quak/src/download/index.ts

import { randomUUID } from "node:crypto";
import { open, rename, rm } from "node:fs/promises";
import type { FileHandle } from "node:fs/promises";
import { dirname, join } from "node:path";
import {
    fromBase64,
    initStreamPull,
    pullStreamChunk,
    STREAM_CHUNK_OVERHEAD,
    STREAM_CHUNK_SIZE,
    streamTagFinal,
} from "../crypto/index.js";
import { TruncatedStreamError } from "../errors.js";
import { withRetry } from "../retry.js";
import type { ApiClient } from "../api/client.js";
import type { EnteFile } from "../model/types.js";

export interface DownloadResult {
    path: string;
    bytesWritten: number;
}

// Fired as decrypted plaintext accumulates, with the running total of
// plaintext bytes recovered so far. Within one download it is non-decreasing
// and its last value equals the final `bytesWritten`. A retry restarts the
// file from byte zero (see `fetchAndDecrypt`), so a fresh attempt begins its
// own count from zero.
export type ProgressCallback = (bytesDone: number) => void;

const ENC_CHUNK_SIZE = STREAM_CHUNK_SIZE + STREAM_CHUNK_OVERHEAD;

// Decrypt a secretstream body, handing each plaintext chunk to `sink` as it is
// produced rather than accumulating the whole file. Peak memory is one
// ciphertext chunk of network buffer plus one plaintext chunk — bounded by
// `STREAM_CHUNK_SIZE` regardless of the file's size — so a multi-gigabyte video
// no longer needs its size again in RAM. Returns the total plaintext length.
//
// The truncation contract is exactly the buffered version's, only the sink is
// new: a body cut short still decrypts and authenticates up to its last whole
// chunk, so the absence of TAG_FINAL is the sole evidence it was cut short, and
// this throws rather than let a caller keep a short file. The sink has already
// seen those chunks by then; the caller (`decryptToTemp`) stages them in a temp
// file that is renamed into place only on a clean return, so a throw leaves
// nothing on disk.
const streamDecrypt = async (
    stream: ReadableStream<Uint8Array>,
    header: Uint8Array,
    key: Uint8Array,
    sink: (plaintext: Uint8Array) => Promise<void>,
    onProgress?: ProgressCallback,
): Promise<number> => {
    const state = initStreamPull(header, key);
    const reader = stream.getReader();
    // Incoming reads are held as-is and only stitched into a contiguous chunk
    // at each `ENC_CHUNK_SIZE` boundary, so every received byte is copied once.
    // Concatenating on each read instead — reallocating the whole accumulator
    // per read — is O(n^2) in the bytes buffered, and for a 4 MiB chunk that
    // memory churn dwarfs the libsodium decryption itself.
    const pending: Uint8Array[] = [];
    let pendingBytes = 0;
    let totalPlain = 0;
    let chunksPulled = 0;
    let lastTag = -1;

    // Remove the first `size` bytes from `pending` as one contiguous buffer.
    // A read that straddles the boundary is split with `subarray` (a view, no
    // copy); its tail stays queued for the next chunk. `size` never exceeds
    // `pendingBytes`, so the queue always holds enough.
    const takeContiguous = (size: number): Uint8Array => {
        const out = new Uint8Array(size);
        let offset = 0;
        while (offset < size) {
            const piece = pending[0]!;
            const need = size - offset;
            if (piece.length <= need) {
                out.set(piece, offset);
                offset += piece.length;
                pending.shift();
            } else {
                out.set(piece.subarray(0, need), offset);
                pending[0] = piece.subarray(need);
                offset += need;
            }
        }
        pendingBytes -= size;
        return out;
    };

    const consume = async (
        plaintext: Uint8Array,
        tag: number,
    ): Promise<void> => {
        await sink(plaintext);
        totalPlain += plaintext.length;
        chunksPulled++;
        lastTag = tag;
        onProgress?.(totalPlain);
    };

    for (;;) {
        const { done, value } = await reader.read();
        if (value && value.length > 0) {
            pending.push(value);
            pendingBytes += value.length;
        }

