Add the build flag useNixCaseHack and set it on Darwin

This build flag allows to publish an image on a case insensitive
FS. Otherwise, hash can differs as detailled in
#37

Fixes !37

default.nix

@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> { } }:
+{ pkgs ? import <nixpkgs> { }, system }:
 let
   l = pkgs.lib // builtins;
 
@@ -21,9 +21,14 @@ let
       );
     };
     vendorSha256 = "sha256-/j4ZHOwU5Xi8CE/fHha+2iZhsLd/y2ovzVhvg8HDV78=";
+    ldflags = pkgs.lib.optional pkgs.stdenv.isDarwin [
+      "-X github.com/nlewo/nix2container/nix.useNixCaseHack=true"
+    ];
+
   };
 
   skopeo-nix2container = pkgs.skopeo.overrideAttrs (old: {
+    EXTRA_LDFLAGS = pkgs.lib.optionalString pkgs.stdenv.isDarwin "-X github.com/nlewo/nix2container/nix.useNixCaseHack=true";
     preBuild = let
       patch = pkgs.fetchurl {
         url = "https://github.com/Mic92/image/commit/b3cb51066518ed2c6f6c8cf0cb4ae1e84f68b5ce.patch";

flake.nix

@@ -9,7 +9,7 @@
       let
         pkgs = nixpkgs.legacyPackages.${system};
         nix2container = import ./. {
-          inherit pkgs;
+          inherit pkgs system;
         };
         examples = import ./examples {
           inherit pkgs;

nix/graph.go

@@ -9,6 +9,11 @@ import (
 	"sort"
 )
 
+// On case insensitive FS (adfs on MacOS for instance), Nix adds a
+// suffix to avoid filename collisions.
+// See https://github.com/NixOS/nix/blob/ba9e69cdcd8022f37e344f2c86e60ee2b9da493f/src/libutil/archive.cc#L90
+var useNixCaseHack string
+
 type fileNode struct {
 	// The file name on the FS
 	srcPath  string
@@ -35,15 +40,21 @@ func initGraph() *fileNode {
 // file tree read on the FS. This means transformations are done during
 // the graph construction.
 func addFileToGraph(root *fileNode, path string, info *os.FileInfo, options *types.PathOptions) error {
-	pathInTar := filePathToTarPath(path, options)
+
+	dstPath := path
+	if useNixCaseHack != "" {
+		dstPath = removeNixCaseHackSuffix(dstPath)
+	}
+
+	dstPath = filePathToTarPath(dstPath, options)
 	// A regex in the options could make the path becoming the
 	// empty string. In this case, we don't want to create
 	// anything in the graph.
-	if pathInTar == "" {
+	if dstPath == "" {
 		return nil
 	}
 
-	parts := splitPath(pathInTar)
+	parts := splitPath(dstPath)
 	current := root
 	for _, part := range parts {
 		if node, exists := current.contents[part]; exists {
@@ -58,13 +69,13 @@ func addFileToGraph(root *fileNode, path string, info *os.FileInfo, options *typ
 
 	if current.srcPath != "" && current.srcPath != path {
 		return fmt.Errorf("The file '%s' already exists in the tar with source path %s but is added again with the source path %s",
-			pathInTar, current.srcPath, path)
+			dstPath, current.srcPath, path)
 	}
 	current.srcPath = path
 
 	if current.options != nil && !reflect.DeepEqual(current.options, options) {
 		return fmt.Errorf("The file '%s' already exists in the tar with options %#v but is overriden with options %#v",
-			pathInTar, current.options, options)
+			dstPath, current.options, options)
 	}
 	current.options = options
 

nix/tar_test.go

@@ -24,3 +24,21 @@ func TestTar(t *testing.T) {
 		t.Errorf("Size is %d while it should be %d", size, expectedSize)
 	}
 }
+
+func TestRemoveNixCaseHackSuffix(t *testing.T) {
+	ret := removeNixCaseHackSuffix("filename~nix~case~hack~1")
+	expected := "filename"
+	if ret != expected {
+		t.Errorf("%s should be %s", ret, expected)
+	}
+	ret = removeNixCaseHackSuffix("/path~nix~case~hack~1/filename")
+	expected = "/path/filename"
+	if ret != expected {
+		t.Errorf("%s should be %s", ret, expected)
+	}
+	ret = removeNixCaseHackSuffix("filename~nix~")
+	expected = "filename~nix~"
+	if ret != expected {
+		t.Errorf("%s should be %s", ret, expected)
+	}
+}

nix/utils.go

@@ -2,11 +2,31 @@ package nix
 
 import (
 	"github.com/nlewo/nix2container/types"
+	"path"
 	"path/filepath"
 	"regexp"
 	"strings"
 )
 
+func removeNixCaseHackSuffix(filepath string) string {
+	caseHackSuffix := "~nix~case~hack~"
+	parts := strings.Split(filepath, "/")
+	cleaned := make([]string, len(parts))
+	for i, part := range parts {
+		idx := strings.Index(part, caseHackSuffix)
+		if idx != -1 {
+			cleaned[i] = part[0:idx]
+		} else {
+			cleaned[i] = part
+		}
+	}
+	var prefix string
+	if strings.HasPrefix(filepath, "/") {
+		prefix = "/"
+	}
+	return prefix + path.Join(cleaned...)
+}
+
 func splitPath(path string) []string {
 	cleaned := filepath.Clean(path)
 	parts := strings.Split(cleaned, "/")