fix: Fix needs to work for upgrades and when selectors are provided (#922)

* fix: Fix `needs` to work for upgrades and when selectors are provided

Fixes #919

* Add test framework for `helmfile apply`

* Various enhancements and fixes to the DAG support

- Make the order of upgrades/deletes more deterministic for testability
- Fix the test framework so that we can validate log outputs and errors
- Add more test cases for `helmfile apply`, along with bug fixes.
- Make sure it fails with an intuitive error when you have non-existent releases referenced from witin "needs"

pkg/testhelper/diff.go

@@ -0,0 +1,103 @@
+package testhelper
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"math"
+	"strings"
+
+	"github.com/aryann/difflib"
+)
+
+func Diff(want, got string, context int) (string, bool) {
+	records := difflib.Diff(
+		strings.Split(want, "\n"),
+		strings.Split(got, "\n"),
+	)
+
+	w := &bytes.Buffer{}
+
+	changed := checkAndPrintRecords(w, records, context)
+
+	return w.String(), changed
+}
+
+func checkAndPrintRecords(w io.Writer, records []difflib.DiffRecord, context int) bool {
+	var changed bool
+	if context >= 0 {
+		distances := calculateDistances(records)
+		omitting := false
+		for i, diff := range records {
+			if diff.Delta != difflib.Common {
+				changed = true
+			}
+			if distances[i] > context {
+				if !omitting {
+					fmt.Fprintln(w, "...")
+					omitting = true
+				}
+			} else {
+				omitting = false
+				fmt.Fprintln(w, formatRecord(diff))
+			}
+		}
+	} else {
+		for _, diff := range records {
+			if diff.Delta != difflib.Common {
+				changed = true
+			}
+			fmt.Fprintln(w, formatRecord(diff))
+		}
+	}
+	return changed
+}
+
+func formatRecord(diff difflib.DiffRecord) string {
+	var prefix string
+	switch diff.Delta {
+	case difflib.RightOnly:
+		prefix = "+ "
+	case difflib.LeftOnly:
+		prefix = "- "
+	case difflib.Common:
+		prefix = "  "
+	}
+
+	return prefix + diff.Payload
+}
+
+// Shamelessly and thankfully copied from https://github.com/databus23/helm-diff/blob/99b8474af7726ca6f57b37b0b8b8f3cd36c991e8/diff/diff.go#L116
+func calculateDistances(diffs []difflib.DiffRecord) map[int]int {
+	distances := map[int]int{}
+
+	// Iterate forwards through diffs, set 'distance' based on closest 'change' before this line
+	change := -1
+	for i, diff := range diffs {
+		if diff.Delta != difflib.Common {
+			change = i
+		}
+		distance := math.MaxInt32
+		if change != -1 {
+			distance = i - change
+		}
+		distances[i] = distance
+	}
+
+	// Iterate backwards through diffs, reduce 'distance' based on closest 'change' after this line
+	change = -1
+	for i := len(diffs) - 1; i >= 0; i-- {
+		diff := diffs[i]
+		if diff.Delta != difflib.Common {
+			change = i
+		}
+		if change != -1 {
+			distance := change - i
+			if distance < distances[i] {
+				distances[i] = distance
+			}
+		}
+	}
+
+	return distances
+}