feat(process): add Logger interface for exec wrapper (#93)

- Define Logger interface with Debug and Error methods
- Add NopLogger as default (no-op implementation)
- Add SetDefaultLogger/DefaultLogger for package-level config
- Add WithLogger method for per-command logger injection
- Log commands at DEBUG level before execution
- Log failures at ERROR level with error details
- Add comprehensive tests for logger functionality

Compatible with pkg/log.Logger and other structured loggers.

Closes #90

Co-authored-by: Claude <noreply@anthropic.com>

pkg/process/exec/exec.go

@@ -37,6 +37,7 @@ type Cmd struct {
 	ctx    context.Context
 	opts   Options
 	cmd    *exec.Cmd
+	logger Logger
 }
 
 // WithDir sets the working directory
@@ -69,17 +70,23 @@ func (c *Cmd) WithStderr(w io.Writer) *Cmd {
 	return c
 }
 
+// WithLogger sets a custom logger for this command.
+// If not set, the package default logger is used.
+func (c *Cmd) WithLogger(l Logger) *Cmd {
+	c.logger = l
+	return c
+}
+
 // Run executes the command and waits for it to finish.
 // It automatically logs the command execution at debug level.
 func (c *Cmd) Run() error {
 	c.prepare()
-
-	// TODO: Use a proper logger interface when available in pkg/process
-	// For now using cli.Debug which might not be visible unless verbose
-	// cli.Debug("Executing: %s %s", c.name, strings.Join(c.args, " "))
+	c.logDebug("executing command")
 
 	if err := c.cmd.Run(); err != nil {
-		return wrapError(err, c.name, c.args)
+		wrapped := wrapError(err, c.name, c.args)
+		c.logError("command failed", wrapped)
+		return wrapped
 	}
 	return nil
 }
@@ -87,9 +94,13 @@ func (c *Cmd) Run() error {
 // Output runs the command and returns its standard output.
 func (c *Cmd) Output() ([]byte, error) {
 	c.prepare()
+	c.logDebug("executing command")
+
 	out, err := c.cmd.Output()
 	if err != nil {
-		return nil, wrapError(err, c.name, c.args)
+		wrapped := wrapError(err, c.name, c.args)
+		c.logError("command failed", wrapped)
+		return nil, wrapped
 	}
 	return out, nil
 }
@@ -97,9 +108,13 @@ func (c *Cmd) Output() ([]byte, error) {
 // CombinedOutput runs the command and returns its combined standard output and standard error.
 func (c *Cmd) CombinedOutput() ([]byte, error) {
 	c.prepare()
+	c.logDebug("executing command")
+
 	out, err := c.cmd.CombinedOutput()
 	if err != nil {
-		return out, wrapError(err, c.name, c.args)
+		wrapped := wrapError(err, c.name, c.args)
+		c.logError("command failed", wrapped)
+		return out, wrapped
 	}
 	return out, nil
 }
@@ -144,3 +159,18 @@ func wrapError(err error, name string, args []string) error {
 	}
 	return fmt.Errorf("failed to execute %q: %w", cmdStr, err)
 }
+
+func (c *Cmd) getLogger() Logger {
+	if c.logger != nil {
+		return c.logger
+	}
+	return defaultLogger
+}
+
+func (c *Cmd) logDebug(msg string) {
+	c.getLogger().Debug(msg, "cmd", c.name, "args", strings.Join(c.args, " "))
+}
+
+func (c *Cmd) logError(msg string, err error) {
+	c.getLogger().Error(msg, "cmd", c.name, "args", strings.Join(c.args, " "), "err", err)
+}

