Only write INI file if changed

internal/mqinimerge/mqinimerge.go

@@ -14,11 +14,12 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-// Package mqini provides information about queue managers
+// Package mqinimerge merges user-supplied INI files into qm.ini and mqat.ini
 package mqinimerge
 
 import (
 	"bufio"
+	"bytes"
 	"errors"
 	"fmt"
 	"io/ioutil"
@@ -318,14 +319,31 @@ func prepareStanzasToMerge(key string, attrList strings.Builder, iniConfigList [
 	return iniConfigList, nil
 }
 
-// writeConfigStanzas writes the INI file updates into corresponding mq ini files.
+// writeFileIfChanged writes the specified data to the specified file path
+// (just like ioutil.WriteFile), but first checks if this is needed
+func writeFileIfChanged(path string, data []byte, perm os.FileMode) error {
+	current, err := ioutil.ReadFile(path)
+	if err != nil {
+		return err
+	}
+	// Only write the new file if the it's different from the current file
+	if !bytes.Equal(current, data) {
+		err = ioutil.WriteFile(path, data, perm)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// writeConfigStanzas writes the INI file updates into corresponding MQ INI files.
 func writeConfigStanzas(qmConfig []string, atConfig []string) error {
-	err := ioutil.WriteFile(filepath.Join(qmgrDir, "qm.ini"), []byte(strings.Join(qmConfig, "\n")), 0644)
+	err := writeFileIfChanged(filepath.Join(qmgrDir, "qm.ini"), []byte(strings.Join(qmConfig, "\n")), 0644)
 	if err != nil {
 		return err
 	}
 
-	err = ioutil.WriteFile(filepath.Join(qmgrDir, "mqat.ini"), []byte(strings.Join(atConfig, "\n")), 0644)
+	err = writeFileIfChanged(filepath.Join(qmgrDir, "mqat.ini"), []byte(strings.Join(atConfig, "\n")), 0644)
 	if err != nil {
 		return err
 	}

internal/mqinimerge/mqinimerge_test.go

@@ -17,9 +17,13 @@ package mqinimerge
 
 import (
 	"bufio"
+	"fmt"
 	"io/ioutil"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestIniFileStanzas(t *testing.T) {
@@ -205,3 +209,48 @@ func checkReturns(stanza string, isqmini bool, shouldexist bool, t *testing.T) {
 		}
 	}
 }
+
+var writeFileIfChangedTests = []struct {
+	before []byte
+	after  []byte
+	same   bool
+}{
+	{[]byte("ABC€"), []byte("ABC€"), true},
+	{[]byte("ABC€"), []byte("ABC$"), false},
+	{[]byte("ABC€"), []byte("BBC€"), false},
+}
+
+func TestWriteFileIfChanged(t *testing.T) {
+	tmpFile := filepath.Join(os.TempDir(), t.Name())
+	t.Logf("Using temp file %v", tmpFile)
+	for i, table := range writeFileIfChangedTests {
+		t.Run(fmt.Sprintf("%v", i), func(t *testing.T) {
+			err := ioutil.WriteFile(tmpFile, table.before, 0600)
+			time.Sleep(time.Second * 1)
+			defer os.Remove(tmpFile)
+			fi, err := os.Stat(tmpFile)
+			if err != nil {
+				t.Fatal(err)
+			}
+			beforeMod := fi.ModTime()
+			err = writeFileIfChanged(tmpFile, table.after, 0600)
+			if err != nil {
+				t.Error(err)
+			}
+			fi, err = os.Stat(tmpFile)
+			if err != nil {
+				t.Error(err)
+			}
+			afterMod := fi.ModTime()
+			if table.same {
+				if beforeMod != afterMod {
+					t.Errorf("Expected file timestamps to be the same (%v); got %v", beforeMod, afterMod)
+				}
+			} else {
+				if beforeMod == afterMod {
+					t.Errorf("Expected file timestamp to be different got %v and %v", beforeMod, afterMod)
+				}
+			}
+		})
+	}
+}