        while (pendingBytes >= ENC_CHUNK_SIZE) {
            const encChunk = takeContiguous(ENC_CHUNK_SIZE);
            // A whole chunk that fails to authenticate while the stream carries
            // on is corruption, not truncation; that error propagates unchanged.
            const { plaintext, tag } = pullStreamChunk(state, encChunk);
            await consume(plaintext, tag);
        }

        if (done) {
            if (pendingBytes > 0) {
                const buffer = takeContiguous(pendingBytes);
                // Whatever is left over once every whole chunk has been
                // consumed must be the stream's final chunk, and a final
                // chunk that actually arrived in full authenticates. If it
                // does not, the body stopped part-way through a chunk — the
                // ordinary shape of a dropped connection. Poly1305 cannot
                // tell a partial chunk from a corrupt one, so this is
                // reported as the truncation it almost always is, with the
                // authentication failure kept as the error's cause. Only the
                // pull is guarded: a sink failure on a chunk that did
                // authenticate is a disk error, not a truncation.
                let pulled;
                try {
                    pulled = pullStreamChunk(state, buffer);
                } catch (err) {
                    throw new TruncatedStreamError(
                        `download: stream truncated: response body ended with ${buffer.length} trailing bytes that did not authenticate as a final chunk (transfer stopped mid-chunk, or the data is corrupt)`,
                        { cause: err },
                    );
                }
                await consume(pulled.plaintext, pulled.tag);
            }
            break;
        }
    }

    // Only the last chunk of a secretstream carries TAG_FINAL. Everything a
    // dropped connection did deliver still decrypts and authenticates, so the
    // absence of TAG_FINAL is the only evidence that the body was cut short.
    if (chunksPulled === 0) {
        throw new TruncatedStreamError(
            "download: stream truncated: response body contained no secretstream chunks",
        );
    }
    const tagFinal = streamTagFinal();
    if (lastTag !== tagFinal) {
        throw new TruncatedStreamError(
            `download: stream truncated: last chunk tag ${lastTag}, expected TAG_FINAL (${tagFinal})`,
        );
    }
    return totalPlain;
};

// Stage a write to `destination` atomically and durably, then rename it into
// place. `fill` writes the contents into the open temp file handle — either the
// whole buffer at once (`writeAtomic`) or chunk by chunk as they decrypt
// (`decryptToTemp`). The temp file is a sibling of the destination (same
// directory, so the rename cannot cross a filesystem boundary), so callers
// never observe a partially written destination, and a pre-existing file is
// replaced only once the new contents are complete on disk.
//
// Durability against a power cut needs two fsyncs. Without them the write can
// return while the data or the rename is still only in the kernel's page
// cache, and a crash then resurrects an empty renamed file — exactly the
// corruption a later backup run treats as a complete download. So the temp
// file's contents are fsynced before the rename, and the containing directory
// is fsynced after it, so both the bytes and the new directory entry are on
// stable storage before this returns.
//
// On any failure — including a `fill` that throws because the stream was
// truncated — the temp file is removed, so the destination is untouched and no
// scratch file is left to fill the disk on repeated failures.
const stageAtomic = async (
    destination: string,
    fill: (handle: FileHandle) => Promise<void>,
): Promise<void> => {
    const dir = dirname(destination);
    // The random suffix keeps concurrent downloads of the same destination
    // from stepping on each other's temporary file.
    const tmpPath = join(dir, `.quak-${randomUUID()}.tmp`);
    try {
        const handle = await open(tmpPath, "w");
        try {
            await fill(handle);
            await handle.sync();
        } finally {
            await handle.close();
        }
        await rename(tmpPath, destination);
        // Fsync the directory so the rename itself survives a crash: renaming
        // over a synced temp file still leaves the new directory entry in the
        // page cache until the directory is synced.
        const dirHandle = await open(dir, "r");
        try {
            await dirHandle.sync();
        } finally {
            await dirHandle.close();
        }
    } catch (err) {
        // Best-effort cleanup. A failure to remove the temporary file must
        // never replace the error that actually explains what went wrong.
        await rm(tmpPath, { force: true }).catch(() => undefined);
        throw err;
    }
};