pkg/process/exec/exec_test.go

@@ -0,0 +1,148 @@
+package exec_test
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"github.com/host-uk/core/pkg/process/exec"
+)
+
+// mockLogger captures log calls for testing
+type mockLogger struct {
+	debugCalls []logCall
+	errorCalls []logCall
+}
+
+type logCall struct {
+	msg     string
+	keyvals []any
+}
+
+func (m *mockLogger) Debug(msg string, keyvals ...any) {
+	m.debugCalls = append(m.debugCalls, logCall{msg, keyvals})
+}
+
+func (m *mockLogger) Error(msg string, keyvals ...any) {
+	m.errorCalls = append(m.errorCalls, logCall{msg, keyvals})
+}
+
+func TestCommand_Run_Good_LogsDebug(t *testing.T) {
+	logger := &mockLogger{}
+	ctx := context.Background()
+
+	err := exec.Command(ctx, "echo", "hello").
+		WithLogger(logger).
+		Run()
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if len(logger.debugCalls) != 1 {
+		t.Fatalf("expected 1 debug call, got %d", len(logger.debugCalls))
+	}
+	if logger.debugCalls[0].msg != "executing command" {
+		t.Errorf("expected msg 'executing command', got %q", logger.debugCalls[0].msg)
+	}
+	if len(logger.errorCalls) != 0 {
+		t.Errorf("expected no error calls, got %d", len(logger.errorCalls))
+	}
+}
+
+func TestCommand_Run_Bad_LogsError(t *testing.T) {
+	logger := &mockLogger{}
+	ctx := context.Background()
+
+	err := exec.Command(ctx, "false").
+		WithLogger(logger).
+		Run()
+	if err == nil {
+		t.Fatal("expected error")
+	}
+
+	if len(logger.debugCalls) != 1 {
+		t.Fatalf("expected 1 debug call, got %d", len(logger.debugCalls))
+	}
+	if len(logger.errorCalls) != 1 {
+		t.Fatalf("expected 1 error call, got %d", len(logger.errorCalls))
+	}
+	if logger.errorCalls[0].msg != "command failed" {
+		t.Errorf("expected msg 'command failed', got %q", logger.errorCalls[0].msg)
+	}
+}
+
+func TestCommand_Output_Good(t *testing.T) {
+	logger := &mockLogger{}
+	ctx := context.Background()
+
+	out, err := exec.Command(ctx, "echo", "test").
+		WithLogger(logger).
+		Output()
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if strings.TrimSpace(string(out)) != "test" {
+		t.Errorf("expected 'test', got %q", string(out))
+	}
+	if len(logger.debugCalls) != 1 {
+		t.Errorf("expected 1 debug call, got %d", len(logger.debugCalls))
+	}
+}
+
+func TestCommand_CombinedOutput_Good(t *testing.T) {
+	logger := &mockLogger{}
+	ctx := context.Background()
+
+	out, err := exec.Command(ctx, "echo", "combined").
+		WithLogger(logger).
+		CombinedOutput()
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if strings.TrimSpace(string(out)) != "combined" {
+		t.Errorf("expected 'combined', got %q", string(out))
+	}
+	if len(logger.debugCalls) != 1 {
+		t.Errorf("expected 1 debug call, got %d", len(logger.debugCalls))
+	}
+}
+
+func TestNopLogger(t *testing.T) {
+	// Verify NopLogger doesn't panic
+	var nop exec.NopLogger
+	nop.Debug("msg", "key", "val")
+	nop.Error("msg", "key", "val")
+}
+
+func TestSetDefaultLogger(t *testing.T) {
+	original := exec.DefaultLogger()
+	defer exec.SetDefaultLogger(original)
+
+	logger := &mockLogger{}
+	exec.SetDefaultLogger(logger)
+
+	if exec.DefaultLogger() != logger {
+		t.Error("default logger not set correctly")
+	}
+
+	// Test nil resets to NopLogger
+	exec.SetDefaultLogger(nil)
+	if _, ok := exec.DefaultLogger().(exec.NopLogger); !ok {
+		t.Error("expected NopLogger when setting nil")
+	}
+}
+
+func TestCommand_UsesDefaultLogger(t *testing.T) {
+	original := exec.DefaultLogger()
+	defer exec.SetDefaultLogger(original)
+
+	logger := &mockLogger{}
+	exec.SetDefaultLogger(logger)
+
+	ctx := context.Background()
+	_ = exec.Command(ctx, "echo", "test").Run()
+
+	if len(logger.debugCalls) != 1 {
+		t.Errorf("expected default logger to receive 1 debug call, got %d", len(logger.debugCalls))
+	}
+}

pkg/process/exec/logger.go

@@ -0,0 +1,30 @@
+package exec
+
+// Logger interface for command execution logging.
+// Compatible with pkg/log.Logger and other structured loggers.
+type Logger interface {
+	Debug(msg string, keyvals ...any)
+	Error(msg string, keyvals ...any)
+}
+
+// NopLogger is a no-op logger that discards all messages.
+type NopLogger struct{}
+
+func (NopLogger) Debug(string, ...any) {}
+func (NopLogger) Error(string, ...any) {}
+
+var defaultLogger Logger = NopLogger{}
+
+// SetDefaultLogger sets the package-level default logger.
+// Commands without an explicit logger will use this.
+func SetDefaultLogger(l Logger) {
+	if l == nil {
+		l = NopLogger{}
+	}
+	defaultLogger = l
+}
+
+// DefaultLogger returns the current default logger.
+func DefaultLogger() Logger {
+	return defaultLogger
+}