What causes "transformVersionStream got nil body" in hashicorp/packer?

Retry the plugin install; transient HTTP issues may yield empty bodies.

transformVersionStream got nil body

Error message

transformVersionStream got nil body

What it means

transformVersionStream converts a GitHub API tag-list response stream into a JSON list of releases; it refuses a nil io.ReadCloser. A nil body means the HTTP response had no body to decode, which the getter treats as an internal/protocol failure rather than silently producing empty versions.

Source

Thrown at packer/plugin-getter/github/getter.go:94

					Filename string `json:"filename"`
				}{
					Checksum: checksumString,
					Filename: checksumFilename,
				}); err != nil {
					return nil, err
				}
			}
		}
		_, _ = buffer.WriteString("]")
		return io.NopCloser(buffer), nil
	}
}

// transformVersionStream get a stream from github tags and transforms it into
// something Packer wants, namely a json list of Release.
func transformVersionStream(in io.ReadCloser) (io.ReadCloser, error) {
	if in == nil {
		return nil, fmt.Errorf("transformVersionStream got nil body")
	}
	defer in.Close()
	dec := json.NewDecoder(in)

	m := []struct {
		Ref string `json:"ref"`
	}{}
	if err := dec.Decode(&m); err != nil {
		return nil, err
	}

	out := []plugingetter.Release{}
	for _, m := range m {
		out = append(out, plugingetter.Release{
			Version: strings.TrimPrefix(m.Ref, "refs/tags/"),
		})
	}

View on GitHub (pinned to eb36e3c3e4)

Solutions

Retry the plugin install; transient HTTP issues may yield empty bodies.
Check proxy/TLS middleboxes that may strip or truncate GitHub API responses.
Update Packer — newer releases handle GitHub API changes in the getter.
Use a protocol:// (file/http) or required_plugins pinning to bypass flaky github discovery.

Defensive patterns

Strategy: retry

Validate before calling

if resp.Body == nil {
    return fmt.Errorf("github tags response has no body; status=%d", resp.StatusCode)
}

Try / catch

versions, err := getter.Get(...)
if err != nil && strings.Contains(err.Error(), "nil body") {
    /* retry once, then fall back to a different source protocol */
}

Prevention

Keep Packer updated so getter changes track GitHub API behavior.
Avoid proxies/middleboxes that alter or truncate response bodies.
Pin plugin sources with explicit version constraints to reduce discovery calls.

When it happens

Trigger: Called after fetching the github tags stream; a nil ReadCloser is passed in — e.g. the HTTP client returned a response with nil Body or a code path constructs the request without a body due to a failed/aborted response.

Common situations: Unusual proxy or TLS interception stripping the response body; a mocked/tested HTTP client returning a bare response; an upstream change in how the tags endpoint responds during `packer init` with a github.com source.

Related errors

AI-assisted analysis of hashicorp/packer@eb36e3c3e4 (2026-09-05). Data as JSON: /api/errors/1f92485bd976313b. Report an issue: GitHub.