Tigraine

Daniel Hoelbling-Inzko talks about programming

Why I love Go error handling

One thing that Go almost forces you to do is to explicitly handle each and every error that any random part of the system might create. This has one very obvious side effect of making even simple code quite long and peppered with if err != nil statements:

func writeFile() error {  

   fileName := "test.txt"  
   f, err := os.Open(fileName)  
   if err != nil {  
      return fmt.Errorf("failed to open file %s: %w", fileName, err)  
   }  
   defer f.Close()  

   d1 := []byte("hello\ngo\n")  
   _, err = f.Write(d1)  
   if err != nil {  
      return fmt.Errorf("unable to write to file %s: %w", fileName, err)  
   }  
   return nil  
}

As you can see a simple open-file-and-write requires 2 error checks that add almost 50% of lines of code to this rather simple method. Because of this most Java/C#/C++ people you show Go code to almost always react with aversion and distaste and never give the language another chance (although I think this is now changing gradually).

But I actually think this is Go's biggest strength and a boon to developers. By having errors be so "in your face" - you have to do something about exceptions. In languages with Exceptions traversing up the call stack it's all too easy to just expect someone up the food chain to catch your exception. All too often that doesn't happen or if it does it's a very generic "catch-everything" block that can only log the problem without having any chance to actually recover from it.

Go in contrast makes you think about every error in detail and how it affects the current control flow. A classic example of this would be a for loop that calls some method. I've seen all too often bugs because people didn't put a try catch inside the loop, so the first problem that arises (and most likely it's a very rare thing that happens) stops execution of the loop and you are then wondering why you're missing half your data or something like that. If you have to really think hard about each error, you're much more likely to also think about how it's affecting the code you're currently writing, so in Go I find myself writing continue and break a lot more frequently than I usually do in Java/Kotlin.

Another case where conscious error handling is very handy in my opinion is when making an application resilient to failures downstream (see my recent post on Bulkheads). Only if you have useful error handling in place on all levels of the application can you start building logic that responds to these errors (without having to go on a archeological excavation of the whole call stack).

Obviously you have to have some discipline in your errors, just doing return err won't do you any favours here. But I find the way Go requires error handling also tends to promote more deliberate throwing of errors that carry actually useful information up the call stack (because you need that info to handle them up there). If that's then in place you can also make much better decisions on how to treat these errors in failure scenarios and when deciding if a CircuitBreaker should trip or not etc.

Filed under golang, errors

Debugging Go IOWait Hang: Sometimes it's really not your code

If something looks like a bug in the Language Runtime, Standard Library or the Operating System I tend to always approach it with caution: It's usually a bug in my code and I'm just not seeing it.

But sometimes it's not me - it's really the compiler and you spend a solid week debugging a Go program until you find out that cross-compiling from OSX to Linux leads to a stdlib Bug that manifests itself with the whole application just hanging in IOWait loops given enough concurrency.

Obviously the whole thing was really frustrating because:

  • The bug only happened on production servers (obviously - anything else would not be fun).
  • Could only be reproduced on a large dataset of 300 million items (so every test also takes quite a while)
  • I had to test if it works without concurrency (which took 2 days and yes it did)

But the important finding from this exercise was that you can print the full stacktrace of all running Goroutines as well as their status for a running/hanging program! You just have to send the kill -ABRT signal to a process! This is similar to what you see when a panic occurs and was massively helpful in hunting down this bug. Kudos to the Go team for that.

An example for this:

package main

func main() {
  for {}
}

The program will obviously hang and do a busy loop, but if you send the kill -ABRT signal you'll get something similar to this printed to stderr:

SIGABRT: abort
PC=0x1056d70 m=0 sigcode=0

goroutine 1 [running]:
main.main()
        /Users/tigraine/projects/test/main.go:4 fp=0xc00003c788 sp=0xc00003c780 pc=0x1056d70
runtime.main()
        /usr/local/Cellar/go/1.14.1/libexec/src/runtime/proc.go:203 +0x212 fp=0xc00003c7e0 sp=0xc00003c788 pc=0x102b3f2
runtime.goexit()
        /usr/local/Cellar/go/1.14.1/libexec/src/runtime/asm_amd64.s:1373 +0x1 fp=0xc00003c7e8 sp=0xc00003c7e0 pc=0x10528f1
...
Filed under golang, go, debugging

Convert a millisecond precision unix timestamp to Time in go

It's no real secret that I do love the programming language Go. So I was really delighted to see that Go apparently does all the right things when it comes to their time package that handles time zones etc correctly by default as opposed to be something bolted on after the fact like most other languages.

But for some unknown reason it's just way too complex to convert a millisecond resolution Unix timestamp to time.Time. The built-in time.Unix() function only supports second and nanosecond precision.

This means that you either have to multiply the millis to nanoseconds or split them into seconds and nanoseconds. So obviously my naive implementation was:

time.Unix(0, timestamp * int64(1000000))

But that code looked ugly to me - especially if you have to do this a few times around the codebase - so I wrote a function.

But for some reason I also decided to benchmark my function as I am working on a performance critical piece of code right now. And it turns out that the simple multiplication to turn millis into nanos is 2x slower than dividing the millis into seconds and then turning the remainder into nanos.

time.Unix(ms/int64(millisInSecond), (ms%int64(millisInSecond))*int64(nsInSecond))

Benchmark:

goos: darwin
goarch: amd64
pkg: github.com/tigraine/go-timemillis
BenchmarkMult-8         2000000000               0.50 ns/op
BenchmarkDiv-8          2000000000               0.25 ns/op

So I packaged my findings into a library which is now available on GitHub: go-timemilli

Filed under golang, time

Golang hidden gems: testing.T.Log

One thing I love about Go is it's build chain and overall ease of use. Some things take time to get used to, but the lightning fast builds and the convention-based testing Go offers are addicting right from the start.

Today I found another hidden Gem I think is just genius: testing.T.Log(). Ok I admit, not the most sexy method to get excited about - but bear with me for a moment. Imagine the following code.

func TestSomething(t *testing.T) {
  t.Log("Hello World")
}

What's the output? If you'd expect Hello World you are mistaken. The output is exactly nothing :)

testing.T.Log() only prints something if a testing.T.Error or testing.T.Fatal occurred. Brilliant! Nothing is more annoying than chatty test suites where your actual problem is buried in 2-3 megabytes of meaningless debug statements! And this solves the problem really elegantly. You can log as much debug info as you want and it will only surface if the test actually failed.

Filed under golang, go, testing

Golang: int is not a type

Today I ran into a very interesting compiler error in my go program: int is not a type. Although the same lines of code that didn't compile worked a few minutes earlier.

It took me 20 minutes to figure it out. I was already halfway through a forum post on the golangbridge forums and trying to put together a minimal example when I noticed the problem. Apparently I had a typo in mit func init() and had inadvertently called it func int()!

The go compiler didn't even complain with me overriding one of it's core types.

Filed under golang

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