Write GC summary file using UTF-8

Tested by adding a unit test for writeJsonSummary.

Fixes #4644.

clients/spark/core/src/main/scala/io/treeverse/clients/GarbageCollector.scala

@@ -661,9 +661,12 @@ object GarbageCollector {
 
     val stream = dstFS.create(dstPath)
     try {
-      stream.writeChars(compact(render(jsonSummary)))
+      val bytes = compact(render(jsonSummary)).getBytes("UTF-8")
+      stream.write(bytes)
     } finally {
       stream.close()
     }
   }
+
+  def writeJsonSummaryForTesting = writeJsonSummary _
 }

clients/spark/core/src/test/scala/io/treeverse/clients/GarbageCollectorSpec.scala

@@ -1,12 +1,29 @@
 package io.treeverse.clients
 
+import scala.collection.JavaConverters._
 import org.scalatest._
 import matchers.should._
 import funspec._
 
+import io.treeverse.lakefs.catalog
+
+import org.apache.commons.io.FileUtils
 import org.apache.spark.{HashPartitioner, SparkConf}
 import org.apache.spark.sql.{Dataset, SparkSession}
-import io.treeverse.lakefs.catalog
+
+import org.json4s._
+import org.json4s.native.JsonMethods
+
+import java.nio.file.{Files, Path, Paths}
+
+trait TempDirectory {
+  def withTempDirectory(testMethod: (Path) => Any) {
+    val tempDir = Files.createTempDirectory("test-gc")
+
+    testMethod(tempDir)
+    FileUtils.deleteDirectory(tempDir.toFile)
+  }
+}
 
 class ARangeGetter(
     val repo: String,
@@ -182,6 +199,38 @@ class GarbageCollectorSpec extends AnyFunSpec with Matchers with SparkSessionSet
   }
 }
 
+class GarbageCollectorJsonOutputSpec extends AnyFunSpec with Matchers with SparkSessionSetup with TempDirectory {
+    describe("writeJsonSummary") {
+      it("should write a summary") {
+        withSparkSession(spark =>
+          withTempDirectory(tempDir => {
+            val sc = spark.sparkContext
+            val configMapper = new ConfigMapper(sc.broadcast(Array[(String, String)]()))
+            val dstRoot = tempDir.resolve("writeJsonSummary/")
+            val numDeletedObjects = 2906
+            val gcRules = "gobble gobble"
+            val time = "I always will remember, 'Twas a year ago November"
+
+            GarbageCollector.writeJsonSummaryForTesting(configMapper, dstRoot.toAbsolutePath.toString, numDeletedObjects, gcRules, time)
+
+            val written = FileUtils.listFiles(dstRoot.toFile, null, true)
+              .asScala
+              .iterator
+              .filter((f) => !f.toString.endsWith(".crc"))
+              .toSeq
+            written.size should be(1)
+            val actualBytes = Files.readAllBytes(Paths.get(written(0).toString))
+            Console.out.println("GOT: " + new String(actualBytes, "UTF-8"))
+            // Explicitly verify that we received UTF-8 encoded data!
+            val actual = JsonMethods.parse(new String(actualBytes, "UTF-8"))
+            (actual \ "gc_rules") should be(JString(gcRules))
+            (actual \ "num_deleted_objects") should be(JInt(numDeletedObjects))
+            // TODO(ariels): Verify dt=${time} in path.
+          }))
+      }
+    }
+}
+
 trait SparkSessionSetup {
   def withSparkSession(testMethod: (SparkSession) => Any) {
     val conf = new SparkConf()