DroidDeck/docs/development/stores.md

Stores: GOG, Epic Games and Amazon Games in the launcher

How the Stores section (app/src/main/java/com/droiddeck/launcher/stores/, ui/Stores*.kt) fits the rest of the app, and the on-disk contract that makes a store game an ordinary added game.

Where a store game lives

<Games storage>/Games/<Store>/<title>/
    <game files>
    .droiddeck-store.json      the sidecar (below)
    .droiddeck-launch.bat      only when the game needs arguments or environment (Epic, Amazon)
    .droiddeck-epic-code       one-shot, written right before an Epic launch, read and deleted by the .bat

The default root is the app's internal storage - the runtime's tree, /root/Games/Stores, no bind needed - whatever the session's Game storage setting says. When a card the app may write is in the device (GameStorage.options), Install asks "Install to" (internal / the card, free space shown; the last pick is only the dialog's default). A card's root is <card app folder>/Games; when the card is the Steam library the library's bind (/mnt/droiddeck-sd) already covers it, otherwise SessionService binds the root at /mnt/droiddeck-stores/<volume uuid>, so a shortcut's guest path - and its appid - is the same from one session to the next. Every root is scanned (StoreInstallRoot.roots): internal, every card, and the Steam library's Games folder for installs an earlier build put there. <Store> is GOG, Epic or Amazon; <title> is the title with unsafe characters dropped, at most 60 characters (StoreInstallRoot.folderName), chosen once; a rerun (repair, update) lands on the folder whose sidecar carries the game's id, wherever it is. An Epic install on a card keeps its in-flight chunks in cacheDir/stores/epic/<id>/; "" keeps the cache beside the game, as an internal install has it. The scratch folder is removed on cancel-with-delete and on uninstall.

Preflight resolves an existing game's folder before selecting its target root, so a resume or repair on a card checks and remembers that volume rather than internal storage. Fresh GOG and Amazon installs can reject insufficient catalog-size space immediately; existing installs defer to manifest checks of the files still needing work (GOG MD5, Amazon SHA-256 where supplied). Free-space display probes the nearest existing ancestor without creating the target folder; if the probe cannot determine space, it reports Free space unknown rather than blocking an install. StoreInstallRoot uses -1 for unknown and preserves a measured StatFs zero as 0 B free. Fresh GOG/Amazon preflight and the backends' own space checks (GOG in Kotlin, Epic and Amazon in the Rust engine) reject known zero when bytes remain. The read-only probe walks existing ancestors without creating directories; a failed StatFs probe can use a positive File.usableSpace fallback, but that API's ambiguous zero is unknown, not a measurement of a full disk. Preflight failures remain failed Downloads rows with Clear and Retry. Retry reruns preflight outside the worker lifecycle and then enqueues the real backend; it never occupies a queue slot with a placeholder job. Failures are kept in the Downloads UI, notification and logs rather than duplicated as transient failure toasts. Active queue persistence and ordinary backend pause/retry are unchanged.

Epic installs run in the Rust epic module of libdroiddeckstores.so (epic/install/), driven by EpicInstaller through EpicNative.install. There is one engine; the old hidden storesEpicMode switch is no longer read. Without the native library the Epic store cannot install (the facade reports "the store engine is not available"). The design, and how it was written from a functional spec only, is in epic-cleanroom.md.

A run, in order: CDN list from the build information (EpicApiClient.getManifestApiJson) → manifest from the first CDN that serves it → parse (binary or JSON) → files selected by the device language's install tags (EpicInstallTags) → staging recovery → the delta pass (a selected file is up to date when it has the manifest size and SHA-1; a file without a manifest hash is always rewritten) → space plan → chunk cache preparation → fetch schedule → download and write → the installed record (StoreInstalls.complete). Resume, update and repair are the same rerun.

