I have a slightly funky issue in golang. Essentially I have a slice of strings which represent file paths. I then run a cat against those filepaths to combine the files before sorting, deduping, etc.
here is the section of code (where 'applicableReductions' is the string slice):
applicableReductions := []string{}
for _, fqFromListName := range fqFromListNames {
filePath := GetFilePath()
//BROKE CODE GOES HERE
}
applicableReductions = append(applicableReductions, filePath)
fileOut, err := os.Create(toListWriteTmpFilePath)
if err != nil {
return err
}
cat := exec.Command("cat", applicableReductions...)
catStdOut, err := cat.StdoutPipe()
if err != nil {
return err
}
go func(cat *exec.Cmd) error {
if err := cat.Start(); err != nil {
return fmt.Errorf("File reduction error (cat) : %s", err)
}
return nil
}(cat)
// Init Writer & write file
writer := bufio.NewWriter(fileOut)
defer writer.Flush()
_, err = io.Copy(writer, catStdOut)
if err != nil {
return err
}
if err = cat.Wait(); err != nil {
return err
}
fDiff.StandardiseData(fileOut, toListUpdateFolderPath, list.Name)
The above works fine. The problem comes when I try to append a new ele to the array. I have a seperate function which creates a new file from db content which is then added to the applicableReductions slice.
func RetrieveDomainsFromDB(collection *Collection, listName, outputPath string) error {
domains, err := domainReviews.GetDomainsForList(listName)
if err != nil {
return err
}
if len(domains) < 1 {
return ErrNoDomainReviewsForList
}
fh, err := os.OpenFile(outputPath, os.O_RDWR, 0774)
if err != nil {
fh, err = os.Create(outputPath)
if err != nil {
return err
}
}
defer fh.Close()
_, err = fh.WriteString(strings.Join(domains, "\n"))
if err != nil {
return err
}
return nil
}
If I call the above function and append the filePath to the applicableReduction slice, it is in there but doesnt get called by cat.
To clarify, when I put the following where it says BROKE CODE GOES HERE:
if dbSource {
err = r.RetrieveDomainsFromDB(collection, ToListName, filePath)
if err != nil {
return err
continue
}
}
The filepath can be seen when doing fmt.Println(applicableReductions) but the content of the files contents are not seen in the cat output file.
I thought perhaps a delay in the file being written so i tried adding a time.wait, tis didnt help. However the solution I found was to sort the slice, e.g this code above the call to exec cat solves the problem but I dont know why:
sort.Strings(applicableReductions)
I have confirmed all files present on both successful and unsucessful runs the only difference is without the sort, the content of the final appended file is missing
An explanation from a go-pro out there would be very much appreciated, let me know if you need more info, debug - happy to oblige to understand
UPDATE It has been suggested that this is the same issue as here: Golang append an item to a slice, I think I understand the issue there and I'm not saying this isnt the same but I cannot see the same thing happenning - the slice in question is not touched from outside the main function (e.g. no editing of the slice in RetrieveDomainsFromDB function), I create the slice before a loop, append to it within a loop and then use it after the loop - Ive added an example at the top to show how the slice is built - please could someone clarify where this slice is being copied if this is the case
