.editorconfig

@@ -12,8 +12,5 @@ tab_width = 4
 [{*.yaml,*.yml}]
 indent_size = 2
 
-[src/test/**/*]
-ktlint_standard_no-wildcard-imports = disabled
-
 [{*.kt,*.kts}]
 ij_kotlin_packages_to_use_import_on_demand = org.junit.jupiter.api,aws.sdk.kotlin.services.s3,kotlinx.coroutines,java.io,ziputils

.github/workflows/publish.yaml

@@ -22,12 +22,6 @@ jobs:
       - name: Setup Gradle
         uses: gradle/gradle-build-action@v2
 
-      - name: Setup AWS Credentials
-        run: |
-          mkdir ~/.aws
-          echo ${{ secrets.aws_config }} | base64 -d > ~/.aws/config
-          echo ${{ secrets.aws_credentials }} | base64 -d > ~/.aws/credentials
-
       - name: Run checks
         run: ./gradlew check
 

.github/workflows/test.yaml

@@ -25,11 +25,5 @@ jobs:
       - name: Setup Gradle
         uses: gradle/gradle-build-action@v2
 
-      - name: Setup AWS Credentials
-        run: |
-          mkdir ~/.aws
-          echo ${{ secrets.aws_config }} | base64 -d > ~/.aws/config
-          echo ${{ secrets.aws_credentials }} | base64 -d > ~/.aws/credentials
-
       - name: Run checks
         run: ./gradlew check

.idea/.name

@@ -1 +0,0 @@
-s3backup-tool

README.md

@@ -3,8 +3,8 @@ This is a small backup utility for uploading/restoring a local directory to/from
 an AWS S3 bucket.
 
 ## Usage
-This tool is released as a JAR in the [releases page](https://git.koval.net/cyclane/teamcity-executors-test-task/releases).
-Use `java -jar s3backup-tool-<version>.jar --help` for more detailed usage instructions.
+This tool is released as a JAR in the [release page](https://git.koval.net/cyclane/teamcity-executors-test-task/releases).
+Use `java -jar <backup-jar-name>.jar --help` for more detailed usage instructions.
 
 ### --help
 ```
@@ -65,29 +65,34 @@ Arguments:
 ```
 
 ## Assumptions
-1. The test task is not interested in re-implementations of common libraries (AWS SDK, Clikt, Gradle Shadow, ...)
-2. The test task is more interested in Kotlin JVM (not Kotlin Native).
+1. This test task is not interested in re-implementations of common libraries (AWS SDK, Clikt, Gradle Shadow, ...)
+2. The last part (restoration of a single file) should be optimised so that only the part of the blob required for this 
+   file is downloaded.
+3. Only this tool is ever used to create backups, so S3 object keys are in the expected format, and ZIP files do not have
+   a comment in the *end of central directory* record (making it a predictable length of 22 bytes).
+   - EOCD64 should similarly not have a comment.
 
