Make sure scrollbars have the right size initially

We refresh the view after reading just enough to fill it, so that we see the
initial content as quickly as possible, but then we continue reading enough
lines so that we can tell how long the scrollbar needs to be, and then we
refresh again. This can result in slight flicker of the scrollbar when it is
first drawn with a bigger size and then jumps to a smaller size; however, that's
a good tradeoff for a solution that provides both good speed and accuracy.

pkg/tasks/tasks_test.go

@@ -4,6 +4,8 @@ import (
 	"bytes"
 	"io"
 	"os/exec"
+	"reflect"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -45,7 +47,7 @@ func TestNewCmdTaskInstantStop(t *testing.T) {
 		return cmd, reader
 	}
 
-	fn := manager.NewCmdTask(start, "prefix\n", 20, onDone)
+	fn := manager.NewCmdTask(start, "prefix\n", LinesToRead{20, -1}, onDone)
 
 	_ = fn(stop)
 
@@ -99,7 +101,7 @@ func TestNewCmdTask(t *testing.T) {
 		return cmd, reader
 	}
 
-	fn := manager.NewCmdTask(start, "prefix\n", 20, onDone)
+	fn := manager.NewCmdTask(start, "prefix\n", LinesToRead{20, -1}, onDone)
 	wg := sync.WaitGroup{}
 	wg.Add(1)
 	go func() {
@@ -134,3 +136,111 @@ func TestNewCmdTask(t *testing.T) {
 		t.Errorf("expected writer to receive the following content: \n%s\n. But instead it received: %s", expectedContent, actualContent)
 	}
 }
+
+// A dummy reader that simply yields as many blank lines as requested. The only
+// thing we want to do with the output is count the number of lines.
+type BlankLineReader struct {
+	totalLinesToYield int
+	linesYielded      int
+}
+
+func (d *BlankLineReader) Read(p []byte) (n int, err error) {
+	if d.totalLinesToYield == d.linesYielded {
+		return 0, io.EOF
+	}
+
+	d.linesYielded += 1
+	p[0] = '\n'
+	return 1, nil
+}
+
+func TestNewCmdTaskRefresh(t *testing.T) {
+	type scenario struct {
+		name                        string
+		totalTaskLines              int
+		linesToRead                 LinesToRead
+		expectedLineCountsOnRefresh []int
+	}
+
+	scenarios := []scenario{
+		{
+			"total < initialRefreshAfter",
+			150,
+			LinesToRead{100, 120},
+			[]int{100, 100},
+		},
+		{
+			"total == initialRefreshAfter",
+			150,
+			LinesToRead{100, 100},
+			[]int{100, 100, 100},
+		},
+		{
+			"total > initialRefreshAfter",
+			150,
+			LinesToRead{100, 50},
+			[]int{50, 100, 100},
+		},
+		{
+			"initialRefreshAfter == -1",
+			150,
+			LinesToRead{100, -1},
+			[]int{100, 100},
+		},
+		{
+			"totalTaskLines < initialRefreshAfter",
+			25,
+			LinesToRead{100, 50},
+			[]int{25},
+		},
+		{
+			"totalTaskLines between total and initialRefreshAfter",
+			75,
+			LinesToRead{100, 50},
+			[]int{50, 75},
+		},
+	}
+
+	for _, s := range scenarios {
+		writer := bytes.NewBuffer(nil)
+		lineCountsOnRefresh := []int{}
+		refreshView := func() {
+			lineCountsOnRefresh = append(lineCountsOnRefresh, strings.Count(writer.String(), "\n"))
+		}
+
+		manager := NewViewBufferManager(
+			utils.NewDummyLog(),
+			writer,
+			func() {},
+			refreshView,
+			func() {},
+			func() {},
+		)
+
+		stop := make(chan struct{})
+		reader := BlankLineReader{totalLinesToYield: s.totalTaskLines}
+		start := func() (*exec.Cmd, io.Reader) {
+			// not actually starting this because it's not necessary
+			cmd := secureexec.Command("blah blah")
+
+			return cmd, &reader
+		}
+
+		fn := manager.NewCmdTask(start, "", s.linesToRead, func() {})
+		wg := sync.WaitGroup{}
+		wg.Add(1)
+		go func() {
+			time.Sleep(100 * time.Millisecond)
+			close(stop)
+			wg.Done()
+		}()
+		_ = fn(stop)
+
+		wg.Wait()
+
+		if !reflect.DeepEqual(lineCountsOnRefresh, s.expectedLineCountsOnRefresh) {
+			t.Errorf("%s: expected line counts on refresh: %v, got %v",
+				s.name, s.expectedLineCountsOnRefresh, lineCountsOnRefresh)
+		}
+	}
+}