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// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Flow control
package http2
// inflowMinRefresh is the minimum number of bytes we'll send for a
// flow control window update.
const inflowMinRefresh = 4 << 10
// inflow accounts for an inbound flow control window.
// It tracks both the latest window sent to the peer (used for enforcement)
// and the accumulated unsent window.
type inflow struct {
avail int32
unsent int32
}
// init sets the initial window.
func (f *inflow) init(n int32) {
f.avail = n
}
// add adds n bytes to the window, with a maximum window size of max,
// indicating that the peer can now send us more data.
// For example, the user read from a {Request,Response} body and consumed
// some of the buffered data, so the peer can now send more.
// It returns the number of bytes to send in a WINDOW_UPDATE frame to the peer.
// Window updates are accumulated and sent when the unsent capacity
// is at least inflowMinRefresh or will at least double the peer's available window.
func (f *inflow) add(n int) (connAdd int32) {
if n < 0 {
panic("negative update")
}
unsent := int64(f.unsent) + int64(n)
// "A sender MUST NOT allow a flow-control window to exceed 2^31-1 octets."
// RFC 7540 Section 6.9.1.
const maxWindow = 1<<31 - 1
if unsent+int64(f.avail) > maxWindow {
panic("flow control update exceeds maximum window size")
}
f.unsent = int32(unsent)
if f.unsent < inflowMinRefresh && f.unsent < f.avail {
// If there aren't at least inflowMinRefresh bytes of window to send,
// and this update won't at least double the window, buffer the update for later.
return 0
}
f.avail += f.unsent
f.unsent = 0
return int32(unsent)
}
// take attempts to take n bytes from the peer's flow control window.
// It reports whether the window has available capacity.
func (f *inflow) take(n uint32) bool {
if n > uint32(f.avail) {
return false
}
f.avail -= int32(n)
return true
}
// takeInflows attempts to take n bytes from two inflows,
// typically connection-level and stream-level flows.
// It reports whether both windows have available capacity.
func takeInflows(f1, f2 *inflow, n uint32) bool {
if n > uint32(f1.avail) || n > uint32(f2.avail) {
return false
}
f1.avail -= int32(n)
f2.avail -= int32(n)
return true
}
// outflow is the outbound flow control window's size.
type outflow struct {
_ incomparable
// n is the number of DATA bytes we're allowed to send.
// An outflow is kept both on a conn and a per-stream.
n int32
// conn points to the shared connection-level outflow that is
// shared by all streams on that conn. It is nil for the outflow
// that's on the conn directly.
conn *outflow
}
func (f *outflow) setConnFlow(cf *outflow) { f.conn = cf }
func (f *outflow) available() int32 {
n := f.n
if f.conn != nil && f.conn.n < n {
n = f.conn.n
}
return n
}
func (f *outflow) take(n int32) {
if n > f.available() {
panic("internal error: took too much")
}
f.n -= n
if f.conn != nil {
f.conn.n -= n
}
}
// add adds n bytes (positive or negative) to the flow control window.
// It returns false if the sum would exceed 2^31-1.
func (f *outflow) add(n int32) bool {
sum := f.n + n
if (sum > n) == (f.n > 0) {
f.n = sum
return true
}
return false
}
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