ACME: poll authorizations instead of challenges.

Previously, we assumed that we should poll the challenge URL until
completion, because all the servers we tested correctly responded to a
POST-as-GET to the challenger URL.
We also made an initial POST-as-GET request to fetch the status and skip
the challenges in "valid" state.

RFC8555 7.5.1 specifies that we should poll the authorization resource
status instead. Additionally, the discussion around Errata ID 6317
implies that it is harmful to initiate multiple challenges, as a single
failure invalidates the whole authorization.

Given the above, we can simplify the code and proceed to poll the
authorization status after initiating the first available challenge.

Author: bavshin-f5

src/acme.rs

@@ -405,18 +405,33 @@ where
         url: http::Uri,
         authorization: types::Authorization,
     ) -> Result<()> {
-        let mut result = Err(anyhow!("no challenges"));
         let identifier = authorization.identifier.as_ref();
 
-        for challenge in authorization.challenges {
-            result = self.do_challenge(order, &identifier, &challenge).await;
+        // Find and set up first supported challenge.
+        let (challenge, solver) = authorization
+            .challenges
+            .iter()
+            .find_map(|x| {
+                let solver = self.find_solver_for(&x.kind)?;
+                solver.register(order, &identifier, x).ok()?;
+                Some((x, solver))
+            })
+            .ok_or(anyhow!("no supported challenge for {identifier:?}"))?;
 
-            if result.is_ok() {
-                break;
-            }
+        scopeguard::defer! {
+            let _ = solver.unregister(&identifier, challenge);
+        };
+
+        let res = self.post(&challenge.url, b"{}").await?;
+        let result: types::Challenge = serde_json::from_slice(res.body())?;
+        if !matches!(
+            result.status,
+            ChallengeStatus::Processing | ChallengeStatus::Valid
+        ) {
+            return Err(anyhow!("unexpected challenge status {:?}", result.status));
         }
 
-        result?;
+        wait_for_retry(&res).await;
 
         let mut tries = 10;
 
@@ -440,63 +455,7 @@ where
         );
 
         if result.status != AuthorizationStatus::Valid {
-            return Err(anyhow!("authorization failed"));
-        }
-
-        Ok(())
-    }
-
-    async fn do_challenge(
-        &self,
-        ctx: &AuthorizationContext<'_>,
-        identifier: &Identifier<&str>,
-        challenge: &types::Challenge,
-    ) -> Result<()> {
-        let res = self.post(&challenge.url, b"").await?;
-        let result: types::Challenge = serde_json::from_slice(res.body())?;
-
-        // Previous challenge result is still valid.
-        // Should not happen as we already skip valid authorizations.
-        if result.status == ChallengeStatus::Valid {
-            return Ok(());
-        }
-
-        let solver = self
-            .find_solver_for(&challenge.kind)
-            .ok_or(anyhow!("no solver for {:?}", challenge.kind))?;
-
-        solver.register(ctx, identifier, challenge)?;
-
-        scopeguard::defer! {
-            let _ = solver.unregister(identifier, challenge);
-        };
-
-        // "{}" in request payload initiates the challenge, "" checks the status.
-        let mut payload: &[u8] = b"{}";
-        let mut tries = 10;
-
-        let result = loop {
-            let res = self.post(&challenge.url, payload).await?;
-            let result: types::Challenge = serde_json::from_slice(res.body())?;
-
-            if !matches!(
-                result.status,
-                ChallengeStatus::Pending | ChallengeStatus::Processing,
-            ) || tries == 0
-            {
-                break result;
-            }
-
-            tries -= 1;
-            payload = b"";
-            wait_for_retry(&res).await;
-        };
-
-        if result.status != ChallengeStatus::Valid {
-            return Err(result
-                .error
-                .map(Into::into)
-                .unwrap_or(anyhow!("unknown error")));
+            return Err(anyhow!("authorization failed ({:?})", result.status));
         }
 
         Ok(())