Handle failure to persist GSON serialized JSON data (EXPOSUREAPP-3777) (#1604)

* Flush the json data after writing it, otherwise if the app process dies, we end up with an empty file.
GSON deserializes this into a null object that looks like a non-null object until it is evaluated some time later.

Force an evaluation via `require(it.size >= 0)` and delete the corrupt data file when catching the exception.

* Show a visible error dialog if we fail to save data.

* Used buffered versions of readers and writer.

Corona-Warn-App/src/main/java/de/rki/coronawarnapp/nearby/modules/calculationtracker/CalculationTrackerStorage.kt

@@ -2,6 +2,8 @@ package de.rki.coronawarnapp.nearby.modules.calculationtracker
 
 import android.content.Context
 import com.google.gson.Gson
+import de.rki.coronawarnapp.exception.ExceptionCategory
+import de.rki.coronawarnapp.exception.reporting.report
 import de.rki.coronawarnapp.util.di.AppContext
 import de.rki.coronawarnapp.util.gson.fromJson
 import de.rki.coronawarnapp.util.gson.toJson
@@ -36,11 +38,13 @@ class CalculationTrackerStorage @Inject constructor(
             if (!storageFile.exists()) return@withLock emptyMap()
 
             gson.fromJson<Map<String, Calculation>>(storageFile).also {
+                require(it.size >= 0)
                 Timber.v("Loaded calculation data: %s", it)
                 lastCalcuationData = it
             }
         } catch (e: Exception) {
             Timber.e(e, "Failed to load tracked calculations.")
+            if (storageFile.delete()) Timber.w("Storage file was deleted.")
             emptyMap()
         }
     }
@@ -55,6 +59,7 @@ class CalculationTrackerStorage @Inject constructor(
             gson.toJson(data, storageFile)
         } catch (e: Exception) {
             Timber.e(e, "Failed to save tracked calculations.")
+            e.report(ExceptionCategory.INTERNAL)
         }
     }
 }

Corona-Warn-App/src/main/java/de/rki/coronawarnapp/util/gson/GsonExtensions.kt

@@ -9,10 +9,11 @@ inline fun <reified T> Gson.fromJson(json: String): T = fromJson(
     object : TypeToken<T>() {}.type
 )
 
-inline fun <reified T> Gson.fromJson(file: File): T = file.reader().use {
+inline fun <reified T> Gson.fromJson(file: File): T = file.bufferedReader().use {
     fromJson(it, object : TypeToken<T>() {}.type)
 }
 
-inline fun <reified T> Gson.toJson(data: T, file: File) = file.writer().use { writer ->
+inline fun <reified T> Gson.toJson(data: T, file: File) = file.bufferedWriter().use { writer ->
     toJson(data, writer)
+    writer.flush()
 }

Corona-Warn-App/src/test/java/de/rki/coronawarnapp/nearby/modules/calculationtracker/CalculationTrackerStorageTest.kt

@@ -130,4 +130,20 @@ class CalculationTrackerStorageTest : BaseIOTest() {
         storedData.getValue("b2b98400-058d-43e6-b952-529a5255248b").isCalculating shouldBe true
         storedData.getValue("aeb15509-fb34-42ce-8795-7a9ae0c2f389").isCalculating shouldBe false
     }
+
+    @Test
+    fun `we catch empty json data and prevent unsafely initialized maps`() = runBlockingTest {
+        storageDir.mkdirs()
+        storageFile.writeText("")
+
+        storageFile.exists() shouldBe true
+
+        createStorage().apply {
+            val value = load()
+            value.size shouldBe 0
+            value shouldBe emptyMap()
+
+            storageFile.exists() shouldBe false
+        }
+    }
 }