v0.18.2 resume rotated files instead of replay, keep quiet SSE alive, refuse HEAD on strem path

internal/source/file/file_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"os"
 	"path/filepath"
+	"syscall"
 	"testing"
 	"time"
 
@@ -44,3 +45,40 @@ func TestRemoveWatcherPreservesReplacement(t *testing.T) {
 		t.Fatal("finished watcher was not removed")
 	}
 }
+
+// A rotated file reappears under its archive name with the same inode. Its
+// replacement watcher resumes where the original stopped, so a `from = "start"`
+// source does not re-emit every record the file already delivered.
+func TestRotatedFileResumesInsteadOfReplaying(t *testing.T) {
+	dir := t.TempDir()
+	active := filepath.Join(dir, "session.jsonl")
+	if err := os.WriteFile(active, []byte("one\ntwo\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	info, err := os.Stat(active)
+	if err != nil {
+		t.Fatal(err)
+	}
+	inode := info.Sys().(*syscall.Stat_t).Ino
+
+	archive := filepath.Join(dir, "session_260916_120000.jsonl")
+	if err := os.Rename(active, archive); err != nil {
+		t.Fatal(err)
+	}
+
+	for name, watcher := range map[string]*fileWatcher{
+		"still tailing the renamed inode": {inode: inode, position: 8},
+		"already moved on from it":        {inode: 99, prevInode: inode, prevPosition: 8},
+	} {
+		source := &FileSource{watchers: map[string]*fileWatcher{active: watcher}}
+		position, ok := source.readPosition(archive)
+		if !ok || position != 8 {
+			t.Errorf("%s: position = %d, ok = %v, want 8, true", name, position, ok)
+		}
+	}
+
+	unrelated := &FileSource{watchers: map[string]*fileWatcher{active: {inode: 99}}}
+	if _, ok := unrelated.readPosition(archive); ok {
+		t.Error("a file no watcher has read was treated as rotated")
+	}
+}