Fix streaming failing with "range not supported" on servers negotiating compression

CHANGELOG.md

@@ -25,6 +25,12 @@ All notable changes to this project will be documented in this file. Take a look
 * `LCPService.init` now requires an explicit `deviceName` parameter. We recommend passing `UIDevice.current.name`. See [the migration guide](docs/Migration%20Guide.md).
 * `LCPDialogAuthentication` no longer takes a `sender` view controller. It now presents its passphrase dialog through a new `LCPDialogAuthenticationDelegate` that you implement and retain for the lifetime of the authentication. See [the Readium LCP guide](docs/Guides/Readium%20LCP.md) and [the migration guide](docs/Migration%20Guide.md).
 
+### Fixed
+
+#### Shared
+
+* [#653](https://github.com/readium/swift-toolkit/issues/653) `DefaultHTTPClient` now sends `Accept-Encoding: identity` with byte range requests. Previously, `URLSession` transparently negotiated a compressed (e.g. gzip) response, for which servers usually cannot serve byte ranges, causing streaming to fail with `HTTPError.rangeNotSupported` even though the server supports ranges.
+
 ### Removed
 
 * The deprecated `ReadiumAdapterGCDWebServer` and `ReadiumAdapterLCPSQLite` adapter packages have been removed.

Sources/Shared/Toolkit/HTTP/DefaultHTTPClient.swift

@@ -246,6 +246,16 @@ public final class DefaultHTTPClient: HTTPClient, Loggable {
             request.userAgent = userAgent
         }
 
+        // `URLSession` transparently negotiates compressed responses (e.g.
+        // gzip), but servers usually cannot serve byte ranges of a compressed
+        // representation and may omit the `Accept-Ranges` header in that case.
+        // Requesting the identity encoding ensures the server advertises and
+        // serves byte ranges properly.
+        // See https://github.com/readium/swift-toolkit/issues/653
+        if request.hasHeader("Range"), !request.hasHeader("Accept-Encoding") {
+            request.headers["Accept-Encoding"] = "identity"
+        }
+
         log(.info, request)
 
         let taskDelegate = TaskDelegate(

Tests/SharedTests/Toolkit/HTTP/DefaultHTTPClientTests.swift

@@ -770,6 +770,63 @@ struct DefaultHTTPClientTests {
             }
         }
 
+        @Test("Range request sends Accept-Encoding: identity to prevent transparent compression")
+        func rangeRequestSendsIdentityAcceptEncoding() async {
+            let receivedAcceptEncoding = Capture<String?>(nil)
+
+            let client = makeClient { request in
+                receivedAcceptEncoding.value = request.value(forHTTPHeaderField: "Accept-Encoding")
+                return .success(
+                    statusCode: 206,
+                    headers: ["Accept-Ranges": "bytes"],
+                    body: Data("partial".utf8)
+                )
+            }
+
+            var httpRequest = HTTPRequest(url: makeURL())
+            httpRequest.setRange(0 ..< 7)
+            _ = await client.fetch(httpRequest)
+
+            #expect(receivedAcceptEncoding.value == "identity")
+        }
+
+        @Test("Range request preserves a custom Accept-Encoding header")
+        func rangeRequestPreservesCustomAcceptEncoding() async {
+            let receivedAcceptEncoding = Capture<String?>(nil)
+
+            let client = makeClient { request in
+                receivedAcceptEncoding.value = request.value(forHTTPHeaderField: "Accept-Encoding")
+                return .success(
+                    statusCode: 206,
+                    headers: ["Accept-Ranges": "bytes"],
+                    body: Data("partial".utf8)
+                )
+            }
+
+            var httpRequest = HTTPRequest(url: makeURL(), headers: ["Accept-Encoding": "gzip"])
+            httpRequest.setRange(0 ..< 7)
+            _ = await client.fetch(httpRequest)
+
+            #expect(receivedAcceptEncoding.value == "gzip")
+        }
+
+        @Test("Regular request does not force the Accept-Encoding header")
+        func regularRequestDoesNotForceAcceptEncoding() async {
+            let receivedAcceptEncoding = Capture<String?>(nil)
+
+            let client = makeClient { request in
+                receivedAcceptEncoding.value = request.value(forHTTPHeaderField: "Accept-Encoding")
+                return .success()
+            }
+
+            _ = await client.fetch(HTTPRequest(url: makeURL()))
+
+            // `URLSession` may transparently add its own `Accept-Encoding`
+            // (e.g. gzip) at a lower level, so we only check that the client
+            // did not force the identity encoding.
+            #expect(receivedAcceptEncoding.value != "identity")
+        }
+
         @Test("Open-ended setRange omits upper bound in Range header")
         func openEndedRangeRequest() async {
             let receivedRange = Capture<String?>(nil)