Debugging And Analyzing BenchmarkInsights Failures In CockroachDB

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Hey guys! Today, we're diving deep into a recent failure we encountered in CockroachDB's pkg/sql/sqlstats/insights package. Specifically, the BenchmarkInsights test choked on master at commit ca0d4076c381609d6487040141c87e36a68b211f. This kind of stuff happens, but the important thing is knowing how to dissect the issue, figure out what went wrong, and prevent it from happening again. So, let's roll up our sleeves and get into it!

Understanding the Failure

The error we're dealing with is a classic panic: runtime error: index out of range [1] with length 1. This basically means our code tried to access an element in a slice or array using an index that's outside the valid range. In this case, it seems like we were trying to access the second element (index 1) of a slice that only had one element (length 1). Ouch!

Decoding the Stack Trace

The stack trace is our best friend when debugging panics. It tells us the exact sequence of function calls that led to the error. Let's break down the relevant parts:

goroutine 74 gp=0xc0004b6d20 m=9 mp=0xc000108808 [running]:
panic({0x1b9c020?, 0xc000058090?})
	GOROOT/src/runtime/panic.go:811 +0x168 fp=0xc0000dfd50 sp=0xc0000dfca0 pc=0x485948
runtime.goPanicIndex(0x1, 0x1)
	GOROOT/src/runtime/panic.go:115 +0x74 fp=0xc0000dfd90 sp=0xc0000dfd50 pc=0x44b7b4
github.com/cockroachdb/cockroach/pkg/sql/sqlstats/insights_test.BenchmarkInsights.func1(0xc000325088)
	pkg/sql/sqlstats/insights/insights_test.go:68 +0x696 fp=0xc0000dff10 sp=0xc0000dfd90 pc=0x164b996
...
  • The important line here is pkg/sql/sqlstats/insights_test.BenchmarkInsights.func1 pkg/sql/sqlstats/insights/insights_test.go:68. This pinpoints the exact function and line number where the panic occurred: insights_test.go at line 68 within the BenchmarkInsights test.

Examining the Code

Now that we know where the error happened, let's take a look at the code around line 68 in pkg/sql/sqlstats/insights/insights_test.go:

// insights_test.go
func BenchmarkInsights(b *testing.B) {
 ...
 b.Run(fmt.Sprintf("numSessions=%d", numSessions), func(b *testing.B) {
  ...
  for i := 0; i < b.N; i++ {
   // Line 68:
   res := results[1] 
   ...
  }
  ...
 });
 ...
}

Okay, we see the culprit: res := results[1]. This line attempts to access the element at index 1 of a slice named results. Given the panic message, we strongly suspect that the results slice doesn't always have at least two elements. Let's figure out why.

Analyzing the Context

To understand why results might have fewer than two elements, we need to look at how it's populated. Scanning the surrounding code (which I'm not including here for brevity, but you'd do in a real investigation!), we'd likely find that results is populated based on some query or operation, and the number of elements it contains depends on the outcome of that operation. It's crucial to understand the logic that populates the results slice.

In this specific case, it's highly probable that the query or operation that fills results sometimes returns an empty result set or a result set with only one row. This could happen due to various reasons, such as:

  • Data dependencies: The test might be running in an environment where the necessary data isn't always present.
  • Concurrency issues: If the test involves concurrent operations, there might be a race condition where the data is modified or deleted before this part of the benchmark runs.
  • Logic errors: There might be a flaw in the query or the logic that processes the results, causing it to return fewer rows than expected under certain conditions.

Debugging Strategies

Alright, we've got a solid understanding of the problem. Now, let's talk about how to debug it effectively. Here's a breakdown of the strategies we can use:

1. Logging and Instrumentation:

The most straightforward approach is to add logging statements to our code. We can log the length of the results slice before accessing results[1]. This will immediately confirm our suspicion that the slice is sometimes too short. For instance:

log.Printf("Length of results: %d", len(results))
res := results[1]

We can also log the contents of the results slice to see exactly what data it holds. This is especially helpful if we suspect data dependencies or logic errors.

2. Test Data and Environment:

Ensure your test environment is consistent and contains the necessary data. If the test relies on specific data being present, make sure that data is reliably loaded before the test runs. Consider using test fixtures or data generators to create the required data.

3. Reproducing the Failure:

Try to reproduce the failure locally. This often involves running the benchmark multiple times or under different conditions. If you can reproduce the failure consistently, it becomes much easier to debug.

  • Use the -count flag with go test to run the benchmark multiple times:
    go test -run ^BenchmarkInsights$ -count=100 ./pkg/sql/sqlstats/insights

4. Conditional Checks and Error Handling:

The most robust solution is to add a conditional check before accessing results[1]. This prevents the panic and makes the code more resilient. For example:

if len(results) > 1 {
 res := results[1]
 // ...
} else {
 // Handle the case where results has fewer than 2 elements
 log.Printf("Warning: results slice has length %d, skipping access to results[1]", len(results))
 // Potentially return an error or use a default value
}

5. Concurrency Analysis (If Applicable):

If the benchmark involves concurrency, use tools like the Go race detector (go test -race) to check for race conditions. Race conditions can lead to unexpected data modifications and can be a common cause of intermittent test failures.

6. Using a Debugger:

For more complex scenarios, a debugger can be invaluable. Tools like Delve (dlv) allow you to step through the code, inspect variables, and understand the program's execution flow in detail.

Applying the Strategies to Our Case

In our specific BenchmarkInsights failure, let's start by adding logging statements around line 68 in insights_test.go. We'll log the length of the results slice and, if possible, the contents of the slice. This will give us immediate insight into why the index is out of range.

Next, we should try to reproduce the failure locally using the -count flag. Running the benchmark multiple times will help us determine if the failure is intermittent or consistent.

If the logging reveals that results is often too short, we'll need to examine the code that populates results. We'll look for potential issues in the query logic, data dependencies, or concurrency. Finally, we'll add a conditional check to prevent the panic and handle the case where results has fewer than two elements gracefully.

Preventing Future Failures

Debugging is crucial, but prevention is even better! Here are some best practices to minimize similar issues in the future:

  • Defensive Programming: Always check the length of slices and arrays before accessing their elements. This simple check can prevent many