What causes "failed to write gzip raw data: %w" in hashicorp/packer?

Retry the packer command — transient memory pressure is the only plausible cause

failed to write gzip raw data: %w

Error message

failed to write gzip raw data: %w

What it means

The `gzip()` HCL2 function compresses a string and the underlying gzip.Writer failed while writing the raw input data. This is a defensive wrap: writes into a bytes.Buffer essentially never fail, so the error is near-unreachable.

Source

Thrown at hcl2template/function/encoding.go:32

)

// Base64GzipFunc constructs a function that compresses a string with gzip and then encodes the result in
// Base64 encoding.
var Base64GzipFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "str",
			Type: cty.String,
		},
	},
	Type: function.StaticReturnType(cty.String),
	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
		s := args[0].AsString()

		var b bytes.Buffer
		gz := gzip.NewWriter(&b)
		if _, err := gz.Write([]byte(s)); err != nil {
			return cty.UnknownVal(cty.String), fmt.Errorf("failed to write gzip raw data: %w", err)
		}
		if err := gz.Flush(); err != nil {
			return cty.UnknownVal(cty.String), fmt.Errorf("failed to flush gzip writer: %w", err)
		}
		if err := gz.Close(); err != nil {
			return cty.UnknownVal(cty.String), fmt.Errorf("failed to close gzip writer: %w", err)
		}
		return cty.StringVal(base64.StdEncoding.EncodeToString(b.Bytes())), nil
	},
})

// Base64Gzip compresses a string with gzip and then encodes the result in
// Base64 encoding.
//
// Packer uses the "standard" Base64 alphabet as defined in RFC 4648 section 4.
//
// Strings in the Packer language are sequences of unicode characters rather
// than bytes, so this function will first encode the characters from the string

View on GitHub (pinned to eb36e3c3e4)

Solutions

Retry the packer command — transient memory pressure is the only plausible cause
Inspect the wrapped error (%w) for the root cause
Reduce template variable sizes if memory is constrained

Defensive patterns

Strategy: try-catch

Try / catch

// error surfaces during template evaluation; wrap the packer run
gz_b64=$(packer console <<< 'gzip("data")' 2>&1) || echo "gzip failed: $gz_b64"

Prevention

Retry on transient memory pressure
Inspect the wrapped %w error for the real cause
Keep gzip inputs to reasonable-size strings

When it happens

Trigger: Calling `gzip(someString)` in a template where gz.Write returns an error — practically only if the internal buffer write fails (out-of-memory-type conditions).

Common situations: Almost never hit in practice; the input is always a string coerced via AsString so no user input directly causes it.

Related errors

AI-assisted analysis of hashicorp/packer@eb36e3c3e4 (2026-09-05). Data as JSON: /api/errors/f91f52c566c8a8f0. Report an issue: GitHub.