[feature] Set/show instance language(s); show post language on frontend (#2362)

* update go text, include text/display

* [feature] Set instance langs, show post lang on frontend

* go fmt

* WebGet

* set language for whole article, don't use FA icon

* mention instance languages + other optional config vars

* little tweak

* put languages in config properly

* warn log language parse

* change some naming around

* tidy up validate a bit

* lint

* rename LanguageTmpl in template

7 files changed, 130 insertions, 52 deletions

diff --git a/internal/config/config.go b/internal/config/config.go
index 7cb31d0a1..b7d2eff36 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -23,6 +23,7 @@ import (
 
 	"codeberg.org/gruf/go-bytesize"
 	"github.com/mitchellh/mapstructure"
+	"github.com/superseriousbusiness/gotosocial/internal/language"
 )
 
 // cfgtype is the reflected type information of Configuration{}.
@@ -76,13 +77,14 @@ type Configuration struct {
 	WebTemplateBaseDir string `name:"web-template-base-dir" usage:"Basedir for html templating files for rendering pages and composing emails."`
 	WebAssetBaseDir    string `name:"web-asset-base-dir" usage:"Directory to serve static assets from, accessible at example.org/assets/"`
 
-	InstanceFederationMode         string `name:"instance-federation-mode" usage:"Set instance federation mode."`
-	InstanceExposePeers            bool   `name:"instance-expose-peers" usage:"Allow unauthenticated users to query /api/v1/instance/peers?filter=open"`
-	InstanceExposeSuspended        bool   `name:"instance-expose-suspended" usage:"Expose suspended instances via web UI, and allow unauthenticated users to query /api/v1/instance/peers?filter=suspended"`
-	InstanceExposeSuspendedWeb     bool   `name:"instance-expose-suspended-web" usage:"Expose list of suspended instances as webpage on /about/suspended"`
-	InstanceExposePublicTimeline   bool   `name:"instance-expose-public-timeline" usage:"Allow unauthenticated users to query /api/v1/timelines/public"`
-	InstanceDeliverToSharedInboxes bool   `name:"instance-deliver-to-shared-inboxes" usage:"Deliver federated messages to shared inboxes, if they're available."`
-	InstanceInjectMastodonVersion  bool   `name:"instance-inject-mastodon-version" usage:"This injects a Mastodon compatible version in /api/v1/instance to help Mastodon clients that use that version for feature detection"`
+	InstanceFederationMode         string             `name:"instance-federation-mode" usage:"Set instance federation mode."`
+	InstanceExposePeers            bool               `name:"instance-expose-peers" usage:"Allow unauthenticated users to query /api/v1/instance/peers?filter=open"`
+	InstanceExposeSuspended        bool               `name:"instance-expose-suspended" usage:"Expose suspended instances via web UI, and allow unauthenticated users to query /api/v1/instance/peers?filter=suspended"`
+	InstanceExposeSuspendedWeb     bool               `name:"instance-expose-suspended-web" usage:"Expose list of suspended instances as webpage on /about/suspended"`
+	InstanceExposePublicTimeline   bool               `name:"instance-expose-public-timeline" usage:"Allow unauthenticated users to query /api/v1/timelines/public"`
+	InstanceDeliverToSharedInboxes bool               `name:"instance-deliver-to-shared-inboxes" usage:"Deliver federated messages to shared inboxes, if they're available."`
+	InstanceInjectMastodonVersion  bool               `name:"instance-inject-mastodon-version" usage:"This injects a Mastodon compatible version in /api/v1/instance to help Mastodon clients that use that version for feature detection"`
+	InstanceLanguages              language.Languages `name:"instance-languages" usage:"BCP47 language tags for the instance. Used to indicate the preferred languages of instance residents (in order from most-preferred to least-preferred)."`
 
 	AccountsRegistrationOpen bool `name:"accounts-registration-open" usage:"Allow anyone to submit an account signup request. If false, server will be invite-only."`
 	AccountsApprovalRequired bool `name:"accounts-approval-required" usage:"Do account signups require approval by an admin or moderator before user can log in? If false, new registrations will be automatically approved."`
diff --git a/internal/config/defaults.go b/internal/config/defaults.go
index 2a7c6f9db..3da489501 100644
--- a/internal/config/defaults.go
+++ b/internal/config/defaults.go
@@ -22,6 +22,7 @@ import (
 
 	"codeberg.org/gruf/go-bytesize"
 	"github.com/coreos/go-oidc/v3/oidc"
+	"github.com/superseriousbusiness/gotosocial/internal/language"
 )
 
