upgrade to etcd/client/v3

go.mod

@@ -3,23 +3,28 @@ module github.com/flatcar/locksmith
 go 1.26.0
 
 require (
-	github.com/coreos/go-systemd/v22 v22.3.2
+	github.com/coreos/go-systemd/v22 v22.7.0
 	github.com/coreos/pkg v0.0.0-20240122114842-bbd7aa9bf6fb
 	github.com/godbus/dbus/v5 v5.2.2
 	github.com/rkt/rkt v1.30.0
-	go.etcd.io/etcd/client/v2 v2.305.33
+	go.etcd.io/etcd/api/v3 v3.7.1
+	go.etcd.io/etcd/client/v3 v3.7.1
 )
 
 require (
-	github.com/coreos/go-semver v0.3.0 // indirect
+	github.com/coreos/go-semver v0.3.1 // indirect
 	github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf // indirect
-	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
+	github.com/golang/protobuf v1.5.4 // indirect
+	github.com/grpc-ecosystem/grpc-gateway/v2 v2.29.0 // indirect
 	github.com/hashicorp/errwrap v1.1.0 // indirect
-	github.com/json-iterator/go v1.1.12 // indirect
-	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
-	github.com/modern-go/reflect2 v1.0.2 // indirect
-	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
-	go.etcd.io/etcd/api/v3 v3.5.33 // indirect
-	go.etcd.io/etcd/client/pkg/v3 v3.5.33 // indirect
+	go.etcd.io/etcd/client/pkg/v3 v3.7.1 // indirect
+	go.uber.org/multierr v1.11.0 // indirect
+	go.uber.org/zap v1.27.1 // indirect
+	golang.org/x/net v0.56.0 // indirect
 	golang.org/x/sys v0.48.0 // indirect
+	golang.org/x/text v0.39.0 // indirect
+	google.golang.org/genproto/googleapis/api v0.0.0-20260414002931-afd174a4e478 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20260414002931-afd174a4e478 // indirect
+	google.golang.org/grpc v1.82.1 // indirect
+	google.golang.org/protobuf v1.36.11 // indirect
 )

