cannot convert fs.FS zip file to io.ReadSeeker (missing Seek)

[Rory Campbell-Lange]

I have a program that needs to work equally for a directory of files or for the contents of a zip file, so I'm using an fs.FS to abstract the two.

Some of the files need to be provided to a PDF importer that accepts an io.ReadSeeker (#1).

Generally this is working fine except when trying to convert a zip file to an io.ReadSeeker. Concisely:

1 package main
2
3 import (
4 "archive/zip"
5 "fmt"
6 "io"
7 )
8
9 func main() {
10 z, _ := zip.OpenReader("a.zip")
11 a, _ := z.Open("a") // a is a file in a.zip
12 f, _ := a.Stat()
13 fmt.Println(f.Name() == "a") // true
14 fmt.Println(a.(io.ReadSeeker))
15 }

Fails on line 14 with

interface conversion: *zip.checksumReader is not io.ReadSeeker: missing method Seek

Advice on how to rectify this would be gratefully received.

#1
https://github.com/phpdave11/gofpdf/blob/b09d9214a2296c8ea10312d9c5a520e27f2148c9/contrib/gofpdi/gofpdi.go#L52

[Dan Kortschak]

On Thu, 2022-09-22 at 00:58 +0100, Rory Campbell-Lange wrote:
> interface conversion: *zip.checksumReader is not io.ReadSeeker:
> missing method Seek
>
> Advice on how to rectify this would be gratefully received.

Would it be acceptable to conditionally copy the reader's contents into
a buffer that implements io.ReadSeeker?

if rs, ok := r.(io.ReadSeeker); ok {
useReadSeeker(rs)
} else {
b, err := io.ReadAll(r)
// handle err
useReadSeeker(bytes.NewReader(b))
}

[Anthony Martin]

Rory Campbell-Lange <ro...@campbell-lange.net> once said:
> interface conversion: *zip.checksumReader is not io.ReadSeeker:
> missing method Seek
>
> Advice on how to rectify this would be gratefully received.

Read the entire zip.File into memory with io.ReadAll and create
a bytes.Reader with the resulting byte slice. That will give you
a type that implements io.ReadSeeker.

Cheers,
Anthony

[robert engels]

fs.FS does not support positional reads. It is always the entire file.

You can create a wrapper struct that implements the Seek() (trivial - just keep a position and adjust Read() accordingly).

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[robert engels]

Others have suggested passing a ByteBuffer - I don’t think that will work because you will be missing other methods that are probably needed (FileInfo to get the name, etc)

[Dan Kortschak]

On Wed, 2022-09-21 at 19:30 -0500, robert engels wrote:
> Others have suggested passing a ByteBuffer - I don’t think that will
> work because you will be missing other methods that are probably
> needed (FileInfo to get the name, etc)

The function that was pointed to takes an ~io.ReadSeeker (oddly a
pointer to the interface) and doesn't look like it expects to
conditionally use other methods. The other details can always be
obtained from the original value.

[robert engels]

Yea - my bad. Should be fine.

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[Matthew Zimmerman]

I've got this same challenge.  The contents of any file within a zip needs to be read as a stream, hence why there is no underlying seek method available (assuming to handle the decompression properly) So if you want to seek in a file within a zip, you need to read into a buffer first.  You don't have to load the whole zip, just the file within the zip you're working on.

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[Rory Campbell-Lange]

On 22/09/22, 'Dan Kortschak' via golang-nuts (golan...@googlegroups.com) wrote:
> On Thu, 2022-09-22 at 00:58 +0100, Rory Campbell-Lange wrote:
> > interface conversion: *zip.checksumReader is not io.ReadSeeker:
> > missing method Seek
>

> Would it be acceptable to conditionally copy the reader's contents into
> a buffer that implements io.ReadSeeker?
>
> if rs, ok := r.(io.ReadSeeker); ok {
> useReadSeeker(rs)
> } else {
> b, err := io.ReadAll(r)
> // handle err
> useReadSeeker(bytes.NewReader(b))
> }

Thanks very much for the suggestion. The following works ok as a test (which is basically your code):

fileAsReadSeeker := func(file fs.File) io.ReadSeeker {
if rs, ok := file.(io.ReadSeeker); ok {
return rs
}
b, err := io.ReadAll(file)
if err != nil {
panic(err) // to fix
}
return bytes.NewReader(b)
}

I'm now thinking of adding in a bytes.Buffer field into the underlying struct holding the fs.File file which can be initialised (and reused) if an io.ReadSeeker is required and provides a clearer error path.

Thanks again,
Rory