assets: add file system layer for zipped embed assets

This commits adds an optional decompression layer to a embed.FS
instance. If a file can't be found during Open, the filename plus a .gz
ending is tried. If this succeeds, the content is decompressed and
served.

Signed-off-by: Jan Fajerski <[email protected]>

README.md

@@ -5,6 +5,7 @@ This repository contains Go libraries that are shared across Prometheus
 components and libraries. They are considered internal to Prometheus, without
 any stability guarantees for external usage.
 
+* **assets**: Embedding of static assets with gzip support
 * **config**: Common configuration structures
 * **expfmt**: Decoding and encoding for the exposition format
 * **model**: Shared data structures

assets/Makefile

@@ -0,0 +1,18 @@
+# Copyright 2018 The Prometheus Authors
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+include ../Makefile.common
+
+.PHONY: test
+	@echo ">> Running assets tests"
+	test:: deps check_license unused common-test

assets/embed_gzip.go

@@ -0,0 +1,123 @@
+// Copyright 2021 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package assets
+
+import (
+	"compress/gzip"
+	"embed"
+	"io"
+	"io/fs"
+	"time"
+)
+
+const (
+	gzipSuffix = ".gz"
+)
+
+type FileSystem struct {
+	embed embed.FS
+}
+
+func New(fs embed.FS) FileSystem {
+	return FileSystem{fs}
+}
+
+// Open implements the fs.FS interface.
+func (compressed FileSystem) Open(path string) (fs.File, error) {
+	// If we have the file in our embed FS, just return that as it could be a dir.
+	var f fs.File
+	if f, err := compressed.embed.Open(path); err == nil {
+		return f, nil
+	}
+
+	f, err := compressed.embed.Open(path + gzipSuffix)
+	if err != nil {
+		return f, err
+	}
+	// Read the decompressed content into a buffer.
+	gr, err := gzip.NewReader(f)
+	if err != nil {
+		return f, err
+	}
+	defer gr.Close()
+
+	c, err := io.ReadAll(gr)
+	if err != nil {
+		return f, err
+	}
+	// Wrap everything in our custom File.
+	return &File{file: f, content: c}, nil
+}
+
+type File struct {
+	// The underlying file.
+	file fs.File
+	// The decrompressed content, needed to return an accurate size.
+	content []byte
+	// Offset for calls to Read().
+	offset int
+}
+
+// Stat implements the fs.File interface.
+func (f File) Stat() (fs.FileInfo, error) {
+	stat, err := f.file.Stat()
+	if err != nil {
+		return stat, err
+	}
+	return FileInfo{stat, int64(len(f.content))}, nil
+}
+
+// Read implements the fs.File interface.
+func (f *File) Read(buf []byte) (int, error) {
+	if len(buf) > len(f.content)-f.offset {
+		buf = buf[0:len(f.content[f.offset:])]
+	}
+	n := copy(buf, f.content[f.offset:])
+	if n == len(f.content)-f.offset {
+		return n, io.EOF
+	}
+	f.offset += n
+	return n, nil
+}
+
+// Close implements the fs.File interface.
+func (f File) Close() error {
+	return f.file.Close()
+}
+
+type FileInfo struct {
+	fi         fs.FileInfo
+	actualSize int64
+}
+
+// Name implements the fs.FileInfo interface.
+func (fi FileInfo) Name() string {
+	name := fi.fi.Name()
+	return name[:len(name)-len(gzipSuffix)]
+}
+
+// Size implements the fs.FileInfo interface.
+func (fi FileInfo) Size() int64 { return fi.actualSize }
+
+// Mode implements the fs.FileInfo interface.
+func (fi FileInfo) Mode() fs.FileMode { return fi.fi.Mode() }
+
+// ModTime implements the fs.FileInfo interface.
+func (fi FileInfo) ModTime() time.Time { return fi.fi.ModTime() }
+
+// IsDir implements the fs.FileInfo interface.
+func (fi FileInfo) IsDir() bool { return fi.fi.IsDir() }
+
+// Sys implements the fs.FileInfo interface.
+func (fi FileInfo) Sys() interface{} { return nil }

assets/embed_gzip_test.go

@@ -0,0 +1,93 @@
+// Copyright 2021 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package assets
+
+import (
+	"embed"
+	"io/ioutil"
+	"strings"
+	"testing"
+)
+
+//go:embed testdata
+var EmbedFS embed.FS
+
+var testFS = New(EmbedFS)
+
+func TestFS(t *testing.T) {
+	cases := []struct {
+		name            string
+		path            string
+		expectedSize    int64
+		expectedContent string
+	}{
+		{
+			name:            "uncompressed file",
+			path:            "testdata/uncompressed",
+			expectedSize:    4,
+			expectedContent: "foo\n",
+		},
+		{
+			name:            "compressed file",
+			path:            "testdata/compressed",
+			expectedSize:    4,
+			expectedContent: "foo\n",
+		},
+		{
+			name:            "both, open uncompressed",
+			path:            "testdata/both",
+			expectedSize:    4,
+			expectedContent: "foo\n",
+		},
+		{
+			name:         "both, open compressed",
+			path:         "testdata/both.gz",
+			expectedSize: 29,
+			// we don't check content for a explicitly compressed file
+			expectedContent: "",
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			f, err := testFS.Open(c.path)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			stat, err := f.Stat()
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			size := stat.Size()
+			if size != c.expectedSize {
+				t.Fatalf("size is wrong, expected %d, got %d", c.expectedSize, size)
+			}
+
+			if strings.HasSuffix(c.path, ".gz") {
+				// don't read the comressed content
+				return
+			}
+
+			content, err := ioutil.ReadAll(f)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if string(content) != c.expectedContent {
+				t.Fatalf("content is wrong, expected %s, got %s", c.expectedContent, string(content))
+			}
+		})
+	}
+}

assets/go.mod

@@ -0,0 +1,3 @@
+module github.com/prometheus/common/assets
+
+go 1.17

assets/testdata/both

@@ -0,0 +1 @@
+foo

assets/testdata/uncompressed

@@ -0,0 +1 @@
+foo