go.sum

@@ -1,46 +1,77 @@
-github.com/coreos/go-semver v0.3.0 h1:wkHLiw0WNATZnSG7epLsujiMCgPAc9xhjJ4tgnAxmfM=
-github.com/coreos/go-semver v0.3.0/go.mod h1:nnelYz7RCh+5ahJtPPxZlU+153eP4D4r3EedlOD2RNk=
+github.com/cespare/xxhash/v2 v2.3.0 h1:UL815xU9SqsFlibzuggzjXhog7bL6oX9BbNZnL2UFvs=
+github.com/cespare/xxhash/v2 v2.3.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
+github.com/coreos/go-semver v0.3.1 h1:yi21YpKnrx1gt5R+la8n5WgS0kCrsPp33dmEyHReZr4=
+github.com/coreos/go-semver v0.3.1/go.mod h1:irMmmIw/7yzSRPWryHsK7EYSg09caPQL03VsM8rvUec=
 github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf h1:iW4rZ826su+pqaw19uhpSCzhj44qo35pNgKFGqzDKkU=
 github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf/go.mod h1:F5haX7vjVVG0kc13fIWeqUViNPyEJxv/OmvnBo0Yme4=
-github.com/coreos/go-systemd/v22 v22.3.2 h1:D9/bQk5vlXQFZ6Kwuu6zaiXJ9oTPe68++AzAJc1DzSI=
-github.com/coreos/go-systemd/v22 v22.3.2/go.mod h1:Y58oyj3AT4RCenI/lSvhwexgC+NSVTIJ3seZv2GcEnc=
+github.com/coreos/go-systemd/v22 v22.7.0 h1:LAEzFkke61DFROc7zNLX/WA2i5J8gYqe0rSj9KI28KA=
+github.com/coreos/go-systemd/v22 v22.7.0/go.mod h1:xNUYtjHu2EDXbsxz1i41wouACIwT7Ybq9o0BQhMwD0w=
 github.com/coreos/pkg v0.0.0-20240122114842-bbd7aa9bf6fb h1:GIzvVQ9UkUlOhSDlqmrQAAAUd6R3E+caIisNEyWXvNE=
 github.com/coreos/pkg v0.0.0-20240122114842-bbd7aa9bf6fb/go.mod h1:E3G3o1h8I7cfcXa63jLwjI0eiQQMgzzUDFVpN/nH/eA=
-github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc h1:U9qPSI2PIWSS1VwoXQT9A3Wy9MM3WgvqSxFWenqJduM=
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/godbus/dbus/v5 v5.0.4/go.mod h1:xhWf0FNVPg57R7Z0UbKHbJfkEywrmjJnf7w5xrFpKfA=
+github.com/go-logr/logr v1.4.3 h1:CjnDlHq8ikf6E492q6eKboGOC0T8CDaOvkHCIg8idEI=
+github.com/go-logr/logr v1.4.3/go.mod h1:9T104GzyrTigFIr8wt5mBrctHMim0Nb2HLGrmQ40KvY=
+github.com/go-logr/stdr v1.2.2 h1:hSWxHoqTgW2S2qGc0LTAI563KZ5YKYRhT3MFKZMbjag=
+github.com/go-logr/stdr v1.2.2/go.mod h1:mMo/vtBO5dYbehREoey6XUKy/eSumjCCveDpRre4VKE=
 github.com/godbus/dbus/v5 v5.2.2 h1:TUR3TgtSVDmjiXOgAAyaZbYmIeP3DPkld3jgKGV8mXQ=
 github.com/godbus/dbus/v5 v5.2.2/go.mod h1:3AAv2+hPq5rdnr5txxxRwiGjPXamgoIHgz9FPBfOp3c=
-github.com/google/gofuzz v1.0.0/go.mod h1:dBl0BpW6vV/+mYPU4Po3pmUjxk6FQPldtuIdl/M65Eg=
+github.com/golang/protobuf v1.5.4 h1:i7eJL8qZTpSEXOPTxNKhASYpMn+8e5Q6AdndVa1dWek=
+github.com/golang/protobuf v1.5.4/go.mod h1:lnTiLA8Wa4RWRcIUkrtSVa5nRhsEGBg48fD6rSs7xps=
+github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
+github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
+github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
+github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/grpc-ecosystem/grpc-gateway/v2 v2.29.0 h1:5VipnvEpbqr2gA2VbM+nYVbkIF28c5ZQfqCBQ5g2xfk=
+github.com/grpc-ecosystem/grpc-gateway/v2 v2.29.0/go.mod h1:Hyl3n6Twe1hvtd9XUXDec4pTvgMSEixRuQKPTMH2bNs=