• Schedule (epic/install/schedule.rs). The pending files' parts form one timeline. A chunk is downloaded at its first use and feeds every later use in the same file directly. If it is used again in a later file it is kept, decompressed and verified, in the chunk cache, which has a byte budget; when keeping a chunk would exceed it, the cached chunk needed furthest in the future is dropped and downloaded again when needed. Chunks a previous run left in the cache are reused, soonest-needed first, as far as the budget allows.
• Writing (epic/install/writer.rs). Files are admitted strictly in order, a little ahead of the download, and a download is dispatched only once the file of its first use is admitted, so the number of half-written files stays small. Each admitted file is written into a unique epic-<pid>-<seq>-<nanos>.stage file in <game>/.droiddeck-epic-staging/; the file SHA-1 is computed in file order as parts land. A complete file is synced, size- and hash-checked, and renamed over its destination. Right before the first rename of the run StoreInstalls.markUpdating switches an installed record to updating; a refusal fails the run with nothing replaced.
• Fetching uses the shared fetch_core (adaptive window, CDN rotation, retries with back-off, stall and 429 cooldowns) with the speed tier's window and decode workers. Every chunk is checked (header, zlib stream end, exact size, SHA-1) before any byte reaches a file or the cache.
• Space (epic/install/space.rs). Free space is probed per volume (statvfs; unknown is never "no space"). The plan: net peak of the pending files in order (each old copy credited at its own swap), a cache budget of half the remaining space (at most 1 GiB, 256 MiB when unknown, 0 is allowed), an overlap budget that limits how many bytes of files replacing existing data are staged at once on an update (at least the largest replaced file), and a 64 MiB transient allowance. A card install keeps its cache in cacheDir/stores/epic/<id>/ when internal storage can hold a useful cache and the game has no non-empty .chunks yet; otherwise the cache is <game>/.chunks. When the full plan does not fit, the run falls back to one file at a time with no cache (needs only the net peak); when even that does not fit the install is refused before any chunk request, naming the storage, the need, the free space and the shortfall.
• Progress: the bar counts pending-file bytes written (not compressed chunk bytes), at most every 50 ms plus once per finished file. The run logs epic: plan … and epic: summary mode=adaptive|narrow files=… bytes_written=… avg_mbps=… fetches=… refetches=… peak_cache_mib=… hosts=….
• Cancel / pause / failure: the cancel flag is forwarded within 250 ms; the run stops its requests, joins its threads, removes every unfinished stage file and .part cache entry, and only then returns. Swapped files stay; unswapped destinations keep their old content; the chunk cache stays for the next run. Success removes the cache and the staging folder (when only its marker is left).

The staging folder is recognised by its owner marker (DroidDeck Epic staging v1 + LF). Before the delta pass the run checks that the folder is a real directory at the expected canonical path, that the marker matches and that every other entry is a plain .stage file directly inside it; only then are those orphans deleted. Anything unexpected fails the run without deleting. A manifest path naming the staging folder, an absolute path or a .. segment is refused. .stage / .part files elsewhere in the game folder are never touched.

Amazon installs run in the Rust amazon module (amazon/install/), driven by AmazonInstaller through AmazonNative.install; sign-in, the library sync and the size lookup call the same module through AmazonNative.call. The design, and how it was written from a functional spec only, is in amazon-cleanroom.md.

An Amazon run, in order: download lookup for the entitlement (GetGameDownload: a CDN base URL with a signed query, and the version id) → manifest.proto from that base (a 4-byte big-endian header length, an opaque header, then an xz or LZMA-alone compressed protobuf) → parse (every file of every package; paths refused when absolute, with a drive letter, .., empty segments, or naming DroidDeck's own root files) → leftover <file>.tmp siblings removed → the check of files on disk (a file is complete when it has the manifest size and SHA-256; the same test drives the space check and the download plan) → space check (Not enough free space: need …, only … free; unknown free space never blocks) → empty files created locally → every other pending file fetched from files/<sha256 hex> under the base, one streamed GET each through the shared fetch core (the speed tier's window; at least half the CPU cores verifying and writing), hashed as it arrives, written to <file>.tmp and renamed over the destination after the size and hash checks → the installed record. Before the first file is put in place the record is switched to updating, so an interrupted repair is offered as "Resume install". There is no partial-file resume: a file is complete or fetched again from its first byte. Files the manifest does not list are left alone.

