Support optional --use-temp-dir in get to download to temp dir and copy, to avoid partial jar if interrupted

src/main/java/me/itzg/helpers/get/GetCommand.java

@@ -134,6 +134,11 @@ public class GetCommand implements Callable<Integer> {
     @Option(names = "--retry-delay", description = "in seconds", defaultValue = "2")
     int retryDelay;
 
+    @Option(names = {"--use-temp-dir"},
+        description = "Specifies the name of a directory to use for downloading to a temporary file, before it is copied to the final destination",
+        paramLabel = "DIR")
+    Path tempDir;
+
     @Parameters(split = OPTION_SPLIT_COMMAS, paramLabel = "URI",
         description = "The URI of the resource to retrieve. When the output is a directory,"
             + " more than one URI can be requested.",
@@ -159,6 +164,10 @@ public Integer call() throws IOException {
             throw new ParameterException(spec.commandLine(), "No URIs were given");
         }
 
+        if (tempDir != null && !Files.isDirectory(tempDir)) {
+            throw new ParameterException(spec.commandLine(), "The supplied temporary directory does not exist");
+        }
+
         final LatchingUrisInterceptor interceptor = new LatchingUrisInterceptor();
 
         try (CloseableHttpClient client = HttpClients.custom()
@@ -209,8 +218,9 @@ null, new JsonPathOutputHandler(
                         stdout.println(outputFile);
                     }
                 } else {
+                    final Path saveToFile = getSaveToFile();
                     final Path file = fetch(uris.get(0))
-                        .toFile(outputFile)
+                        .toFile(saveToFile)
                         .skipUpToDate(skipUpToDate)
                         .acceptContentTypes(acceptContentTypes)
                         .handleDownloaded((uri, f, contentSizeBytes) -> {
@@ -219,6 +229,9 @@ null, new JsonPathOutputHandler(
                             }
                         })
                         .execute();
+                    if (tempDir != null) {
+                        Files.move(saveToFile, outputFile);
+                    }
                     if (this.outputFilename) {
                         stdout.println(file);
                     }
@@ -477,5 +490,8 @@ private static URI removeUserInfo(URI uri) throws URISyntaxException {
         );
     }
 
+    private Path getSaveToFile() {
+        return tempDir == null ? outputFile : tempDir.resolve(outputFile.getFileName());
+    }
 
 }

src/test/java/me/itzg/helpers/get/GetCommandTest.java

@@ -12,9 +12,12 @@
 import java.net.MalformedURLException;
 import java.net.URI;
 import java.net.URISyntaxException;
+import java.nio.file.Files;
+import java.nio.file.Path;
 import me.itzg.helpers.TestLoggingAppender;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 import picocli.CommandLine;
 import picocli.CommandLine.ExitCode;
 
@@ -222,4 +225,49 @@ void tryHttps(WireMockRuntimeInfo wmInfo) throws Exception {
         // See https://wiremock.org/docs/https/#common-https-issues
         assertThat(err).contains("unable to find valid certification path to requested target");
     }
+
+    @Test
+    void failWhenSuppliedTempDirDoesntExist() throws Exception {
+        final String stderr = tapSystemErrNormalized(() -> {
+
+            final int status =
+                new CommandLine(new GetCommand())
+                    .execute(
+                        "--use-temp-dir",
+                        "non-existent-temp-directory",
+                        "-o",
+                        "unused.out",
+                        "http://unused"
+                    );
+
+            assertThat(status).isEqualTo(ExitCode.USAGE);
+        });
+
+        assertThat(stderr)
+            .contains("The supplied temporary directory does not exist");
+    }
+
+    @Test
+    void failWhenSuppliedTempDirIsAFile(@TempDir Path tempDir) throws Exception {
+        final Path expectedFile = tempDir.resolve("isafile");
+        Files.createFile(expectedFile);
+
+        final String stderr = tapSystemErrNormalized(() -> {
+
+            final int status =
+                new CommandLine(new GetCommand())
+                    .execute(
+                        "--use-temp-dir",
+                        expectedFile.toString(),
+                        "-o",
+                        "unused.out",
+                        "http://unused"
+                    );
+
+            assertThat(status).isEqualTo(ExitCode.USAGE);
+        });
+
+        assertThat(stderr)
+            .contains("The supplied temporary directory does not exist");
+    }
 }

src/test/java/me/itzg/helpers/get/OutputToFileTest.java

@@ -230,4 +230,30 @@ void skipsUpToDate_butDownloadsWhenAbsent(@TempDir Path tempDir) throws IOExcept
     assertThat(fileToDownload).hasContent("New content");
   }
 
+  @Test
+  void successfulWithTemporaryDirectory(@TempDir Path tempDir) throws MalformedURLException, IOException {
+    mock.expectRequest("GET", "/downloadsToFile.txt",
+        response()
+            .withBody("Response content to file", MediaType.TEXT_PLAIN)
+    );
+
+    final Path expectedFile = tempDir.resolve("out.txt");
+    final Path downloadingDir = tempDir.resolve("downloading");
+    Files.createDirectory(downloadingDir);
+
+    final int status =
+        new CommandLine(new GetCommand())
+            .execute(
+                "-o",
+                expectedFile.toString(),
+                "--use-temp-dir",
+                downloadingDir.toString(),
+                mock.buildMockedUrl("/downloadsToFile.txt").toString()
+            );
+
+    assertThat(status).isEqualTo(0);
+    assertThat(expectedFile).exists();
+    assertThat(expectedFile).hasContent("Response content to file");
+  }
+
 }