Cannot COPY file to existing directory without trailing slash

[Zetten]

Actual behavior
A Dockerfile with content

RUN mkdir -p /path/to/foo
COPY sourcefile /path/to/foo

will fail to build in Kaniko, with the error:
error building image: error building stage: open /path/to/foo: is a directory

Expected behavior
docker build is rather more lenient with the target for COPY commands, and will create the file in the destination directory: /path/to/foo/sourcefile.

Additional Information
I've tried tracking down the problem through the DestinationFilepath function in command_util.go. This checks whether the destination is a directory using the IsDestDir function. However IsDestDir is quite naive and string-based, and only checks for the existence of a trailing slash or cwd marker - which are not present in the example above.

Changing IsDestDir seems like it may have side effects in other places, but I managed to test successfully with a new function, replacing the IsDestDir call in DestinationFilepath:

func IsDirectory(path string) bool {
	fileInfo, err := os.Stat(path)
	if err != nil{
		return false
	}
	return fileInfo.IsDir()
}

It's not clear to me whether this is the best approach, nor how to properly test it. I'm happy to create a PR with some feedback!

[priyawadhwa]

@Zetten I agree that currently IsDestDir is naive and should be improved. I think the string parsing is necessary in the case where we have something like

COPY file /maybe/a/dir

and /maybe/a/dir doesn't exist yet, so the trailing slash tells us if it's meant to be a directory or a file.

Perhaps we should check if the directory exists first, and if not then check the string? If you're still interested in opening a PR for this, that would be great!

[Zetten]

The PR tries to avoid any repercussions of changing IsDestDir, but it's used in a few places and not always to check real filesystems. I'm not so hot with go but I've tried to keep it as simple and defensive as I can. For the same reason, I've also missed out testing this - it hasn't broken the existing stuff, but with some guidance I can try adding a new test.