Add internal engine relocation candidate

internal/engine/source_file_test.go

@@ -0,0 +1,48 @@
+package engine
+
+import (
+	"io/fs"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// Files disappearing between filesystem operations are not deterministic with
+// real concurrent deletion. Exercise the private seam that controls optionality.
+func TestRegularFileMissingFailurePhases(t *testing.T) {
+	name := filepath.Join(t.TempDir(), "input.yml")
+	if err := os.WriteFile(name, []byte("{}"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	for _, test := range []struct {
+		name    string
+		stat    func(string) (fs.FileInfo, error)
+		open    func(string) (*os.File, error)
+		missing bool
+	}{
+		{"preflight", func(string) (fs.FileInfo, error) { return nil, fs.ErrNotExist }, os.Open, true},
+		{"open", os.Stat, func(string) (*os.File, error) { return nil, fs.ErrNotExist }, true},
+		{"post-open stat", os.Stat, func(name string) (*os.File, error) {
+			file, err := os.Open(name)
+			if err == nil {
+				err = file.Close()
+			}
+			return file, err
+		}, false},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			data, failure := readRegularFile(name, test.stat, test.open, 16)
+			if data != nil || failure == nil || failure.kind != sourceReadIO || failure.missingTarget != test.missing {
+				t.Fatalf("data = %v, failure = %#v", data, failure)
+			}
+		})
+	}
+	_, failure := readBoundedSource(missingDuringRead{}, 16)
+	if failure == nil || failure.kind != sourceReadIO || failure.missingTarget {
+		t.Fatalf("late read failure = %#v", failure)
+	}
+}
+
+type missingDuringRead struct{}
+
+func (missingDuringRead) Read([]byte) (int, error) { return 0, fs.ErrNotExist }