fix(output): route formatter errors through an injectable sink

internal/output/csv.go

@@ -7,7 +7,6 @@ import (
 	"encoding/csv"
 	"fmt"
 	"io"
-	"os"
 )
 
 // FormatAsCSV formats data as CSV (with header) and writes it to w.
@@ -67,10 +66,10 @@ func writeCSVRows(w io.Writer, rows []map[string]string, cols []string, writeHea
 	flushCSV(cw)
 }
 
-// flushCSV flushes the csv.Writer and reports any write error to stderr.
+// flushCSV flushes the csv.Writer and reports any write error to ErrorSink.
 func flushCSV(cw *csv.Writer) {
 	cw.Flush()
 	if err := cw.Error(); err != nil {
-		fmt.Fprintf(os.Stderr, "csv write error: %v\n", err)
+		writeInternalError("csv write error: %v\n", err)
 	}
 }

internal/output/csv_test.go

@@ -5,6 +5,7 @@ package output
 
 import (
 	"bytes"
+	"errors"
 	"strings"
 	"testing"
 )
@@ -150,3 +151,26 @@ func TestFormatAsCSV_SingleObject(t *testing.T) {
 		t.Errorf("output should contain 'Alice', got:\n%s", out)
 	}
 }
+
+type failingCSVWriter struct{}
+
+func (failingCSVWriter) Write(p []byte) (int, error) {
+	return 0, errors.New("boom")
+}
+
+func TestFormatAsCSV_WriteError_UsesErrorSink(t *testing.T) {
+	origSink := ErrorSink
+	var errBuf bytes.Buffer
+	ErrorSink = &errBuf
+	defer func() { ErrorSink = origSink }()
+
+	data := []interface{}{
+		map[string]interface{}{"name": "Alice"},
+	}
+
+	FormatAsCSV(failingCSVWriter{}, data)
+
+	if got := errBuf.String(); got == "" || !strings.Contains(got, "csv write error: boom") {
+		t.Fatalf("expected csv write error in ErrorSink, got: %q", got)
+	}
+}

internal/output/print.go

@@ -12,12 +12,20 @@ import (
 	"github.com/larksuite/cli/internal/validate"
 )
 
+// ErrorSink receives best-effort internal output formatting errors.
+// Tests and embedders may override it to capture these messages.
+var ErrorSink io.Writer = os.Stderr
+
+func writeInternalError(format string, args ...interface{}) {
+	fmt.Fprintf(ErrorSink, format, args...)
+}
+
 // PrintJson prints data as formatted JSON to w.
 func PrintJson(w io.Writer, data interface{}) {
 	injectNotice(data)
 	b, err := json.MarshalIndent(data, "", "  ")
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "json marshal error: %v\n", err)
+		writeInternalError("json marshal error: %v\n", err)
 		return
 	}
 	fmt.Fprintln(w, string(b))
@@ -53,7 +61,7 @@ func PrintNdjson(w io.Writer, data interface{}) {
 	emit := func(item interface{}) {
 		b, err := json.Marshal(item)
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "ndjson marshal error: %v\n", err)
+			writeInternalError("ndjson marshal error: %v\n", err)
 			return
 		}
 		fmt.Fprintln(w, string(b))

internal/output/print_test.go

@@ -99,3 +99,37 @@ func TestPrintJson_NoNotice(t *testing.T) {
 		t.Error("expected no _notice when PendingNotice is nil")
 	}
 }
+
+func TestPrintJson_MarshalError_UsesErrorSink(t *testing.T) {
+	origSink := ErrorSink
+	var errBuf bytes.Buffer
+	ErrorSink = &errBuf
+	defer func() { ErrorSink = origSink }()
+
+	var out bytes.Buffer
+	PrintJson(&out, map[string]interface{}{"bad": make(chan int)})
+
+	if out.Len() != 0 {
+		t.Fatalf("expected no stdout output on marshal error, got: %q", out.String())
+	}
+	if got := errBuf.String(); got == "" || !bytes.Contains(errBuf.Bytes(), []byte("json marshal error:")) {
+		t.Fatalf("expected json marshal error in ErrorSink, got: %q", got)
+	}
+}
+
+func TestPrintNdjson_MarshalError_UsesErrorSink(t *testing.T) {
+	origSink := ErrorSink
+	var errBuf bytes.Buffer
+	ErrorSink = &errBuf
+	defer func() { ErrorSink = origSink }()
+
+	var out bytes.Buffer
+	PrintNdjson(&out, map[string]interface{}{"bad": make(chan int)})
+
+	if out.Len() != 0 {
+		t.Fatalf("expected no stdout output on marshal error, got: %q", out.String())
+	}
+	if got := errBuf.String(); got == "" || !bytes.Contains(errBuf.Bytes(), []byte("ndjson marshal error:")) {
+		t.Fatalf("expected ndjson marshal error in ErrorSink, got: %q", got)
+	}
+}