// Write `plaintext` to `destination` atomically and durably. Exported so the
// metadata store can reuse the same durable write for small whole-buffer
// payloads; originals go through `decryptToTemp` instead so they never buffer.
export const writeAtomic = async (
    destination: string,
    plaintext: Uint8Array,
): Promise<void> =>
    stageAtomic(destination, (handle) => handle.writeFile(plaintext));

// Decrypt `stream` straight to `destination`, one plaintext chunk at a time,
// under the atomic writer's temp-then-rename discipline. Memory stays bounded
// by the chunk size: each decrypted chunk is written to the temp file and
// dropped. The rename happens only after the stream authenticates as terminated
// on TAG_FINAL; a truncated stream throws and leaves the destination untouched.
// Returns the plaintext length written.
const decryptToTemp = async (
    destination: string,
    stream: ReadableStream<Uint8Array>,
    header: Uint8Array,
    key: Uint8Array,
    onProgress?: ProgressCallback,
): Promise<number> => {
    let bytesWritten = 0;
    await stageAtomic(destination, async (handle) => {
        bytesWritten = await streamDecrypt(
            stream,
            header,
            key,
            async (plaintext) => {
                await handle.write(plaintext);
            },
            onProgress,
        );
    });
    return bytesWritten;
};

// Fetch a stream and decrypt it to `destination`, retrying the whole sequence.
//
// The request is only the first third of a download. `getXStream` returns as
// soon as headers arrive, and the bytes are pulled here, so a socket reset
// mid-body — the dominant failure mode for multi-megabyte photos over a CDN —
// throws in `streamDecrypt` and never reaches `ApiClient` at all. Retrying the
// request alone would miss it entirely.
//
// The client's own retry is therefore switched off for these two calls: with
// both layers active the budgets would multiply, and the library default of
// four attempts would mean sixteen requests for one file. The policy comes
// from the client so a caller that configured one gets it here too.
//
// Because the plaintext is streamed to disk rather than buffered, the atomic
// write is part of the retried unit. A retry starts the file over from byte
// zero — the secretstream pull state is not resumable and there is no Range
// support — staging into a fresh temp file each time: a failed attempt writes
// and then removes its own temp file, and only the attempt that reaches
// TAG_FINAL renames one into place, so a download that needed three tries still
// performs exactly one rename over the destination.
const fetchAndDecrypt = async (
    api: ApiClient,
    openStream: () => Promise<ReadableStream<Uint8Array>>,
    header: Uint8Array,
    key: Uint8Array,
    destination: string,
    onProgress?: ProgressCallback,
): Promise<number> =>
    withRetry(async () => {
        const stream = await openStream();
        return decryptToTemp(destination, stream, header, key, onProgress);
    }, api.getRetryOptions());

export const downloadFile = async (
    api: ApiClient,
    file: EnteFile,
    outPath?: string,
    onProgress?: ProgressCallback,
): Promise<DownloadResult> => {
    const resolvedPath = outPath ?? file.metadata.title;
    const header = fromBase64(file.file.decryptionHeader);
    const bytesWritten = await fetchAndDecrypt(
        api,
        () => api.getFileStream(file.id, { retry: false }),
        header,
        file.key,
        resolvedPath,
        onProgress,
    );
    return { path: resolvedPath, bytesWritten };
};

export const downloadThumbnail = async (
    api: ApiClient,
    file: EnteFile,
    outPath?: string,
    onProgress?: ProgressCallback,
): Promise<DownloadResult> => {
    const resolvedPath = outPath ?? `thumb_${file.metadata.title}`;
    const header = fromBase64(file.thumbnail.decryptionHeader);
    const bytesWritten = await fetchAndDecrypt(
        api,
        () => api.getThumbnailStream(file.id, { retry: false }),
        header,
        file.key,
        resolvedPath,
        onProgress,
    );
    return { path: resolvedPath, bytesWritten };
};