Gradle: Implement smart downloads which avoid download if the file exists and is valid.

buildSrc/build.gradle.kts

@@ -12,6 +12,7 @@ dependencies {
   implementation("org.jetbrains.kotlin:kotlin-serialization:1.6.10")
   implementation("gradle.plugin.com.github.johnrengelman:shadow:7.1.1")
   implementation("com.google.code.gson:gson:2.8.9")
+  implementation("org.bouncycastle:bcprov-jdk15on:1.70")
 }
 
 java.sourceCompatibility = JavaVersion.VERSION_1_8

buildSrc/src/main/kotlin/cloud/kubelet/foundation/gradle/SetupPaperServer.kt

@@ -59,14 +59,11 @@ open class SetupPaperServer : DefaultTask() {
     val build = builds.last()
     val download = build.downloads["application"]!!
     val url = paperVersionClient.resolveDownloadUrl(build, download)
-
-    ant.invokeMethod(
-      "get", mapOf(
-        "src" to url.toString(),
-        "dest" to paperJarFile.absolutePath
-      )
-    )
-
-    logger.lifecycle("Installed Paper Server ${build.version} build ${build.build}")
+    val downloader = SmartDownload(paperJarFile.toPath(), url, download.sha256)
+    if (downloader.download()) {
+      logger.lifecycle("Installed Paper Server ${build.version} build ${build.build}")
+    } else {
+      logger.lifecycle("Paper Server ${build.version} build ${build.build} is up-to-date")
+    }
   }
 }

buildSrc/src/main/kotlin/cloud/kubelet/foundation/gradle/SmartDownload.kt

@@ -0,0 +1,62 @@
+package cloud.kubelet.foundation.gradle
+
+import java.net.URI
+import java.nio.file.Files
+import java.nio.file.Path
+import java.security.MessageDigest
+
+class SmartDownload(val localFilePath: Path, val remoteDownloadUrl: URI, val sha256: String) {
+  fun download(): Boolean {
+    if (!checkLocalFileHash()) {
+      downloadRemoteFile()
+      return true
+    }
+    return false
+  }
+
+  private fun downloadRemoteFile() {
+    val url = remoteDownloadUrl.toURL()
+    val remoteFileStream = url.openStream()
+    val localFileStream = Files.newOutputStream(localFilePath)
+    remoteFileStream.transferTo(localFileStream)
+    if (!checkLocalFileHash()) {
+      throw RuntimeException("Download of ${remoteDownloadUrl} did not result in valid hash.")
+    }
+  }
+
+  private fun checkLocalFileHash(): Boolean {
+    if (!Files.exists(localFilePath)) {
+      return false
+    }
+
+    val digest = MessageDigest.getInstance("SHA-256")
+    val localFileStream = Files.newInputStream(localFilePath)
+    val buffer = ByteArray(16 * 1024)
+
+    while (true) {
+      val size = localFileStream.read(buffer)
+      if (size <= 0) {
+        break
+      }
+
+      val bytes = buffer.take(size).toByteArray()
+      digest.update(bytes)
+    }
+
+    val sha256Bytes = digest.digest()
+    val localSha256Hash = bytesToHex(sha256Bytes)
+    return localSha256Hash.equals(sha256, ignoreCase = true)
+  }
+
+  private fun bytesToHex(hash: ByteArray): String {
+    val hexString = StringBuilder(2 * hash.size)
+    for (i in hash.indices) {
+      val hex = Integer.toHexString(0xff and hash[i].toInt())
+      if (hex.length == 1) {
+        hexString.append('0')
+      }
+      hexString.append(hex)
+    }
+    return hexString.toString()
+  }
+}