config_tmpl.go

// Copyright 2017-2023 The Cloudprober 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 config provides parser for cloudprober configs.

Example Usage:

	c, err := config.ParseTemplate(config, sysvars.SysVars())

ParseTemplate processes a config file as a Go text template using the provided
variable map (usually GCP metadata variables) and some predefined macros.

# Macros

Cloudprober configs support some macros to make configs construction easier:

	env
		Get the value of an environment variable.

		Example:

		probe {
		  name: "dns_google_jp"
		  type: DNS
		  targets {
		    host_names: "1.1.1.1"
		  }
		  dns_probe {
		    resolved_domain: "{{env "TEST_DOM"}}"
		  }
		}

		# Then run cloudprober as:
		TEST_DOM=google.co.jp ./cloudprober --config_file=cloudprober.cfg

	gceCustomMetadata
	 	Get value of a GCE custom metadata key. It first looks for the given key in
		the instance's custom metadata and if it is not found there, it looks for it
		in the project's custom metaata.

		# Get load balancer IP from metadata.
		probe {
		  name: "http_lb"
		  type: HTTP
		  targets {
		    host_names: "{{gceCustomMetadata "lb_ip"}}"
		  }
		}

	extractSubstring
		Extract substring from a string using regex. Example use in config:

		# Sharded VM-to-VM connectivity checks over internal IP
		# Instance name format: ig-<zone>-<shard>-<random-characters>, e.g. ig-asia-east1-a-00-ftx1
		{{$shard := .instance | extractSubstring "[^-]+-[^-]+-[^-]+-[^-]+-([^-]+)-.*" 1}}
		probe {
		  name: "vm-to-vm-{{$shard}}"
		  type: PING
		  targets {
		    gce_targets {
		      instances {}
		    }
		    regex: "{{$targets}}"
		  }
		  run_on: "{{$run_on}}"
		}

	mkMap
		Returns a map built from the arguments. It's useful as Go templates take only
		one argument. With this function, we can create a map of multiple values and
		pass it to a template. Example use in config:

		{{define "probeTmpl"}}
		probe {
		  type: {{.typ}}
		  name: "{{.name}}"
		  targets {
		    host_names: "www.google.com"
		  }
		}
		{{end}}

		{{template "probeTmpl" mkMap "typ" "PING" "name" "ping_google"}}
		{{template "probeTmpl" mkMap "typ" "HTTP" "name" "http_google"}}


	mkSlice
		Returns a slice consisting of the arguments. It can be used with the built-in
		'range' function to replicate text.


		{{with $regions := mkSlice "us=central1" "us-east1"}}
		{{range $_, $region := $regions}}

		probe {
		  name: "service-a-{{$region}}"
		  type: HTTP
		  targets {
		    host_names: "service-a.{{$region}}.corp.xx.com"
		  }
		}

		{{end}}
		{{end}}
*/
package config

import (
	"bytes"
	"fmt"
	"regexp"
	"text/template"

	"cloud.google.com/go/compute/metadata"
	"github.com/Masterminds/sprig/v3"
	configpb "github.com/cloudprober/cloudprober/config/proto"
	"google.golang.org/protobuf/encoding/prototext"
)

// readFromGCEMetadata returns the value of GCE custom metadata variables. To
// allow for instance level as project level variables, it looks for metadata
// variable in the following order:
//
// a. If the given key is set in the instance's custom metadata, its value is
// returned.
//
// b. If (and only if), the key is not found in the step above, we look for
// the same key in the project's custom metadata.
var readFromGCEMetadata = func(metadataKeyName string) (string, error) {
	val, err := metadata.InstanceAttributeValue(metadataKeyName)
	// If instance level config found, return
	if _, notFound := err.(metadata.NotDefinedError); !notFound {
		return val, err
	}
	// Check project level config next
	return metadata.ProjectAttributeValue(metadataKeyName)
}

// DefaultConfig returns the default config string.
func DefaultConfig() string {
	b, _ := prototext.Marshal(&configpb.ProberConfig{})
	return string(b)
}

// parseTemplate processes a config file as a Go text template.
func parseTemplate(config string, sysVars map[string]string, getGCECustomMetadata func(string) (string, error)) (string, error) {
	if getGCECustomMetadata == nil {
		getGCECustomMetadata = readFromGCEMetadata
	}

	gceCustomMetadataFunc := func(v string) (string, error) {
		// We return "undefined" if metadata variable is not defined.
		val, err := getGCECustomMetadata(v)
		if err != nil {
			if _, notFound := err.(metadata.NotDefinedError); notFound {
				return "undefined", nil
			}
			return "", err
		}
		return val, nil
	}

	funcMap := map[string]interface{}{
		"gceCustomMetadata": gceCustomMetadataFunc,

		// extractSubstring allows us to extract substring from a string using
		// regex matching groups.
		"extractSubstring": func(re string, n int, s string) (string, error) {
			r, err := regexp.Compile(re)
			if err != nil {
				return "", err
			}
			matches := r.FindStringSubmatch(s)
			if len(matches) <= n {
				return "", fmt.Errorf("match number %d not found. Regex: %s, String: %s", n, re, s)
			}
			return matches[n], nil
		},
		"envSecret": func(s string) string { return "**$" + s + "**" },
	}

	for name, f := range sprig.TxtFuncMap() {
		funcMap[name] = f
	}
	funcMap["mkSlice"] = funcMap["list"]
	funcMap["mkMap"] = funcMap["dict"]

	configTmpl, err := template.New("cloudprober_cfg").Funcs(funcMap).Parse(config)
	if err != nil {
		return "", err
	}
	var b bytes.Buffer
	if err := configTmpl.Execute(&b, sysVars); err != nil {
		return "", err
	}
	return b.String(), nil
}