go/neo/interal += xpath, xfilepath

The xfilepath package supports resolving filepaths with a user prefix
to absolute paths: it converts '~' and '~username' to $HOME of user
(as it's done by for  instance bash). No builtin golang module supports this
functionality [1]. We need this functionality in order to imitate the
behaviour of NEO/py in NEO/go [2].

---

[1] https://stackoverflow.com/questions/47261719/how-can-i-resolve-a-relative-path-to-absolute-path-in-golang)
[2] nexedi/slapos!1307 (comment 17574)

/reviewed-by @kirr
/reviewed-on kirr/neo!1

go/neo/internal/xpath/xfilepath/xfilepath.go

+// Copyright (C) 2023  Nexedi SA and Contributors
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+// Package xfilepath provides addons to std package filepath.
+package xfilepath
+
+import (
+	"os/user"
+	path_ "path"
+	"strings"
+)
+
+// ExpandUser does '~' and '~username' expansion.
+//
+// It resolves '~' into the $HOME directory of current user
+// and '~user' into the $HOME directory of 'user'.
+//
+// If the provided path doesn't start with '~', ExpandUser returns the path
+// without any changes.
+//
+// ExpandUser does NOT support bashs '~+' and '~-' prefix (for '$PWD' and '$OLDPWD')
+// and it does NOT support bashs '~N', '~+N' and '~-N' prefix.
+//
+// Please see here for bash tilde expansion documentation:
+//   https://www.gnu.org/software/bash/manual/html_node/Tilde-Expansion.html
+func ExpandUser(path string) string {
+	if strings.HasPrefix(path, "~") {
+		s := strings.SplitN(path[1:], "/", 2)
+		if home, err := homeOf(s[0]); err == nil {
+			var remainder string
+			if len(s) > 1 { remainder = s[1] }
+			return path_.Join(home, remainder)
+		}
+		// We ignore any error returned by 'homeOf' and simply
+		// return path as it is.
+		// We do so to be compatible with the behaviour of bash and
+		// of pythons 'os.path.expanduser' function.
+	}
+	return path
+}
+
+// homeOf returns $HOME of user with given username.
+// If username is an empty string (e.g. ""), homeOf returns
+// $HOME of the current user.
+func homeOf(username string) (string, error) {
+	usr, err := lookupUser(username)
+	if err != nil {
+		return "", err
+	}
+	return usr.HomeDir, nil
+}
+
+// lookupUser returns User with username.
+//
+// If username is an empty string (""), lookupUser
+// returns the current user.
+func lookupUser(username string) (*user.User, error) {
+	if username == "" {
+		return user.Current()
+	} else {
+		return user.Lookup(username)
+	}
+}

go/neo/internal/xpath/xfilepath/xfilepath_test.go

+// Copyright (C) 2023  Nexedi SA and Contributors.
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+package xfilepath
+
+import (
+	"os/user"
+	"testing"
+
+	"lab.nexedi.com/kirr/neo/go/internal/xtesting"
+)
+
+func TestExpandUser(t *testing.T) {
+	X := xtesting.FatalIf(t)
+	xhomeOf := func(username string) string {
+		t.Helper()
+		home, err := homeOf(username); X(err)
+		return home
+	}
+	home := xhomeOf("")
+	homeRoot := xhomeOf("root")
+	me, err := user.Current(); X(err)
+	testv := []struct{ path, expandOk string }{
+		{"", ""},
+		{"~", home},
+		{"~/", home},
+		{"~" + me.Username, home},
+		{"~/abc", home + "/abc"},
+		{"~root/", homeRoot},
+		{"~root", homeRoot},
+		{"abc/", "abc/"},
+		{"abc/~", "abc/~"},
+		{"/abc/", "/abc/"},
+	}
+	for _, tCase := range testv {
+		expanded := ExpandUser(tCase.path)
+		if expanded != tCase.expandOk {
+			t.Errorf("%s  -> %s  ; want %s", tCase.path, expanded, tCase.expandOk)
+		}
+	}
+}