Finding and fixing memory leaks in Go

This post reviews how I found a memory leak, how I fixed it, how I fixed similar issues in Google's sample Go code, and how we're improving our libraries to prevent this in the future.

The Google Cloud Client Libraries for Go generally use gRPC under the hood to connect with Google Cloud APIs. When you create an API client, the library initializes a connection to the API then leaves that connection open until you call Close on the Client.

client, err := api.NewClient()
// Check err.
defer client.Close()

Clients are safe to use concurrently, so you should keep the same Client around until you're done with it. But, what happens if you don't Close the client when you should?

You get a memory leak. The underlying connections never get cleaned up.

Google has a bunch of GitHub automation bots to help manage hundreds of GitHub repos. Some of our bots proxy their requests through a Go server running on Cloud Run. Our memory usage looked like a classic sawtooth memory leak:

I started debugging by adding the pprof.Index handler to the server:

mux.HandleFunc("/debug/pprof/", pprof.Index)

pprof provides runtime profiling data, like memory usage. See Profiling Go Programs on the Go Blog for more information.

Then, I built and started the server locally:

$ go build
$ PROJECT_ID=my-project PORT=8080 ./serverless-scheduler-proxy

Next, I sent a some requests requests to the server:

for i in {1..5}; do
curl --header "Content-Type: application/json" --request POST --data '{"name": "HelloHTTP", "type": "testing", "location": "us-central1"}' localhost:8080/v0/cron
echo " -- $i"
done

The exact payload and endpoint is specific to our server and is irrelevant for this post.

To get a baseline for what memory is being used, I collected some initial pprof data:

curl http://localhost:8080/debug/pprof/heap > heap.0.pprof

Inspecting the output, you can see some memory usage, but nothing immediately stands out as a large issue (which is good! We just started the server!):

$ go tool pprof heap.0.pprof
File: serverless-scheduler-proxy
Type: inuse_space
Time: May 4, 2021 at 9:33am (EDT)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top10
Showing nodes accounting for 2129.67kB, 100% of 2129.67kB total
Showing top 10 nodes out of 30
      flat  flat%   sum%        cum   cum%
 1089.33kB 51.15% 51.15%  1089.33kB 51.15%  google.golang.org/grpc/internal/transport.newBufWriter (inline)
  528.17kB 24.80% 75.95%   528.17kB 24.80%  bufio.NewReaderSize (inline)
  512.17kB 24.05%   100%   512.17kB 24.05%  google.golang.org/grpc/metadata.Join
         0     0%   100%   512.17kB 24.05%  cloud.google.com/go/secretmanager/apiv1.(*Client).AccessSecretVersion
         0     0%   100%   512.17kB 24.05%  cloud.google.com/go/secretmanager/apiv1.(*Client).AccessSecretVersion.func1
         0     0%   100%   512.17kB 24.05%  github.com/googleapis/gax-go/v2.Invoke
         0     0%   100%   512.17kB 24.05%  github.com/googleapis/gax-go/v2.invoke
         0     0%   100%   512.17kB 24.05%  google.golang.org/genproto/googleapis/cloud/secretmanager/v1.(*secretManagerServiceClient).AccessSecretVersion
         0     0%   100%   512.17kB 24.05%  google.golang.org/grpc.(*ClientConn).Invoke
         0     0%   100%  1617.50kB 75.95%  google.golang.org/grpc.(*addrConn).createTransport

The next step was sending a bunch of requests to the server and seeing if we could (1) reproduce the seeming memory leak and (2) identify what the leak is.

Sending 500 requests:

for i in {1..500}; do
curl --header "Content-Type: application/json" --request POST --data '{"name": "HelloHTTP", "type": "testing", "location": "us-central1"}' localhost:8080/v0/cron
echo " -- $i"
done

Collecting and analyzing more pprof data:

$ curl http://localhost:8080/debug/pprof/heap > heap.6.pprof
$ go tool pprof heap.6.pprof
File: serverless-scheduler-proxy
Type: inuse_space
Time: May 4, 2021 at 9:50am (EDT)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top10
Showing nodes accounting for 94.74MB, 94.49% of 100.26MB total
Dropped 26 nodes (cum <= 0.50MB)
Showing top 10 nodes out of 101
      flat  flat%   sum%        cum   cum%
   51.59MB 51.46% 51.46%    51.59MB 51.46%  google.golang.org/grpc/internal/transport.newBufWriter
   19.60MB 19.55% 71.01%    19.60MB 19.55%  bufio.NewReaderSize
    6.02MB  6.01% 77.02%     6.02MB  6.01%  bytes.makeSlice
    4.51MB  4.50% 81.52%    10.53MB 10.51%  crypto/tls.(*Conn).readHandshake
       4MB  3.99% 85.51%     4.50MB  4.49%  crypto/x509.parseCertificate
       3MB  2.99% 88.51%        3MB  2.99%  crypto/tls.Client
    2.50MB  2.49% 91.00%     2.50MB  2.49%  golang.org/x/net/http2/hpack.(*headerFieldTable).addEntry
    1.50MB  1.50% 92.50%     1.50MB  1.50%  google.golang.org/grpc/internal/grpcsync.NewEvent
       1MB     1% 93.50%        1MB     1%  runtime.malg
       1MB     1% 94.49%        1MB     1%  encoding/json.(*decodeState).literalStore

