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). 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. Active queue persistence and ordinary backend pause/retry are unchanged.

Epic installs use the same bounded Java writer whether the native engine is available or not. It fetches one chunk at a time, verifies it, and writes the required parts into the current game file. Chunks are deleted after their last use; reusable chunks are retained up to 64 MiB, with at most one additional incoming chunk before eviction. Evicted shared chunks may be downloaded again. This trades parallel-download throughput and potentially extra network traffic for bounded temporary disk usage. The native batch APIs remain available but are not used by this install path.

The bounded Epic download bar, measured rate and ETA count pending file bytes written, not unique compressed chunks fetched. Shared chunks therefore cannot complete the bar before later files are written, and an evicted chunk's refetch is reflected in the time spent doing that work. Streaming stays in the Download stage until all files are written and verified, then enters Install for registration; it does not bounce between stages for each file.

The space check follows the manifest delta pass and simulates file writes, chunk fetches, and evictions, rather than adding the entire cache to the entire game. It accounts for space reclaimable from outdated outputs without deleting them until both game and cache volume checks pass. An insufficient-space rejection preserves existing game files. Completed files and verified cached chunks are accounted for on resume. On a card, game and cache requirements are checked separately. File SHA-1 hashes are checked where supplied by the manifest; failed or cancelled output files are removed, and an interrupted process is repaired by the next delta pass. A successful run removes the cache. A resume within a large incomplete file restarts that file, not its individual parts.

The sidecar

{
  "version": 1,
  "store": "epic",                    // gog | epic | amazon
  "id": "Samorost3",                  // the store's own id: GOG product id, Epic app name, Amazon product id
  "title": "Samorost 3",
  "exe": "Samorost3.exe",             // relative to the folder, forward slashes; what the icon is read from
  "launcher": ".droiddeck-launch.bat",// optional; what the shortcut runs instead of exe
  "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. 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.

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.
• 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 each manager runs its Java fetch loop.

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.