gitutil: fix HOME=NUL failure on Windows ARM64

util/gitutil/git_cli.go

@@ -219,9 +219,9 @@ func (cli *GitCLI) Run(ctx context.Context, args ...string) (_ []byte, err error
 			}
 		} else {
 			cmd.Env = append(cmd.Env,
-				"GIT_CONFIG_NOSYSTEM=1",         // Disable reading from system gitconfig.
-				"HOME="+os.DevNull,              // Disable reading from user gitconfig.
-				"GIT_CONFIG_GLOBAL="+os.DevNull, // Disable reading from global gitconfig.
+				"GIT_CONFIG_NOSYSTEM=1",               // Disable reading from system gitconfig.
+				"HOME="+noConfigHome(),                // Disable reading from user gitconfig.
+				"GIT_CONFIG_GLOBAL="+noConfigGlobal(), // Disable reading from global gitconfig.
 			)
 		}
 		for _, ev := range proxyEnvVars {

util/gitutil/git_cli_test.go

@@ -35,8 +35,8 @@ func TestGitCLIConfigEnv(t *testing.T) {
 		_, err := cli.Run(context.Background(), "status")
 		require.NoError(t, err)
 		require.Contains(t, got, "GIT_CONFIG_NOSYSTEM=1")
-		require.Contains(t, got, "HOME="+os.DevNull)
-		require.Contains(t, got, "GIT_CONFIG_GLOBAL="+os.DevNull)
+		require.Contains(t, got, "HOME="+noConfigHome())
+		require.Contains(t, got, "GIT_CONFIG_GLOBAL="+noConfigGlobal())
 		require.NotContains(t, got, "HOME=/tmp/home")
 		require.NotContains(t, got, "XDG_CONFIG_HOME=/tmp/xdg")
 		require.NotContains(t, got, "GIT_CONFIG_GLOBAL=/tmp/global-gitconfig")
@@ -55,8 +55,8 @@ func TestGitCLIConfigEnv(t *testing.T) {
 		_, err := cli.Run(context.Background(), "status")
 		require.NoError(t, err)
 		require.NotContains(t, got, "GIT_CONFIG_NOSYSTEM=1")
-		require.NotContains(t, got, "HOME="+os.DevNull)
-		require.NotContains(t, got, "GIT_CONFIG_GLOBAL="+os.DevNull)
+		require.NotContains(t, got, "HOME="+noConfigHome())
+		require.NotContains(t, got, "GIT_CONFIG_GLOBAL="+noConfigGlobal())
 		require.Contains(t, got, "HOME=/tmp/home")
 		require.Contains(t, got, "XDG_CONFIG_HOME=/tmp/xdg")
 		require.Contains(t, got, `USERPROFILE=C:\Users\tester`)

util/gitutil/git_noconfig_unix.go

@@ -0,0 +1,18 @@
+//go:build !windows
+
+package gitutil
+
+import "os"
+
+// noConfigHome returns a path to use for HOME that prevents git from
+// reading user-level configuration. On Unix, /dev/null works because
+// git silently fails to read /dev/null/.gitconfig.
+func noConfigHome() string {
+	return os.DevNull
+}
+
+// noConfigGlobal returns a path to use for GIT_CONFIG_GLOBAL that
+// prevents git from reading the global gitconfig.
+func noConfigGlobal() string {
+	return os.DevNull
+}

util/gitutil/git_noconfig_windows.go

@@ -0,0 +1,47 @@
+//go:build windows
+
+package gitutil
+
+import (
+	"os"
+	"path/filepath"
+	"sync"
+)
+
+var (
+	noConfigOnce sync.Once
+	noConfigDir  string
+	noConfigFile string
+)
+
+func initNoConfig() {
+	// On Windows ARM64, git cannot use the NUL device as HOME or a config
+	// path ("fatal: unable to access 'NUL': Invalid argument"). Create a
+	// temporary empty directory for HOME (so ~/.gitconfig is absent) and an
+	// empty file for GIT_CONFIG_GLOBAL.
+	dir, err := os.MkdirTemp("", "buildkit-git-noconfig")
+	if err != nil {
+		// Fallback: use os.DevNull (works on AMD64, may fail on ARM64).
+		noConfigDir = os.DevNull
+		noConfigFile = os.DevNull
+		return
+	}
+	noConfigDir = dir
+	noConfigFile = filepath.Join(dir, "empty-gitconfig")
+	// Best-effort create; an empty file is a valid (empty) gitconfig.
+	os.WriteFile(noConfigFile, nil, 0o600)
+}
+
+// noConfigHome returns a path to use for HOME that prevents git from
+// reading user-level configuration.
+func noConfigHome() string {
+	noConfigOnce.Do(initNoConfig)
+	return noConfigDir
+}
+
+// noConfigGlobal returns a path to use for GIT_CONFIG_GLOBAL that
+// prevents git from reading the global gitconfig.
+func noConfigGlobal() string {
+	noConfigOnce.Do(initNoConfig)
+	return noConfigFile
+}