Read the ENS cache from disk before the first lookup

app/src/main/java/com/pombo/android/core/EnsStore.kt

@@ -6,6 +6,8 @@ import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
 import kotlinx.coroutines.withContext
 import org.json.JSONObject
 import java.io.File
@@ -39,8 +41,20 @@ class EnsStore(context: Context) {
     val avatarUrls: StateFlow<Map<String, String>> = _avatarUrls.asStateFlow()
 
     private val file = File(context.filesDir, "ens-cache.json")
+    private val loadLock = Mutex()
+    @Volatile private var loaded = false
+
+    /** Reads the disk cache, once; [name] and [avatar] wait for it. */
+    suspend fun warmUp() {
+        if (loaded) return
+        loadLock.withLock {
+            if (loaded) return
+            readDisk()
+            loaded = true
+        }
+    }
 
-    suspend fun warmUp() = withContext(Dispatchers.IO) {
+    private suspend fun readDisk() = withContext(Dispatchers.IO) {
         try {
             if (!file.exists()) return@withContext
             val root = JSONObject(file.readText())
@@ -92,6 +106,7 @@ class EnsStore(context: Context) {
      * cold or stale. Concurrent callers share one lookup.
      */
     suspend fun name(address: String, lookup: suspend () -> String?): String? {
+        warmUp()
         val key = address.lowercase()
         names[key]?.let { if (fresh(it)) return it.value }
 
@@ -117,6 +132,7 @@ class EnsStore(context: Context) {
 
     /** Same contract for the avatar text record. */
     suspend fun avatar(address: String, lookup: suspend () -> String?): String? {
+        warmUp()
         val key = address.lowercase()
         avatars[key]?.let { if (fresh(it)) return it.value }
 

app/src/test/java/com/pombo/android/core/EnsStoreTest.kt

@@ -3,6 +3,9 @@ package com.pombo.android.core
 import android.content.Context
 import io.mockk.every
 import io.mockk.mockk
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.async
+import kotlinx.coroutines.awaitAll
 import kotlinx.coroutines.runBlocking
 import org.json.JSONObject
 import org.junit.Assert.assertEquals
@@ -45,4 +48,29 @@ class EnsStoreTest {
         val store = EnsStore(context).apply { warmUp() }
         assertEquals("found.eth", store.name("0xabc") { "found.eth" })
     }
+
+    @Test
+    fun `a lookup made before warmUp sees what is on disk`() = runBlocking {
+        storeOnDisk(JSONObject().put("name", "on.disk.eth").put("at", twentyMinutesAgo))
+        var lookups = 0
+        assertEquals("on.disk.eth", EnsStore(context).name("0xabc") { lookups++; "net.eth" })
+        assertEquals(0, lookups)
+    }
+
+    @Test
+    fun `concurrent first lookups all wait for the disk`() = runBlocking {
+        storeOnDisk(
+            JSONObject().put("name", JSONObject.NULL).put("nameConfirmed", true).put("at", twentyMinutesAgo)
+                .put("avatar", "https://a.example/a.png").put("avatarAt", twentyMinutesAgo)
+        )
+        val store = EnsStore(context)
+        var lookups = 0
+        val results = listOf(
+            async(Dispatchers.Default) { store.name("0xabc") { lookups++; "net.eth" } },
+            async(Dispatchers.Default) { store.name("0xABC") { lookups++; "net.eth" } },
+            async(Dispatchers.Default) { store.avatar("0xabc") { lookups++; "https://net/a.png" } }
+        ).awaitAll()
+        assertEquals(listOf(null, null, "https://a.example/a.png"), results)
+        assertEquals(0, lookups)
+    }
 }