config: Don't treat EPERM as error when signallingwc-prune-reused-pid

When pruning Gitaly's runtime directory we send the equivalent of
`kill -0 <PID>` to validate the process has ended. However, if the pid
has been reused by the kernel for a process owned by another user we
will receive `EPERM`, causing Gitaly to exit with an error.

On systems where dozens or hundreds of old runtime directories have
accumulated over months, the odds of our hitting a reused pid are quite
high.

Let's treat `EPERM` as acceptable in this scenario, as we know that the
process we tried to signal can't be Gitaly if it's owned by another
user.

Changelog: fixed

2 files changed, 9 insertions, 1 deletions

diff --git a/internal/gitaly/config/config.go b/internal/gitaly/config/config.go
index 460c6114a..0e458b0ba 100644
--- a/internal/gitaly/config/config.go
+++ b/internal/gitaly/config/config.go
@@ -586,7 +586,9 @@ func PruneRuntimeDirectories(log log.FieldLogger, runtimeDir string) error {
 			}()
 
 			if err := process.Signal(syscall.Signal(0)); err != nil {
-				if !errors.Is(err, os.ErrProcessDone) {
+				// Either the process does not exist, or the pid has been re-used by for a
+				// process owned by another user and is not a Gitaly process.
+				if !errors.Is(err, os.ErrProcessDone) && !errors.Is(err, syscall.EPERM) {
 					return fmt.Errorf("signal: %w", err)
 				}
 
diff --git a/internal/gitaly/config/config_test.go b/internal/gitaly/config/config_test.go
index b5690bf98..132bdc68d 100644
--- a/internal/gitaly/config/config_test.go
+++ b/internal/gitaly/config/config_test.go
@@ -1336,6 +1336,12 @@ func TestPruneRuntimeDirectories(t *testing.T) {
 			expectedLogs[staleRuntimeDir] = "removing leftover runtime directory"
 		}
 
+		// Setup runtime directory with pid of process not owned by git user
+		rootRuntimeDir, err := SetupRuntimeDirectory(cfg, 1)
+		require.NoError(t, err)
+		expectedLogs[rootRuntimeDir] = "removing leftover runtime directory"
+		prunableDirs = append(prunableDirs, rootRuntimeDir)
+
 		// Create an unexpected file in the runtime directory
 		unexpectedFilePath := filepath.Join(baseDir, "unexpected-file")
 		require.NoError(t, os.WriteFile(unexpectedFilePath, []byte(""), os.ModePerm))