vault-link/frontend/sync-client/src/services/fetch-controller.ts

import type { Logger } from "../tracing/logger";
import { SyncResetError } from "../errors/sync-reset-error";

/**
 * Offers a resettable fetch implementation that waits until syncing is enabled
 * and aborts outstanding requests when a reset is started.
 */
export class FetchController {
    private static readonly UNTIL_RESOLUTION = Symbol();

    private isResetting = false;

    // Promise resolves on the next state change: sync enabled/disabled or reset started/ended
    private until: Promise<symbol>;
    private resolveUntil: (value: symbol | PromiseLike<symbol>) => void;
    private rejectUntil: (reason?: unknown) => void;

    public constructor(
        private _canFetch: boolean,
        private readonly logger: Logger
    ) {
        ({
            promise: this.until,
            resolve: this.resolveUntil,
            reject: this.rejectUntil
        } = Promise.withResolvers<symbol>());
    }

    /**
     * Whether the fetch implementation can immediately send requests once outside of a reset.
     */
    public get canFetch(): boolean {
        return this._canFetch;
    }

    /**
     * Allow or disallow fetching. The changes only take effect if not resetting.
     * When called during a reset, its effect is deferred until the reset is finished.
     *
     * @param canFetch Whether fetching is enabled
     */
    public set canFetch(canFetch: boolean) {
        this._canFetch = canFetch;

        if (!this.isResetting) {
            const previousResolve = this.resolveUntil;
            ({
                promise: this.until,
                resolve: this.resolveUntil,
                reject: this.rejectUntil
            } = Promise.withResolvers<symbol>());
            previousResolve(FetchController.UNTIL_RESOLUTION);
        }
    }

    private static getUrlFromInput(input: RequestInfo | URL): string {
        if (input instanceof URL) {
            return input.href;
        }
        if (typeof input === "string") {
            return input;
        }
        return input.url;
    }

    /**
     * Starts a reset, causing all ongoing and future fetches to be rejected
     * with a SyncResetError until finishReset is called.
     */
    public startReset(): void {
        this.isResetting = true;
        this.rejectUntil(new SyncResetError());
        // Catch unhandled rejection if no fetches are waiting
        this.until.catch(() => {
            // Intentionally ignore - this rejection is handled by waiting fetches
        });
    }

    /**
     * Finishes a reset, allowing fetches to proceed or wait again depending on
     * the current sync settings.
     */
    public finishReset(): void {
        if (!this.isResetting) {
            return;
        }

        this.isResetting = false;
        ({
            promise: this.until,
            resolve: this.resolveUntil,
            reject: this.rejectUntil
        } = Promise.withResolvers<symbol>());
    }

    /**
     *
     * |------------------|---------------|-----------------------------------------------------|
     * |                  | Sync enabled  |                 Sync disabled                       |
     * |------------------|-------------- |-----------------------------------------------------|
     * | During reset     |        Rejects with SyncResetError without sending request          |
     * |------------------|-------------- |-----------------------------------------------------|
     * | Outside of reset | Same as fetch | Blocks until sync is enabled and then same as fetch |
     * |------------------|---------------|-----------------------------------------------------|
     *
     * @param logger for errors
     * @param fetch to wrap
     * @returns a wrapped fetch implementation affected by the FetchController state
     */
    public getControlledFetchImplementation(
        logger: Logger,
        fetch: typeof globalThis.fetch = globalThis.fetch
    ): typeof globalThis.fetch {
        return async (
            input: RequestInfo | URL,
            init?: RequestInit
        ): Promise<Response> => {
            while (!this.canFetch || this.isResetting) {
                await this.until;
            }

            try {
                // https://github.com/jonbern/fetch-retry/blob/8684ef4e688375f623bd76f13add76dbc1d67cfb/index.js#L67C1-L70C21
                const _input =
                    typeof Request !== "undefined" && input instanceof Request
                        ? input.clone()
                        : input;

                const fetchPromise = fetch(_input, init);

                // We only want to catch rejections from `this.until`
                let result: symbol | Response | undefined = undefined;
                do {
                    result = await Promise.race([this.until, fetchPromise]);
                } while (result === FetchController.UNTIL_RESOLUTION);

                const fetchResult: Response = result as Response; // eslint-disable-line @typescript-eslint/no-unsafe-type-assertion

                if (!fetchResult.ok) {
                    this.logger.warn(
                        `Fetch for ${FetchController.getUrlFromInput(
                            input
                        )}, got status ${fetchResult.status}`
                    );
                }

                return fetchResult;
            } catch (error) {
                logger.warn(
                    `Fetch for ${FetchController.getUrlFromInput(
                        input
                    )}, got error: ${error}`
                );
                throw error;
            }
        };
    }
}