Adds checks for null

Backend sends data as null, frontend
code was not prepared and failed silently

app/assets/javascripts/jobs/store/mutations.js

@@ -26,7 +26,7 @@ export default {
 
     if (log.append) {
       if (isNewJobLogActive()) {
-        state.trace = updateIncrementalTrace(log.lines, state.trace);
+        state.trace = log.lines ? updateIncrementalTrace(log.lines, state.trace) : state.trace;
       } else {
         state.trace += log.html;
       }
@@ -35,9 +35,9 @@ export default {
       // When the job still does not have a trace
       // the trace response will not have a defined
       // html or size. We keep the old value otherwise these
-      // will be set to `undefined`
+      // will be set to `null`
       if (isNewJobLogActive()) {
-        state.trace = logLinesParser(log.lines) || state.trace;
+        state.trace = log.lines ? logLinesParser(log.lines) : state.trace;
       } else {
         state.trace = log.html || state.trace;
       }

app/assets/javascripts/jobs/store/utils.js

@@ -147,13 +147,15 @@ export const findOffsetAndRemove = (newLog = [], oldParsed = []) => {
 
   const firstNew = newLog[0];
 
-  if (last.offset === firstNew.offset || (last.line && last.line.offset === firstNew.offset)) {
-    cloneOldLog.splice(lastIndex);
-  } else if (last.lines && last.lines.length) {
-    const lastNestedIndex = last.lines.length - 1;
-    const lastNested = last.lines[lastNestedIndex];
-    if (lastNested.offset === firstNew.offset) {
-      last.lines.splice(lastNestedIndex);
+  if (last && firstNew) {
+    if (last.offset === firstNew.offset || (last.line && last.line.offset === firstNew.offset)) {
+      cloneOldLog.splice(lastIndex);
+    } else if (last.lines && last.lines.length) {
+      const lastNestedIndex = last.lines.length - 1;
+      const lastNested = last.lines[lastNestedIndex];
+      if (lastNested.offset === firstNew.offset) {
+        last.lines.splice(lastNestedIndex);
+      }
     }
   }
 
@@ -170,7 +172,7 @@ export const findOffsetAndRemove = (newLog = [], oldParsed = []) => {
  * @param array oldLog
  * @param array newLog
  */
-export const updateIncrementalTrace = (newLog, oldParsed = []) => {
+export const updateIncrementalTrace = (newLog = [], oldParsed = []) => {
   const parsedLog = findOffsetAndRemove(newLog, oldParsed);
 
   return logLinesParser(newLog, parsedLog);

spec/frontend/jobs/store/mutations_spec.js

@@ -80,6 +80,81 @@ describe('Jobs Store Mutations', () => {
       expect(stateCopy.traceSize).toEqual(511846);
       expect(stateCopy.isTraceComplete).toEqual(true);
     });
+
+    describe('with new job log', () => {
+      let stateWithNewLog;
+      beforeEach(() => {
+        gon.features = gon.features || {};
+        gon.features.jobLogJson = true;
+
+        stateWithNewLog = state();
+      });
+
+      afterEach(() => {
+        gon.features.jobLogJson = false;
+      });
+
+      describe('log.lines', () => {
+        describe('when append is true', () => {
+          it('sets the parsed log ', () => {
+            mutations[types.RECEIVE_TRACE_SUCCESS](stateWithNewLog, {
+              append: true,
+              size: 511846,
+              complete: true,
+              lines: [
+                {
+                  offset: 1,
+                  content: [{ text: 'Running with gitlab-runner 11.12.1 (5a147c92)' }],
+                },
+              ],
+            });
+
+            expect(stateWithNewLog.trace).toEqual([
+              {
+                offset: 1,
+                content: [{ text: 'Running with gitlab-runner 11.12.1 (5a147c92)' }],
+                lineNumber: 0,
+              },
+            ]);
+          });
+        });
+
+        describe('when it is defined', () => {
+          it('sets the parsed log ', () => {
+            mutations[types.RECEIVE_TRACE_SUCCESS](stateWithNewLog, {
+              append: false,
+              size: 511846,
+              complete: true,
+              lines: [
+                { offset: 0, content: [{ text: 'Running with gitlab-runner 11.11.1 (5a147c92)' }] },
+              ],
+            });
+
+            expect(stateWithNewLog.trace).toEqual([
+              {
+                offset: 0,
+                content: [{ text: 'Running with gitlab-runner 11.11.1 (5a147c92)' }],
+                lineNumber: 0,
+              },
+            ]);
+          });
+        });
+
+        describe('when it is null', () => {
+          it('sets the default value', () => {
+            mutations[types.RECEIVE_TRACE_SUCCESS](stateWithNewLog, {
+              append: true,
+              html,
+              size: 511846,
+              complete: false,
+              lines: null,
+            });
+
+            expect(stateWithNewLog.trace).toEqual([]);
+          });
+        });
+      });
+    });
   });
 
   describe('STOP_POLLING_TRACE', () => {

spec/frontend/jobs/store/utils_spec.js

@@ -291,6 +291,13 @@ describe('Jobs Store Utils', () => {
         });
       });
     });
+
+    describe('when no data is provided', () => {
+      it('returns an empty array', () => {
+        const result = findOffsetAndRemove();
+        expect(result).toEqual([]);
+      });
+    });
   });
 
   describe('getIncrementalLineNumber', () => {