internal/mmap: Add tests

2 files changed, 112 insertions, 1 deletions

diff --git a/internal/mmap/mmap.go b/internal/mmap/mmap.go
index d437b12d..a8644a3b 100644
--- a/internal/mmap/mmap.go
+++ b/internal/mmap/mmap.go
@@ -74,7 +74,7 @@ func (mmap *Mmap) Data() []byte {
 // Close unmaps the mapping,
 // invalidating previously returned data.
 //
-// Labels: Idem-Yes.
+// Labels: Idem-Yes, MT-Unsafe.
 func (mmap *Mmap) Close() error {
 	if mmap.data == nil {
 		return nil
diff --git a/internal/mmap/mmap_test.go b/internal/mmap/mmap_test.go
new file mode 100644
index 00000000..814c23a3
--- /dev/null
+++ b/internal/mmap/mmap_test.go
@@ -0,0 +1,111 @@
+//go:build unix
+
+package mmap_test
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"lindenii.org/go/furgit/internal/mmap"
+)
+
+// makeTempFileWithContents creates one file with content and opens it for reading.
+func makeTempFileWithContents(t *testing.T, content []byte) *os.File {
+	t.Helper()
+
+	path := filepath.Join(t.TempDir(), "data")
+
+	err := os.WriteFile(path, content, 0o600)
+	if err != nil {
+		t.Fatalf("WriteFile: %v", err)
+	}
+
+	file, err := os.Open(path) //nolint:gosec
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+
+	t.Cleanup(func() { _ = file.Close() })
+
+	return file
+}
+
+// makeLargeTempFileLongerThanPage creates and opens one file
+// whose content repeats seed
+// until it spans more than one OS page.
+func makeLargeTempFileLongerThanPage(t *testing.T, seed []byte) (*os.File, []byte) {
+	t.Helper()
+
+	content := bytes.Repeat(seed, os.Getpagesize()/len(seed)+2)
+
+	return makeTempFileWithContents(t, content), content
+}
+
+func TestOpenData(t *testing.T) {
+	t.Parallel()
+
+	file, content := makeLargeTempFileLongerThanPage(t, []byte("mapped bytes\n"))
+
+	mapping, err := mmap.Open(file)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+
+	// The mapping must survive closing the originating file.
+	err = file.Close()
+	if err != nil {
+		t.Fatalf("file Close: %v", err)
+	}
+
+	if !bytes.Equal(mapping.Data(), content) {
+		t.Fatalf("Data mismatch")
+	}
+
+	err = mapping.Close()
+	if err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+}
+
+func TestOpenEmpty(t *testing.T) {
+	t.Parallel()
+
+	file := makeTempFileWithContents(t, nil)
+
+	mapping, err := mmap.Open(file)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+
+	if len(mapping.Data()) != 0 {
+		t.Fatalf("Data = %d bytes, want empty", len(mapping.Data()))
+	}
+
+	err = mapping.Close()
+	if err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+}
+
+func TestCloseIdempotent(t *testing.T) {
+	t.Parallel()
+
+	file, _ := makeLargeTempFileLongerThanPage(t, []byte("once"))
+
+	mapping, err := mmap.Open(file)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+
+	err = mapping.Close()
+	if err != nil {
+		t.Fatalf("first Close: %v", err)
+	}
+
+	err = mapping.Close()
+	if err != nil {
+		t.Fatalf("second Close: %v", err)
+	}
+}