The sidecar

{
  "version": 1,
  "store": "epic",
  "id": "Samorost3",
  "title": "Samorost 3",
  "exe": "Samorost3.exe",
  "launcher": ".droiddeck-launch.bat",
  "args": ["-EpicPortal", "-epicusername=\"Name\"", "..."],
  "env": {"FUEL_DIR": "C:\\ProgramData\\Amazon Games Services\\Legacy"},
  "installVersion": "1.4.0",
  "installedAt": 1759900000000,
  "cover": "https://...", "hero": "https://...",
  "extra": {"namespace": "...", "catalogItemId": "..."}
}

An install writes the sidecar at its start with "state": "installing" (store, id, title, no exe) and rewrites it finished at its end (exe, launcher, no state field = installed). A folder under a store root whose sidecar says installing, or that has no sidecar at all, is an unfinished install: AddedGames.scan leaves it out (it is neither a game nor a Custom folder), and the Stores card and page offer Resume install, which reuses that folder - and an Epic .chunks beside it - instead of starting a fresh one (StoreInstallRoot.existingFolder). A finished install being repaired keeps its finished sidecar until Epic reaches the first verified file swap. Immediately before that swap Epic atomically changes the record to "state": "updating", preserves launch choices and identifiers, and clears installVersion. Network/hash failures or cancellation before that point leave the old installed record and files intact. A later failure can leave a mixture of verified new files and untouched old files: replacement is atomic per file, not a transactional whole-game update or rollback. The updating folder is excluded from installed scans and offered for resume. Cancellation with deletion clears caches but retains that folder and its verified swaps; retry preserves the updating record's metadata. If marking or swapping fails, no destructive fallback is attempted (the conservative updating state can remain even when a swap fails). Completion writes the installed record with the new version only after all selected files succeed. An already registered live Steam shortcut is not revoked at each file swap; do not launch an in-progress update. The launcher and the finished sidecar are written before the art and the Steam registration; a failure writing them fails the download with its message.

StoreGameSidecar.parse refuses anything whose exe or launcher would point outside the folder. Every store install is a Steam shortcut; an addToSteam field in a sidecar from an earlier build is read past and ignored.

Registration with Steam

AddedGames.scan walks the store folders as it walks the user's added folders, and AddedGames.scanGame reads the sidecar: the exe (the folder's guess only when that file is gone), the title as the shortcut's name, the store as the game's source. From there the path is the one an added folder takes: session/added-games.json → droiddeck-steam-shortcuts at the client's next start. StoreInstalls.register rewrites the listing at install time, so a client that restarts inside the running session already has it, and asks a running client to add the shortcut live over its DevTools port (SteamLiveShortcuts: SteamClient.Apps.AddShortcut, our tag, the compat tool). The appid Steam derives (CRC32 of the quoted exe plus the name, high bit set) is the one the listing carries, so the live add and the writer never disagree. Uninstall removes the folder and the listing drops the entry; the live client is asked to remove it too.

Library.SteamGame.source is steam, gog, epic, amazon or added; the Games tab shows it as a chip on every row and in the hero.

Launch

A Steam shortcut names an exe and nothing more, and the listing carries no launch options, so a game that needs arguments or environment gets .droiddeck-launch.bat (StoreLaunch.launcherText): cd into the exe's folder, set each variable, start the exe with the arguments and wait. Proton's steam.exe shim hands a .bat to Wine's cmd. For Epic the script also reads .droiddeck-epic-code when present, deletes it, and passes -AUTH_LOGIN=unused -AUTH_PASSWORD=<code> -AUTH_TYPE=exchangecode on the game's command line only; the variable it read the code into is cleared on that same line, so nothing of it is left in the game's environment. A launcher from an earlier build is rewritten before the next Epic launch.

The code is minted for every launch, wherever it starts. The Games tab and Stores call StoreLaunch.prepare before they launch; the compat tool (droiddeck-proton, droiddeck-proton-wrap) runs droiddeck-store-launch before Proton on every real launch, the Steam client's Play button included. It finds the store launcher among Steam's arguments, reads the sidecar beside it, and for an Epic game asks the app through <session>/stores/req and resp (StoreLaunchRequests → StoreLaunch.epicCode), waiting up to 5 s; the app writes the code into the game's folder itself. Each attempt logs epic launch id=… code=yes|no reason=…, never the code.

An Epic game's launch choices are the Epic card in the Games tab's launch settings - "Epic sign-in (EOS)" (on) and "Launch offline" (off), stored in the sidecar's epic block so both launch paths read the same thing - and "Resolve Epic sign-in", which opens Epic's account page behind its sign-in in Android's browser.

