app: write config atomically

A crash mid-write corrupted config.json, and every later load then
threw on parse, crash-looping the app until its data was cleared,
losing the key and pinned host. Write to a temp file and rename.

1 files changed, 7 insertions, 1 deletions

diff --git a/app/src/main/java/invalid/lena/rsend/Config.kt b/app/src/main/java/invalid/lena/rsend/Config.kt
index 8c9c22c..d7f17eb 100644
--- a/app/src/main/java/invalid/lena/rsend/Config.kt
+++ b/app/src/main/java/invalid/lena/rsend/Config.kt
@@ -4,6 +4,7 @@ import android.content.Context
 import org.json.JSONArray
 import org.json.JSONObject
 import java.io.File
+import java.io.IOException
 
 // Config is rsend's whole state: the remote target, the schedule, and the
 // folders to push. It is stored as plain JSON in app-private storage.
@@ -64,8 +65,13 @@ data class Config(
             return fromJson(JSONObject(f.readText()))
         }
 
+        // save writes tmp-then-rename so a crash mid-write cannot corrupt the
+        // config and brick every later load.
         fun save(ctx: Context, c: Config) {
-            file(ctx).writeText(c.toJson().toString(2))
+            val f = file(ctx)
+            val tmp = File(f.path + ".tmp")
+            tmp.writeText(c.toJson().toString(2))
+            if (!tmp.renameTo(f)) throw IOException("rename ${tmp.path} failed")
         }
 
         fun fromJson(o: JSONObject): Config {