014.error

Go Errors

Concept

Errors in Go are values. Effective error handling involves using simple errors, sentinel errors (package-level variables), custom error types for structured data, and error wrapping to preserve context. Use errors.Is and errors.As to inspect errors.

References

• https://go.dev/blog/go1.13-errors
• https://gobyexample.com/errors
• https://pkg.go.dev/errors

Quest

Objective

Implement a file validation system using sentinel errors, custom error types, error wrapping, and error inspection.

Requirements

Variables & Types

• Sentinel Errors (Package-level):
  • ErrEmptyFilename: "filename cannot be empty"
  • ErrFileTooLarge: "file size exceeds limit"
• Custom Error: ValidationError struct with fields Filename (string) and Reason (string).
  • Implement Error() string: Return "validation failed for '<Filename>': <Reason>".

Functions

1. ValidateFilename(filename string) error

   • Return ErrEmptyFilename if filename is empty.
   • Return nil otherwise.

2. ValidateFileSize(size int64, maxSize int64) error

   • Return ErrFileTooLarge if size > maxSize.
   • Return new error "file size cannot be negative" if size < 0.
   • Return nil otherwise.

3. ValidateFileExtension(filename string, allowedExts []string) error

   • Check if filename ends with any string in allowedExts.
   • If valid, return nil.
   • If invalid, return a *ValidationError with Filename set and Reason set to "unsupported file extension".

4. ValidateFile(filename string, size int64, maxSize int64) error

   • Call ValidateFilename. If error, wrap it with "file validation failed: %w".
   • Call ValidateFileSize. If error, wrap it with "file validation failed: %w".
   • Return nil if success.

5. CanRetry(err error) bool

   • Return true if err wraps a *ValidationError (unsupported extension can be fixed).
   • Return false if err is nil, or wraps ErrEmptyFilename, ErrFileTooLarge, or any other error.

Inputs

• Varies by function.

Outputs

• Errors (Sentinel, Custom, Wrapped, or simple) and Booleans.

Examples

• ValidateFilename("") → ErrEmptyFilename
• ValidateFileSize(200, 100) → ErrFileTooLarge
• ValidateFileExtension("doc.exe", [".txt"]) → &ValidationError{"doc.exe", "unsupported file extension"}

Testing

To run the tests, execute the following command from the root directory:

go test -v ./quests/014.error

Or from the quest directory:

go test -v

Expected output:

=== RUN   TestValidateFilename
--- PASS: TestValidateFilename (0.00s)
=== RUN   TestValidateFileSize
--- PASS: TestValidateFileSize (0.00s)
=== RUN   TestValidateFileExtension
--- PASS: TestValidateFileExtension (0.00s)
=== RUN   TestValidateFileWrapping
--- PASS: TestValidateFileWrapping (0.00s)
=== RUN   TestCanRetry
--- PASS: TestCanRetry (0.00s)
PASS