Inadvertent multiple returns - go

Question

I'm building an application where I get muslim prayer data from multiple sources. The first being S3, the second being aladhan (a public api). I only want to get data from aladhan if it's not available in S3. If I do have to get the data from the public source then I upload it to my s3.
Here is the code:
This is my interface loop code. I've put in print statements to show that I'm running into the return statement twice, once with data in my return struct, the second time the struct is nil.
// prayeriface.go
package prayer
import (
"fmt"
)
type MonthPrayerIface interface {
GetMonthPrayer(input *LookupInput) (*PreCurrNextMonthPrayer, error)
}
type PreCurrNextMonthPrayer struct {
custData *LookupInput
CurrentMonthData *PCal
PreviousMonthData *PCal
NextMonthData *PCal
prayers []MonthPrayerIface
}
func (p *PreCurrNextMonthPrayer) GetMonthPrayers() (*PreCurrNextMonthPrayer, error) {
var err error
var monthlyData *PreCurrNextMonthPrayer
defer func() {
fmt.Printf("return monthlyData address & value = %p %v\n", monthlyData, monthlyData)
}()
for k, data := range p.prayers {
fmt.Printf("loop = %v, data= %T %v\n", k, monthlyData, monthlyData)
monthlyData, err = data.GetMonthPrayer(p.custData)
fmt.Printf("\terr= %v\n", err)
fmt.Printf("\tmonthlyData= %p %v\n", monthlyData, monthlyData)
if err == nil {
fmt.Printf("loop-return: err == nil \n")
return monthlyData, nil
}
}
if err == nil {
fmt.Printf("post-loop:\n")
fmt.Printf("\tmonthlyData= %p %v\n", monthlyData, monthlyData)
return monthlyData, nil
}
return nil, fmt.Errorf("unable to get prayer data from all sources %s", err)
}
func NewMonthPrayer(input *LookupInput, prayers ...MonthPrayerIface) (*PreCurrNextMonthPrayer, error) {
var err error
t := &PreCurrNextMonthPrayer{
custData: input,
prayers: prayers,
}
t, err = t.GetMonthPrayers()
if err != nil {
return nil, err
}
return t, nil
}
As you can see, I'm looping over an interface struct method called GetMonthPrayer
This is my s3 source
// s3.go
package prayer
import (
"bytes"
"encoding/json"
"errors"
"fmt"
"github.com/aws/aws-sdk-go/aws"
"github.com/aws/aws-sdk-go/service/s3"
"io"
awsservices "prayer-times/src/aws"
)
// S3Store s3 storage object for prayer calendars
type S3Store struct {
data *PCal
}
// GetMonthPrayer retrieves prayer data from s3, otherwise from aladhan
func (s *S3Store) GetMonthPrayer(input *LookupInput) (*PreCurrNextMonthPrayer, error) {
mPrayer := new(PreCurrNextMonthPrayer)
fmt.Println("attempting to retrieve prayer data from s3")
s3Client := awsservices.NewS3Service()
pMonthInput := &LookupInput{
Country: input.Country,
ZipCode: input.ZipCode,
custTime: input.custTime.AddDate(0, -1, 0),
}
nMonthInput := &LookupInput{
Country: input.Country,
ZipCode: input.ZipCode,
custTime: input.custTime.AddDate(0, 1, 0),
}
// s3Pdata retrieves data from S3 and
s3pData := func(input *LookupInput) (*PCal, error) {
pCalendar := new(PCal)
data, err := s3Client.GetObject(&s3.GetObjectInput{
Bucket: aws.String(bucket),
Key: aws.String(
fmt.Sprintf(
"%s/%d/%d/%d",
input.Country,
input.ZipCode,
input.custTime.Year(),
input.custTime.Month())),
})
if err != nil {
return nil, err
}
if data == nil {
return nil, errors.New("error data from s3 is nil")
