mgsh/shellcomplete.go

package main

// shellcomplete.go — Tab completion for the '!' shell escape.
//
// `!vi <Tab>` should behave like it does in a shell: the first word completes
// against the executables on PATH, everything after it against the filesystem.
// Paths resolve relative to the active project directory, because that is where
// forwardShell runs the command.
//
// Word splitting here is whitespace only. Quoting and backslash escapes are the
// shell's business at execution time; getting them right for completion too
// would buy little for a one-off escape hatch.

import (
	"os"
	"path/filepath"
	"sort"
	"strings"
	"sync"
)

// shellCommandList supplies the executable names for the command position. A
// variable so tests can hand over a fixed set instead of whatever happens to be
// installed on the machine running them.
var shellCommandList = pathExecutables

// completeShellLine returns the candidates and the prefix they replace for a
// line that is headed for a shell. ok is false for any other line, which is
// then left to the builtin command tree.
func completeShellLine(typed string) (cands []string, prefix string, ok bool) {
	body, allowCommand, ok := shellLine(typed)
	if !ok {
		return nil, "", false
	}
	cands, prefix = shellCandidates(body, allowCommand)
	return cands, prefix, true
}

// shellLine works out which part of a typed line will reach a shell, and
// whether its command word is still open for completion. Two things get there:
// a '!' escape, and an alias that expands to one — `alias ll '!ls -la'` makes
// everything after `ll` a shell argument just as surely.
//
// Only the alias itself is inspected, not what its expansion might expand to
// again: an alias chain can rewrite its arguments, and guessing at that would
// offer candidates for a command line that is not the one being built.
func shellLine(typed string) (body string, allowCommand, ok bool) {
	trimmed := strings.TrimLeft(typed, " \t")
	if rest, found := strings.CutPrefix(trimmed, "!"); found {
		return rest, true, true
	}

	// the alias name has to be complete — while it is still being typed there
	// is no way to know what it will turn out to be
	sep := strings.IndexAny(trimmed, " \t")
	if sep < 0 {
		return "", false, false
	}
	name := trimmed[:sep]
	if isBuiltin(name) { // a builtin can never be shadowed by an alias
		return "", false, false
	}
	expansion, defined := aliases[name]
	if !defined || !strings.HasPrefix(strings.TrimSpace(expansion), "!") {
		return "", false, false
	}
	// the command comes from the alias body, so only arguments are left to complete
	return trimmed[sep:], false, true
}

// shellCandidates completes the last word of a shell command line. allowCommand
// says whether its first word may still be completed against PATH.
func shellCandidates(body string, allowCommand bool) (cands []string, prefix string) {
	word := body[strings.LastIndexAny(body, " \t")+1:]
	inCommand := allowCommand && strings.TrimLeft(body[:len(body)-len(word)], " \t") == ""

	// a command word without a separator names something on PATH; with one it
	// is a path like ./script, exactly as a shell reads it
	if inCommand && !strings.ContainsRune(word, '/') {
		if word == "" {
			return nil, "" // every executable on the machine helps nobody
		}
		return matchPrefix(shellCommandList(), word), word
	}

	dir, base := splitPathToken(word)
	return matchPrefix(pathEntries(dir), base), base
}

// splitPathToken splits a path token into the directory part, kept exactly as
// typed, and the basename being completed. Completing only the basename is what
// keeps the candidate list readable: "src/ma<Tab>" offers "main.go", not the
// whole path again.
func splitPathToken(word string) (dir, base string) {
	if i := strings.LastIndexByte(word, '/'); i >= 0 {
		return word[:i+1], word[i+1:]
	}
	return "", word
}

// pathEntries lists what a directory token points at. Directories come back
// with a trailing slash, so completing one leads straight into it.
func pathEntries(dir string) []string {
	root := DIR
	switch {
	case strings.HasPrefix(dir, "~/"):
		home, err := os.UserHomeDir()
		if err != nil {
			return nil
		}
		root, dir = home, dir[2:]
	case strings.HasPrefix(dir, "/"):
		root = ""
	}

	entries, err := os.ReadDir(filepath.Join(root, dir))
	if err != nil {
		return nil
	}
	out := make([]string, 0, len(entries))
	for _, e := range entries {
		name := e.Name()
		if e.IsDir() {
			name += "/"
		}
		out = append(out, name)
	}
	return out
}

// matchPrefix keeps the candidates starting with prefix, sorted and without
// duplicates. A hidden entry only shows up once the prefix asks for it, as in a
// shell.
func matchPrefix(cands []string, prefix string) []string {
	wantHidden := strings.HasPrefix(prefix, ".")
	seen := map[string]bool{}
	var out []string
	for _, c := range cands {
		if !strings.HasPrefix(c, prefix) || seen[c] {
			continue
		}
		if !wantHidden && strings.HasPrefix(c, ".") {
			continue
		}
		seen[c] = true
		out = append(out, c)
	}
	sort.Strings(out)
	return out
}

// pathExecutables lists the executable names on PATH. The scan happens once per
// session: PATH cannot change from inside mgsh, and a few thousand directory
// entries are not worth walking on every Tab.
var pathExecutables = sync.OnceValue(func() []string {
	var out []string
	seen := map[string]bool{}
	for _, dir := range filepath.SplitList(os.Getenv("PATH")) {
		if dir == "" {
			dir = "."
		}
		entries, err := os.ReadDir(dir)
		if err != nil {
			continue
		}
		for _, e := range entries {
			name := e.Name()
			if seen[name] {
				continue // the first one on PATH is the one that would run
			}
			fi, err := e.Info()
			if err != nil || fi.IsDir() || fi.Mode().Perm()&0o111 == 0 {
				continue
			}
			seen[name] = true
			out = append(out, name)
		}
	}
	sort.Strings(out)
	return out
})