KTOR-1464 Fix client Digest auth realm handling

Fix client digest auth realm handling to use the realm provided by the server if the realm is not configured in the client feature

ktor-client/ktor-client-features/ktor-client-auth/common/src/io/ktor/client/features/auth/Auth.kt

@@ -50,6 +50,7 @@ public class Auth(
 
                     val request = HttpRequestBuilder()
                     request.takeFromWithExecutionContext(context)
+                    request.attributes.put(AuthHeaderAttribute, authHeader)
                     provider.addRequestHeaders(request)
                     request.attributes.put(circuitBreaker, Unit)
 
@@ -67,3 +68,11 @@ public class Auth(
 public fun HttpClientConfig<*>.Auth(block: Auth.() -> Unit) {
     install(Auth, block)
 }
+
+/**
+ * AuthHeader from the previous unsuccessful request. This actually should be passed as
+ * parameter to AuthProvider.addRequestHeaders instead in the future and the attribute will
+ * be removed after that.
+ */
+@PublicAPICandidate("1.6.0")
+internal val AuthHeaderAttribute = AttributeKey<HttpAuthHeader>("AuthHeader")

ktor-client/ktor-client-features/ktor-client-auth/common/src/io/ktor/client/features/auth/providers/DigestAuthProvider.kt

@@ -55,14 +55,18 @@ public class DigestAuthProvider(
 
     override fun isApplicable(auth: HttpAuthHeader): Boolean {
         if (auth !is HttpAuthHeader.Parameterized ||
-            auth.parameter("realm") != realm ||
             auth.authScheme != AuthScheme.Digest
         ) return false
 
         val newNonce = auth.parameter("nonce") ?: return false
         val newQop = auth.parameter("qop")
         val newOpaque = auth.parameter("opaque")
 
+        val newRealm = auth.parameter("realm") ?: return false
+        if (newRealm != realm && realm != null) {
+            return false
+        }
+
         serverNonce.value = newNonce
         qop.value = newQop
         opaque.value = newOpaque
@@ -90,6 +94,9 @@ public class DigestAuthProvider(
         }
 
         val token = makeDigest(tokenSequence.joinToString(":"))
+        val realm = realm ?: request.attributes.getOrNull(AuthHeaderAttribute)?.let { auth ->
+            (auth as? HttpAuthHeader.Parameterized)?.parameter("realm")
+        }
 
         val auth = HttpAuthHeader.Parameterized(
             AuthScheme.Digest,

ktor-client/ktor-client-features/ktor-client-auth/common/test/io/ktor/client/features/auth/DigestProviderTest.kt

@@ -56,6 +56,32 @@ class DigestProviderTest {
         checkStandardFields(authHeader)
     }
 
+    @Test
+    fun addRequestHeadersMissingRealm() = testSuspend {
+        if (!PlatformUtils.IS_JVM) return@testSuspend
+
+        val providerWithoutRealm = DigestAuthProvider("username", "pass", null)
+        val authHeader = parseAuthorizationHeader(authAllFields)!!
+        requestBuilder.attributes.put(AuthHeaderAttribute, authHeader)
+
+        assertTrue(providerWithoutRealm.isApplicable(authHeader))
+        providerWithoutRealm.addRequestHeaders(requestBuilder)
+
+        val resultAuthHeader = requestBuilder.headers[HttpHeaders.Authorization]!!
+        checkStandardFields(resultAuthHeader)
+    }
+
+    @Test
+    fun addRequestHeadersChangedRealm() = testSuspend {
+        if (!PlatformUtils.IS_JVM) return@testSuspend
+
+        val providerWithoutRealm = DigestAuthProvider("username", "pass", "wrong!")
+        val authHeader = parseAuthorizationHeader(authAllFields)!!
+        requestBuilder.attributes.put(AuthHeaderAttribute, authHeader)
+
+        assertFalse(providerWithoutRealm.isApplicable(authHeader))
+    }
+
     @Test
     fun addRequestHeadersOmitsQopAndOpaqueWhenMissing() = testSuspend {
         if (!PlatformUtils.IS_JVM) return@testSuspend