}
defer func() {
err := data.Body.Close()
if err != nil {
fmt.Printf("unable to close s3 body: %s", err)
}
}()
s3buf := bytes.NewBuffer(nil)
if _, err := io.Copy(s3buf, data.Body); err != nil {
return nil, err
}
dataBytes := s3buf.Bytes()
decoder := json.NewDecoder(bytes.NewReader(dataBytes))
err = decoder.Decode(&pCalendar)
if err != nil {
fmt.Printf("unable to decode json: %s", err)
}
return pCalendar, nil
}
aladhanData := new(AladhanStore)
getAladhanData := func(input *LookupInput) (*PreCurrNextMonthPrayer, error) {
data, err := aladhanData.GetMonthPrayer(input)
if err != nil {
return nil, err
}
return data, nil
}
// Get current data from s3, if not s3, then get all three from aladhan
cMonthS3Data, err := s3pData(input)
pMonthS3Data, err := s3pData(pMonthInput)
nMonthS3Data, err := s3pData(nMonthInput)
if err != nil {
adata, err := getAladhanData(input)
if err != nil {
fmt.Printf("err: %s", err)
return nil, err
}
return adata, nil
}
mPrayer.CurrentMonthData = cMonthS3Data
// Get previous month data from s3, if not s3, then get all three from aladhan
mPrayer.PreviousMonthData = pMonthS3Data
// Get next month data from s3, if not s3, then get all three from aladhan
mPrayer.NextMonthData = nMonthS3Data
return mPrayer, nil
}
Here is my aladhan source
// aladhan.go
package prayer
import (
"bytes"
"encoding/json"
"fmt"
"github.com/aws/aws-sdk-go/aws"
"github.com/aws/aws-sdk-go/service/s3/s3manager"
"golang.org/x/sync/errgroup"
"io"
"log"
awsservices "prayer-times/src/aws"
"prayer-times/src/urljsonoutput"
"prayer-times/src/zipcoordinates"
)
var (
aladhanURL string = "https://api.aladhan.com/v1/calendar?"
)
// PCal contains the prayer times of the month as well as the return code
type PCal struct {
Code int `json:"code"`
Status string `json:"status"`
Data []struct {
Timings DailyPrayers
}
}
/*
AladhanData returns the total monthly prayers of given month, coordinates, and zip from aladhan.
https://api.aladhan.com/v1/calendar?latitude=51.508515&longitude=-0.1254872&method=1&month=4&year=2017
*/
func AladhanData(input *LookupInput) *PCal {
coordinates := zipcoordinates.HereCoordinates(&zipcoordinates.GeoLocationInput{
PostalCode: input.ZipCode,
CountryCode: input.Country,
})
respStruct := new(PCal)
_, err := urljsonoutput.GetURLJSON(fmt.Sprintf(
"%slatitude=%v&longitude=%v&method=%v&month=%v&year=%v",
aladhanURL,
coordinates.Items[0].Position.Latitude,
coordinates.Items[0].Position.Longitude,
input.Method,
int(input.custTime.Month()),
input.custTime.Year()), respStruct)
if err != nil {
log.Fatalf("unable to pull monthly prayer data %v", err)
}
return respStruct
}
// AladhanStore struct to interact with interface for GetMonthPrayer
type AladhanStore struct {
data *PCal
}
// GetMonthPrayer Pulls prayer data from aladhan
func (a *AladhanStore) GetMonthPrayer(input *LookupInput) (*PreCurrNextMonthPrayer, error) {
mPrayer := new(PreCurrNextMonthPrayer)
// Return prayer data from aladhan
custPMonthTime := input.custTime.AddDate(0, -1, 0)
pMonthLookupInput := new(LookupInput)
pMonthLookupInput.custTime = custPMonthTime
pMonthLookupInput.ZipCode = input.ZipCode
pMonthLookupInput.Country = input.Country
custNMonthTime := input.custTime.AddDate(0, 1, 0)
nMonthLookupInput := new(LookupInput)
