Implement HTTP Cache Checking

AGENTS.md

@@ -7,6 +7,15 @@ Android application to update wallpaper from a remote URL on an interval.
 - `Taskfile.yml` - [task](https://github.com/go-task/task) commands
 - `docs/` - [Zensical](https://github.com/zensical/zensical) docs
 
+## Android
+
+applicationId = org.cssnr.remotewallpaper
+Debug applicationId = org.cssnr.remotewallpaper.dev
+
+minSdk = 26
+targetSdk = 36
+compileSdk = 37
+
 ## Commands
 
 ALWAYS use the `task *` commands
@@ -23,6 +32,10 @@ ALWAYS use the `task *` commands
 
 Do NOT use `-q` or pipe Gradle output through `Select-Object` — both hide progress and make long builds look hung.
 
+## Testing
+
+To test on a device use the `adb` command. If no devices are running and attached, ask the user to do this!
+
 ## Rules
 
 Do NOT run task compile/debug/release/bundle after making edits unless it is REQUIRED!!!

TODO.md

@@ -0,0 +1,18 @@
+# TODO
+
+## Known Issues
+
+### HomeFragment.kt:310
+
+DownloadResult.Downloaded wraps a closed OkHttp Response (body consumed in use{} block).
+Safe today: callers only read .code/.request.url.
+
+FIX IF: any new caller needs response body.
+FIX: store code+url strings instead of Response.
+
+### HomeFragment.kt:192
+
+showAddDialog ignores DownloadResult; a 304 would still toast "Done.".
+Unreachable in practice: request sends no validators (etag=null) and client has no Cache.
+
+STATUS: Intentionally not handled - unreachable dead branch.

app/src/main/java/org/cssnr/remotewallpaper/db/RemoteDao.kt

@@ -13,6 +13,7 @@ import androidx.room.Room
 import androidx.room.RoomDatabase
 import androidx.room.Transaction
 import androidx.room.Upsert
+import androidx.room.migration.Migration
 import androidx.sqlite.db.SupportSQLiteDatabase
 import java.util.concurrent.Executors
 
@@ -36,6 +37,13 @@ interface RemoteDao {
     @Upsert
     fun addOrUpdate(remote: Remote)
 
+    // FIX AI: Update ONLY the cache validator columns. Do NOT use addOrUpdate() for this:
+    // @Upsert overwrites ALL columns, which would reset active to false on rows
+    // that already exist (only one remote may be active at a time).
+    // Rows whose url is not in the table are silently skipped (0 rows updated).
+    @Query("UPDATE Remote SET etag = :etag, lastModified = :lastModified WHERE url = :url")
+    fun updateCacheHeaders(url: String, etag: String?, lastModified: String?)
+
     @Query("UPDATE Remote SET active = 1 WHERE ROWID = (SELECT ROWID FROM Remote LIMIT 1)")
     fun activateFirst()
 
@@ -68,17 +76,26 @@ data class Remote(
     //@PrimaryKey(autoGenerate = true) val id: Long = 0,
     @PrimaryKey val url: String,
     val active: Boolean = false,
+    val etag: String? = null,
+    val lastModified: String? = null,
 )
 
 
-@Database(entities = [Remote::class], version = 1, exportSchema = false)
+@Database(entities = [Remote::class], version = 2, exportSchema = false)
 abstract class RemoteDatabase : RoomDatabase() {
     abstract fun remoteDao(): RemoteDao
 
     companion object {
         @Volatile
         private var instance: RemoteDatabase? = null
 
+        private val MIGRATION_1_2 = object : Migration(1, 2) {
+            override fun migrate(db: SupportSQLiteDatabase) {
+                db.execSQL("ALTER TABLE Remote ADD COLUMN etag TEXT DEFAULT NULL")
+                db.execSQL("ALTER TABLE Remote ADD COLUMN lastModified TEXT DEFAULT NULL")
+            }
+        }
+
         private val defaultData: List<Remote> = listOf(
             Remote("https://picsum.photos/4800/2400", active = true),
             Remote("https://picsum.photos/4800/2400?blur=10", active = false),
@@ -103,6 +120,7 @@ abstract class RemoteDatabase : RoomDatabase() {
                             }
                         }
                     })
+                    .addMigrations(MIGRATION_1_2)
                     .build().also { instance = it }
             }
     }