An Amazon game's exe and arguments come from its fuel.json (Main.Command, used when it names an existing file; Main.Args, used even when it does not), else from a score over every .exe below the folder (Unreal shipping binaries and Binaries/Win64 first, crash handlers, setups, launchers and tiny stubs last; the rules see paths relative to the install folder), else the shared picker. Its launcher sets five FuelPump variables frozen at install time: FUEL_DIR, AMAZON_GAMES_SDK_PATH, AMAZON_GAMES_FUEL_ENTITLEMENT_ID, AMAZON_GAMES_FUEL_PRODUCT_SKU and AMAZON_GAMES_FUEL_DISPLAY_NAME (the account name, Player without one). No Amazon Games SDK files are deployed into the prefix and fuel.json PostInstall steps are not run.

A game's web links reach Android: droiddeck-store-launch sets the prefix's HKCU\Software\Wine\WineBrowser Browsers to /usr/local/bin/droiddeck-open-url, which is also xdg-open first on the game's PATH, and hands http(s) addresses to the app, which opens them in Android's browser. EOS uses this when a sign-in needs a one-time step ("corrective action"): it falls back to a browser, the user approves on the phone, and the game signs in. BL_DEBUG_BROWSER=1 in droiddeck-env adds WINEDEBUG=+shell,+winebrowser. Epic's EOS overlay is not provisioned (with it games exited while loading); an OverlayPath an earlier build wrote is removed at launch. The parked overlay work is on the branch park/epic-eos-overlay.

Cloud saves

GOG and Epic games sync their saves with the store's cloud (CloudSaves, after Bannerlator's managers): before a launch the cloud copy comes down, after the game exits what changed goes up. CloudPlan decides per file against a baseline - each file's local MD5 and cloud MD5/time as of this device's last sync:

• No upload before a download has completed once on this device (no baseline): a first launch creates new profile files that must never replace the real saves in the cloud.
• The first download lets the cloud win where the two differ.
• A file changed on both sides since the baseline is a conflict: neither side is overwritten; Manage saves shows "Cloud conflict" with Keep cloud / Keep local.
• Otherwise the side that changed wins; bytes that match move nothing.

Both sides are backed up before they are replaced: the local folder before a download writes into it, the cloud copies before an upload replaces them, under files/stores/cloud-backups/<store>-<id>/local-* and cloud-* (the last three of each).

A game launched before Proton has made its prefix (a first launch) gets its download deferred: the request is answered at once (result=deferred reason=no-prefix) and the download runs as soon as the prefix's user folder appears, within a minute. Every path logs cloud <store> <id> down|up result=ok|skipped|deferred|failed files= bytes= reason=.

• The folder: GOG's public remote-config gives a location template per game (by its Galaxy client id), e.g. <?APPLICATION_DATA_LOCAL_LOW?>/Hyperstrange/ELDERBORN; Epic's catalog gives CloudSaveFolder, e.g. {AppData}/Game/Saved/SaveGames. CloudSavePaths expands it inside the game's prefix (compatdata/<shortcut appid>/pfx/drive_c/users/steamuser) or its install folder, case-insensitively, and refuses a path that leaves its boundary. A game whose store gives none has no cloud saves.
• The services: GOG cloudstorage.gog.com/v1/<user>/<client> with a token issued to the game's own client (the client secret is kept at install; the Galaxy token is the fallback); Epic's savesync data storage with signed read and write links (EpicCloud → EpicSaveSync → the Rust epic/savesync.rs; the template's root token and {EpicId} / {AppName} are read by EpicSaveTemplate).
• Which launches: droiddeck-store-launch recognises a store game by the launcher .bat Steam runs or, for a game without one (most GOG games), by its exe - the folders above it are searched for the sidecar.
• When: every launch runs droiddeck-store-launch, which asks the app (cloud-down) and waits up to 15 s; for a store game with cloud saves it returns 10, and the compat tool then waits for Proton instead of exec'ing it and runs droiddeck-store-launch --exited after, which asks for cloud-up without waiting. Each sync logs cloud <store> <id> down|up files= bytes= result=.
• Uploads that do not depend on one hook: the pre-launch step marks the game dirty in the app's storage; the upload runs on the first of the compat tool's exit request (trigger=exit), the session's end (trigger=session-end, after teardown, reading the prefix directly) or the app's next start for a mark a killed app left (trigger=recovery, not while a session runs). The mark goes only after an upload that went through or a final skip (off, no cloud saves, no baseline, not installed, nothing local).
• Per game: a GOG or Epic game's hero button reads "Cloud saves" and opens its saves view: the Cloud saves switch (on; the sidecar's "cloud", read by both launch paths), the last sync, Upload / Download and the conflict actions, or "No cloud saves".

