Attempt to fix TestPipeWritesAccumulate

Test fails because it's reading 4KB instead of the
expected 64KB. Changed the test to read pipe buffer
size instead of hardcode and added some logging in
case the reason for failure was not pipe buffer size.

PiperOrigin-RevId: 253916040

pkg/sentry/fs/fdpipe/pipe_test.go

@@ -358,9 +358,9 @@ func TestPipeReadAheadBuffer(t *testing.T) {
 	}
 }
 
-// This is very important for pipes in general because they can return EWOULDBLOCK and for
-// those that block they must continue until they have read all of the data (and report it
-// as such.
+// This is very important for pipes in general because they can return
+// EWOULDBLOCK and for those that block they must continue until they have read
+// all of the data (and report it as such).
 func TestPipeReadsAccumulate(t *testing.T) {
 	fds := make([]int, 2)
 	if err := syscall.Pipe(fds); err != nil {
@@ -393,8 +393,8 @@ func TestPipeReadsAccumulate(t *testing.T) {
 		t.Fatalf("write to pipe got (%d, %v), want (%d, nil)", n, err, len(data))
 	}
 
-	// Construct a segment vec that is a bit more than we have written so we trigger
-	// an EWOULDBLOCK.
+	// Construct a segment vec that is a bit more than we have written so we
+	// trigger an EWOULDBLOCK.
 	wantBytes := len(data) + 1
 	readBuffer := make([]byte, wantBytes)
 	iov := usermem.BytesIOSequence(readBuffer)
@@ -446,8 +446,8 @@ func TestPipeWritesAccumulate(t *testing.T) {
 		wfile.Close()
 		t.Fatalf("newPipeOperations got error %v, want nil", err)
 	}
-	// Don't forget to remove the fd from the fd notifier.  Otherwise other tests will
-	// likely be borked, because it's global :(
+	// Don't forget to remove the fd from the fd notifier. Otherwise other tests
+	// will likely be borked, because it's global :(
 	defer p.Release()
 
 	inode := fs.NewMockInode(ctx, fs.NewMockMountSource(nil), fs.StableAttr{
@@ -455,32 +455,48 @@ func TestPipeWritesAccumulate(t *testing.T) {
 	})
 	file := fs.NewFile(ctx, fs.NewDirent(ctx, inode, "pipe"), fs.FileFlags{Read: true}, p)
 
-	// Construct a segment vec that is larger than the pipe size to trigger an EWOULDBLOCK.
-	wantBytes := 65536 * 2
+	pipeSize, _, errno := syscall.Syscall(syscall.SYS_FCNTL, uintptr(wfile.FD()), syscall.F_GETPIPE_SZ, 0)
+	if errno != 0 {
+		t.Fatalf("fcntl(F_GETPIPE_SZ) failed: %v", errno)
+	}
+	t.Logf("Pipe buffer size: %d", pipeSize)
+
+	// Construct a segment vec that is larger than the pipe size to trigger an
+	// EWOULDBLOCK.
+	wantBytes := int(pipeSize) * 2
 	writeBuffer := make([]byte, wantBytes)
 	for i := 0; i < wantBytes; i++ {
 		writeBuffer[i] = 'a'
 	}
 	iov := usermem.BytesIOSequence(writeBuffer)
 	n, err := p.Write(ctx, file, iov, 0)
-	total := n
-	iov = iov.DropFirst64(n)
 	if err != syserror.ErrWouldBlock {
 		t.Fatalf("Writev got error %v, want %v", err, syserror.ErrWouldBlock)
 	}
+	if n != int64(pipeSize) {
+		t.Fatalf("Writev partial write, got: %v, want %v", n, pipeSize)
+	}
+	total := n
+	iov = iov.DropFirst64(n)
 
 	// Read the entire pipe buf size to make space for the second half.
-	throwAway := make([]byte, 65536)
-	if n, err := syscall.Read(fds[0], throwAway); n != len(throwAway) || err != nil {
-		t.Fatalf("write to pipe got (%d, %v), want (%d, nil)", n, err, len(throwAway))
+	readBuffer := make([]byte, n)
+	if n, err := syscall.Read(fds[0], readBuffer); n != len(readBuffer) || err != nil {
+		t.Fatalf("write to pipe got (%d, %v), want (%d, nil)", n, err, len(readBuffer))
+	}
+	if !bytes.Equal(readBuffer, writeBuffer[:len(readBuffer)]) {
+		t.Fatalf("wrong data read from pipe, got: %v, want: %v", readBuffer, writeBuffer)
 	}
 
 	// This time we should not block.
 	n, err = p.Write(ctx, file, iov, 0)
-	total += n
 	if err != nil {
 		t.Fatalf("Writev got error %v, want nil", err)
 	}
+	if n != int64(pipeSize) {
+		t.Fatalf("Writev partial write, got: %v, want %v", n, pipeSize)
+	}
+	total += n
 
 	// Assert that the result we got back is cumulative.
 	if total != int64(wantBytes) {