From a2d253f1e92578ccea95f962bbd1a1aebf190de1 Mon Sep 17 00:00:00 2001
From: Paul Buetow <paul@buetow.org>
Date: Fri, 6 Mar 2026 16:03:00 +0200
Subject: fix: make flags string cache concurrent-safe (task 386)

---
 internal/file/flags.go      | 11 ++++++++---
 internal/file/flags_test.go | 40 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+), 3 deletions(-)
 create mode 100644 internal/file/flags_test.go

(limited to 'internal')

diff --git a/internal/file/flags.go b/internal/file/flags.go
index ca749e1..c06c27b 100644
--- a/internal/file/flags.go
+++ b/internal/file/flags.go
@@ -3,12 +3,13 @@ package file
 import (
 	"os"
 	"strings"
+	"sync"
 	"syscall"
 )
 
 type Flags int32
 
-var flagsToHumanCache = map[Flags]string{}
+var flagsToHumanCache sync.Map
 var unknownFlag = Flags(-1)
 
 type tuple struct {
@@ -49,12 +50,16 @@ func (f Flags) Is(flag int) bool {
 }
 
 func (f Flags) BuildString(sb *strings.Builder) {
-	if str, ok := flagsToHumanCache[f]; ok {
+	if cached, ok := flagsToHumanCache.Load(f); ok {
+		str, _ := cached.(string)
 		sb.WriteString(str)
 		return
 	}
 	str := f.String()
-	flagsToHumanCache[f] = str
+	cached, loaded := flagsToHumanCache.LoadOrStore(f, str)
+	if loaded {
+		str, _ = cached.(string)
+	}
 	sb.WriteString(str)
 }
 
diff --git a/internal/file/flags_test.go b/internal/file/flags_test.go
new file mode 100644
index 0000000..120e432
--- /dev/null
+++ b/internal/file/flags_test.go
@@ -0,0 +1,40 @@
+package file
+
+import (
+	"strings"
+	"sync"
+	"syscall"
+	"testing"
+)
+
+func TestFlagsBuildStringConcurrent(t *testing.T) {
+	flagsToHumanCache = sync.Map{}
+
+	const workers = 32
+	const iterations = 500
+	const want = "O_WRONLY|O_APPEND"
+	flag := Flags(syscall.O_WRONLY | syscall.O_APPEND)
+
+	var wg sync.WaitGroup
+	errs := make(chan string, workers)
+	for i := 0; i < workers; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for j := 0; j < iterations; j++ {
+				var sb strings.Builder
+				flag.BuildString(&sb)
+				if got := sb.String(); got != want {
+					errs <- got
+					return
+				}
+			}
+		}()
+	}
+	wg.Wait()
+	close(errs)
+
+	for got := range errs {
+		t.Fatalf("unexpected BuildString output %q, want %q", got, want)
+	}
+}
-- 
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