Downloads

DownloadQueue runs 1–3 jobs at a time (Setup / the Downloads page), each a store's whole install (DownloadJob.run), with stages Manifest → Download → Verify → Install. Pause stops the job and keeps the files; every store's install skips complete, verified files on the rerun, so resume is a rerun. Cancel deletes the folder. StoreDownloadService holds a foreground notification while anything runs. The native engine (libdroiddeckstores.so, StoresNative, GogNative, EpicNative, AmazonNative; contract in app/src/main/rust/stores/JNI.md) is loaded on first use; without it the GOG manager runs its Java fetch loop, and the Epic and Amazon stores (all of whose service calls and installs run in the Rust epic and amazon modules) are unavailable: sign-in, sync and install report "the store engine is not available".

Settings

Setting	Pref	Where
Show Stores in the rail	SessionPrefs.gameStoresEnabled (off)	Setup › Stores
Open a store on: Library · Store	SessionPrefs.storesOpenTab (library)	the chip row's cog, Setup › Stores
Download speed tier	SessionPrefs.gameStoresSpeedTier (fast)	the cog, Setup › Stores, Downloads
Downloads at a time	SessionPrefs.gameStoresParallel (1)	Downloads
Install to (last pick, dialog default only)	SessionPrefs.storesInstallTarget	the Install dialog

Store log lines (the engines' included) go through StoresState.logLine, which redacts them (StoreLog.redactLine) and writes them to logcat and to filesDir/logs/stores/stores-<date>.log (StoreLogFiles: one file a day, seven days kept, ~2 MB each before it rolls to .1). Nothing shows them in the app; the session's Share logs zip carries them under stores/.

Store URLs are logged through StoreLog.redactUrl, which drops the query string where the signed tokens live.

Security

• Sign-ins: filesDir/stores/<store>/credentials.json only, read and written through StoreAccounts alone (GogAuth, EpicCredentialStore, AmazonCredentialStore, cloud saves all call it). The file is an envelope {"v":1,"alg":"AES/GCM","iv":...,"ct":...} (base64): AES-256-GCM (CredentialCipher) under a key generated in the AndroidKeyStore (alias droiddeck-store-credentials; encrypt/decrypt, GCM, no padding, not exportable, no user authentication so background downloads and launch-time exchange codes work with the screen off; StrongBox where present, else the TEE). Written through a temp file and a rename, mode 600.
• Earlier plain files: sealed at app start (StoreAccounts.encryptAll, and on any read), verified by opening the result, the plain file replaced; the log says stores: credentials encrypted <store>. A plain .tmp left by an older build is deleted.
• No Keystore (some ROMs): the file stays plain, the log says stores: keystore unavailable, credentials stay plain (<exception class>) once, and the next start tries again.
• A file that can never open - it does not authenticate (AEADBadTagException: tampered, or another key after data was cleared or a backup restored on another device), the key is permanently invalidated or gone from the Keystore, or it is not an envelope: deleted, and the store reads as signed out, so its sign-in card shows.
• A failure that may pass (KeyStoreException, ProviderException, any other InvalidKeyException, an I/O error, StrongBox busy): the file is kept, the store stays signed in (no sign-in card) and is marked unavailable (StoreAccounts.isUnavailable), a store action shows <Store> sign-in could not be read: try again., the log says stores: credentials unreadable this start (<exception class>) once, and the next access tries again.
• The Linux session: the app's files directory is bound into every session, so the guest can see credentials.json - as the envelope only; the key never leaves the Keystore. Nothing in the guest reads the files: droiddeck-store-launch asks the app over <session>/stores/req|resp for an Epic exchange code, which the app writes beside the game's launcher; the launcher reads it into a variable and deletes it before the game starts (single use, expires in minutes). Steam's own client files are outside this.