Initial source code

a/a.go

@@ -0,0 +1,6 @@
+package a
+
+import "embed"
+
+//go:embed *
+var Embed embed.FS

a/hello.txt

@@ -0,0 +1 @@
+Empty

a/just-a.txt

@@ -0,0 +1 @@
+A

b/b.go

@@ -0,0 +1,6 @@
+package b
+
+import "embed"
+
+//go:embed *
+var Embed embed.FS

b/hello.txt

@@ -0,0 +1 @@
+Hello World!

b/just-b.txt

@@ -0,0 +1 @@
+B

go.mod

@@ -0,0 +1,11 @@
+module github.com/1f349/overlapfs
+
+go 1.21.1
+
+require github.com/stretchr/testify v1.8.4
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)

go.sum

@@ -0,0 +1,10 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.8.4 h1:CcVxjf3Q8PM0mHUKJCdn+eZZtm5yQwehR5yeSVQQcUk=
+github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

overlapfs.go

@@ -0,0 +1,89 @@
+package overlapfs
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+)
+
+// OverlapFS layers B on top of A
+//
+// Open and Stat reimplemented to show B unless ErrNotExist is returned then show A.
+//
+// Glob and ReadDir are reimplemented to uniquely merge the returned lists.
+//
+// For ReadFile and Sub use the standard fs.ReadFile and fs.Sub implementations
+type OverlapFS struct {
+	A, B fs.FS
+}
+
+var _ interface {
+	fs.FS
+	fs.StatFS
+	fs.GlobFS
+	fs.ReadDirFS
+} = &OverlapFS{}
+
+func (o OverlapFS) Open(name string) (fs.File, error) {
+	fmt.Println("Open", name)
+	open, err := o.B.Open(name)
+	switch {
+	case err == nil:
+		return open, nil
+	case errors.Is(err, fs.ErrNotExist):
+		return o.A.Open(name)
+	}
+	return nil, err
+}
+
+func (o OverlapFS) Stat(name string) (fs.FileInfo, error) {
+	stat, err := fs.Stat(o.B, name)
+	switch {
+	case err == nil:
+		return stat, nil
+	case errors.Is(err, fs.ErrNotExist):
+		return fs.Stat(o.A, name)
+	}
+	return nil, err
+}
+
+func (o OverlapFS) Glob(pattern string) ([]string, error) {
+	aSlice, err := fs.Glob(o.B, pattern)
+	if err != nil {
+		return nil, err
+	}
+	bSlice, err := fs.Glob(o.A, pattern)
+	if err != nil {
+		return nil, err
+	}
+
+	mergeUnique(aSlice, bSlice, func(s string) string { return s })
+	return aSlice, nil
+}
+
+func (o OverlapFS) ReadDir(name string) ([]fs.DirEntry, error) {
+	aSlice, err := fs.ReadDir(o.B, name)
+	if err != nil {
+		return nil, err
+	}
+	bSlice, err := fs.ReadDir(o.A, name)
+	if err != nil {
+		return nil, err
+	}
+
+	mergeUnique(aSlice, bSlice, func(entry fs.DirEntry) string { return entry.Name() })
+	return aSlice, nil
+}
+
+// mergeUnique puts unique values from b into a
+func mergeUnique[T any](a, b []T, key func(T) string) {
+	m := make(map[string]struct{})
+	for _, i := range a {
+		m[key(i)] = struct{}{}
+	}
+	for _, i := range b {
+		if _, ok := m[key(i)]; !ok {
+			a = append(a, i)
+		}
+	}
+}

overlapfs_test.go

@@ -0,0 +1,31 @@
+package overlapfs
+
+import (
+	"github.com/1f349/overlapfs/a"
+	"github.com/1f349/overlapfs/b"
+	"github.com/stretchr/testify/assert"
+	"io"
+	"testing"
+)
+
+func TestOverlapFS_Open(t *testing.T) {
+	o := OverlapFS{A: a.Embed, B: b.Embed}
+	open, err := o.Open("hello.txt")
+	assert.NoError(t, err)
+	stat, err := open.Stat()
+	assert.NoError(t, err)
+	assert.Equal(t, "hello.txt", stat.Name())
+	bAll, err := io.ReadAll(open)
+	assert.Equal(t, "Hello World!", string(bAll))
+}
+
+func TestOverlapFS_Stat(t *testing.T) {
+	o := OverlapFS{A: a.Embed, B: b.Embed}
+	open, err := o.Stat("hello.txt")
+	assert.NoError(t, err)
+	stat, err := open.Stat()
+	assert.NoError(t, err)
+	assert.Equal(t, "hello.txt", stat.Name())
+	bAll, err := io.ReadAll(open)
+	assert.Equal(t, "Hello World!", string(bAll))
+}