3 files changed, 134 insertions, 0 deletions
diff --git a/internal/config/atomicfile/README.md b/internal/config/atomicfile/README.md
new file mode 100644
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+++ b/internal/config/atomicfile/README.md
@@ -0,0 +1,36 @@
+# Atomicfile
+
+This package was copied from: https://github.com/tailscale/tailscale/blob/21460a5b14e9fb883fedd6071ff53729ed68370c/atomicfile
+
+# License:
+
+```
+BSD 3-Clause License
+
+Copyright (c) 2020 Tailscale Inc & AUTHORS.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this
+   list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice,
+   this list of conditions and the following disclaimer in the documentation
+   and/or other materials provided with the distribution.
+
+3. Neither the name of the copyright holder nor the names of its
+   contributors may be used to endorse or promote products derived from
+   this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+```
diff --git a/internal/config/atomicfile/atomicfile.go b/internal/config/atomicfile/atomicfile.go
new file mode 100644
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+++ b/internal/config/atomicfile/atomicfile.go
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+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package atomicfile contains code related to writing to filesystems
+// atomically.
+//
+// This package should be considered internal; its API is not stable.
+package atomicfile // import "tailscale.com/atomicfile"
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+)
+
+// WriteFile writes data to filename+some suffix, then renames it into filename.
+// The perm argument is ignored on Windows. If the target filename already
+// exists but is not a regular file, WriteFile returns an error.
+func WriteFile(filename string, data []byte, perm os.FileMode) (err error) {
+	fi, err := os.Stat(filename)
+	if err == nil && !fi.Mode().IsRegular() {
+		return fmt.Errorf("%s already exists and is not a regular file", filename)
+	}
+	f, err := os.CreateTemp(filepath.Dir(filename), filepath.Base(filename)+".tmp")
+	if err != nil {
+		return err
+	}
+	tmpName := f.Name()
+	defer func() {
+		if err != nil {
+			f.Close()
+			os.Remove(tmpName)
+		}
+	}()
+	if _, err := f.Write(data); err != nil {
+		return err
+	}
+	if runtime.GOOS != "windows" {
+		if err := f.Chmod(perm); err != nil {
+			return err
+		}
+	}
+	if err := f.Sync(); err != nil {
+		return err
+	}
+	if err := f.Close(); err != nil {
+		return err
+	}
+	return os.Rename(tmpName, filename)
+}
diff --git a/internal/config/atomicfile/atomicfile_test.go b/internal/config/atomicfile/atomicfile_test.go
new file mode 100644
index 0000000..78c93e6
--- /dev/null
+++ b/internal/config/atomicfile/atomicfile_test.go
@@ -0,0 +1,47 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !js && !windows
+
+package atomicfile
+
+import (
+	"net"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func TestDoesNotOverwriteIrregularFiles(t *testing.T) {
+	// Per tailscale/tailscale#7658 as one example, almost any imagined use of
+	// atomicfile.Write should likely not attempt to overwrite an irregular file
+	// such as a device node, socket, or named pipe.
+
+	const filename = "TestDoesNotOverwriteIrregularFiles"
+	var path string
+	// macOS private temp does not allow unix socket creation, but /tmp does.
+	if runtime.GOOS == "darwin" {
+		path = filepath.Join("/tmp", filename)
+		t.Cleanup(func() { os.Remove(path) })
+	} else {
+		path = filepath.Join(t.TempDir(), filename)
+	}
+
+	// The least troublesome thing to make that is not a file is a unix socket.
+	// Making a null device sadly requires root.
+	l, err := net.ListenUnix("unix", &net.UnixAddr{Name: path, Net: "unix"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer l.Close()
+
+	err = WriteFile(path, []byte("hello"), 0644)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !strings.Contains(err.Error(), "is not a regular file") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}