feat: Create session with password if refresh token is invalid

darkfeed/src/main/kotlin/Main.kt

···
             verifyAndUpdateFeedGeneratorRecord(bskyApi)
         } catch (error: Exception) {
             log.warn("Failed to update feed generator record: {}", error.message)
+        } catch (error: NotImplementedError) {
+            log.warn("Unimplemented path hit while updating feed generator record:\n{}", error.stackTraceToString())
         }
 
         // Serve the feed generator.

darkfeed/src/main/kotlin/api/AuthManager.kt

···
                 }
             }
 
+            HttpStatusCode.BadRequest -> throw RefreshTokenInvalidException()
+
             else -> {
                 TODO("Handle failures")
             }
···
     /** Create `AuthTokens` from an `AuthSession`. */
     fun into(): AuthTokens = AuthTokens(this.accessJwt, this.refreshJwt, this.did)
 }
+
+/**
+ * Base class for errors specific to the [AuthManager].
+ *
+ * @param message Description of the error.
+ * @param cause Underlying exception that caused the error, if any.
+ */
+open class AuthManagerException(
+    message: String,
+    cause: Throwable? = null,
+) : RuntimeException(message, cause)
+
+/**
+ * Stored refresh token is invalid.
+ */
+class RefreshTokenInvalidException() : AuthManagerException(message = "Refresh token is invalid")

darkfeed/src/main/kotlin/api/AuthPlugin.kt

···
                         // other requests will block when they try to get the
                         // tokens.
                         authMutex.withLock {
-                            authManager.refreshSession()
+                            try {
+                                authManager.refreshSession()
+                            } catch (error: RefreshTokenInvalidException) {
+                                // Create new session using username and password.
+                                log.debug("Refresh token is invalid. Creating session from username and password.")
+                                authManager.createSession()
+                            }
                             authManager.authTokens?.accessToken
                         }
                     }