super basic chainfs

Author: Am Laher

LICENSE

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+MIT License
+
+Copyright (c) 2021 Amir Laher
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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+# Chainfs
+
+A go package which chains together filesystems so that you can combine fs.FS filesystems.
+
+`chainfs.FS` keeps looking through a slice of `fs.FS` filesytems in order to find a given file. It returns the first match.

chainfs.go

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+package chainfs
+
+import (
+	"io/fs"
+	"os"
+)
+
+type FS struct {
+	Filesystems []fs.FS
+}
+
+// Open opens the named file.
+func (cfs FS) Open(name string) (fs.File, error) {
+	for _, fs := range cfs.Filesystems {
+		file, err := fs.Open(name)
+		if err == nil {
+			return file, nil
+		}
+	}
+	return nil, os.ErrNotExist
+}

chainfs_test.go

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+package chainfs
+
+import (
+	"io/fs"
+	"testing"
+	"testing/fstest"
+)
+
+func TestChainFS(t *testing.T) {
+	a := fstest.MapFS{"a": &fstest.MapFile{Data: []byte("text")}}
+	b := fstest.MapFS{"b": &fstest.MapFile{Data: []byte("text")}}
+	fs := FS{Filesystems: []fs.FS{a, b}}
+
+	if _, err := fs.Open("a"); err != nil {
+		t.Fatalf("file should exist")
+	}
+	if _, err := fs.Open("b"); err != nil {
+		t.Fatalf("file should exist")
+	}
+}

go.mod

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+module github.com/laher/chainfs
+
+go 1.15