fix(pkg/stash): close etcd sessions

Athens creates a new session for each 'Stash' request and then never closes it.
The sessions (etcd leases) are consequently held forever and eventually leads
to resource exhaustion on the etcd cluster.

This has been fixed by closing the acquired session at the end of the request.

Fixes: gomods#1886

.golangci.yml

@@ -6,6 +6,10 @@ linters-settings:
   cyclop:
     max-complexity: 12
     skip-tests: true
+  errcheck:
+    exclude-functions:
+      - (*go.etcd.io/etcd/client/v3/concurrency.Mutex).Unlock
+      - (*go.etcd.io/etcd/client/v3/concurrency.Session).Close
   gofumpt:
     extra-rules: true
 
@@ -50,15 +54,15 @@ linters:
 issues:
   exclude-use-default: false
   exclude:
-    - 'package-comments: should have a package comment'
-    - 'ST1000: at least one file in a package should have a package comment'
-    - 'G204: Subprocess launched with a potential tainted input or cmd arguments'
-    - 'G204: Subprocess launched with variable'
-    - 'G402: TLS MinVersion too low.'
-    - 'const `op` is unused'
+    - "package-comments: should have a package comment"
+    - "ST1000: at least one file in a package should have a package comment"
+    - "G204: Subprocess launched with a potential tainted input or cmd arguments"
+    - "G204: Subprocess launched with variable"
+    - "G402: TLS MinVersion too low."
+    - "const `op` is unused"
   exclude-rules:
     - path: cmd/proxy/main.go
-      text: 'G108: Profiling endpoint is automatically exposed on /debug/pprof'
+      text: "G108: Profiling endpoint is automatically exposed on /debug/pprof"
     - path: pkg/stash/stasher.go
       linters:
         - contextcheck

pkg/stash/with_etcd.go

@@ -56,33 +56,31 @@ func (s *etcd) Stash(ctx context.Context, mod, ver string) (newVer string, err e
 	const op errors.Op = "etcd.Stash"
 	ctx, span := observ.StartSpan(ctx, op.String())
 	defer span.End()
-	sesh, err := concurrency.NewSession(s.client)
+
+	sess, err := concurrency.NewSession(s.client)
 	if err != nil {
-		return ver, errors.E(op, err)
+		return "", errors.E(op, err)
 	}
-	mv := config.FmtModVer(mod, ver)
-	mu := concurrency.NewMutex(sesh, mv)
-	err = mu.Lock(ctx)
-	if err != nil {
-		return ver, errors.E(op, err)
+	defer sess.Close()
+
+	m := concurrency.NewMutex(sess, config.FmtModVer(mod, ver))
+	if err := m.Lock(ctx); err != nil {
+		return "", errors.E(op, err)
 	}
-	defer func() {
-		const op errors.Op = "etcd.Unlock"
-		lockErr := mu.Unlock(ctx)
-		if err == nil && lockErr != nil {
-			err = errors.E(op, lockErr)
-		}
-	}()
+	defer m.Unlock(ctx)
+
 	ok, err := s.checker.Exists(ctx, mod, ver)
 	if err != nil {
-		return ver, errors.E(op, err)
+		return "", errors.E(op, err)
 	}
+
 	if ok {
 		return ver, nil
 	}
+
 	newVer, err = s.stasher.Stash(ctx, mod, ver)
 	if err != nil {
-		return ver, errors.E(op, err)
+		return "", errors.E(op, err)
 	}
 	return newVer, nil
 }