 github.com/hashicorp/errwrap v1.1.0 h1:OxrOeh75EUXMY8TBjag2fzXGZ40LB6IKw45YeGUDY2I=
 github.com/hashicorp/errwrap v1.1.0/go.mod h1:YH+1FKiLXxHSkmPseP+kNlulaMuP3n2brvKWEqk/Jc4=
-github.com/json-iterator/go v1.1.12 h1:PV8peI4a0ysnczrg+LtxykD8LfKY9ML6u2jnxaEnrnM=
-github.com/json-iterator/go v1.1.12/go.mod h1:e30LSqwooZae/UwlEbR2852Gd8hjQvJoHmT4TnhNGBo=
-github.com/modern-go/concurrent v0.0.0-20180228061459-e0a39a4cb421/go.mod h1:6dJC0mAP4ikYIbvyc7fijjWJddQyLn8Ig3JB5CqoB9Q=
-github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd h1:TRLaZ9cD/w8PVh93nsPXa1VrQ6jlwL5oN8l14QlcNfg=
-github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd/go.mod h1:6dJC0mAP4ikYIbvyc7fijjWJddQyLn8Ig3JB5CqoB9Q=
-github.com/modern-go/reflect2 v1.0.2 h1:xBagoLtFs94CBntxluKeaWgTMpvLxC4ur3nMaC9Gz0M=
-github.com/modern-go/reflect2 v1.0.2/go.mod h1:yWuevngMOJpCy52FWWMvUC8ws7m/LJsjYzDa0/r8luk=
-github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 h1:Jamvg5psRIccs7FGNTlIRMkT8wgtp5eCXdBlqhYGL6U=
 github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/rkt/rkt v1.30.0 h1:ZI5RQtSibfjicSttV/HLiHuWreYClEJA2Or5XKAdJb0=
 github.com/rkt/rkt v1.30.0/go.mod h1:V5VwmwHe6x1kflB4uXl1XJwXTgRISEMt2lZE6m6lXd0=
-github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
-github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
 github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
 github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
-go.etcd.io/etcd/api/v3 v3.5.33 h1:mUYXLPBlddsrnoJClluMOvUiamVfkHZrpjcGn0P/E7A=
-go.etcd.io/etcd/api/v3 v3.5.33/go.mod h1:HWqrOfdxawIb0emEazUt6pwL/dFYw5spr770r4qqPMM=
-go.etcd.io/etcd/client/pkg/v3 v3.5.33 h1:x9J84BdbVtb+47fQFsXniTA2sZEdy9S5+dQ9q5urpqM=
-go.etcd.io/etcd/client/pkg/v3 v3.5.33/go.mod h1:sngbNbeexiATS2+HxALIeJ7yoKofPzRDn5k9HqROgJ4=
-go.etcd.io/etcd/client/v2 v2.305.33 h1:cgnmUT79NRRF6IsjEqwZhGik/uv/+Ixqgk9uHtW/2gw=
-go.etcd.io/etcd/client/v2 v2.305.33/go.mod h1:SD4bz1U5a5KF5QzS/R9Gf7wiiGY7LD6IQpfZsX3+Qw4=
+go.etcd.io/etcd/api/v3 v3.7.1 h1:KJG0/DcWGfe3Y1otDf/fsBf0TSSgpxZ5RO/L8SFt73E=
+go.etcd.io/etcd/api/v3 v3.7.1/go.mod h1:8bXIpCMeV7E3/XL0Ix123ATn3dB+0V7d9zklHbB0m78=
+go.etcd.io/etcd/client/pkg/v3 v3.7.1 h1:rKYsj3pRkR0eK3yjT3XOgrhqfmIfj9pzNgxjh7mfFv4=
+go.etcd.io/etcd/client/pkg/v3 v3.7.1/go.mod h1:cnzZGIUzSfjEwLC6UBVsSXlEK1eepS/JUD7wE6PLRT0=
+go.etcd.io/etcd/client/v3 v3.7.1 h1:0PEMMC0KuZmVIN+RAbdqfkZ45pYTgKVtmBEbRCvZFUg=
+go.etcd.io/etcd/client/v3 v3.7.1/go.mod h1:ffNqALa8tRCYhYo1F9oR489y23K39Gz+BSR3ApAGYq0=
+go.opentelemetry.io/auto/sdk v1.2.1 h1:jXsnJ4Lmnqd11kwkBV2LgLoFMZKizbCi5fNZ/ipaZ64=
+go.opentelemetry.io/auto/sdk v1.2.1/go.mod h1:KRTj+aOaElaLi+wW1kO/DZRXwkF4C5xPbEe3ZiIhN7Y=
+go.opentelemetry.io/otel v1.43.0 h1:mYIM03dnh5zfN7HautFE4ieIig9amkNANT+xcVxAj9I=
+go.opentelemetry.io/otel v1.43.0/go.mod h1:JuG+u74mvjvcm8vj8pI5XiHy1zDeoCS2LB1spIq7Ay0=
