http: superfluous response.WriteHeader call StatusOK - go

Question

In my code, I've a loop that processing set of files (based on what available at pre-specified folder), and based on the output of each processed file, some info is sent to the client, so I wrote the below:
for i, file := range files {
uniqueSlice := unique(matches)
output = Output{MSG: "ok", File: file, Skills: uniqueSlice}
data, err := json.Marshal(output)
if err != nil {
panic(err)
}
w.Header().Set("Content-Type", "application/json")
w.WriteHeader(http.StatusOK) // -< Error from here
w.Write(data)
}
Above working correctly if the folder has a single file, but if has more than one, I got the error: http: superfluous response.WriteHeader call
I understood the error is due to using w.WriteHeader(http.StatusOK) which can not be used more than once to be set, but I need it to be set for the client to process the returned data.
How can I fix this code, so that I can return data directly to the client upon processing each file.
UPDATE
If I remove http.StatusOK as recommended in the comments below, then I get the returned as plain text not as JSON!

Answer 1

You can't just concatenate JSON documents together and expect the result to be valid json encoded. You'll have to put your output objects in an array and then output that array once at the end, otherwise the response won't be valid json.
If you output objects individually like your code did, the final data will look like
{"MSG": "ok", "File": "...", "Skills": [...]}{"MSG": "ok", "File": "...", "Skills": [...]}{"MSG": "ok", "File": "...", "Skills": [...]}
Each one of those outputs is valid by itself, but the entire output with the objects just concatenated together, is not.
Ideally, when outputting json to a stream like an HTTP response, instead of storing it in an intermediate buffer (data) for you, use json.NewEncoder(w) where w is the http response writer. Streaming is almost always better than rendering to a variable.
var outputs = make([]Output,0,len(files)
for i, file := range files {
uniqueSlice := unique(matches)
outputs = append(outputs, Output{MSG: "ok", File: file, Skills: uniqueSlice})
}
w.Header().Set("Content-Type", "application/json")
w.WriteHeader(http.StatusOK)
if err := json.NewEncoder(w).Encode(outputs); err != nil {
panic(err)
}