proposal: archive/zip: enhance ErrFormat error string with more granular details of failure

[niksw7]

Proposal Details

The current code for reading zip
read,err := zip.OpenReader("filename.zip")
In cases of error due to headers etc, it's very difficult to debug without changing the golang source code(And adding logs there). The error just states "zip: not a valid zip file".
It would be beneficial to know why the zip is not valid. So adding a proposal to make the errors here more clear with exact reason for the zip file being invalid

[seankhliao]

what sort of API (custom error type?) is being proposed here?

[nnzv]

@niksw7, to provide a clearer understanding of the issue, it's advisable to utilize the unzip utility to decompress the file. There are multiple approaches to replicating the error, such as creating an empty file and attempting to read the zip file.

% touch foo.zip

package main

import (
    "fmt"
    "os"
    "archive/zip"
)

func main() {
    _, err := zip.OpenReader("foo.zip")
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }
}

% go run main.go
zip: not a valid zip file
exit status 1

% unzip foo.zip
Archive:  foo.zip
  End-of-central-directory signature not found.  Either this file is not
  a zipfile, or it constitutes one disk of a multi-part archive.  In the
  latter case the central directory and zipfile comment will be found on
  the last disk(s) of this archive.
unzip:  cannot find zipfile directory in one of foo.zip or
        foo.zip.zip, and cannot find foo.zip.ZIP, period.

[niksw7]

@nnzv ,
I am speaking from the archive/zip package error messages.
There are lot of use cases, where from a golang code we have to copy few files from zip. So a more clear error message would help in debugging. I will try to add few test cases for it to ellaborate