golib/ioutils/iowrapper/doc.go

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/*
Package iowrapper provides a flexible I/O wrapper that enables customization and interception
of read, write, seek, and close operations on any underlying I/O object without modifying
its implementation.

# Philosophy

The iowrapper package follows these core principles:

  - Transparency: Wrap any I/O object without changing its interface
  - Flexibility: Customize operations at runtime with custom functions
  - Thread Safety: All operations and updates are thread-safe via atomic operations
  - Zero Overhead: Minimal performance cost when no customization is used
  - Composability: Wrappers can be chained for complex transformations

# Architecture

The package consists of two main components:

1. IOWrapper Interface: Implements all standard Go I/O interfaces (io.Reader, io.Writer,
io.Seeker, io.Closer) and provides methods to set custom functions for each operation.

2. Custom Functions: Four function types (FuncRead, FuncWrite, FuncSeek, FuncClose) that
allow intercepting and transforming I/O operations.

The wrapper stores custom functions using atomic values from github.com/nabbar/golib/atomic,
ensuring thread-safe updates without mutex locks. When no custom function is set, operations
are delegated to the underlying object if it implements the corresponding interface.

# Operation Flow

For each I/O operation:

1. Load the custom function atomically
2. If a custom function is set, call it
3. Otherwise, check if the underlying object implements the required interface
4. If yes, delegate to the underlying object
5. If no, return an appropriate error (io.ErrUnexpectedEOF for Read/Write/Seek, nil for Close)

# Error Handling

The package uses a specific error handling strategy:

  - Custom functions returning nil indicate an error condition, causing Read/Write to return io.ErrUnexpectedEOF
  - Empty slice return ([]byte{}) from custom functions indicates 0 bytes processed without error
  - Underlying object errors are encapsulated in the return values (nil for errors, partial data otherwise)
  - Close operations return nil for non-closeable objects (graceful degradation)

# Thread Safety

All operations are thread-safe:

  - Custom functions are stored and loaded using atomic operations
  - Multiple goroutines can safely call Read, Write, Seek, or Close concurrently
  - Custom functions can be updated (SetRead, SetWrite, etc.) while I/O operations are in progress
  - The underlying object reference is immutable after wrapper creation

# Performance Characteristics

The wrapper introduces minimal overhead:

  - Wrapper creation: ~5.7ms per 10,000 operations
  - Read/Write (default): ~0-100ns per operation
  - Read/Write (custom): ~0-100ns + custom function cost
  - Function updates: ~100ns per operation
  - Memory: ~64 bytes per wrapper instance
  - Allocations: 0 per I/O operation (unless custom function allocates)

# Advantages

  - Non-invasive: No modification to underlying I/O objects
  - Flexible: Change behavior at runtime without code changes
  - Composable: Chain multiple wrappers for complex transformations
  - Efficient: Near-zero overhead for delegation
  - Safe: Thread-safe by design using atomic operations
  - Standard: Implements all standard Go I/O interfaces

# Limitations

  - Error Context: Underlying errors are not directly propagated; they are encapsulated in return values
  - No Context Support: Operations do not support context.Context for cancellation
  - Single Object: Wraps a single underlying object; cannot multiplex multiple sources
  - No Buffering: No built-in buffering; use bufio if needed

# Typical Use Cases

1. Logging and Monitoring

Track I/O operations for debugging or metrics without modifying application code.

2. Data Transformation

Apply transformations (encryption, compression, encoding) transparently during I/O.

3. Rate Limiting

Control throughput by intercepting operations and applying rate limits.

4. Validation

Verify data integrity or format during read/write operations.

5. Checksumming

Calculate checksums (MD5, SHA256) while reading or writing data.

6. Testing

Mock I/O behavior for unit tests without complex test fixtures.

7. Wrapper Chaining

Compose multiple transformations by wrapping wrappers (e.g., log → compress → encrypt).

# Example Usage Patterns

Basic wrapping and reading:

	buf := bytes.NewBuffer([]byte("hello"))
	wrapper := iowrapper.New(buf)
	data := make([]byte, 5)
	n, _ := wrapper.Read(data)

Custom read with transformation:

	wrapper.SetRead(func(p []byte) []byte {
	    n, _ := reader.Read(p)
	    // Transform data to uppercase
	    for i := 0; i < n; i++ {
	        if p[i] >= 'a' && p[i] <= 'z' {
	            p[i] -= 32
	        }
	    }
	    return p[:n]
	})

Logging wrapper:

	var cnt atomic.Int64
	wrapper.SetRead(func(p []byte) []byte {
	    n, _ := reader.Read(p)
	    cnt.Add(int64(n))
	    log.Printf("Read %d bytes (total: %d)", n, cnt.Load())
	    return p[:n]
	})

Wrapper chaining:

	// Chain: file → logging → compression → encryption
	logged := iowrapper.New(file)
	logged.SetRead(logFunc)

	compressed := iowrapper.New(logged)
	compressed.SetRead(compressFunc)

	encrypted := iowrapper.New(compressed)
	encrypted.SetRead(encryptFunc)

# Package Dependencies

The package depends on:

  - Standard library: io (for interfaces)
  - Internal: github.com/nabbar/golib/atomic (for thread-safe value storage)

# Design Decisions

1. Atomic Operations over Mutexes

The package uses atomic operations instead of mutexes for custom function storage.
This provides better performance for high-frequency I/O operations and simpler
lock-free concurrency.

2. Slice-Based Function Signatures

Custom functions use []byte return values instead of (int, error). This simplifies
the implementation and allows functions to return partial data (via slice length)
while using nil to signal errors. This design trades some Go idiomaticity for
simplicity and flexibility.

3. Immutable Underlying Object

The underlying object is set at creation and cannot be changed. This ensures
predictable behavior and avoids race conditions when accessing the underlying object.

4. Graceful Degradation

Operations on non-implementing objects return sensible defaults (error for Read/Write/Seek,
nil for Close) rather than panicking, allowing wrappers to work with partial I/O objects.

# Testing

The package has 100% test coverage with 114 specifications covering:

  - Basic I/O operations
  - Custom function registration and execution
  - Edge cases and boundary conditions
  - Error handling and propagation
  - Thread safety and concurrency
  - Integration scenarios (logging, transformation, checksumming)
  - Performance benchmarking

Run tests with:

	go test -v -cover .
	go test -race .  # With race detector

# Thread Safety Guarantees

  - All exported methods are safe for concurrent use
  - Custom functions can be updated while I/O operations are in progress
  - No data races when accessed from multiple goroutines
  - Atomic operations ensure visibility across CPU cores

# Best Practices

1. Minimize allocations in custom functions by reusing the provided buffer
2. Return nil from custom functions to signal errors (results in io.ErrUnexpectedEOF)
3. Use atomic types (atomic.Int64, etc.) for counters in custom functions
4. Chain wrappers for complex transformations instead of one large custom function
5. Reset custom functions to nil when returning to default behavior
6. Be aware that underlying errors are encapsulated, not directly propagated

# See Also

  - io.Reader, io.Writer, io.Seeker, io.Closer: Standard Go I/O interfaces
  - github.com/nabbar/golib/atomic: Thread-safe atomic value storage
  - bufio: For buffered I/O on top of wrappers
*/
package iowrapper