 // Defaults contains a populated Configuration with reasonable defaults. Note that
@@ -62,6 +63,7 @@ var Defaults = Configuration{
 	InstanceExposeSuspended:        false,
 	InstanceExposeSuspendedWeb:     false,
 	InstanceDeliverToSharedInboxes: true,
+	InstanceLanguages:              make(language.Languages, 0),
 
 	AccountsRegistrationOpen: true,
 	AccountsApprovalRequired: true,
diff --git a/internal/config/flags.go b/internal/config/flags.go
index b29d0fe04..2dc583abc 100644
--- a/internal/config/flags.go
+++ b/internal/config/flags.go
@@ -88,6 +88,7 @@ func (s *ConfigState) AddServerFlags(cmd *cobra.Command) {
 		cmd.Flags().Bool(InstanceExposeSuspendedFlag(), cfg.InstanceExposeSuspended, fieldtag("InstanceExposeSuspended", "usage"))
 		cmd.Flags().Bool(InstanceExposeSuspendedWebFlag(), cfg.InstanceExposeSuspendedWeb, fieldtag("InstanceExposeSuspendedWeb", "usage"))
 		cmd.Flags().Bool(InstanceDeliverToSharedInboxesFlag(), cfg.InstanceDeliverToSharedInboxes, fieldtag("InstanceDeliverToSharedInboxes", "usage"))
+		// cmd.Flags().StringSlice(InstanceLanguagesFlag(), cfg.InstanceLanguages.TagStrs(), fieldtag("InstanceLanguages", "usage"))
 
 		// Accounts
 		cmd.Flags().Bool(AccountsRegistrationOpenFlag(), cfg.AccountsRegistrationOpen, fieldtag("AccountsRegistrationOpen", "usage"))
diff --git a/internal/config/gen/gen.go b/internal/config/gen/gen.go
index d9fb30904..96e036d98 100644
--- a/internal/config/gen/gen.go
+++ b/internal/config/gen/gen.go
@@ -67,6 +67,7 @@ func main() {
 	fmt.Fprint(output, "import (\n")
 	fmt.Fprint(output, "\t\"time\"\n\n")
 	fmt.Fprint(output, "\t\"codeberg.org/gruf/go-bytesize\"\n")
+	fmt.Fprint(output, "\t\"github.com/superseriousbusiness/gotosocial/internal/langs\"\n")
 	fmt.Fprint(output, ")\n\n")
 	generateFields(output, nil, reflect.TypeOf(config.Configuration{}))
 	_ = output.Close()
diff --git a/internal/config/helpers.gen.go b/internal/config/helpers.gen.go
index 6e9b71812..a008092d3 100644
--- a/internal/config/helpers.gen.go
+++ b/internal/config/helpers.gen.go
@@ -22,6 +22,7 @@ import (
 	"time"
 
 	"codeberg.org/gruf/go-bytesize"
+	"github.com/superseriousbusiness/gotosocial/internal/language"
 )
 
 // GetLogLevel safely fetches the Configuration value for state's 'LogLevel' field
@@ -924,6 +925,31 @@ func GetInstanceInjectMastodonVersion() bool { return global.GetInstanceInjectMa
 // SetInstanceInjectMastodonVersion safely sets the value for global configuration 'InstanceInjectMastodonVersion' field
 func SetInstanceInjectMastodonVersion(v bool) { global.SetInstanceInjectMastodonVersion(v) }
 
+// GetInstanceLanguages safely fetches the Configuration value for state's 'InstanceLanguages' field
+func (st *ConfigState) GetInstanceLanguages() (v language.Languages) {
+	st.mutex.RLock()
+	v = st.config.InstanceLanguages
+	st.mutex.RUnlock()
+	return
+}
+
+// SetInstanceLanguages safely sets the Configuration value for state's 'InstanceLanguages' field
+func (st *ConfigState) SetInstanceLanguages(v language.Languages) {
+	st.mutex.Lock()
+	defer st.mutex.Unlock()
+	st.config.InstanceLanguages = v
+	st.reloadToViper()
+}
+
+// InstanceLanguagesFlag returns the flag name for the 'InstanceLanguages' field
+func InstanceLanguagesFlag() string { return "instance-languages" }
+
+// GetInstanceLanguages safely fetches the value for global configuration 'InstanceLanguages' field
+func GetInstanceLanguages() language.Languages { return global.GetInstanceLanguages() }
+
+// SetInstanceLanguages safely sets the value for global configuration 'InstanceLanguages' field
+func SetInstanceLanguages(v language.Languages) { global.SetInstanceLanguages(v) }
+
 // GetAccountsRegistrationOpen safely fetches the Configuration value for state's 'AccountsRegistrationOpen' field
 func (st *ConfigState) GetAccountsRegistrationOpen() (v bool) {
 	st.mutex.RLock()
diff --git a/internal/config/validate.go b/internal/config/validate.go
index 45cdc4eee..d79d83b9d 100644
--- a/internal/config/validate.go
+++ b/internal/config/validate.go
@@ -18,85 +18,131 @@
 package config
 
 import (
-	"errors"
 	"fmt"
-	"strings"
 
