plugin/file: track zone mtime for reload_by_mtime (#8556)

* plugin/file: track zone mtime for reload_by_mtime

Signed-off-by: git-jxj <65210887+git-jxj@users.noreply.github.com>

* plugin/file: capture initial mtime before parsing

Signed-off-by: git-jxj <65210887+git-jxj@users.noreply.github.com>

* plugin/file: keep unloaded zones retryable after open error

Signed-off-by: git-jxj <65210887+git-jxj@users.noreply.github.com>

---------

Signed-off-by: git-jxj <65210887+git-jxj@users.noreply.github.com>

plugin/file/setup_test.go

@@ -1,8 +1,10 @@
 package file
 
 import (
+	"bytes"
 	"context"
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -247,3 +249,131 @@ func TestFileParseRelativeZones(t *testing.T) {
 		}
 	}
 }
+
+func TestFileParseReloadByMtimeInitializesMtime(t *testing.T) {
+	name, rm, err := test.TempFile(".", dbMiekNL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer rm()
+
+	c := caddy.NewTestController("dns", "file "+name+" miek.nl. {\n\treload_by_mtime\n}")
+	zones, _, err := fileParse(c)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	fi, err := os.Stat(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	z := zones.Z["miek.nl."]
+	if z.file_mtime.IsZero() {
+		t.Fatal("file mtime was not initialized for reload_by_mtime")
+	}
+	if !z.file_mtime.Equal(fi.ModTime()) {
+		t.Fatalf("file mtime = %s, want %s", z.file_mtime, fi.ModTime())
+	}
+}
+
+func TestFileParseReloadByMtimeMissingFileLeavesBaselineUnset(t *testing.T) {
+	name := filepath.Join(t.TempDir(), "missing.db")
+	c := caddy.NewTestController("dns", "file "+name+" example.org. {\n\treload_by_mtime\n}")
+	zones, _, err := fileParse(c)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !zones.Z["example.org."].file_mtime.IsZero() {
+		t.Fatal("missing initial file must leave the mtime baseline unset")
+	}
+}
+
+func TestFileParseReloadByMtimeRecoversLaterZoneAfterMissingFile(t *testing.T) {
+	dir := t.TempDir()
+	missing := filepath.Join(dir, "missing.db")
+	existing := filepath.Join(dir, "existing.db")
+	if err := os.WriteFile(existing, []byte(dbMiekNL), 0600); err != nil {
+		t.Fatal(err)
+	}
+
+	corefile := fmt.Sprintf(`file %s example.org. {
+	reload 10ms
+	reload_by_mtime
+}
+file %s miek.nl. {
+	reload 10ms
+	reload_by_mtime
+}`, missing, existing)
+	zones, _, err := fileParse(caddy.NewTestController("dns", corefile))
+	if err != nil {
+		t.Fatal(err)
+	}
+	z := zones.Z["miek.nl."]
+	if z == nil {
+		t.Fatal("later zone was not configured")
+	}
+	if err := z.Reload(nil); err != nil {
+		t.Fatal(err)
+	}
+	defer z.OnShutdown()
+
+	const wantSerial = 1282630057
+	for start := time.Now(); time.Since(start) < 2*time.Second; {
+		if z.SOASerialIfDefined() == wantSerial {
+			return
+		}
+		time.Sleep(2 * time.Millisecond)
+	}
+	t.Fatalf("later zone never loaded: SOA serial = %d, want %d", z.SOASerialIfDefined(), wantSerial)
+}
+
+func TestFileParseReloadByMtimeUsesOpenedFileMtime(t *testing.T) {
+	name, rm, err := test.TempFile(".", dbRelative)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer rm()
+
+	initialMtime := time.Now().Add(-2 * time.Hour).Truncate(time.Second)
+	if err := os.Chtimes(name, initialMtime, initialMtime); err != nil {
+		t.Fatal(err)
+	}
+
+	c := caddy.NewTestController("dns", "file "+name+" example.org. {\n\treload_by_mtime\n}")
+	zones, _, err := fileParseWithParser(c, func(reader io.Reader, origin, fileName string, serial int64) (*Zone, error) {
+		// Simulate the opened reader consuming the old zone before a writer
+		// replaces its contents while the initial load is still in progress.
+		oldContents, err := io.ReadAll(reader)
+		if err != nil {
+			return nil, err
+		}
+		newContents := strings.Replace(dbRelative, "192.0.2.1", "192.0.2.2", 1)
+		if err := os.WriteFile(fileName, []byte(newContents), 0644); err != nil {
+			return nil, err
+		}
+		newMtime := initialMtime.Add(time.Hour)
+		if err := os.Chtimes(fileName, newMtime, newMtime); err != nil {
+			return nil, err
+		}
+		return Parse(bytes.NewReader(oldContents), origin, fileName, serial)
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	z := zones.Z["example.org."]
+	entry, ok := z.Search("foo.example.org.")
+	if !ok || len(entry.Type(dns.TypeA)) != 1 || entry.Type(dns.TypeA)[0].(*dns.A).A.String() != "192.0.2.1" {
+		t.Fatal("initial load did not retain the old zone record")
+	}
+	if !z.file_mtime.Equal(initialMtime) {
+		t.Fatalf("initial mtime = %s, want opened file's %s", z.file_mtime, initialMtime)
+	}
+	fi, err := os.Stat(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !fi.ModTime().After(z.file_mtime) {
+		t.Fatalf("changed zone mtime %s must be newer than loaded baseline %s", fi.ModTime(), z.file_mtime)
+	}
+}