interp: implement the enable, compgen, history and logout builtins

enable -n disables a builtin so the name falls through to the exec
handler, which is the point of the builtin in bash too — running the
real /bin/echo, say. `builtin` still bypasses that, as it does in bash,
and subshells inherit the set of disabled builtins.

compgen generates completions from the shell's own aliases, functions,
variables, keywords and $PATH — the interpreter is the only thing that
knows them, so a line editor above it had no way to ask before. The -A
job and -A signal actions yield nothing for now; they fill in when the
job control builtins land.

history reads the list the embedder supplies with the new
interp.History option, since the interpreter never reads input lines
itself and so has no history of its own. logout is bash's error for a
shell that is not a login shell.

Author: 0pcom

interp/api.go

@@ -234,6 +234,16 @@ type Runner struct {
 	// TODO: close a file once "exec" replaces it, like other shells do.
 	keptFiles []io.Closer
 
+	// disabledBuiltins holds the names turned off by `enable -n`, which are
+	// then looked up as external commands like bash does.
+	disabledBuiltins map[string]bool
+
+	// historyList and historyClear back the history builtin. They are nil
+	// unless the embedder supplied them with [History]; this interpreter does
+	// not read input lines itself, so only a line editor above it knows them.
+	historyList  func() []string
+	historyClear func()
+
 	// Fake signal callbacks
 	callbackErr  string
 	callbackExit string
@@ -476,6 +486,20 @@ func Dir(path string) RunnerOption {
 	}
 }
 
+// History supplies the command history to the history builtin. list returns the
+// history oldest first, and clear, which may be nil, discards it.
+//
+// The interpreter never reads input lines itself, so it has no history of its
+// own; only the line editor above it has one. Without this option, history
+// reports that no history list is available.
+func History(list func() []string, clear func()) RunnerOption {
+	return func(r *Runner) error {
+		r.historyList = list
+		r.historyClear = clear
+		return nil
+	}
+}
+
 // Interactive configures the interpreter to behave like an interactive shell,
 // akin to Bash. Currently, this only enables the expansion of aliases,
 // but later on it should also change other behavior.
@@ -1023,6 +1047,8 @@ func (r *Runner) Reset() {
 		accessHandler:        r.accessHandler,
 		procSubstHandler:     r.procSubstHandler,
 		procSubsts:           r.procSubsts,
+		historyList:          r.historyList,
+		historyClear:         r.historyClear,
 
 		// These can be set by functions like [Dir] or [Params], but
 		// builtins can overwrite them; reset the fields to whatever the
@@ -1301,9 +1327,12 @@ func (r *Runner) subshell(background bool) *Runner {
 		evalDepth:            r.evalDepth,
 		stmtDepth:            r.stmtDepth,
 		procSubstUses:        slices.Clip(r.procSubstUses),
+		historyList:          r.historyList,
+		historyClear:         r.historyClear,
 
 		origStdout: r.origStdout, // used for process substitutions
 	}
+	r2.disabledBuiltins = maps.Clone(r.disabledBuiltins)
 	r2.writeEnv = newOverlayEnviron(r.writeEnv, background)
 	// Funcs are copied, since they might be modified.
 	r2.Funcs = maps.Clone(r.Funcs)