ToolsInstaller: write to temporary file, fsync, rename

Reported-by: Andre Christanto <christantoandre@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

app/src/main/java/com/wireguard/android/util/SharedLibraryLoader.java

@@ -54,6 +54,7 @@ public final class SharedLibraryLoader {
                     while ((len = in.read(buffer)) != -1) {
                         out.write(buffer, 0, len);
                     }
+                    out.getFD().sync();
                 }
                 return true;
             }

app/src/main/java/com/wireguard/android/util/ToolsInstaller.java

@@ -149,18 +149,22 @@ public final class ToolsInstaller {
     public boolean extract() throws IOException {
         localBinaryDir.mkdirs();
         final File files[] = new File[EXECUTABLES.length];
+        final File tempFiles[] = new File[EXECUTABLES.length];
         boolean allExist = true;
         for (int i = 0; i < files.length; ++i) {
             files[i] = new File(localBinaryDir, EXECUTABLES[i]);
+            tempFiles[i] = new File(localBinaryDir, EXECUTABLES[i] + ".tmp");
             allExist &= files[i].exists();
         }
         if (allExist)
             return false;
         for (int i = 0; i < files.length; ++i) {
-            if (!SharedLibraryLoader.extractLibrary(context, EXECUTABLES[i], files[i]))
+            if (!SharedLibraryLoader.extractLibrary(context, EXECUTABLES[i], tempFiles[i]))
                 throw new FileNotFoundException("Unable to find " + EXECUTABLES[i]);
-            if (!files[i].setExecutable(true, false))
-                throw new IOException("Unable to mark " + files[i].getAbsolutePath() + " as executable");
+            if (!tempFiles[i].setExecutable(true, false))
+                throw new IOException("Unable to mark " + tempFiles[i].getAbsolutePath() + " as executable");
+            if (!tempFiles[i].renameTo(files[i]))
+                throw new IOException("Unable to rename " + tempFiles[i].getAbsolutePath() + " to " + files[i].getAbsolutePath());
         }
         return true;
     }