+go.opentelemetry.io/otel/metric v1.43.0 h1:d7638QeInOnuwOONPp4JAOGfbCEpYb+K6DVWvdxGzgM=
+go.opentelemetry.io/otel/metric v1.43.0/go.mod h1:RDnPtIxvqlgO8GRW18W6Z/4P462ldprJtfxHxyKd2PY=
+go.opentelemetry.io/otel/sdk v1.43.0 h1:pi5mE86i5rTeLXqoF/hhiBtUNcrAGHLKQdhg4h4V9Dg=
+go.opentelemetry.io/otel/sdk v1.43.0/go.mod h1:P+IkVU3iWukmiit/Yf9AWvpyRDlUeBaRg6Y+C58QHzg=
+go.opentelemetry.io/otel/sdk/metric v1.43.0 h1:S88dyqXjJkuBNLeMcVPRFXpRw2fuwdvfCGLEo89fDkw=
+go.opentelemetry.io/otel/sdk/metric v1.43.0/go.mod h1:C/RJtwSEJ5hzTiUz5pXF1kILHStzb9zFlIEe85bhj6A=
+go.opentelemetry.io/otel/trace v1.43.0 h1:BkNrHpup+4k4w+ZZ86CZoHHEkohws8AY+WTX09nk+3A=
+go.opentelemetry.io/otel/trace v1.43.0/go.mod h1:/QJhyVBUUswCphDVxq+8mld+AvhXZLhe+8WVFxiFff0=
+go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
+go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
+go.uber.org/multierr v1.11.0 h1:blXXJkSxSSfBVBlC76pxqeO+LN3aDfLQo+309xJstO0=
+go.uber.org/multierr v1.11.0/go.mod h1:20+QtiLqy0Nd6FdQB9TLXag12DsQkrbs3htMFfDN80Y=
+go.uber.org/zap v1.27.1 h1:08RqriUEv8+ArZRYSTXy1LeBScaMpVSTBhCeaZYfMYc=
+go.uber.org/zap v1.27.1/go.mod h1:GB2qFLM7cTU87MWRP2mPIjqfIDnGu+VIO4V/SdhGo2E=
+golang.org/x/net v0.56.0 h1:Rw8j/hFzGvJUZwNBXnAtf5sVDVt+65SK2C7IxCxZt5o=
+golang.org/x/net v0.56.0/go.mod h1:D3Ku6r+V6JROoZK144D2XfMHFcMq/0zSfLelVTCFKec=
 golang.org/x/sys v0.48.0 h1:bbX/i/6MgT9BVLM9RT1thmxL04yeTAhbEz4SyadbXoo=
 golang.org/x/sys v0.48.0/go.mod h1:hNLxWAXmnKAxqDtdwIYC4bM9oQPEecfsnNMuSxOs3og=
-gopkg.in/yaml.v2 v2.4.0 h1:D8xgwECY7CYvx+Y2n4sBz93Jn9JRvxdiyyo8CTfuKaY=
-gopkg.in/yaml.v2 v2.4.0/go.mod h1:RDklbk79AGWmwhnvt/jBztapEOGDOx6ZbXqjP6csGnQ=
+golang.org/x/text v0.39.0 h1:UbZz4pLOvn600D6Oh6GGEI6VAmndrEBLv8/6BEXzyus=
+golang.org/x/text v0.39.0/go.mod h1:3UwRclnC2g0TU9x8PZiyfOajCd1zaUNHF9cvqcQZ+ZM=
+gonum.org/v1/gonum v0.17.0 h1:VbpOemQlsSMrYmn7T2OUvQ4dqxQXU+ouZFQsZOx50z4=
+gonum.org/v1/gonum v0.17.0/go.mod h1:El3tOrEuMpv2UdMrbNlKEh9vd86bmQ6vqIcDwxEOc1E=
+google.golang.org/genproto/googleapis/api v0.0.0-20260414002931-afd174a4e478 h1:yQugLulqltosq0B/f8l4w9VryjV+N/5gcW0jQ3N8Qec=
+google.golang.org/genproto/googleapis/api v0.0.0-20260414002931-afd174a4e478/go.mod h1:C6ADNqOxbgdUUeRTU+LCHDPB9ttAMCTff6auwCVa4uc=
+google.golang.org/genproto/googleapis/rpc v0.0.0-20260414002931-afd174a4e478 h1:RmoJA1ujG+/lRGNfUnOMfhCy5EipVMyvUE+KNbPbTlw=
+google.golang.org/genproto/googleapis/rpc v0.0.0-20260414002931-afd174a4e478/go.mod h1:4Hqkh8ycfw05ld/3BWL7rJOSfebL2Q+DVDeRgYgxUU8=
+google.golang.org/grpc v1.82.1 h1:NnAxzGRA0677vCa4BUkOAnO5+FfQqVl9iUXeD0IqcGE=
+google.golang.org/grpc v1.82.1/go.mod h1:yzTZ1TB1Z3SG+LIYaI+WiE8D5+PZ3ArnrSp8zF3+/ZA=
+google.golang.org/protobuf v1.36.11 h1:fV6ZwhNocDyBLK0dj+fg8ektcVegBBuEolpbTQyBNVE=
+google.golang.org/protobuf v1.36.11/go.mod h1:HTf+CrKn2C3g5S8VImy6tdcUvCska2kB7j23XfzDpco=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