 ## Design decisions
-- Backups may be large, so we want to use **multipart uploads** if possible (< 100mb is recommended).
+- Backups may be large, so we want to use multipart uploads if possible (< 100mb is recommended).
   https://docs.aws.amazon.com/AmazonS3/latest/userguide/mpuoverview.html
   - The Java SDK has high-level support for this via [S3TransferManager](https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/transfer/s3/S3TransferManager.html),
     but unfortunately when the content is too small, the HTTP `Content-Length` is not automatically calculated resulting
     in an error response from the API.
-    - I'm not sure whether this is intended behaviour or a bug, but decided to manually implement multipart uploads using
-      the Kotlin SDK instead anyway.
-- **ZIP files** are used so that the backups can be stored in a very common format which also provides compression.
-  - Allows future expansion to allow for encryption as well.
-  - [Java ZIP specification](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/zip/package-summary.html): 
+  - I'm not sure whether this is intended behaviour or a bug, but decided to manually implement multipart uploads using
+    the Kotlin SDK instead anyway.
+  - **Note**: I could have just used a temporary file (with a known `Content-Length`), but I wanted to play around with
+    streams and kotlin concurrency a bit, which is why I went with the more scalable way using streams.
+- Zip files are used so that the backups can be stored in a very common format which also provides compression.
+  - Java zip specification: https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/zip/package-summary.html
     - ZIP64 implementation is optional, but possible, so we'll handle it.
   - The End of Central Directory record is also useful for locating the exact positions of files in the blob, so that
     single files can be downloaded using the HTTP `Range` header.
-  - End of Central Directory comment must be blank. Otherwise, the EOCD length is unpredictable, and so we
+  - End of Central Directory comment must be blank (assumption 3). Otherwise, the EOCD length is unpredictable and so we
     cannot use just a single request the HTTP `Range` header to get the entire EOCD.
-    - *Alternative*: use S3 object tags to store the EOCD size, fallback to 22 bytes otherwise. This could be
-      interesting if we want the backup tool to be able to import existing ZIPs (which could potentially have a comment),
-      but that is beyond the scope of the instructions.
-- Only the `BackupClient` is tested, since by testing it, all other (internal) classes are functions are tested as well.
+    - Alternative: use S3 object tags to store the EOCD offset, but this way the blob itself would no longer contain all
+      the data required by this backup tool.
+    - Alternative: store the EOCD offset in the EOCD comment or the beginning of the file, but this makes a similar, but
+      more strict assumption anyway.
 
 ## Instructions
 Create a backup utility that copies files to AWS S3. The utility should take a local directory with files and put it into AWS S3 in the form of one blob file. The reverse behavior should also be possible. We should be able to specify what backup we want to restore and where it should put the files on the local system. The utility should be able to restore one individual file from a backup.

build.gradle.kts

@@ -14,7 +14,7 @@ repositories {
 
 dependencies {
     implementation("aws.sdk.kotlin:s3:1.0.25")
-    implementation("org.slf4j:slf4j-simple:2.0.9") // AWS SDK wants a SLF4J provider.
+    implementation("org.slf4j:slf4j-simple:2.0.9")
     implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.7.3")
     implementation("com.github.ajalt.clikt:clikt:4.2.1")
     testImplementation("org.jetbrains.kotlin:kotlin-test")

settings.gradle.kts

@@ -1,4 +1,4 @@
 plugins {
     id("org.gradle.toolchains.foojay-resolver-convention") version "0.5.0"
 }
-rootProject.name = "s3backup-tool"
+rootProject.name = "teamcity-executors-test-task"

