Support the upgrade flag --reset-then-reuse-values added in Helm v3.14.0 (#634)

* Allow fetching only user-supplied existing values

* Add support for the --reset-then-reuse-values flag

This flag was added in Helm v3.14.0; for details see
helm/helm#9653.

* Document --reset-then-reuse-values in the README

* Add a comment referencing the new flag's origin

* Move min Helm version check to narrower scope

* Fix unintended name shadowing

Author: trk9001

README.md

@@ -103,6 +103,7 @@ Flags:
       --repo string                      specify the chart repository url to locate the requested chart
       --reset-values                     reset the values to the ones built into the chart and merge in any new values
       --reuse-values                     reuse the last release's values and merge in any new values. If '--reset-values' is specified, this is ignored
+      --reset-then-reuse-values          reset the values to the ones built into the chart, apply the last release's values and merge in any new values. If '--reset-values' or '--reuse-values' is specified, this is ignored
       --set stringArray                  set values on the command line (can specify multiple or separate values with commas: key1=val1,key2=val2)
       --set-file stringArray             set values from respective files specified via the command line (can specify multiple or separate values with commas: key1=path1,key2=path2)
       --set-string stringArray           set STRING values on the command line (can specify multiple or separate values with commas: key1=val1,key2=val2)
@@ -198,6 +199,7 @@ Flags:
       --repo string                      specify the chart repository url to locate the requested chart
       --reset-values                     reset the values to the ones built into the chart and merge in any new values
       --reuse-values                     reuse the last release's values and merge in any new values. If '--reset-values' is specified, this is ignored
+      --reset-then-reuse-values          reset the values to the ones built into the chart, apply the last release's values and merge in any new values. If '--reset-values' or '--reuse-values' is specified, this is ignored
       --set stringArray                  set values on the command line (can specify multiple or separate values with commas: key1=val1,key2=val2)
       --set-file stringArray             set values from respective files specified via the command line (can specify multiple or separate values with commas: key1=path1,key2=path2)
       --set-string stringArray           set STRING values on the command line (can specify multiple or separate values with commas: key1=val1,key2=val2)

cmd/helm3.go

@@ -16,8 +16,11 @@ import (
 var (
 	helmVersionRE  = regexp.MustCompile(`Version:\s*"([^"]+)"`)
 	minHelmVersion = semver.MustParse("v3.1.0-rc.1")
-	// See https://github.com/helm/helm/pull/9426
+	// See https://github.com/helm/helm/pull/9426.
 	minHelmVersionWithDryRunLookupSupport = semver.MustParse("v3.13.0")
+	// The --reset-then-reuse-values flag for `helm upgrade` was added in
+	// https://github.com/helm/helm/pull/9653 and released as part of Helm v3.14.0.
+	minHelmVersionWithResetThenReuseValues = semver.MustParse("v3.14.0")
 )
 
 func getHelmVersion() (*semver.Version, error) {
@@ -132,15 +135,29 @@ func (d *diffCmd) template(isUpgrade bool) ([]byte, error) {
 	// Let's simulate that in helm-diff.
 	// See https://medium.com/@kcatstack/understand-helm-upgrade-flags-reset-values-reuse-values-6e58ac8f127e
 	shouldDefaultReusingValues := isUpgrade && len(d.values) == 0 && len(d.stringValues) == 0 && len(d.stringLiteralValues) == 0 && len(d.jsonValues) == 0 && len(d.valueFiles) == 0 && len(d.fileValues) == 0
-	if (d.reuseValues || shouldDefaultReusingValues) && !d.resetValues && d.clusterAccessAllowed() {
+	if (d.reuseValues || d.resetThenReuseValues || shouldDefaultReusingValues) && !d.resetValues && d.clusterAccessAllowed() {
 		tmpfile, err := os.CreateTemp("", "existing-values")
 		if err != nil {
 			return nil, err
 		}
 		defer func() {
 			_ = os.Remove(tmpfile.Name())
 		}()
-		if err := d.writeExistingValues(tmpfile); err != nil {
+		// In the presence of --reuse-values (or --reset-values), --reset-then-reuse-values is ignored.
+		if d.resetThenReuseValues && !d.reuseValues {
+			var supported bool
+			supported, err = isHelmVersionAtLeast(minHelmVersionWithResetThenReuseValues)
+			if err != nil {
+				return nil, err
+			}
+			if !supported {
+				return nil, fmt.Errorf("Using --reset-then-reuse-values requires at least helm version %s", minHelmVersionWithResetThenReuseValues.String())
+			}
+			err = d.writeExistingValues(tmpfile, false)
+		} else {
+			err = d.writeExistingValues(tmpfile, true)
+		}
+		if err != nil {
 			return nil, err
 		}
 		flags = append(flags, "--values", tmpfile.Name())
@@ -308,8 +325,12 @@ func (d *diffCmd) template(isUpgrade bool) ([]byte, error) {
 	return filter(out), err
 }
 
-func (d *diffCmd) writeExistingValues(f *os.File) error {
-	cmd := exec.Command(os.Getenv("HELM_BIN"), "get", "values", d.release, "--all", "--output", "yaml")
+func (d *diffCmd) writeExistingValues(f *os.File, all bool) error {
+	args := []string{"get", "values", d.release, "--output", "yaml"}
+	if all {
+		args = append(args, "--all")
+	}
+	cmd := exec.Command(os.Getenv("HELM_BIN"), args...)
 	debugPrint("Executing %s", strings.Join(cmd.Args, " "))
 	defer func() {
 		_ = f.Close()

cmd/upgrade.go

@@ -56,6 +56,7 @@ type diffCmd struct {
 	fileValues               []string
 	reuseValues              bool
 	resetValues              bool
+	resetThenReuseValues     bool
 	allowUnreleased          bool
 	noHooks                  bool
 	includeTests             bool
@@ -242,6 +243,7 @@ func newChartCommand() *cobra.Command {
 	f.StringArrayVar(&diff.fileValues, "set-file", []string{}, "set values from respective files specified via the command line (can specify multiple or separate values with commas: key1=path1,key2=path2)")
 	f.BoolVar(&diff.reuseValues, "reuse-values", false, "reuse the last release's values and merge in any new values. If '--reset-values' is specified, this is ignored")
 	f.BoolVar(&diff.resetValues, "reset-values", false, "reset the values to the ones built into the chart and merge in any new values")
+	f.BoolVar(&diff.resetThenReuseValues, "reset-then-reuse-values", false, "reset the values to the ones built into the chart, apply the last release's values and merge in any new values. If '--reset-values' or '--reuse-values' is specified, this is ignored")
 	f.BoolVar(&diff.allowUnreleased, "allow-unreleased", false, "enables diffing of releases that are not yet deployed via Helm")
 	f.BoolVar(&diff.install, "install", false, "enables diffing of releases that are not yet deployed via Helm (equivalent to --allow-unreleased, added to match \"helm upgrade --install\" command")
 	f.BoolVar(&diff.noHooks, "no-hooks", false, "disable diffing of hooks")