Response size clamp (#786)

*) Limit response http body or json response size to FN_MAX_RESPONSE_SIZE (default unlimited)
*) If limits are exceeded 502 is returned with 'body too large' in the error message

api/common/io_utils.go

@@ -0,0 +1,34 @@
+package common
+
+import (
+	"io"
+)
+
+type clampWriter struct {
+	w           io.Writer
+	remaining   int64
+	overflowErr error
+}
+
+func NewClampWriter(buf io.Writer, maxResponseSize uint64, overflowErr error) io.Writer {
+	if maxResponseSize != 0 {
+		return &clampWriter{w: buf, remaining: int64(maxResponseSize), overflowErr: overflowErr}
+	}
+	return buf
+}
+
+func (g *clampWriter) Write(p []byte) (int, error) {
+	if g.remaining <= 0 {
+		return 0, g.overflowErr
+	}
+	if int64(len(p)) > g.remaining {
+		p = p[0:g.remaining]
+	}
+
+	n, err := g.w.Write(p)
+	g.remaining -= int64(n)
+	if g.remaining <= 0 {
+		err = g.overflowErr
+	}
+	return n, err
+}