Stop failing if artifact file exists, but empty

Empty output artifacts are a valid and desired use case. (Think of `grep` or any other filtering program where one of the outputs can be empty.)
Argo failing on empty outputs was a pretty bad surprise for us.

Fixes argoproj#1642
The problem was introduced in https://github.com/argoproj/argo/pulls/1247

util/file/fileutil.go

@@ -30,7 +30,7 @@ func ExistsInTar(sourcePath string, tarReader TarReader) bool {
  if hdr.FileInfo().IsDir() && strings.Contains(sourcePath, strings.Trim(hdr.Name, "/")) {
  return true
  }
- if strings.Contains(sourcePath, hdr.Name) && hdr.Size > 0 {
+ if strings.Contains(sourcePath, hdr.Name) {
  return true
  }
  }

util/file/fileutil_test.go

@@ -97,16 +97,14 @@ func TestExistsInTar(t *testing.T) {
  },
  },
  {
- sourcePath: "/empty.txt", expected: false,
+ sourcePath: "/empty.txt", expected: true,
  files: []fakeFile{
- // fails because empty.txt is empty
  {name: "empty.txt", body: ""},
  },
  },
  {
- sourcePath: "/tmp/empty.txt", expected: false,
+ sourcePath: "/tmp/empty.txt", expected: true,
  files: []fakeFile{
- // fails because empty.txt is empty
  {name: "empty.txt", body: ""},
  },
  },

workflow/executor/docker/docker.go

@@ -61,7 +61,7 @@ func (d *DockerExecutor) CopyFile(containerID string, sourcePath string, destPat
  return err
  }
  if !file.ExistsInTar(sourcePath, tar.NewReader(gzipReader)) {
- errMsg := fmt.Sprintf("path %s does not exist (or %s is empty) in archive %s", sourcePath, sourcePath, destPath)
+ errMsg := fmt.Sprintf("path %s does not exist in archive %s", sourcePath, destPath)
  log.Warn(errMsg)
  return errors.Errorf(errors.CodeNotFound, errMsg)
  }