Fix plain text log output dropping trailing partial lines

core/src/main/java/hudson/console/AnnotatedLargeText.java

@@ -227,7 +227,15 @@ protected boolean delegateToWriteLogTo(StaplerRequest2 req, StaplerResponse2 rsp
     @CheckReturnValue
     @Override
     public long writeLogTo(long start, OutputStream out) throws IOException {
-        return super.writeLogTo(start, new PlainTextConsoleOutputStream(out));
+        PlainTextConsoleOutputStream ptos = new PlainTextConsoleOutputStream(out);
+        long r = super.writeLogTo(start, ptos);
+        if (isComplete()) {
+            // The client is not expected to perform any further reads after this one. Make sure that we have flushed the line buffer.
+            ptos.forceEol();
+        }
+        // Back-track any pending bytes in the line buffer.
+        r -= ptos.lineBufferSize();
+        return r;
     }
 
     /**

core/src/test/java/hudson/console/AnnotatedLargeTextPartialLineTest.java

@@ -0,0 +1,124 @@
+/*
+ * The MIT License
+ *
+ * Copyright 2026 CloudBees, Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package hudson.console;
+
+import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.is;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import org.junit.jupiter.api.Test;
+import org.kohsuke.stapler.StaplerRequest2;
+import org.kohsuke.stapler.framework.io.ByteBuffer;
+
+/**
+ * Tests that {@link AnnotatedLargeText#writeLogTo(long, java.io.OutputStream)} does not silently drop
+ * a trailing partial line, mirroring the behavior of {@link AnnotatedLargeText#writeHtmlTo(long, java.io.Writer)}.
+ */
+class AnnotatedLargeTextPartialLineTest {
+
+    private static final String COMPLETE_LINE = "first line\n";
+    private static final String PARTIAL_LINE = "second partial";
+
+    @Test
+    void completedLogEmitsTrailingPartialLine() throws Exception {
+        ByteBuffer memory = buffer(COMPLETE_LINE + PARTIAL_LINE);
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+
+        long next = new AnnotatedLargeText<>(memory, UTF_8, true, null).writeLogTo(0, out);
+
+        assertThat(out.toString(UTF_8), is(COMPLETE_LINE + PARTIAL_LINE));
+        assertThat(next, is(memory.length()));
+    }
+
+    @Test
+    void completedLogEndingWithNewlineIsUnaffected() throws Exception {
+        ByteBuffer memory = buffer(COMPLETE_LINE);
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+
+        long next = new AnnotatedLargeText<>(memory, UTF_8, true, null).writeLogTo(0, out);
+
+        assertThat(out.toString(UTF_8), is(COMPLETE_LINE));
+        assertThat(next, is(memory.length()));
+    }
+
+    @Test
+    void streamedIncompleteLogWithholdsTrailingPartialLine() throws Exception {
+        ByteBuffer memory = buffer(COMPLETE_LINE + PARTIAL_LINE);
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+
+        // The streaming branch of LargeText writes in bulk rather than up to the last newline,
+        // so the partial line stays in the line buffer and must not be counted as delivered.
+        long next = new StreamingAnnotatedLargeText(memory).writeLogTo(0, out);
+
+        assertThat(out.toString(UTF_8), is(COMPLETE_LINE));
+        assertThat(next, is((long) COMPLETE_LINE.length()));
+    }
+
+    @Test
+    void streamedIncompleteLogResumesFromWithheldOffset() throws Exception {
+        ByteBuffer memory = buffer(COMPLETE_LINE + PARTIAL_LINE);
+        StreamingAnnotatedLargeText text = new StreamingAnnotatedLargeText(memory);
+        long next = text.writeLogTo(0, new ByteArrayOutputStream());
+
+        memory.write("\nthird line\n".getBytes(UTF_8));
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        long end = text.writeLogTo(next, out);
+
+        assertThat(out.toString(UTF_8), is(PARTIAL_LINE + "\nthird line\n"));
+        assertThat(end, is(memory.length()));
+    }
+
+    @Test
+    void incompleteLogStopsAtLastNewline() throws Exception {
+        ByteBuffer memory = buffer(COMPLETE_LINE + PARTIAL_LINE);
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+
+        long next = new AnnotatedLargeText<>(memory, UTF_8, false, null).writeLogTo(0, out);
+
+        assertThat(out.toString(UTF_8), is(COMPLETE_LINE));
+        assertThat(next, is((long) COMPLETE_LINE.length()));
+    }
+
+    private static ByteBuffer buffer(String text) throws IOException {
+        ByteBuffer memory = new ByteBuffer();
+        memory.write(text.getBytes(UTF_8));
+        return memory;
+    }
+
+    /** Simulates {@code Accept: multipart/form-data} without a Stapler request being bound to the thread. */
+    private static final class StreamingAnnotatedLargeText extends AnnotatedLargeText<Void> {
+
+        StreamingAnnotatedLargeText(ByteBuffer memory) {
+            super(memory, UTF_8, false, null);
+        }
+
+        @Override
+        public boolean isStreamingRequest(StaplerRequest2 req) {
+            return true;
+        }
+    }
+}