config: preserve ownership and permissions on configfile

When running `docker login` or `docker logout`, the CLI updates
the configuration file by creating a temporary file, to replace
the old one (if exists).

When using `sudo`, this caused the file to be created as `root`,
making it inaccessible to the current user.

This patch updates the CLI to fetch permissions and ownership of
the existing configuration file, and applies those permissions
to the new file, so that it has the same permissions as the
existing file (if any).

Currently, only done for "Unix-y" systems (Mac, Linux), but
can be implemented for Windows in future if there's a need.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/config/configfile/file.go

@@ -196,6 +196,9 @@ func (configFile *ConfigFile) Save() error {
 		os.Remove(temp.Name())
 		return err
 	}
+	// Try copying the current config file (if any) ownership and permissions
+	copyFilePermissions(configFile.Filename, temp.Name())
+
 	return os.Rename(temp.Name(), configFile.Filename)
 }
 

cli/config/configfile/file_unix.go

@@ -0,0 +1,35 @@
+// +build !windows
+
+package configfile
+
+import (
+	"os"
+	"syscall"
+)
+
+// copyFilePermissions copies file ownership and permissions from "src" to "dst",
+// ignoring any error during the process.
+func copyFilePermissions(src, dst string) {
+	var (
+		mode     os.FileMode = 0600
+		uid, gid int
+	)
+
+	fi, err := os.Stat(src)
+	if err != nil {
+		return
+	}
+	if fi.Mode().IsRegular() {
+		mode = fi.Mode()
+	}
+	if err := os.Chmod(dst, mode); err != nil {
+		return
+	}
+
+	uid = int(fi.Sys().(*syscall.Stat_t).Uid)
+	gid = int(fi.Sys().(*syscall.Stat_t).Gid)
+
+	if uid > 0 && gid > 0 {
+		_ = os.Chown(dst, uid, gid)
+	}
+}

cli/config/configfile/file_windows.go

@@ -0,0 +1,5 @@
+package configfile
+
+func copyFilePermissions(src, dst string) {
+	// TODO implement for Windows
+}