Gracefully handle HTTP 405 errors

ldp/src/error.rs

@@ -26,4 +26,7 @@ pub enum Error {
     /// The RDF data failed to parse.
     #[error(transparent)]
     InvalidRdfSyntax(#[from] oxigraph::io::RdfSyntaxError),
+
+    #[error(transparent)]
+    ToStr(#[from] http::header::ToStrError),
 }

ldp/src/resource.rs

@@ -2,6 +2,7 @@ use crate::rdf_source::RdfSource;
 use crate::vocab;
 use bytes::Bytes;
 use futures::Stream;
+use http::Method;
 use oxigraph::io::{RdfFormat, RdfParser};
 use oxigraph::model::{GraphNameRef, NamedNodeRef, Quad};
 use reqwest_middleware::reqwest::{Client, Response, StatusCode, Url, header};
@@ -34,6 +35,7 @@ pub struct ResourceRequestBuilder {
     client: ClientWithMiddleware,
     url: Url,
     follow_described_by: bool,
+    validate_support: bool,
     formats: Vec<RdfFormat>,
 }
 
@@ -52,17 +54,30 @@ impl ResourceRequestBuilder {
             client,
             url,
             follow_described_by: true,
+            validate_support: true,
             formats: Vec::new(),
         }
     }
 
     /// If the HTTP response includes a [describedby](https://www.w3.org/TR/ldp/#link-relation-describedby) link rel, then it will be followed.
+    ///
+    /// Default value: `true`.
     #[must_use]
     pub fn follow_described_by(mut self, value: bool) -> Self {
         self.follow_described_by = value;
         self
     }
 
+    /// Require the presence of a `Link: <http://www.w3.org/ns/ldp#Resource>; rel="type"` response header.
+    /// This is described in [section 4.2.1.4](https://www.w3.org/TR/ldp/#ldpr-resource) of the spec.
+    ///
+    /// Default value: `true`.
+    #[must_use]
+    pub fn validate_support(mut self, value: bool) -> Self {
+        self.validate_support = value;
+        self
+    }
+
     /// Restrict the request to the given RDF format.
     ///
     /// Repeated calls are additive. By default, all formats — even non-RDF formats — are accepted.
@@ -150,19 +165,48 @@ impl ResourceRequest {
         request_builder
     }
 
+    fn is_method_allowed(response: &Response, method: Method) -> crate::Result<bool> {
+        for header in response.headers().get_all(header::ALLOW) {
+            for value in header.to_str()?.replace(" ", "").split(',') {
+                if value == method {
+                    return Ok(true);
+                }
+            }
+        }
+        Ok(false)
+    }
+
     /// Send the request.
     ///
-    /// There are two stages to this process. First, a HEAD request is made to determine whether
+    /// There are two stages to this process. First, a `HEAD` request is made to determine whether
     /// the requested resource is described by an RDF graph at another location. If it is, and if
     /// the user permits it, then the URL in the
     /// [describedby](https://www.w3.org/TR/ldp/#link-relation-describedby) header is used.
     /// Otherwise, the original URL is used.
     ///
-    /// During the second stage, a GET request is made. No parsing occurs at this time.
+    /// During the second stage, a `GET` request is made. No parsing occurs at this time.
+    ///
+    /// If the `HEAD` method is not allowed on this URI (a violation of the LDP spec), the `Allow`
+    /// header is inspected for `GET`. If absent, this method will return an error.
     pub async fn send(&self) -> crate::Result<Resource> {
         let request_builder = self.builder.client.head(self.builder.url.clone());
-        let mut response = request_builder.send().await?.error_for_status()?;
-        Self::ensure_ldp_support(&response)?;
+        let mut response = request_builder.send().await?;
+
+        match response.error_for_status_ref() {
+            Ok(response) => {
+                if self.builder.validate_support {
+                    Self::ensure_ldp_support(&response)?;
+                }
+            }
+            Err(err) if err.status() == Some(StatusCode::METHOD_NOT_ALLOWED) => {
+                if !Self::is_method_allowed(&response, Method::GET)? {
+                    return Err(err.into());
+                }
+            }
+            err => {
+                err?;
+            }
+        }
 
         let url_to_get;
         let described_by = Self::extract_described_by(&response);
@@ -178,7 +222,10 @@ impl ResourceRequest {
         request_builder = self.add_media_types(request_builder);
 
         response = request_builder.send().await?.error_for_status()?;
-        Self::ensure_ldp_support(&response)?;
+
+        if self.builder.validate_support {
+            Self::ensure_ldp_support(&response)?;
+        }
 
         let state_token = response
             .headers()