src/shared/network/remoteImageFetch.ts

import {
  type OutboundUrlGuardMode,
  getProviderOutboundGuard,
  parseAndValidatePublicUrl,
  parseOutboundUrl,
} from "@/shared/network/outboundUrlGuard";

const DEFAULT_MAX_REMOTE_IMAGE_BYTES = 20 * 1024 * 1024;
const DEFAULT_MAX_REDIRECTS = 3;
const DEFAULT_TIMEOUT_MS = 15000;

export interface RemoteImageFetchOptions {
  fetchImpl?: typeof fetch;
  guard?: OutboundUrlGuardMode;
  maxBytes?: number;
  maxRedirects?: number;
  signal?: AbortSignal;
  timeoutMs?: number;
}

export interface RemoteImageFetchResult {
  buffer: Buffer;
  contentType: string;
  url: string;
}

function validateRemoteImageUrl(input: string | URL, guard: OutboundUrlGuardMode) {
  return guard === "public-only" ? parseAndValidatePublicUrl(input) : parseOutboundUrl(input);
}

function combineSignals(signal: AbortSignal | undefined, timeoutMs: number) {
  const timeoutSignal = AbortSignal.timeout(timeoutMs);
  if (!signal) return timeoutSignal;
  return AbortSignal.any([signal, timeoutSignal]);
}

async function readResponseBuffer(response: Response, maxBytes: number) {
  const contentLengthHeader = response.headers.get("content-length");
  const contentLength = contentLengthHeader ? Number.parseInt(contentLengthHeader, 10) : null;
  if (contentLength !== null && Number.isFinite(contentLength) && contentLength > maxBytes) {
    throw new Error(`Remote image exceeds ${maxBytes} byte limit`);
  }

  if (!response.body) {
    const buffer = Buffer.from(await response.arrayBuffer());
    if (buffer.byteLength > maxBytes) {
      throw new Error(`Remote image exceeds ${maxBytes} byte limit`);
    }
    return buffer;
  }

  const reader = response.body.getReader();
  const chunks: Buffer[] = [];
  let totalBytes = 0;

  try {
    while (true) {
      const { done, value } = await reader.read();
      if (done) break;

      const chunk = Buffer.from(value);
      totalBytes += chunk.byteLength;
      if (totalBytes > maxBytes) {
        await reader.cancel();
        throw new Error(`Remote image exceeds ${maxBytes} byte limit`);
      }
      chunks.push(chunk);
    }
  } finally {
    reader.releaseLock();
  }

  return Buffer.concat(chunks, totalBytes);
}

export async function fetchRemoteImage(
  input: string | URL,
  options: RemoteImageFetchOptions = {}
): Promise<RemoteImageFetchResult> {
  const fetchImpl = options.fetchImpl ?? fetch;
  const guard = options.guard ?? getProviderOutboundGuard();
  const maxBytes = options.maxBytes ?? DEFAULT_MAX_REMOTE_IMAGE_BYTES;
  const maxRedirects = options.maxRedirects ?? DEFAULT_MAX_REDIRECTS;
  const signal = combineSignals(options.signal, options.timeoutMs ?? DEFAULT_TIMEOUT_MS);

  let currentUrl = validateRemoteImageUrl(input, guard);
  for (let redirectCount = 0; redirectCount <= maxRedirects; redirectCount++) {
    const response = await fetchImpl(currentUrl.toString(), {
      method: "GET",
      redirect: "manual",
      signal,
    });

    if (response.status >= 300 && response.status < 400) {
      const location = response.headers.get("location");
      if (!location) {
        throw new Error(`Remote image redirect missing Location header (${response.status})`);
      }
      if (redirectCount >= maxRedirects) {
        throw new Error(`Remote image exceeded ${maxRedirects} redirect limit`);
      }
      currentUrl = validateRemoteImageUrl(new URL(location, currentUrl), guard);
      continue;
    }

    if (!response.ok) {
      throw new Error(`Remote image fetch error ${response.status}`);
    }

    return {
      buffer: await readResponseBuffer(response, maxBytes),
      contentType: response.headers.get("content-type") || "application/octet-stream",
      url: currentUrl.toString(),
    };
  }

  throw new Error(`Remote image exceeded ${maxRedirects} redirect limit`);
}