Avoid including unrelated content in step log after parse errors

Previously, when not resetting the position on Long parse error or
non-positive index change, the position of the next transition was
mistaken for the end of the current section and the entire content since
the last transition was emitted.

The added tests are failing without this patch: The logs of step 1
contained the "between1\n" line -> "1\nbetween1\n". The atEnd tests
would contain the "after" part.

Author: das7pad

src/main/java/org/jenkinsci/plugins/workflow/log/FileLogStorage.java

@@ -292,6 +292,7 @@ private void maybeFlush() {
                     // Note that NumberFormatException is nonfatal in the case of the overall build log:
                     // the whole-build HTML output always includes exactly what is in the main log file,
                     // at worst with some missing or inaccurate startStep/endStep annotations.
+                    pos = -1;
                     continue;
                 }
                 if (pos == -1) {
@@ -317,7 +318,10 @@ private void maybeFlush() {
                     raf.readFully(data);
                     buf.write(data);
                     pos = -1;
-                } // else some sort of mismatch
+                } else {
+                    // Some sort of mismatch. Do not emit this section.
+                    pos = -1;
+                }
             }
             if (pos != -1 && /* otherwise race condition? */ end > pos) {
                 // In case the build is ongoing and we are still actively writing content for this step,

src/test/java/org/jenkinsci/plugins/workflow/log/FileLogStorageTest.java

@@ -28,6 +28,8 @@
 
 import hudson.model.TaskListener;
 import java.io.File;
+import java.nio.charset.StandardCharsets;
+import org.apache.commons.io.FileUtils;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
@@ -55,6 +57,48 @@ public class FileLogStorageTest extends LogStorageTestBase {
         assertOverallLog(0, lines("stuff"), true);
     }
 
+    @Test public void corruptIndex() throws Exception {
+        FileUtils.writeStringToFile(log, "before\n1\nbetween1\n2\nbetween2\n3\nafter", StandardCharsets.UTF_8);
+        FileUtils.writeStringToFile(new File(log + "-index"), "7 1\n?\n18 2\n20\n29 3\n31", StandardCharsets.UTF_8);
+        assertStepLog("1", 0, "", false);
+        assertStepLog("2", 0, "2\n", false);
+        assertStepLog("3", 0, "3\n", false);
+    }
+
+    @Test public void samePositionInIndex() throws Exception {
+        FileUtils.writeStringToFile(log, "before\n1\nbetween1\n2\nbetween2\n3\nafter", StandardCharsets.UTF_8);
+        FileUtils.writeStringToFile(new File(log + "-index"), "7 1\n7\n18 2\n20\n29 3\n31", StandardCharsets.UTF_8);
+        assertStepLog("1", 0, "", false);
+        assertStepLog("2", 0, "2\n", false);
+        assertStepLog("3", 0, "3\n", false);
+    }
+
+    @Test public void decrementedPositionInIndex() throws Exception {
+        FileUtils.writeStringToFile(log, "before\n1\nbetween1\n2\nbetween2\n3\nafter", StandardCharsets.UTF_8);
+        FileUtils.writeStringToFile(new File(log + "-index"), "7 1\n0\n18 2\n20\n29 3\n31", StandardCharsets.UTF_8);
+        assertStepLog("1", 0, "", false);
+        assertStepLog("2", 0, "2\n", false);
+        assertStepLog("3", 0, "3\n", false);
+    }
+
+    @Test public void corruptIndexAtEnd() throws Exception {
+        FileUtils.writeStringToFile(log, "before\n1\nafter", StandardCharsets.UTF_8);
+        FileUtils.writeStringToFile(new File(log + "-index"), "7 1\n?", StandardCharsets.UTF_8);
+        assertStepLog("1", 0, "", false);
+    }
+
+    @Test public void samePositionInIndexAtEnd() throws Exception {
+        FileUtils.writeStringToFile(log, "before\n1\nafter", StandardCharsets.UTF_8);
+        FileUtils.writeStringToFile(new File(log + "-index"), "7 1\n7", StandardCharsets.UTF_8);
+        assertStepLog("1", 0, "", false);
+    }
+
+    @Test public void decrementedPositionInIndexAtEnd() throws Exception {
+        FileUtils.writeStringToFile(log, "before\n1\nafter", StandardCharsets.UTF_8);
+        FileUtils.writeStringToFile(new File(log + "-index"), "7 1\n0", StandardCharsets.UTF_8);
+        assertStepLog("1", 0, "", false);
+    }
+
     @Test public void interruptionDoesNotCloseStream() throws Exception {
         LogStorage ls = createStorage();
         TaskListener overall = ls.overallListener();