src/main/kotlin/backup/BackupClient.kt

@@ -214,8 +214,10 @@ class BackupClient(
                 bucket = bucketName
                 key = backupKey
                 range = "bytes=$localHeaderOffset-${
-                    // Over-fetch a bit so that ZipInputStream can see the next header (otherwise it EOFs, even though
-                    // it knows exactly how much data is should read, so not sure why it reads ahead).
+                    // Add CEN min size (46 bytes) so that the next CEN / LOC header is seen by the ZipInputStream
+                    // and so it can see the current entry has stopped.
+                    // Note: yes ZipInputStream should know the exact content length from the LOC, but it was still sending
+                    // EOF errors. Perhaps due to fetching multiples of a power of two, or something else. But this helps.
                     localHeaderOffset + cen.size.toULong() + compressedSize + CentralDirectoryFileHeader.SIZE.toULong()
                 }"
             }
@@ -235,7 +237,7 @@ class BackupClient(
  * @param number The part number that was used for this part upload.
  * @return The CompletedPart object.
  */
-internal fun UploadPartResponse.toCompletedPart(number: Int): CompletedPart {
+private fun UploadPartResponse.toCompletedPart(number: Int): CompletedPart {
     val part = this
     return CompletedPart {
         partNumber = number
@@ -252,7 +254,7 @@ internal fun UploadPartResponse.toCompletedPart(number: Int): CompletedPart {
  * @param n The number of items to take.
  * @return A ByteArray of the first `n` items.
  */
-internal fun ByteArray.take(n: Int) =
+private fun ByteArray.take(n: Int) =
     if (n == size) {
         this // No copy
     } else {
@@ -263,7 +265,7 @@ internal fun ByteArray.take(n: Int) =
  * Compress a file or directory as a ZIP file to an `OutputStream`.
  * @param outputStream The `OutputStream` to write the ZIP file contents to.
  */
-internal fun File.compressToZip(outputStream: OutputStream) =
+private fun File.compressToZip(outputStream: OutputStream) =
     ZipOutputStream(outputStream).use { zipStream ->
         val parentDir = this.canonicalFile.parent + "/"
         val fileQueue = ArrayDeque<File>()
@@ -292,7 +294,7 @@ internal fun File.compressToZip(outputStream: OutputStream) =
  * @param bufSize The buffer size to use for writing the decompressed files.
  * @param entryNameToPath A function to convert ZIP entry names to destination `Path`s.
  */
-internal fun ZipInputStream.decompress(
+private fun ZipInputStream.decompress(
     bufSize: Int = 1024 * 1024,
     limit: Int? = null,
     entryNameToPath: (String) -> Path,
@@ -326,7 +328,7 @@ internal fun ZipInputStream.decompress(
  * @param entry The `ZipEntry` to set attributes of.
  * @param path The `Path` of the file to get the attributes from.
  */
-internal fun setZipAttributes(
+private fun setZipAttributes(
     entry: ZipEntry,
     path: Path,
 ) {
@@ -344,7 +346,7 @@ internal fun setZipAttributes(
  * @param entry The `ZipEntry` to get the attributes from.
  * @param path The `Path` of the file to set the attributes of.
  */
-internal fun applyZipAttributes(
+private fun applyZipAttributes(
     entry: ZipEntry,
     path: Path,
 ) {

