: Sq, c: certify::Command)
         options.push(GetKeysOptions::AllowRevoked);
     }
 
-    let keys = sq.get_certification_keys(&[certifier], Some(&options))?;
-    assert_eq!(
-        keys.len(), 1,
-        "Expect exactly one result from get_certification_keys()"
-    );
-    let mut signer = keys.into_iter().next().unwrap().0;
+    let mut signer = sq.get_certification_key(certifier, Some(&options))?.0;
 
     // Create the certifications.
     let mut new_packets: Vec<Packet> = Vec::new();

src/commands/pki/link.rs

@@ -478,13 +478,8 @@ pub fn add(sq: Sq, c: link::AddCommand)
     };
 
     // Sign it.
-    let keys = sq.get_certification_keys(&[trust_root], None)
-        .context("Looking up local trust root")?;
-    assert!(
-        keys.len() == 1,
-        "Expect exactly one result from get_certification_keys()"
-    );
-    let mut signer = keys.into_iter().next().unwrap().0;
+    let mut signer = sq.get_certification_key(trust_root, None)
+        .context("Looking up local trust root")?.0;
 
     let certifications = active_certification(
             &sq, &vc.fingerprint(), userids,
@@ -645,13 +640,8 @@ pub fn retract(sq: Sq, c: link::RetractCommand)
     };
 
     // Sign it.
-    let keys = sq.get_certification_keys(&[trust_root], None)
-        .context("Looking up local trust root")?;
-    assert!(
-        keys.len() == 1,
-        "Expect exactly one result from get_certification_keys()"
-    );
-    let mut signer = keys.into_iter().next().unwrap().0;
+    let mut signer = sq.get_certification_key(trust_root, None)
+        .context("Looking up local trust root")?.0;
 
     let certifications = active_certification(
             &sq, &cert.fingerprint(), userids, signer.public())

src/common/revoke.rs

@@ -38,12 +38,8 @@ pub fn get_secret_signer<'a>(
     secret: Option<&'a Cert>,
 ) -> Result<(Cert, Box<dyn Signer + Send + Sync>)> {
     if let Some(secret) = secret {
-        if let Ok(keys) = sq.get_certification_keys(&[secret.clone()], None) {
-            assert!(
-                keys.len() == 1,
-                "Expect exactly one result from get_certification_keys()"
-            );
-            Ok((secret.clone(), keys.into_iter().next().expect("have one").0))
+        if let Ok((key, _password)) = sq.get_certification_key(secret, None) {
+            Ok((secret.clone(), key))
         } else {
             if ! sq.time_is_now {
                 return Err(anyhow!(
@@ -63,12 +59,8 @@ does not contain a certification key with secret key material"
             }
         }
     } else {
-        if let Ok(keys) = sq.get_certification_keys(&[cert], None) {
-            assert!(
-                keys.len() == 1,
-                "Expect exactly one result from get_certification_keys()"
-            );
-            Ok((cert.clone(), keys.into_iter().next().expect("have one").0))
+        if let Ok((key, _password)) = sq.get_certification_key(cert, None) {
+            Ok((cert.clone(), key))
         } else {
             if ! sq.time_is_now {
                 return Err(anyhow!(

src/sq.rs

@@ -1204,6 +1204,23 @@ impl<'store: 'rstore, 'rstore> Sq<'store, 'rstore> {
                       options)
     }
 
+    /// Returns suitable certification keys from a given list of Certs.
+    ///
+    /// This returns one key for each Cert.  If a Cert doesn't have an
+    /// appropriate key, then this returns an error.
+    pub fn get_certification_key<C>(&self, cert: C,
+                                    options: Option<&[GetKeysOptions]>)
+        -> Result<(Box<dyn crypto::Signer + Send + Sync>, Option<Password>)>
+    where C: std::borrow::Borrow<Cert>
+    {
+        let keys = self.get_certification_keys(&[cert], options)?;
+        assert!(
+            keys.len() == 1,
+            "Expected exactly one result from get_certification_keys()"
+        );
+        Ok(keys.into_iter().next().unwrap())
+    }
+
     /// Prints additional information in verbose mode.
     pub fn info(&self, msg: fmt::Arguments) {
         if self.verbose {