LOGBACK-992: do not delete the original file when compression fails

logback-core/src/main/java/ch/qos/logback/core/rolling/helper/CompressionStrategyBase.java

@@ -29,4 +29,10 @@ void createMissingTargetDirsIfNecessary(File file) {
             addError("Failed to create parent directories for [" + file.getAbsolutePath() + "]");
         }
     }
+
+    void discardPartialCompressedFile(File file) {
+        if (file.exists() && !file.delete()) {
+            addWarn("Could not delete partially written compressed file [" + file.getAbsolutePath() + "]");
+        }
+    }
 }

logback-core/src/main/java/ch/qos/logback/core/rolling/helper/GZCompressionStrategy.java

@@ -62,6 +62,8 @@ public void compress(String originalFileName, String compressedFileName, String
             addInfo("Done GZ compressing [" + file2gz + "] as [" + gzedFile + "]");
         } catch (Exception e) {
             addStatus(new ErrorStatus("Error occurred while compressing [" + originalFileName + "] into [" + compressedFileName + "].", this, e));
+            discardPartialCompressedFile(gzedFile);
+            return;
         }
 
         if (!file2gz.delete()) {

logback-core/src/main/java/ch/qos/logback/core/rolling/helper/XZCompressionStrategy.java

@@ -64,6 +64,8 @@ public void compress(String nameOfFile2xz, String nameOfxzedFile, String innerEn
             }
         } catch (Exception e) {
             addError("Error occurred while compressing [" + nameOfFile2xz + "] into [" + nameOfxzedFile + "].", e);
+            discardPartialCompressedFile(xzedFile);
+            return;
         }
 
         if (!file2xz.delete()) {

logback-core/src/main/java/ch/qos/logback/core/rolling/helper/ZipCompressionStrategy.java

@@ -78,6 +78,8 @@ public void compress(String originalFileName, String compressedFileName, String
             addInfo("Done ZIP compressing [" + file2zip + "] as [" + zippedFile + "]");
         } catch (Exception e) {
             addStatus(new ErrorStatus("Error occurred while compressing [" + originalFileName + "] into [" + compressedFileName + "].", this, e));
+            discardPartialCompressedFile(zippedFile);
+            return;
         }
         if (!file2zip.delete()) {
             addStatus(new WarnStatus("Could not delete [" + originalFileName + "].", this));

logback-core/src/test/java/ch/qos/logback/core/rolling/helper/CompressionStrategyFailureTest.java

@@ -0,0 +1,86 @@
+/*
+ * Logback: the reliable, generic, fast and flexible logging framework.
+ * Copyright (C) 1999-2026, QOS.ch. All rights reserved.
+ *
+ * This program and the accompanying materials are dual-licensed under
+ * either the terms of the Eclipse Public License v2.0 as published by
+ * the Eclipse Foundation
+ *
+ *   or (per the licensee's choosing)
+ *
+ * under the terms of the GNU Lesser General Public License version 2.1
+ * as published by the Free Software Foundation.
+ */
+package ch.qos.logback.core.rolling.helper;
+
+import ch.qos.logback.core.Context;
+import ch.qos.logback.core.ContextBase;
+import ch.qos.logback.core.status.Status;
+import ch.qos.logback.core.status.testUtil.StatusChecker;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+import java.io.FileWriter;
+import java.io.IOException;
+
+import static ch.qos.logback.core.testUtil.CoreTestConstants.OUTPUT_DIR_PREFIX;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * When compression fails, the original (uncompressed) file must NOT be
+ * deleted, otherwise the rolled log data is lost with only a corrupt
+ * archive left behind. See LOGBACK-992.
+ */
+public class CompressionStrategyFailureTest {
+
+    Context context = new ContextBase();
+    File outputDir;
+    File originalFile;
+    File blockingFile;
+
+    @BeforeEach
+    public void setUp() throws IOException {
+        outputDir = new File(OUTPUT_DIR_PREFIX + "compressFailure-" + System.nanoTime());
+        assertTrue(outputDir.mkdirs());
+        originalFile = new File(outputDir, "app.log");
+        try (FileWriter fw = new FileWriter(originalFile)) {
+            fw.write("hello world\n");
+        }
+        // plain file later used as the parent "directory" of the compression
+        // target, guaranteeing that opening the target stream fails
+        blockingFile = new File(outputDir, "blocking");
+        try (FileWriter fw = new FileWriter(blockingFile)) {
+            fw.write("x");
+        }
+    }
+
+    private String failingTarget(String suffix) {
+        return new File(blockingFile, "app.log" + suffix).getPath();
+    }
+
+    @Test
+    public void originalFileSurvivesFailedGZCompression() {
+        GZCompressionStrategy gz = new GZCompressionStrategy();
+        gz.setContext(context);
+        gz.compress(originalFile.getPath(), failingTarget(".gz"), null);
+
+        StatusChecker checker = new StatusChecker(context);
+        checker.assertContainsMatch(Status.ERROR, "Error occurred while compressing");
+        assertTrue(originalFile.exists(), "original file should survive a failed GZ compression");
+        assertFalse(new File(failingTarget(".gz")).exists());
+    }
+
+    @Test
+    public void originalFileSurvivesFailedZipCompression() {
+        ZipCompressionStrategy zip = new ZipCompressionStrategy();
+        zip.setContext(context);
+        zip.compress(originalFile.getPath(), failingTarget(".zip"), "app.log");
+
+        StatusChecker checker = new StatusChecker(context);
+        checker.assertContainsMatch(Status.ERROR, "Error occurred while compressing");
+        assertTrue(originalFile.exists(), "original file should survive a failed ZIP compression");
+        assertFalse(new File(failingTarget(".zip")).exists());
+    }
+}