src/test/kotlin/backup/BackupClientTest.kt

@@ -1,165 +0,0 @@
-package backup
-
-import aws.sdk.kotlin.services.s3.*
-import kotlinx.coroutines.runBlocking
-import org.junit.jupiter.api.*
-import org.junit.jupiter.api.Assertions.*
-import java.io.File
-import kotlin.io.path.*
-import kotlin.random.Random
-
-val bucketName = System.getenv("BACKUP_BUCKET") ?: "teamcity-executors-test-task"
-
-class BackupClientTest {
-    lateinit var s3: S3Client
-    lateinit var backupClient: BackupClient
-
-    @BeforeEach
-    fun `before each`() =
-        runBlocking {
-            s3 = S3Client.fromEnvironment {}
-            backupClient = BackupClient(s3, bucketName, 1024 * 1024 * 10)
-        }
-
-    @AfterEach
-    fun `after each`() {
-        s3.close()
-    }
-
-    @TestFactory
-    fun `round-trip tests`() =
-        listOf(
-            "empty directory" to {
-                listOf(
-                    Path("_test").createDirectory(),
-                )
-            },
-            "single file" to {
-                listOf(
-                    Path("_test.txt").apply { writeText("Hello World!") },
-                )
-            },
-            "directory structure" to {
-                listOf(
-                    Path("_test").createDirectory(),
-                    Path("_test/a.txt").apply { writeText("This is file A!\nAnother line here.") },
-                    Path("_test/folder").createDirectory(),
-                    Path("_test/another-folder").createDirectory(),
-                    Path("_test/another-folder/b").apply { writeText("This is file B\n") },
-                    Path("_test/another-folder/c.txt").createFile(),
-                    Path("_test/README.md").apply { writeText("# This is a test directory structure.") },
-                )
-            },
-            "single large file" to {
-                val bytes = ByteArray(1024 * 1024 * 32)
-                Random.nextBytes(bytes)
-                listOf(
-                    Path("_test.txt").apply { writeBytes(bytes) },
-                )
-            },
-            "large directory structure" to {
-                val bytes1 = ByteArray(1024 * 1024 * 32)
-                val bytes2 = ByteArray(1024 * 1024 * 48)
-                listOf(
-                    Path("_test").createDirectory(),
-                    Path("_test/a.txt").apply { writeBytes(bytes1) },
-                    Path("_test/folder").createDirectory(),
-                    Path("_test/another-folder").createDirectory(),
-                    Path("_test/another-folder/b").apply { writeText("This is file B\n") },
-                    Path("_test/another-folder/c.txt").createFile(),
-                    Path("_test/README.md").apply { writeBytes(bytes2) },
-                )
-            },
-        ).map { (name, pathsGen) ->
-            DynamicTest.dynamicTest(name) {
-                val paths = pathsGen()
-                val backupKey =
-                    assertDoesNotThrow("should upload files") {
-                        runBlocking {
-                            backupClient.upload(paths.first().toFile())
-                        }
-                    }
-                val restoreDir = Path("_test_restore").createDirectory()
-                assertDoesNotThrow("should recover files") {
-                    runBlocking {
-                        backupClient.restore(restoreDir, backupKey)
-                    }
-                }
-                assertEquals(
-                    paths.size,
-                    restoreDir.toFile().countEntries() - 1,
-                    "number of files in backup restore should be equal to original",
-                )
-                val individualRestoreDir = Path("_test_restore_individual").createDirectory()
-                paths.forEach { path ->
-                    if (path.isDirectory()) {
-                        individualRestoreDir.resolve(path).createDirectory()
-                    } else {
-                        assertDoesNotThrow("should recover file '$path'") {
-                            runBlocking {
-                                backupClient.restoreFile(
-                                    individualRestoreDir.resolve(path).parent,
-                                    backupKey,
-                                    path.toString(),
-                                )
-                            }
-                        }
-                    }
-                }
-                assertEquals(
-                    paths.size,
-                    individualRestoreDir.toFile().countEntries() - 1,
-                    "number of files in individual backup restore should be equal to original",
-                )
-                paths.asReversed().forEach { path ->
-                    val restorePath = restoreDir.resolve(path)
-                    val individualRestorePath = individualRestoreDir.resolve(path)
-                    if (path.isDirectory()) {
-                        assertTrue(restorePath.exists(), "'$path' should exist in backup")
-                        assertTrue(restorePath.isDirectory(), "'$path' should be a directory in backup")
-                        assertTrue(individualRestorePath.exists(), "'$path' should exist in backup (individual)")
-                        assertTrue(
-                            individualRestorePath.isDirectory(),
-                            "'$path' should be a directory in backup (individual)",
-                        )
-                    } else {
-                        val originalBytes = path.toFile().readBytes().asList()
-                        assertEquals(
-                            originalBytes,
-                            restorePath.toFile().readBytes().asList(),
-                            "File contents of '$path' should equal",
-                        )
-                        assertEquals(
-                            originalBytes,
-                            individualRestorePath.toFile().readBytes().asList(),
-                            "File contents of '$path' should equal (individual)",
-                        )
-                    }
-                    // cleanup
-                    path.deleteExisting()
-                    restorePath.deleteExisting()
-                    individualRestorePath.deleteExisting()
-                }
-                // cleanup
-                restoreDir.deleteExisting()
-                individualRestoreDir.deleteExisting()
-                runBlocking {
-                    s3.deleteObject {
-                        bucket = bucketName
-                        key = backupKey
-                    }
-                }
-            }
-        }
-}
-
-internal fun File.countEntries(): Int {
-    val queue = ArrayDeque<File>()
-    queue.add(this)
-    return queue.count { file ->
-        if (file.isDirectory) {
-            queue.addAll(file.listFiles()!!) // shouldn't ever be null, since we know it's a directory
-        }
-        true
-    }
-}