Kailun2047/6.824: src/mapreduce/common_map.go; 12 LoC

Found a possible issue in [Kailun2047/6.824](https://www.github.com/Kailun2047/6.824) at [src/mapreduce/common_map.go](https://github.com/Kailun2047/6.824/blob/9f2fb6c3866cf00d83eb911a4135e25a147930d3/src/mapreduce/common_map.go#L66-L77)

Below is the message reported by the analyzer for this snippet of code. Beware that the analyzer only reports the first
issue it finds, so please do not limit your consideration to the contents of the below message.

> function call at line 75 may store a reference to kv

[Click here to see the code in its original context.](https://github.com/Kailun2047/6.824/blob/9f2fb6c3866cf00d83eb911a4135e25a147930d3/src/mapreduce/common_map.go#L66-L77)

Click here to show the 12 line(s) of Go which triggered the analyzer.

```go
for _, kv := range kvs {
k := kv.Key
reduceTask := ihash(k) % nReduce
intermFile := reduceName(jobName, mapTask, reduceTask)
f, err := os.OpenFile(intermFile, os.O_RDWR|os.O_CREATE|os.O_APPEND, 0755)
if err != nil {
log.Println("Failed to open intermediate file")
}
enc := json.NewEncoder(f)
enc.Encode(&kv)
f.Close()
}

```

Click here to show extra information the analyzer produced.

```
The following paths through the callgraph could lead to a function which writes a pointer argument:
no paths found; call ended in third-party code; stay tuned for diagnostics
```

Leave a reaction on this issue to contribute to the project by classifying this instance as a **Bug** :-1:, **Mitigated** :+1:, or **Desirable Behavior** :rocket:
See the descriptions of the classifications [here](https://github.com/github-vet/rangeclosure-findings#how-can-i-help) for more information.

commit ID: 9f2fb6c3866cf00d83eb911a4135e25a147930d3