Omit duplicate passwd and group entries in syncuser overlay

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

internal/pkg/overlay/funcmap.go

@@ -130,3 +130,34 @@ func importSoftlink(lnk string) string {
 func softlink(target string) string {
 	return fmt.Sprintf("{{ /* softlink \"%s\" */ }}", target)
 }
+
+// UniqueField returns a filtered version of a multi-line input string. input is
+// expected to be a field-separated format with one record per line (terminated
+// by `\n`). Order of lines is preserved, with the first matching line taking
+// precedence.
+//
+// For example, parsing /etc/passwd filter /etc/passwd for unique user names:
+//
+// Lines without the index field (e.g., blank lines) are always included in the
+// output.
+//
+// UniqueField(":", 0, passwdContent)
+func UniqueField(sep string, index int, input string) string {
+	inputLines := strings.Split(input, "\n")
+	var outputLines []string
+	found := make(map[string]bool)
+	for _, line := range inputLines {
+		inputFields := strings.Split(line, sep)
+		if len(inputFields) > index {
+			field := inputFields[index]
+			if field != "" {
+				if found[field] {
+					continue
+				}
+				found[field] = true
+			}
+		}
+		outputLines = append(outputLines, line)
+	}
+	return strings.Join(outputLines, "\n")
+}

internal/pkg/overlay/funcmap_test.go

@@ -59,3 +59,50 @@ func Test_createIgnitionJson(t *testing.T) {
 	node := nodeInfos[0]
 	assert.JSONEq(t, expected_json, createIgnitionJson(&node))
 }
+
+func Test_UniqueField(t *testing.T) {
+	tests := map[string]struct {
+		input  string
+		sep    string
+		field  int
+		output string
+	}{
+		"empty": {
+			input:  ``,
+			sep:    ":",
+			field:  0,
+			output: ``,
+		},
+
+		"unique input": {
+			input: `
+name1:aaaa
+name2:bbbb
+`,
+			sep:   ":",
+			field: 0,
+			output: `
+name1:aaaa
+name2:bbbb
+`,
+		},
+
+		"duplicate field": {
+			input: `
+name1: aaaa
+name1: bbbb
+`,
+			sep:   ":",
+			field: 0,
+			output: `
+name1: aaaa
+`,
+		},
+	}
+
+	for name, tt := range tests {
+		t.Run(name, func(t *testing.T) {
+			assert.Equal(t, tt.output, UniqueField(tt.sep, tt.field, tt.input))
+		})
+	}
+}

internal/pkg/overlay/overlay.go

@@ -500,6 +500,7 @@ func RenderTemplateFile(fileName string, data TemplateStruct) (
 			backupFile = false
 			return ""
 		},
+		"UniqueField": UniqueField,
 	}
 
 	// Merge sprig.FuncMap with our FuncMap