internal/poll: add SPLICE_F_NONBLOCK flag for splice to avoid inconsistency with O_NONBLOCK

For #59041

Details: https://github.com/golang/go/issues/59041#issuecomment-1766610087

Change-Id: Id3fc1df6d86b7c4cc383d09f9465fa8f4cc2a559
Reviewed-on: https://go-review.googlesource.com/c/go/+/536015
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/internal/poll/splice_linux.go

@@ -13,6 +13,12 @@ import (
 )
 
 const (
+	// spliceNonblock doesn't make the splice itself necessarily nonblocking
+	// (because the actual file descriptors that are spliced from/to may block
+	// unless they have the O_NONBLOCK flag set), but it makes the splice pipe
+	// operations nonblocking.
+	spliceNonblock = 0x2
+
 	// maxSpliceSize is the maximum amount of data Splice asks
 	// the kernel to move in a single call to splice(2).
 	// We use 1MB as Splice writes data through a pipe, and 1MB is the default maximum pipe buffer size,
@@ -89,7 +95,11 @@ func spliceDrain(pipefd int, sock *FD, max int) (int, error) {
 		return 0, err
 	}
 	for {
-		n, err := splice(pipefd, sock.Sysfd, max, 0)
+		// In theory calling splice(2) with SPLICE_F_NONBLOCK could end up an infinite loop here,
+		// because it could return EAGAIN ceaselessly when the write end of the pipe is full,
+		// but this shouldn't be a concern here, since the pipe buffer must be sufficient for
+		// this data transmission on the basis of the workflow in Splice.
+		n, err := splice(pipefd, sock.Sysfd, max, spliceNonblock)
 		if err == syscall.EINTR {
 			continue
 		}
@@ -127,7 +137,14 @@ func splicePump(sock *FD, pipefd int, inPipe int) (int, error) {
 	}
 	written := 0
 	for inPipe > 0 {
-		n, err := splice(sock.Sysfd, pipefd, inPipe, 0)
+		// In theory calling splice(2) with SPLICE_F_NONBLOCK could end up an infinite loop here,
+		// because it could return EAGAIN ceaselessly when the read end of the pipe is empty,
+		// but this shouldn't be a concern here, since the pipe buffer must contain inPipe size of
+		// data on the basis of the workflow in Splice.
+		n, err := splice(sock.Sysfd, pipefd, inPipe, spliceNonblock)
+		if err == syscall.EINTR {
+			continue
+		}
 		// Here, the condition n == 0 && err == nil should never be
 		// observed, since Splice controls the write side of the pipe.
 		if n > 0 {