os, net, internal/poll: combine unix sendfile implementations

The internal/poll/sendfile_{bsd,linux,solaris}.go implementations
have more in common than not. Combine into a single sendfile_unix.go.

The net and os packages have redundant code dealing with sendfile
quirks on non-Linux Unix systems, such as the need to determine the
size of the source file before sending. Move the common code into
internal/poll.

Remove some obsolete or incorrect behaviors:

Drop the maximum sendfile chunk size. If we ask the kernel
to copy more data than it is willing to send, it'll copy up to
its limit.

There was a comment in net/sendfile_unix_alt.go indicating that
copying more bytes than a file contains results in the kernel
looping back to the start of the file. I am unable to replicate
this behavior anywhere. Dropped the comment, the workarounds,
and added a test covering this case.

Darwin, Dragonfly, and FreeBSD all support copying the entire
contents of a file by passing 0 for the copy limit.
Take advantage of this.

Change-Id: I9f707ac7a27c165020ae02a6b5bb8f6f16f3c530
Reviewed-on: https://go-review.googlesource.com/c/go/+/621416
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/src/net/sendfile_test.go b/src/net/sendfile_test.go
index 4f3411565b..64b33a54e2 100644
--- a/src/net/sendfile_test.go
+++ b/src/net/sendfile_test.go
@@ -69,7 +69,10 @@ func expectSendfile(t *testing.T, wantConn Conn, f func()) {
 	}
 }
 
-func TestSendfile(t *testing.T) {
+func TestSendfile(t *testing.T)                        { testSendfile(t, 0) }
+func TestSendfileWithExactLimit(t *testing.T)          { testSendfile(t, newtonLen) }
+func TestSendfileWithLimitLargerThanFile(t *testing.T) { testSendfile(t, newtonLen*2) }
+func testSendfile(t *testing.T, limit int64) {
 	ln := newLocalListener(t, "tcp")
 	defer ln.Close()
 
@@ -104,7 +107,14 @@ func TestSendfile(t *testing.T) {
 				sbytes, err = io.Copy(conn, f)
 			default:
 				expectSendfile(t, conn, func() {
-					sbytes, err = io.Copy(conn, f)
+					if limit > 0 {
+						sbytes, err = io.CopyN(conn, f, limit)
+						if err == io.EOF && limit > newtonLen {
+							err = nil
+						}
+					} else {
+						sbytes, err = io.Copy(conn, f)
+					}
 				})
 			}
 			if err != nil {