 	"github.com/miekg/dns"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/language"
 	"github.com/superseriousbusiness/gotosocial/internal/log"
 )
 
-// Validate validates global config settings which don't have defaults, to make sure they are set sensibly.
+// Validate validates global config settings.
 func Validate() error {
-	errs := []error{}
+	// Gather all validation errors in
+	// easily readable format for admins.
+	var (
+		errs gtserror.MultiError
+		errf = func(format string, a ...any) {
+			errs = append(errs, fmt.Errorf(format, a...))
+		}
+	)
 
-	// host
+	// `host`
 	host := GetHost()
 	if host == "" {
-		errs = append(errs, fmt.Errorf("%s must be set", HostFlag()))
+		errf("%s must be set", HostFlag())
 	}
 
-	// accountDomain; only check if host was set, otherwise there's no point
+	// If `account-domain` and `host`
+	// are set, `host` must be a valid
+	// subdomain of `account-domain`.
 	if host != "" {
-		switch ad := GetAccountDomain(); ad {
-		case "":
+		ad := GetAccountDomain()
+		if ad == "" {
+			// `account-domain` not set, fall
+			// back by setting it to `host`.
 			SetAccountDomain(GetHost())
-		default:
-			if !dns.IsSubDomain(ad, host) {
-				errs = append(errs, fmt.Errorf("%s was %s and %s was %s, but %s is not a valid subdomain of %s", HostFlag(), host, AccountDomainFlag(), ad, host, ad))
-			}
+		} else if !dns.IsSubDomain(ad, host) {
+			errf(
+				"%s %s is not a valid subdomain of %s %s",
+				AccountDomainFlag(), ad, HostFlag(), host,
+			)
 		}
 	}
 
-	// protocol
+	// Ensure `protocol` sensibly set.
 	switch proto := GetProtocol(); proto {
 	case "https":
-		// no problem
-		break
+		// No problem.
+
 	case "http":
-		log.Warnf(nil, "%s was set to 'http'; this should *only* be used for debugging and tests!", ProtocolFlag())
+		log.Warnf(
+			nil,
+			"%s was set to 'http'; this should *only* be used for debugging and tests!",
+			ProtocolFlag(),
+		)
+
 	case "":
-		errs = append(errs, fmt.Errorf("%s must be set", ProtocolFlag()))
+		errf("%s must be set", ProtocolFlag())
+
 	default:
-		errs = append(errs, fmt.Errorf("%s must be set to either http or https, provided value was %s", ProtocolFlag(), proto))
+		errf(
+			"%s must be set to either http or https, provided value was %s",
+			ProtocolFlag(), proto,
+		)
 	}
 
-	// federation mode
-	switch federationMode := GetInstanceFederationMode(); federationMode {
+	// `federation-mode` should be
+	// "blocklist" or "allowlist".
+	switch fediMode := GetInstanceFederationMode(); fediMode {
 	case InstanceFederationModeBlocklist, InstanceFederationModeAllowlist:
-		// no problem
-		break
+		// No problem.
+
 	case "":
-		errs = append(errs, fmt.Errorf("%s must be set", InstanceFederationModeFlag()))
+		errf("%s must be set", InstanceFederationModeFlag())
+
 	default:
-		errs = append(errs, fmt.Errorf("%s must be set to either blocklist or allowlist, provided value was %s", InstanceFederationModeFlag(), federationMode))
+		errf(
+			"%s must be set to either blocklist or allowlist, provided value was %s",
+			InstanceFederationModeFlag(), fediMode,
+		)
 	}
 
