concurrency/dir & WriteIdentityToFile (#112)

* concurrency/dir & WriteIdentityToFile

Adds concurrency/dir package to handle atomically writing files to a
directory.

Adds support for spiffe to optionally write the identity certificate,
private key and trust bundle to a given directory.

Signed-off-by: joshvanl <[email protected]>

* Adds Dir comment

Signed-off-by: joshvanl <[email protected]>

---------

Signed-off-by: joshvanl <[email protected]>

concurrency/dir/dir.go

@@ -0,0 +1,90 @@
+/*
+Copyright 2025 The Dapr 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 dir
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/dapr/kit/logger"
+)
+
+type Options struct {
+	Log    logger.Logger
+	Target string
+}
+
+// Dir atomically writes files to a given directory.
+type Dir struct {
+	log logger.Logger
+
+	base      string
+	target    string
+	targetDir string
+
+	prev *string
+}
+
+func New(opts Options) *Dir {
+	return &Dir{
+		log:       opts.Log,
+		base:      filepath.Dir(opts.Target),
+		target:    opts.Target,
+		targetDir: filepath.Base(opts.Target),
+	}
+}
+
+func (d *Dir) Write(files map[string][]byte) error {
+	newDir := filepath.Join(d.base, fmt.Sprintf("%d-%s", time.Now().UTC().UnixNano(), d.targetDir))
+
+	if err := os.MkdirAll(d.base, os.ModePerm); err != nil {
+		return err
+	}
+
+	if err := os.MkdirAll(newDir, os.ModePerm); err != nil {
+		return err
+	}
+
+	for file, b := range files {
+		path := filepath.Join(newDir, file)
+		if err := os.WriteFile(path, b, os.ModePerm); err != nil {
+			return err
+		}
+		d.log.Infof("Written file %s", file)
+	}
+
+	if err := os.Symlink(newDir, d.target+".new"); err != nil {
+		return err
+	}
+
+	d.log.Infof("Syslink %s to %s.new", newDir, d.target)
+
+	if err := os.Rename(d.target+".new", d.target); err != nil {
+		return err
+	}
+
+	d.log.Infof("Atomic write to %s", d.target)
+
+	if d.prev != nil {
+		if err := os.RemoveAll(*d.prev); err != nil {
+			return err
+		}
+	}
+
+	d.prev = &newDir
+
+	return nil
+}

crypto/spiffe/spiffe.go

@@ -28,6 +28,9 @@ import (
 	"github.com/spiffe/go-spiffe/v2/svid/x509svid"
 	"k8s.io/utils/clock"
 
+	"github.com/dapr/kit/concurrency/dir"
+	"github.com/dapr/kit/crypto/pem"
+	"github.com/dapr/kit/crypto/spiffe/trustanchors"
 	"github.com/dapr/kit/logger"
 )
 
@@ -38,6 +41,14 @@ type (
 type Options struct {
 	Log           logger.Logger
 	RequestSVIDFn RequestSVIDFn
+
+	// WriteIdentityToFile is used to write the identity private key and
+	// certificate chain to file. The certificate chain and private key will be
+	// written to the `tls.cert` and `tls.key` files respectively in the given
+	// directory.
+	WriteIdentityToFile *string
+
+	TrustAnchors trustanchors.Interface
 }
 
 // SPIFFE is a readable/writeable store of a SPIFFE X.509 SVID.
@@ -46,6 +57,9 @@ type SPIFFE struct {
 	currentSVID   *x509svid.SVID
 	requestSVIDFn RequestSVIDFn
 
+	dir          *dir.Dir
+	trustAnchors trustanchors.Interface
+
 	log     logger.Logger
 	lock    sync.RWMutex
 	clock   clock.Clock
@@ -54,8 +68,18 @@ type SPIFFE struct {
 }
 
 func New(opts Options) *SPIFFE {
+	var sdir *dir.Dir
+	if opts.WriteIdentityToFile != nil {
+		sdir = dir.New(dir.Options{
+			Log:    opts.Log,
+			Target: *opts.WriteIdentityToFile,
+		})
+	}
+
 	return &SPIFFE{
 		requestSVIDFn: opts.RequestSVIDFn,
+		dir:           sdir,
+		trustAnchors:  opts.TrustAnchors,
 		log:           opts.Log,
 		clock:         clock.RealClock{},
 		readyCh:       make(chan struct{}),
@@ -165,6 +189,31 @@ func (s *SPIFFE) fetchIdentityCertificate(ctx context.Context) (*x509svid.SVID,
 		return nil, fmt.Errorf("error parsing spiffe id from newly signed certificate: %w", err)
 	}
 
+	if s.dir != nil {
+		pkPEM, err := pem.EncodePrivateKey(key)
+		if err != nil {
+			return nil, err
+		}
+
+		certPEM, err := pem.EncodeX509Chain(workloadcert)
+		if err != nil {
+			return nil, err
+		}
+
+		td, err := s.trustAnchors.CurrentTrustAnchors(ctx)
+		if err != nil {
+			return nil, err
+		}
+
+		if err := s.dir.Write(map[string][]byte{
+			"key.pem":  pkPEM,
+			"cert.pem": certPEM,
+			"ca.pem":   td,
+		}); err != nil {
+			return nil, err
+		}
+	}
+
 	return &x509svid.SVID{
 		ID:           spiffeID,
 		Certificates: workloadcert,