Don't try to copy files over themselves

When copying a config module, make sure the full path for src and dst
files don't match, and also check the inode in case we resolved a
different path to the same file.

Make a note about the unsafe usage of reusing a tempDir path.

config/module/copy_dir.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"syscall"
 )
 
 // copyDir copies the src directory contents into dst. Both directories
@@ -19,6 +20,7 @@ func copyDir(dst, src string) error {
 		if err != nil {
 			return err
 		}
+
 		if path == src {
 			return nil
 		}
@@ -36,6 +38,19 @@ func copyDir(dst, src string) error {
 		// destination with the path without the src on it.
 		dstPath := filepath.Join(dst, path[len(src):])
 
+		// we don't want to try and copy the same file over itself.
+		if path == dstPath {
+			return nil
+		}
+
+		// We still might have the same file through a link, so check the
+		// inode if we can
+		if eq, err := sameInode(path, dstPath); eq {
+			return nil
+		} else if err != nil {
+			return err
+		}
+
 		// If we have a directory, make that subdirectory, then continue
 		// the walk.
 		if info.IsDir() {
@@ -74,3 +89,36 @@ func copyDir(dst, src string) error {
 
 	return filepath.Walk(src, walkFn)
 }
+
+// sameInode looks up the inode for paths a and b and returns if they are
+// equal. On windows this will always return false.
+func sameInode(a, b string) (bool, error) {
+	var aIno, bIno uint64
+	aStat, err := os.Stat(a)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return false, nil
+		}
+		return false, err
+	}
+	if st, ok := aStat.Sys().(*syscall.Stat_t); ok {
+		aIno = st.Ino
+	}
+
+	bStat, err := os.Stat(b)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return false, nil
+		}
+		return false, err
+	}
+	if st, ok := bStat.Sys().(*syscall.Stat_t); ok {
+		bIno = st.Ino
+	}
+
+	if aIno > 0 && aIno == bIno {
+		return true, nil
+	}
+
+	return false, nil
+}

config/module/get.go

@@ -37,6 +37,8 @@ func GetCopy(dst, src string) error {
 	if err != nil {
 		return err
 	}
+	// FIXME: This isn't completely safe. Creating and removing our temp path
+	//        exposes where to race to inject files.
 	if err := os.RemoveAll(tmpDir); err != nil {
 		return err
 	}