nMonthLookupInput.custTime = custNMonthTime
nMonthLookupInput.ZipCode = input.ZipCode
nMonthLookupInput.Country = input.Country
prayerData := AladhanData(input)
pMonthPData := AladhanData(pMonthLookupInput)
nMonthPData := AladhanData(nMonthLookupInput)
// Save prayer data into io.Reader to save to s3
var Marshal = func(data interface{}) (io.ReadSeeker, error) {
mdata, err := json.MarshalIndent(data, "", "\t")
if err != nil {
return nil, err
}
return bytes.NewReader(mdata), nil
}
rmData, err := Marshal(prayerData)
pRmData, err := Marshal(pMonthPData)
nRmData, err := Marshal(nMonthPData)
if err != nil {
return nil, err
}
// Save prayer data into s3
g := new(errgroup.Group)
s3Upload := func(rawData *io.ReadSeeker, input *LookupInput) func() error {
return func() error {
s3Client := s3manager.NewUploaderWithClient(awsservices.NewS3Service())
_, err = s3Client.Upload(&s3manager.UploadInput{
Bucket: aws.String(bucket),
Key: aws.String(
fmt.Sprintf(
"%s/%d/%d/%d",
input.Country,
input.ZipCode,
input.custTime.Year(),
int(input.custTime.Month()))),
Body: *rawData,
})
if err != nil {
return err
}
return nil
}
}
g.Go(s3Upload(&pRmData, pMonthLookupInput))
g.Go(s3Upload(&rmData, input))
g.Go(s3Upload(&nRmData, nMonthLookupInput))
if err := g.Wait(); err == nil {
mPrayer.PreviousMonthData = pMonthPData
mPrayer.CurrentMonthData = prayerData
mPrayer.NextMonthData = nMonthPData
return mPrayer, nil
}
return nil, err
}
Here is my test file.
func TestPrayer(t *testing.T) {
p, err := NewMonthPrayer(
&input,
&S3Store{},
&AladhanStore{},
)
if err != nil {
t.Errorf("error: %s", err)
}
data, err := p.GetMonthPrayers()
if err != nil {
t.Errorf("error: %s", err)
}
t.Logf("Test address: %p", data)
t.Logf("data THIS SHOULDN'T BE NIL: %v", data)
t.Logf("ERROR: %s", err)
}
These are my results. Ignore the pass result, the data is first not nil and second nil.
=== RUN TestPrayer
loop = 0, data= *prayer.PreCurrNextMonthPrayer <nil>
attempting to retrieve prayer data from s3
err= <nil>
monthlyData= 0xc000131180 &{<nil> 0xc0002612f0 0xc00051e780 0xc00011cea0 []}
loop-return: err == nil
return monthlyData address & value = 0xc000131180 &{<nil> 0xc0002612f0 0xc00051e780 0xc00011cea0 []}
post-loop:
monthlyData= 0x0 <nil>
return monthlyData address & value = 0x0 <nil>
prayer_test.go:53: Test address: 0x0
prayer_test.go:55: data THIS SHOULDN'T BE NIL: <nil>
prayer_test.go:56: ERROR: %!s(<nil>)
--- PASS: TestPrayer (0.32s)
PASS

Answer 1

The duplicate was due to the GetMonthPrayer call from NewMonthPrayer, which shouldn't have been the case to begin with. It was called first but returned second, thus overwriting the existing data.
func NewMonthPrayer(input *LookupInput, prayers ...MonthPrayerIface) (*PreCurrNextMonthPrayer, error) {
var err error
t := &PreCurrNextMonthPrayer{
custData: input,
prayers: prayers,
}
t, err = t.GetMonthPrayers()
if err != nil {
return nil, err
}
return t, nil
}
I removed the NewMonthPrayer entirely as it was unnecessary, I also removed the function call in the process, thus fixing the initial problem.
// NewPrayer instantiates a prayer type object with the required input
func NewPrayer(input *LookupInput, prayers ...MonthPrayerIface) *Prayer {
return &Prayer{
custData: input,
prayers: prayers,
}
}