apache/beam · error
malformed input for decoding: %s
Error message
malformed input for decoding: %s
What it means
hooks.Decode parses a hook-args string as CSV ("name,arg1,arg2") and panics with this wrapped error if csv.Reader cannot parse it — e.g. unbalanced quotes or bare quotation marks in the input. Unlike most of this package it panics rather than returning an error, so callers must ensure the string is valid CSV.
Source
Thrown at sdks/go/pkg/beam/core/util/hooks/hooks.go:302
func Encode(name string, opts []string) string {
var cfg bytes.Buffer
w := csv.NewWriter(&cfg)
// This should never happen since a bytes.Buffer doesn't fail to write.
if err := w.Write(append([]string{name}, opts...)); err != nil {
panic(errors.Wrap(err, "error encoding arguments"))
}
w.Flush()
return cfg.String()
}
// Decode decodes a hook name and its arguments from a single string.
// This is a convenience function for users of this package that are composing
// hooks.
func Decode(in string) (string, []string) {
r := csv.NewReader(strings.NewReader(in))
s, err := r.Read()
if err != nil {
panic(errors.Wrapf(err, "malformed input for decoding: %s", in))
}
return s[0], s[1:]
}
View on GitHub (pinned to 12126d8942)
Solutions
- Ensure the string is valid CSV: quote fields containing quotes/commas, e.g. name,"arg with ""quotes"""
- Sanitize or validate the input string before calling Decode; use csv.Reader yourself to get a returnable error instead of the panic
- Trim whitespace and reject empty input before calling Decode
- If encoding your own args, use hooks.Encode (or csv.Writer) so round-trips are valid
Example fix
// before
name, args := hooks.Decode(userInput) // panics on `hook "x`
// after
r := csv.NewReader(strings.NewReader(userInput))
rec, err := r.Read()
if err != nil || len(rec) == 0 {
return fmt.Errorf("invalid hook args %q: %w", userInput, err)
}
name, args := rec[0], rec[1:] Defensive patterns
Strategy: validation
Validate before calling
func safeDecode(in string) (name string, args []string, err error) {
if strings.TrimSpace(in) == "" {
return "", nil, fmt.Errorf("empty hook spec")
}
rec, err := csv.NewReader(strings.NewReader(in)).Read()
if err != nil || len(rec) == 0 {
return "", nil, fmt.Errorf("malformed hook spec %q: %v", in, err)
}
return rec[0], rec[1:], nil
} Try / catch
// Decode panics, so recover at the boundary if you must call it directly
func decodeSafe(in string) (s string, args []string) {
defer func() { _ = recover() }()
return hooks.Decode(in)
} Prevention
- Validate or produce hook-args strings with csv.Writer/Reader round-trips, never string concatenation
- Reject empty and whitespace-only input before Decode
- Escape double quotes in argument values per CSV rules ("" inside quoted fields)
- Recover() around Decode when input comes from users, since it panics instead of returning an error
When it happens
Trigger: Calling hooks.Decode(in) with a string containing malformed CSV: unmatched double quotes (e.g. `hook "arg`), stray quote characters, or an empty/multiline string that csv.Read rejects (including io.EOF for empty input).
Common situations: Passing user-supplied hook arguments from job options or CLI flags that contain quotes; encoding a name with commas/quotes without proper CSV quoting; passing an empty string.
Understand the failure class
Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.
- Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.
Related errors
- invalid bundle processing state: %d
- panic in ElementManager.Bundles watermark evaluation gorouti
- pipeline panicked: %v Stacktrace: %s
- panic in stage.Execute bundle processing goroutine: %v, stag
- %v
AI-assisted analysis of apache/beam@12126d8942 (2026-09-13).
Data as JSON: /api/errors/5ba0c0b52cc9bfff.
Report an issue: GitHub.