Add utilities for detecting linux distro based on /etc/os-release (#13786)

Only AMZL/RHEL/Ubuntu/Debian are needed for now as those are the
target distros of EC2 discovery

lib/utils/distro.go

@@ -0,0 +1,52 @@
+/*
+Copyright 2022 Gravitational, Inc.
+
+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 utils
+
+import (
+	"bufio"
+	"io"
+	"strings"
+
+	"strconv"
+
+	"github.com/gravitational/trace"
+)
+
+func OSRelease(rel io.Reader) (map[string]string, error) {
+	scanner := bufio.NewScanner(rel)
+	result := make(map[string]string)
+	for scanner.Scan() {
+		kvLine := scanner.Text()
+		if kvLine == "" || kvLine[0] == '#' {
+			continue
+		}
+		key, value, found := strings.Cut(kvLine, "=")
+		if !found {
+			continue
+		}
+		// unquote value if it is quoted (rhel, amzl)
+		if value[0] == '"' {
+			var err error
+			value, err = strconv.Unquote(value)
+			if err != nil {
+				return nil, trace.Wrap(err)
+			}
+		}
+		result[key] = value
+	}
+	return result, nil
+}

lib/utils/distro_test.go

@@ -0,0 +1,91 @@
+/*
+Copyright 2022 Gravitational, Inc.
+
+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 utils
+
+import (
+	"bytes"
+	"strconv"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestOSRelease(t *testing.T) {
+	for _, tc := range []struct {
+		name     string
+		in       string
+		expected map[string]string
+		err      error
+	}{
+		{
+			name: "amazon example, all quoted",
+			in: `
+NAME="Amazon Linux"
+VERSION="2"
+ID="amzn"
+ID_LIKE="centos rhel fedora"
+VERSION_ID="2"
+PRETTY_NAME="Amazon Linux 2"
+ANSI_COLOR="0;33"
+CPE_NAME="cpe:2.3:o:amazon:amazon_linux:2"
+HOME_URL="https://amazonlinux.com/"
+`,
+			expected: map[string]string{
+				"NAME":        "Amazon Linux",
+				"VERSION":     "2",
+				"ID":          "amzn",
+				"ID_LIKE":     "centos rhel fedora",
+				"VERSION_ID":  "2",
+				"PRETTY_NAME": "Amazon Linux 2",
+				"ANSI_COLOR":  "0;33",
+				"CPE_NAME":    "cpe:2.3:o:amazon:amazon_linux:2",
+				"HOME_URL":    "https://amazonlinux.com/",
+			},
+			err: nil,
+		},
+		{
+			name: "quoted and unquoted",
+			in: `
+PRETTY_NAME="Debian GNU/Linux bookworm/sid"
+ID=debian
+`,
+			expected: map[string]string{
+				"PRETTY_NAME": "Debian GNU/Linux bookworm/sid",
+				"ID":          "debian",
+			},
+			err: nil,
+		},
+		{
+			name: "improperly quoted",
+			in:   `PRETTY_NAME="Debian GNU/Linux bookworm/sid`,
+			err:  strconv.ErrSyntax,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			reader := bytes.NewReader([]byte(tc.in))
+			got, err := OSRelease(reader)
+			if err != nil {
+				if tc.err != nil {
+					require.ErrorIs(t, err, tc.err)
+				} else {
+					require.Failf(t, "unexpected error", err.Error())
+				}
+			}
+			require.EqualValues(t, tc.expected, got)
+		})
+	}
+}