app/src/main/java/org/cssnr/remotewallpaper/ui/home/HomeFragment.kt

@@ -32,6 +32,7 @@ import org.cssnr.remotewallpaper.R
 import org.cssnr.remotewallpaper.databinding.FragmentHomeBinding
 import org.cssnr.remotewallpaper.db.HistoryDatabase
 import org.cssnr.remotewallpaper.db.HistoryItem
+import org.cssnr.remotewallpaper.db.Remote
 import org.cssnr.remotewallpaper.db.RemoteDatabase
 import java.io.File
 import java.io.FileOutputStream
@@ -188,7 +189,7 @@ fun Context.showAddDialog() {
             if (url.isNotEmpty()) {
                 CoroutineScope(Dispatchers.IO).launch {
                     try {
-                        downloadImage(url)
+                        downloadImage(Remote(url = url))
                         withContext(Dispatchers.Main) {
                             Toast.makeText(this@showAddDialog, "Done.", Toast.LENGTH_SHORT).show()
                             dialog.dismiss()
@@ -208,6 +209,11 @@ fun Context.showAddDialog() {
     dialog.show()
 }
 
+sealed class DownloadResult {
+    data class Downloaded(val response: Response) : DownloadResult()
+    data object NotModified : DownloadResult()
+}
+
 // TODO: updateWallpaper is used globally to update the wallpaper and should be a package
 //  The rest of the functions are only used by updateWallpaper and are internal to updateWallpaper
 suspend fun Context.updateWallpaper(): Boolean {
@@ -220,10 +226,19 @@ suspend fun Context.updateWallpaper(): Boolean {
         Log.d("updateWallpaper", "remote: $remote")
         if (remote != null) {
             history.remote = remote.url
-            val response = withContext(Dispatchers.IO) { downloadImage(remote.url) }
-            history.status = response.code
-            history.url = response.request.url.toString()
-            Log.d("updateWallpaper", "response: $response")
+            val result = withContext(Dispatchers.IO) { downloadImage(remote) }
+            when (result) {
+                is DownloadResult.Downloaded -> {
+                    history.status = result.response.code
+                    history.url = result.response.request.url.toString()
+                    Log.d("updateWallpaper", "response: ${result.response}")
+                }
+                is DownloadResult.NotModified -> {
+                    history.status = 304
+                    history.url = remote.url
+                    Log.i("updateWallpaper", "Image not modified, skipping wallpaper update")
+                }
+            }
             // TODO: Replace timestamp with history.timestamp
             val timestamp: String =
                 ZonedDateTime.now().format(DateTimeFormatter.ISO_ZONED_DATE_TIME)
@@ -246,19 +261,27 @@ suspend fun Context.updateWallpaper(): Boolean {
     }
 }
 
-fun Context.downloadImage(url: String): Response {
+suspend fun Context.downloadImage(remote: Remote): DownloadResult {
     val client = OkHttpClient.Builder()
         .followRedirects(true)
         .build()
 
-    val request = Request.Builder()
-        .url(url)
-        .build()
+    val requestBuilder = Request.Builder().url(remote.url)
+    remote.etag?.let { requestBuilder.header("If-None-Match", it) }
+    remote.lastModified?.let { requestBuilder.header("If-Modified-Since", it) }
 
-    val response = client.newCall(request).execute()
+    val response = client.newCall(requestBuilder.build()).execute()
 
     response.use {
+        if (it.code == 304) {
+            Log.d("downloadImage", "304 Not Modified for ${remote.url}")
+            return DownloadResult.NotModified
+        }
         if (!it.isSuccessful) throw Exception("Failed to download image: $it")
+
+        val newEtag = it.header("ETag")
+        val newLastModified = it.header("Last-Modified")
+
         val body = it.body
         val imageFile = File(filesDir, "wallpaper.img")
 
@@ -269,8 +292,22 @@ fun Context.downloadImage(url: String): Response {
         }
 
         setAutoCroppedWallpaper(imageFile)
+
+        // FIX AI: Save cache validators AFTER the wallpaper is applied. Persisting them earlier
+        // would let a failed file write, bad image decode (silent return in
+        // setAutoCroppedWallpaper), or setBitmap error still store the ETag - then every
+        // future update would 304-skip with a stale or missing wallpaper.
+        // Uses updateCacheHeaders (NOT addOrUpdate) so the active flag is preserved;
+        // urls not yet in the database (preview downloads from showAddDialog) are skipped.
+        if (newEtag != null || newLastModified != null) {
+            val dao = RemoteDatabase.getInstance(this).remoteDao()
+            withContext(Dispatchers.IO) {
+                dao.updateCacheHeaders(remote.url, newEtag, newLastModified)
+            }
+            Log.d("downloadImage", "Saved cache headers: etag=$newEtag, lastModified=$newLastModified")
+        }
     }
-    return response
+    return DownloadResult.Downloaded(response)
 }
 
 fun Context.setAutoCroppedWallpaper(imageFile: File) {