google.golang.org/grpc/internal/transport.newBufWriter really stands out as using a ton of memory! That's the first indication of what the leak is related to: gRPC. Looking at our application source code, the only place we were using gRPC was for Google Cloud Secret Manager:

client, err := secretmanager.NewClient(ctx) 
if err != nil { 
    return nil, fmt.Errorf("failed to create secretmanager client: %v", err) 
}

We never called client.Close() and created a Client on every request! So, I added a Close call and the problem went away:

defer client.Close()

I submitted the fix, it automatically deployed, and the sawtooth went away immediately!

Woohoo! 🎉🎉🎉

Around the same time, a user filed an issue on our Go sample repo for Cloud, which contains most of the Go samples for docs on cloud.google.com. The user noticed we forgot to Close the Client in one of our samples!

I had seen the same thing pop up a few other times, so I decided to investigate the entire repo.

I started with a rough estimate of how many affected files there were. Using grep, we can get a list of all files containing a NewClient style call, then pass that list to another invocation of grep to only list the files that don't contain Close, ignoring test files:

$ grep -L Close $(grep -El 'New[^(]*Client' **/*.go) | grep -v test

Oops! There were 207 files… For context, we have about 1300 .go files in the GoogleCloudPlatform/golang-samples repo.

Given the scale of the problem, I thought some automation would be worth it to get a rough start. I didn't want to write a full on Go program to edit the files, so I stuck with Bash:

$ grep -L Close $(grep -El 'New[^(]*Client' **/*.go) | grep -v test | xargs sed -i '/New[^(]*Client/,/}/s/}/}\ndefer client.Close()/'

Is it perfect? No. Did it make a huge dent in the amount of work? Yes!

The first part (up until test) is the exact same as above -- get a list of all of the possibly affected files (the ones that seem to create a Client but never call Close).

Then, I passed that list of files to sed for actual editing. xargs invokes the command you give it with each line of stdin being passed as an argument to the given command.

To understand the sed command, it helps to see what a sample usually looks like in the golang-samples repo (omitting imports and everything after client initialization):

// accessSecretVersion accesses the payload for the given secret version if one
// exists. The version can be a version number as a string (e.g. "5") or an
// alias (e.g. "latest").
func accessSecretVersion(w io.Writer, name string) error {
    // name := "projects/my-project/secrets/my-secret/versions/5"
    // name := "projects/my-project/secrets/my-secret/versions/latest"
    // Create the client.
    ctx := context.Background()
    client, err := secretmanager.NewClient(ctx)
    if err != nil {
        return fmt.Errorf("failed to create secretmanager client: %v", err)
    }
    // ...
}

At a high level, we initialize the client and check if there was an error. Whenever you check the error, there is a closing curly brace (}). I used that information to automate the editing.

The sed command is still a douzy, though:

sed -i '/New[^(]*Client/,/}/s/}/}\ndefer client.Close()/'

The -i says to edit the files in place. I'm OK with this because git can save me if I mess up.

Next, I used the s command to insert defer client.Close() right after the presumed closing curly brace (}) from checking the error.

But, I don't want to replace every }, I only want the first one after a call to NewClient. To do that, you can give an address range for sed to search.

An address range can include the start and end patterns to match before applying whatever command comes next. In this case, the start is /New[^(]*Client/, matching NewClient type calls, and the end (separated by a ,) is /}/, matching the next curly brace. That means our search and replace will only apply between the call to NewClient and the closing curly brace!

From knowing the error handling pattern above, the closing brace of the if err != nil condition is exactly where we want to insert our Close call.

Once I had automatically edited all of the samples, I ran goimports to fix the formatting. Then, I went through each edited file to make sure it did the right thing:

• In server applications, should we actually close the client, or should we keep it around for future requests?
• Is the name of the Client actually client or is it something else?
• Is there more than one Client to Close?

Once that was done, I was left with 180 files edited.

The last order of business is trying to make it so this doesn't happen to users anymore. There are a few ways we have in mind:

1. Better samples. See above.
2. Better GoDoc. We updated our library generator to include a comment in the generated libraries saying to Close the Client when you're done with it. See https://github.com/googleapis/google-cloud-go/issues/3031.
3. Better libraries. Is there a way we can automatically Close clients? Finalizers? Have an idea of how we can do this better? Let us know on https://github.com/googleapis/google-cloud-go/issues/4498.

I hope you learned a bit about Go, memory leaks, pprof, gRPC, and Bash. I'd love to hear your stories about memory leaks you've found and what it took to fix them! If you have ideas about how we can improve our libraries or samples, let us know by filing an issue.