test: CheckForExistingPostmaster implementation easier to test (cloudnative-pg#370)

We weren't testing the positive case in our unit tests, that is when the
`postmaster` process actually exists.

In order to do that, the implementation of `CheckForExistingPostmaster`
had to be changed to allow the code to inject a custom process name.

Closes cloudnative-pg#369

Signed-off-by: Leonardo Cecchi <leonardo.cecchi@enterprisedb.com>

Author: leonardoce

pkg/management/postgres/instance.go

@@ -396,7 +396,7 @@ func (instance *Instance) Reload() error {
 // Run this instance returning an OS process needed
 // to control the instance execution
 func (instance Instance) Run() (*execlog.StreamingCmd, error) {
-	process, err := instance.CheckForExistingPostmaster()
+	process, err := instance.CheckForExistingPostmaster(postgresName)
 	if err != nil {
 		return nil, err
 	}

pkg/management/postgres/pidfile.go

@@ -23,6 +23,7 @@ import (
 	"strings"
 
 	"github.com/mitchellh/go-ps"
+	"k8s.io/utils/strings/slices"
 
 	"github.com/cloudnative-pg/cloudnative-pg/pkg/fileutils"
 	"github.com/cloudnative-pg/cloudnative-pg/pkg/management/log"
@@ -39,7 +40,12 @@ const PostgresqlPidFile = "postmaster.pid" //wokeignore:rule=master
 // directory and check the existence of the relative process. If the
 // process exists, then that process entry is returned.
 // If it doesn't exist then the PID file is stale and is removed.
-func (instance *Instance) CheckForExistingPostmaster() (*os.Process, error) {
+//
+// We are requiring a list of PostgreSQL executables that is used
+// to check if the process with the ID read from the PID file
+// is really a postmaster. This isn't really needed per se, but
+// allows this function to be easier to test.
+func (instance *Instance) CheckForExistingPostmaster(postgresExecutables ...string) (*os.Process, error) {
 	pidFile := path.Join(instance.PgData, PostgresqlPidFile)
 	contextLog := log.WithValues("file", pidFile)
 	pidFileContents, pid, err := instance.GetPostmasterPidFromFile(pidFile)
@@ -64,7 +70,7 @@ func (instance *Instance) CheckForExistingPostmaster() (*os.Process, error) {
 		return nil, instance.CleanUpStalePid()
 	}
 
-	if process.Executable() != postgresName {
+	if !slices.Contains(postgresExecutables, process.Executable()) {
 		// The process is not running PostgreSQL and this PID file is stale
 		contextLog.Info("The PID file is stale (executable mismatch), deleting it",
 			"pidFileContents", pidFileContents, "postgresExecutable", process.Executable())

pkg/management/postgres/pidfile_test.go

@@ -22,6 +22,8 @@ import (
 	"os"
 	"path/filepath"
 
+	"github.com/mitchellh/go-ps"
+
 	"github.com/cloudnative-pg/cloudnative-pg/pkg/fileutils"
 
 	. "github.com/onsi/ginkgo/v2"
@@ -49,7 +51,7 @@ var _ = Describe("the detection of a postmaster process using the pid file", fun
 		instance := NewInstance()
 		instance.PgData = pgdata
 		instance.SocketDirectory = socketDir
-		process, err := instance.CheckForExistingPostmaster()
+		process, err := instance.CheckForExistingPostmaster(postgresName)
 		Expect(err).ShouldNot(HaveOccurred())
 		Expect(process).To(BeNil())
 	})
@@ -67,7 +69,7 @@ var _ = Describe("the detection of a postmaster process using the pid file", fun
 		err = ioutil.WriteFile(filepath.Join(socketDir, ".s.PGSQL.5432.lock"), []byte("1234"), 0o400)
 		Expect(err).ShouldNot(HaveOccurred())
 
-		process, err := instance.CheckForExistingPostmaster()
+		process, err := instance.CheckForExistingPostmaster(postgresName)
 		Expect(err).ShouldNot(HaveOccurred())
 		Expect(process).To(BeNil())
 
@@ -89,10 +91,16 @@ var _ = Describe("the detection of a postmaster process using the pid file", fun
 			filepath.Join(pgdata, PostgresqlPidFile),
 			[]byte(fmt.Sprintf("%v", myPid)), 0o400)
 		Expect(err).ShouldNot(HaveOccurred())
+		myProcess, err := ps.FindProcess(myPid)
+		Expect(err).ShouldNot(HaveOccurred())
+		myExecutable := myProcess.Executable()
 
-		process, err := instance.CheckForExistingPostmaster()
+		process, err := instance.CheckForExistingPostmaster(myExecutable)
 		Expect(err).ShouldNot(HaveOccurred())
-		// to be null because executable file name is not postgres
-		Expect(process).To(BeNil())
+		Expect(process).ToNot(BeNil())
+
+		_, err = instance.CheckForExistingPostmaster(myExecutable, "not_existent_executable")
+		Expect(err).ShouldNot(HaveOccurred())
+		Expect(process).ToNot(BeNil())
 	})
 })