🔥 Feature (v3): Add buffered streaming support

ctx.go

@@ -5,6 +5,7 @@
 package fiber
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"crypto/tls"
@@ -1671,6 +1672,13 @@ func (c *DefaultCtx) SendStream(stream io.Reader, size ...int) error {
 	return nil
 }
 
+// SendStreamWriter sets response body stream writer
+func (c *DefaultCtx) SendStreamWriter(streamWriter func(*bufio.Writer)) error {
+	c.fasthttp.Response.SetBodyStreamWriter(fasthttp.StreamWriter(streamWriter))
+
+	return nil
+}
+
 // Set sets the response's HTTP header field to the specified key, value.
 func (c *DefaultCtx) Set(key, val string) {
 	c.fasthttp.Response.Header.Set(key, val)

ctx_test.go

@@ -24,6 +24,7 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 	"text/template"
 	"time"
@@ -4447,6 +4448,73 @@ func Test_Ctx_SendStream(t *testing.T) {
 	require.Equal(t, "Hello bufio", string(c.Response().Body()))
 }
 
+// go test -run Test_Ctx_SendStreamWriter
+func Test_Ctx_SendStreamWriter(t *testing.T) {
+	t.Parallel()
+	app := New()
+	c := app.AcquireCtx(&fasthttp.RequestCtx{})
+
+	err := c.SendStreamWriter(func(w *bufio.Writer) {
+		w.WriteString("Don't crash please") //nolint:errcheck, revive // It is fine to ignore the error
+	})
+	require.NoError(t, err)
+	require.Equal(t, "Don't crash please", string(c.Response().Body()))
+
+	err = c.SendStreamWriter(func(w *bufio.Writer) {
+		for lineNum := 1; lineNum <= 5; lineNum++ {
+			fmt.Fprintf(w, "Line %d\n", lineNum) //nolint:errcheck, revive // It is fine to ignore the error
+			if err := w.Flush(); err != nil {
+				t.Errorf("unexpected error: %s", err)
+				return
+			}
+		}
+	})
+	require.NoError(t, err)
+	require.Equal(t, "Line 1\nLine 2\nLine 3\nLine 4\nLine 5\n", string(c.Response().Body()))
+
+	err = c.SendStreamWriter(func(_ *bufio.Writer) {})
+	require.NoError(t, err)
+	require.Empty(t, c.Response().Body())
+}
+
+// go test -run Test_Ctx_SendStreamWriter_Interrupted
+func Test_Ctx_SendStreamWriter_Interrupted(t *testing.T) {
+	t.Parallel()
+	app := New()
+	c := app.AcquireCtx(&fasthttp.RequestCtx{})
+
+	var mutex sync.Mutex
+	startChan := make(chan bool)
+	interruptStreamWriter := func() {
+		<-startChan
+		time.Sleep(5 * time.Millisecond)
+		mutex.Lock()
+		c.Response().CloseBodyStream() //nolint:errcheck // It is fine to ignore the error
+		mutex.Unlock()
+	}
+	err := c.SendStreamWriter(func(w *bufio.Writer) {
+		go interruptStreamWriter()
+
+		startChan <- true
+		for lineNum := 1; lineNum <= 5; lineNum++ {
+			mutex.Lock()
+			fmt.Fprintf(w, "Line %d\n", lineNum) //nolint:errcheck, revive // It is fine to ignore the error
+			mutex.Unlock()
+
+			if err := w.Flush(); err != nil {
+				if lineNum < 3 {
+					t.Errorf("unexpected error: %s", err)
+				}
+				return
+			}
+
+			time.Sleep(1500 * time.Microsecond)
+		}
+	})
+	require.NoError(t, err)
+	require.Equal(t, "Line 1\nLine 2\nLine 3\n", string(c.Response().Body()))
+}
+
 // go test -run Test_Ctx_Set
 func Test_Ctx_Set(t *testing.T) {
 	t.Parallel()

docs/api/ctx.md

@@ -1852,6 +1852,47 @@ app.Get("/", func(c fiber.Ctx) error {
 })
 ```
 
+## SendStreamWriter
+
+Sets the response body stream writer.
+
+:::note
+The argument `streamWriter` represents a function that populates
+the response body using a buffered stream writer.
+:::
+
+```go title="Signature"
+func (c Ctx) SendStreamWriter(streamWriter func(*bufio.Writer)) error
+```
+
+```go title="Example"
+app.Get("/", func (c fiber.Ctx) error {
+  return c.SendStreamWriter(func(w *bufio.Writer) {
+    fmt.Fprintf(w, "Hello, World!\n")
+  })
+  // => "Hello, World!"
+})
+```
+
+:::info
+To flush data before the function returns, you can call `w.Flush()`
+on the provided writer. Otherwise, the buffered stream flushes after
+`streamWriter` returns.
+:::
+
+```go title="Example"
+app.Get("/wait", func(c fiber.Ctx) error {
+  return c.SendStreamWriter(func(w *bufio.Writer) {
+    fmt.Fprintf(w, "Waiting for 10 seconds\n")
+    if err := w.Flush(); err != nil {
+      log.Print("User quit early")
+    }
+    time.Sleep(10 * time.Second)
+    fmt.Fprintf(w, "Done!\n")
+  })
+})
+```
+
 ## Set
 
 Sets the response’s HTTP header field to the specified `key`, `value`.