+	// Parse `instance-languages`, and
+	// set enriched version into config.
+	parsedLangs, err := language.InitLangs(GetInstanceLanguages().TagStrs())
+	if err != nil {
+		errf(
+			"%s could not be parsed as an array of valid BCP47 language tags: %v",
+			InstanceLanguagesFlag(), err,
+		)
+	} else {
+		// Parsed successfully, put enriched
+		// versions in config immediately.
+		SetInstanceLanguages(parsedLangs)
+	}
+
+	// `web-assets-base-dir`.
 	webAssetsBaseDir := GetWebAssetBaseDir()
 	if webAssetsBaseDir == "" {
-		errs = append(errs, fmt.Errorf("%s must be set", WebAssetBaseDirFlag()))
+		errf("%s must be set", WebAssetBaseDirFlag())
 	}
 
-	tlsChain := GetTLSCertificateChain()
-	tlsKey := GetTLSCertificateKey()
-	tlsChainFlag := TLSCertificateChainFlag()
-	tlsKeyFlag := TLSCertificateKeyFlag()
+	// Custom / LE TLS settings.
+	//
+	// Only one of custom certs or LE can be set,
+	// and if using custom certs then all relevant
+	// values must be provided.
+	var (
+		tlsChain     = GetTLSCertificateChain()
+		tlsKey       = GetTLSCertificateKey()
+		tlsChainFlag = TLSCertificateChainFlag()
+		tlsKeyFlag   = TLSCertificateKeyFlag()
+	)
 
 	if GetLetsEncryptEnabled() && (tlsChain != "" || tlsKey != "") {
-		errs = append(errs, fmt.Errorf("%s cannot be enabled when %s and/or %s are also set", LetsEncryptEnabledFlag(), tlsChainFlag, tlsKeyFlag))
+		errf(
+			"%s cannot be true when %s and/or %s are also set",
+			LetsEncryptEnabledFlag(), tlsChainFlag, tlsKeyFlag,
+		)
 	}
 
 	if (tlsChain != "" && tlsKey == "") || (tlsChain == "" && tlsKey != "") {
-		errs = append(errs, fmt.Errorf("%s and %s need to both be set or unset", tlsChainFlag, tlsKeyFlag))
-	}
-
-	if len(errs) > 0 {
-		errStrings := []string{}
-		for _, err := range errs {
-			errStrings = append(errStrings, err.Error())
-		}
-		return errors.New(strings.Join(errStrings, "; "))
+		errf(
+			"%s and %s need to both be set or unset",
+			tlsChainFlag, tlsKeyFlag,
+		)
 	}
 
-	return nil
+	return errs.Combine()
 }
diff --git a/internal/config/validate_test.go b/internal/config/validate_test.go
index 72c268c16..1ae3d6c78 100644
--- a/internal/config/validate_test.go
+++ b/internal/config/validate_test.go
@@ -80,7 +80,7 @@ func (suite *ConfigValidateTestSuite) TestValidateAccountDomainNotSubdomain1() {
 	config.SetAccountDomain("example.com")
 
 	err := config.Validate()
-	suite.EqualError(err, "host was gts.example.org and account-domain was example.com, but gts.example.org is not a valid subdomain of example.com")
+	suite.EqualError(err, "account-domain example.com is not a valid subdomain of host gts.example.org")
 }
 
 func (suite *ConfigValidateTestSuite) TestValidateAccountDomainNotSubdomain2() {
@@ -90,7 +90,7 @@ func (suite *ConfigValidateTestSuite) TestValidateAccountDomainNotSubdomain2() {
 	config.SetAccountDomain("gts.example.org")
 
 	err := config.Validate()
-	suite.EqualError(err, "host was example.org and account-domain was gts.example.org, but example.org is not a valid subdomain of gts.example.org")
+	suite.EqualError(err, "account-domain gts.example.org is not a valid subdomain of host example.org")
 }
 
 func (suite *ConfigValidateTestSuite) TestValidateConfigNoProtocol() {
@@ -118,7 +118,7 @@ func (suite *ConfigValidateTestSuite) TestValidateConfigNoProtocolOrHost() {
 	config.SetProtocol("")
 
 	err := config.Validate()
-	suite.EqualError(err, "host must be set; protocol must be set")
+	suite.EqualError(err, "host must be set\nprotocol must be set")
 }
 
 func (suite *ConfigValidateTestSuite) TestValidateConfigBadProtocol() {
@@ -137,7 +137,7 @@ func (suite *ConfigValidateTestSuite) TestValidateConfigBadProtocolNoHost() {
 	config.SetProtocol("foo")
 
 	err := config.Validate()
-	suite.EqualError(err, "host must be set; protocol must be set to either http or https, provided value was foo")
+	suite.EqualError(err, "host must be set\nprotocol must be set to either http or https, provided value was foo")
 }
 
 func TestConfigValidateTestSuite(t *testing.T) {