lock/etcd.go

@@ -18,10 +18,19 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"net/url"
 	"path"
 
-	"go.etcd.io/etcd/client/v2"
+	client "go.etcd.io/etcd/client/v3"
+)
+
+// ErrNotFound is used when a key is not found - which means
+// it returns 0 value.
+var (
+	ErrNotFound         = errors.New("key not found")
+	errNilSemaphore     = errors.New("cannot set nil semaphore")
+	errSemaphoreUpdated = errors.New("semaphore got updated in the meantime")
 )
 
 const (
@@ -32,14 +41,17 @@ const (
 	SemaphorePrefix = keyPrefix + "/" + semaphoreBranch
 )
 
-// KeysAPI is the minimum etcd client.KeysAPI interface EtcdLockClient needs
+// KeysAPI is the minimum etcd client.KV interface EtcdLockClient needs
 // to do its job.
 type KeysAPI interface {
-	Get(ctx context.Context, key string, opts *client.GetOptions) (*client.Response, error)
-	Set(ctx context.Context, key, value string, opts *client.SetOptions) (*client.Response, error)
-	Create(ctx context.Context, key, value string) (*client.Response, error)
+	Get(ctx context.Context, key string, opts ...client.OpOption) (*client.GetResponse, error)
+	Txn(ctx context.Context) client.Txn
 }
 
+// To make sure that KeysAPI interface is indeed a subset of client.KV
+// interface.
+var _ KeysAPI = client.KV(nil)
+
 // EtcdLockClient is a wrapper around the etcd client that provides
 // simple primitives to operate on the internal semaphore and holders
 // structs through etcd.
@@ -59,64 +71,91 @@ func NewEtcdLockClient(keyapi KeysAPI, group string) (*EtcdLockClient, error) {
 
 	elc := &EtcdLockClient{keyapi, key}
 	if err := elc.Init(); err != nil {
-		return nil, err
+		return nil, fmt.Errorf("unable to init etcd lock client: %w", err)
 	}
 
 	return elc, nil
 }
 
 // Init sets an initial copy of the semaphore if it doesn't exist yet.
+// So we first try to get the value, if the value is not found we create the key
+// with a default semaphore value.
 func (c *EtcdLockClient) Init() error {
 	sem := newSemaphore()
-	b, err := json.Marshal(sem)
+	payload, err := json.Marshal(sem)
 	if err != nil {
-		return err
+		return fmt.Errorf("marshalling initial semaphore: %w", err)
 	}
 
-	if _, err := c.keyapi.Create(context.Background(), c.keypath, string(b)); err != nil {
-		eerr, ok := err.(client.Error)
-		if ok && eerr.Code == client.ErrorCodeNodeExist {
-			return nil
-		}
-
-		return err
+	if _, err := c.keyapi.Txn(context.Background()).
+		If(
+			// If key has zero as Version, it means that
+			// the key is absent. Existing keys always
+			// have version greater than zero.
+			client.Compare(client.Version(c.keypath), "=", 0),
+		).
+		Then(
+			client.OpPut(c.keypath, string(payload)),
+		).
+		Commit(); err != nil {
+		return fmt.Errorf("committing initial transaction: %w", err)
 	}
 
 	return nil
 }
 
 // Get fetches the Semaphore from etcd.
 func (c *EtcdLockClient) Get() (*Semaphore, error) {
-	resp, err := c.keyapi.Get(context.Background(), c.keypath, nil)
+	resp, err := c.keyapi.Get(context.Background(), c.keypath)
 	if err != nil {
 		return nil, err
 	}
 
+	// There is no proper way to handle non-existing value for a
+	// given key.
+	// See https://github.com/etcd-io/etcd/issues/6089 for more details.
+	if resp.Count == 0 {
+		return nil, ErrNotFound
+	}
+
+	kv := resp.Kvs[0]
+
 	sem := &Semaphore{}
-	err = json.Unmarshal([]byte(resp.Node.Value), sem)
+	err = json.Unmarshal(kv.Value, sem)
 	if err != nil {
 		return nil, err
 	}
 
-	sem.Index = resp.Node.ModifiedIndex
+	sem.Index = uint64(kv.ModRevision)
 
 	return sem, nil
 }
 
 // Set sets a Semaphore in etcd.
 func (c *EtcdLockClient) Set(sem *Semaphore) error {
 	if sem == nil {
-		return errors.New("cannot set nil semaphore")
+		return errNilSemaphore
 	}
 	b, err := json.Marshal(sem)
 	if err != nil {
 		return err
 	}
 
-	setopts := &client.SetOptions{
-		PrevIndex: sem.Index,
+	response, err := c.keyapi.Txn(context.Background()).
+		If(
+			client.Compare(client.ModRevision(c.keypath), "=", int64(sem.Index)),
+		).
+		Then(
+			client.OpPut(c.keypath, string(b)),
+		).
+		Commit()
+	if err != nil {
+		return fmt.Errorf("making transaction: %w", err)
 	}
 
-	_, err = c.keyapi.Set(context.Background(), c.keypath, string(b), setopts)
-	return err
+	if !response.Succeeded {
+		return errSemaphoreUpdated
+	}
+
+	return nil
 }