proposal: archive/zip: add support for appending files

[james-tech]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   1.6.2
2. What operating system and processor architecture are you using (go env)?
   mac
3. What did you do?
   If possible, provide a recipe for reproducing the error.
   A complete runnable program is good.
   A link on play.golang.org is best.
4. What did you expect to see?
   I found that the archive/zip package release realization is uncompleted.
   I want to append a dir or some files into an exists zip file without unzip it, but the functions are not supported.
5. What did you see instead?
   I use the exec.Command to call zip -r command in the system. may be is not the good way.
   I hope the next version archive/zip will support it.

[dsnet]

The proposal #14386 switches the object format to be using zip files and requires the ability to append to files. Currently the proposal seems to indicate that it will use some internal logic to handle appending to zip files.

[odeke-em]

@james-tech for starters here is something that might solve the problem for you https://github.com/rsc/zipmerge by @rsc.

[kokes]

There is a fork of std/zip by @rogpeppe that supports this. Some discussion on the initial implementation is here: https://groups.google.com/forum/#!topic/golang-dev/ZSubqlF2G4k

[alecha]

Any news on if/when this can be included?

[bradfitz]

@alecha, any news would be posted here. There is no news. If @rogpeppe or others wish to contribute this to the standard library, they're welcome to.

[dsnet]

Following up on Russ' comment in the linked email thread, it would be preferable if any suggested API allowed replacing and deleting existing files (since the appending a new central directory record can simply omit references to "deleted" files). True deletion would require rewriting the entire ZIP file, so there is no need for a new API for that.

[yanolab]

I'm working on this and I have two proposals. First, adding new method func OpenAppender(name string) (*Writer, error) for simply appending to zip.
This gets rid of EOCD and writing new entry from that.
Second, adding new method func (w *Writer) Copy(f *File) error to zip.Writer for coping file without recompressing. This is the same as that implemented in zipmerge (by @rsc).
I think that deleting and replacing should be realized on the user side using suggested API.

For example, deleting file is bellow.

.
.
.
zw := NewWriter(w)
for _, f := range rc.File {
    if _, ok := delMap[f.Name]; ok {
        continue
    }
    zw.Copy(f)
}

In the replacing case, comparing header then copy or write.

I believe that these can cover most cases.

[smasher164]

Assuming that the API isn't concerned with true deletion, one possibility is to mimick that of libzip. Adapted for Go:

// Append appends a file with the specified name to the end of the archive.
// If overwrite is true, any file with the specified name is overwritten.
func (rc *ReadCloser) Append(name string, w io.Writer, overwrite bool) *Writer

// Replace replaces the file at the specified offset.
func (rc *ReadCloser) Replace(offset int64, w io.Writer) *Writer

// Delete marks the file at the specified offset as deleted.
func (rc *ReadCloser) Delete(offset int64) *Writer

[dsnet]

Marking as proposal, so this goes through the appropriate process.