Allow for configuration policies

Summary:
Implements a rough interface for validating configuration against
arbitrary policy rules. Policies are provided as YAML and passed via the
command line as file paths or remote URIs.

The format of policies is:

    enforcements:
      - path: <path>
        rule: <rule>

Where `<path>` is a YAML-ish path to a config field and `<rule>` is any
expression our config validator understands (expressions built in by the
validator library and custom tags defined in `config.validation.go`).

Example policy:

    enforcements:
      - path: variants.production.base
        rule: oneof=debian:jessie debian:stretch
      - path: variants.production.runs.as
        rule: ne=foo
      - path: variants.production.node.dependencies
        rule: isfalse

Command flag parsing was implemented in `main.go` to support the new
`--policy=uri` flag and improve existing handling of `--version` and the
usage statement.

Test Plan: Run `go test ./...`.

Reviewers: thcipriani, demon, hashar, mmodell, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D999

1 files changed, 45 insertions, 7 deletions

diff --git a/main.go b/main.go
index 9257532..bb16b89 100644
--- a/main.go
+++ b/main.go
@@ -7,35 +7,73 @@ import (
 	"log"
 	"os"
 
+	"github.com/pborman/getopt/v2"
+
 	"phabricator.wikimedia.org/source/blubber/config"
 	"phabricator.wikimedia.org/source/blubber/docker"
 	"phabricator.wikimedia.org/source/blubber/meta"
 )
 
+const parameters = "config.yaml variant"
+
+var (
+	showHelp    *bool   = getopt.BoolLong("help", 'h', "show help/usage")
+	showVersion *bool   = getopt.BoolLong("version", 'v', "show version information")
+	policyURI   *string = getopt.StringLong("policy", 'p', "", "policy file URI", "uri")
+)
+
 func main() {
-	if len(os.Args) > 1 && os.Args[1] == "--version" {
+	getopt.SetParameters(parameters)
+	getopt.Parse()
+
+	if *showHelp {
+		getopt.Usage()
+		os.Exit(1)
+	}
+
+	if *showVersion {
 		fmt.Println(meta.FullVersion())
 		os.Exit(0)
 	}
 
-	if len(os.Args) < 3 {
-		fmt.Println("Usage: blubber config.yaml variant")
+	args := getopt.Args()
+
+	if len(args) < 2 {
+		getopt.Usage()
 		os.Exit(1)
 	}
 
-	cfg, err := config.ReadConfigFile(os.Args[1])
+	cfgPath, variant := args[0], args[1]
+
+	cfg, err := config.ReadConfigFile(cfgPath)
 
 	if err != nil {
 		if config.IsValidationError(err) {
-			log.Printf("Your config is invalid:\n%v", config.HumanizeValidationError(err))
+			log.Printf("%s is invalid:\n%v", cfgPath, config.HumanizeValidationError(err))
 			os.Exit(4)
 		} else {
-			log.Printf("Error reading config: %v\n", err)
+			log.Printf("Error reading %s: %v\n", cfgPath, err)
 			os.Exit(2)
 		}
 	}
 
-	dockerFile, err := docker.Compile(cfg, os.Args[2])
+	if *policyURI != "" {
+		policy, err := config.ReadPolicyFromURI(*policyURI)
+
+		if err != nil {
+			log.Printf("Error loading policy from %s: %v\n", *policyURI, err)
+			os.Exit(5)
+		}
+
+		err = policy.Validate(*cfg)
+
+		if err != nil {
+			log.Printf("Configuration fails policy check against:\npolicy: %s\nviolation: %v\n", *policyURI, err)
+			os.Exit(6)
+		}
+	}
+
+	dockerFile, err := docker.Compile(cfg, variant)
 
 	if err != nil {
 		log.Printf("Error compiling config: %v\n", err)