Parse and save Retry-After in errors

Closes #7.

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/).
 - Configurable `Tenant` for `Auth`, this allows login into applications that
   accept only personal accounts.
 - Expose `Auth::client`.
+- `Retry-After` response header on rate limited or service unavailability is
+  now parsed and returned via `Error::retry_after`.
 
 ### Changed
 

src/error.rs

@@ -1,3 +1,5 @@
+use std::time::Duration;
+
 use crate::resource::{ErrorResponse, OAuth2ErrorResponse};
 use reqwest::StatusCode;
 use thiserror::Error;
@@ -25,18 +27,28 @@ enum ErrorKind {
     ErrorResponse {
         status: StatusCode,
         response: ErrorResponse,
+        retry_after: Option<u32>,
     },
     #[error("OAuth2 error with {status}: ({}) {}", .response.error, .response.error_description)]
     OAuth2Error {
         status: StatusCode,
         response: OAuth2ErrorResponse,
+        retry_after: Option<u32>,
     },
 }
 
 impl Error {
-    pub(crate) fn from_error_response(status: StatusCode, response: ErrorResponse) -> Self {
+    pub(crate) fn from_error_response(
+        status: StatusCode,
+        response: ErrorResponse,
+        retry_after: Option<u32>,
+    ) -> Self {
         Self {
-            inner: Box::new(ErrorKind::ErrorResponse { status, response }),
+            inner: Box::new(ErrorKind::ErrorResponse {
+                status,
+                response,
+                retry_after,
+            }),
         }
     }
 
@@ -49,9 +61,14 @@ impl Error {
     pub(crate) fn from_oauth2_error_response(
         status: StatusCode,
         response: OAuth2ErrorResponse,
+        retry_after: Option<u32>,
     ) -> Self {
         Self {
-            inner: Box::new(ErrorKind::OAuth2Error { status, response }),
+            inner: Box::new(ErrorKind::OAuth2Error {
+                status,
+                response,
+                retry_after,
+            }),
         }
     }
 
@@ -84,6 +101,21 @@ impl Error {
             }
         }
     }
+
+    /// Get the retry delay hint on rate limited (HTTP 429) or server unavailability, if any.
+    ///
+    /// This is parsed from response header `Retry-After`.
+    /// See: <https://learn.microsoft.com/en-us/graph/throttling>
+    #[must_use]
+    pub fn retry_after(&self) -> Option<Duration> {
+        match &*self.inner {
+            ErrorKind::ErrorResponse { retry_after, .. }
+            | ErrorKind::OAuth2Error { retry_after, .. } => {
+                Some(Duration::from_secs((*retry_after)?.into()))
+            }
+            _ => None,
+        }
+    }
 }
 
 impl From<reqwest::Error> for Error {

src/util.rs

@@ -2,7 +2,7 @@ use crate::{
     error::{Error, Result},
     resource::{DriveId, ErrorResponse, ItemId, OAuth2ErrorResponse},
 };
-use reqwest::{RequestBuilder, Response, StatusCode};
+use reqwest::{header, RequestBuilder, Response, StatusCode};
 use serde::{de, Deserialize};
 use url::PathSegmentsMut;
 
@@ -310,8 +310,9 @@ pub(crate) async fn handle_error_response(resp: Response) -> Result<Response> {
     if status.is_success() || status.is_redirection() {
         Ok(resp)
     } else {
+        let retry_after = parse_retry_after_sec(&resp);
         let resp: Resp = resp.json().await?;
-        Err(Error::from_error_response(status, resp.error))
+        Err(Error::from_error_response(status, resp.error, retry_after))
     }
 }
 
@@ -320,7 +321,21 @@ pub(crate) async fn handle_oauth2_error_response(resp: Response) -> Result<Respo
     if status.is_success() {
         Ok(resp)
     } else {
+        let retry_after = parse_retry_after_sec(&resp);
         let resp: OAuth2ErrorResponse = resp.json().await?;
-        Err(Error::from_oauth2_error_response(status, resp))
+        Err(Error::from_oauth2_error_response(status, resp, retry_after))
     }
 }
+
+/// The documentation said it is in seconds:
+/// <https://learn.microsoft.com/en-us/graph/throttling#best-practices-to-handle-throttling>.
+/// And HTTP requires it to be a non-negative integer:
+/// <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After>.
+fn parse_retry_after_sec(resp: &Response) -> Option<u32> {
+    resp.headers()
+        .get(header::RETRY_AFTER)?
+        .to_str()
+        .ok()?
+        .parse()
+        .ok()
+}