sh: Expand assignment-like words specially for export/readonly/local.

Examples:
  export x=~
now expands the tilde
  local y=$1
is now safe, even if $1 contains IFS characters or metacharacters.

For a word to "look like an assignment", it must start with a name followed
by an equals sign, none of which may be quoted.

The special treatment applies when the first word (potentially after
"command") is "export", "readonly" or "local". There may be quoting
characters but no expansions. If "local" is overridden with a function there
is no special treatment ("export" and "readonly" cannot be overridden with a
function).

If things like
  local arr=(1 2 3)
are ever allowed in the future, they cannot call a "local" function. This
would either be a run-time error or it would call the builtin.

This matches Austin Group bug #351, planned for the next issue of POSIX.1.

PR:		bin/166771

bin/sh/eval.c

@@ -672,6 +672,52 @@ evalbackcmd(union node *n, struct backcmd *result)
 		result->fd, result->buf, result->nleft, result->jp));
 }
 
+static int
+mustexpandto(const char *argtext, const char *mask)
+{
+	for (;;) {
+		if (*argtext == CTLQUOTEMARK || *argtext == CTLQUOTEEND) {
+			argtext++;
+			continue;
+		}
+		if (*argtext == CTLESC)
+			argtext++;
+		else if (BASESYNTAX[(int)*argtext] == CCTL)
+			return (0);
+		if (*argtext != *mask)
+			return (0);
+		if (*argtext == '\0')
+			return (1);
+		argtext++;
+		mask++;
+	}
+}
+
+static int
+isdeclarationcmd(struct narg *arg)
+{
+	int have_command = 0;
+
+	if (arg == NULL)
+		return (0);
+	while (mustexpandto(arg->text, "command")) {
+		have_command = 1;
+		arg = &arg->next->narg;
+		if (arg == NULL)
+			return (0);
+		/*
+		 * To also allow "command -p" and "command --" as part of
+		 * a declaration command, add code here.
+		 * We do not do this, as ksh does not do it either and it
+		 * is not required by POSIX.
+		 */
+	}
+	return (mustexpandto(arg->text, "export") ||
+	    mustexpandto(arg->text, "readonly") ||
+	    (mustexpandto(arg->text, "local") &&
+		(have_command || !isfunc("local"))));
+}
+
 /*
  * Check if a builtin can safely be executed in the same process,
  * even though it should be in a subshell (command substitution).
@@ -743,11 +789,12 @@ evalcommand(union node *cmd, int flags, struct backcmd *backcmd)
 	exitstatus = 0;
 	for (argp = cmd->ncmd.args ; argp ; argp = argp->narg.next) {
 		if (varflag && isassignment(argp->narg.text)) {
-			expandarg(argp, &varlist, EXP_VARTILDE);
+			expandarg(argp, varflag == 1 ? &varlist : &arglist,
+			    EXP_VARTILDE);
 			continue;
-		}
+		} else if (varflag == 1)
+			varflag = isdeclarationcmd(&argp->narg) ? 2 : 0;
 		expandarg(argp, &arglist, EXP_FULL | EXP_TILDE);
-		varflag = 0;
 	}
 	*arglist.lastp = NULL;
 	*varlist.lastp = NULL;

bin/sh/exec.c

@@ -648,6 +648,19 @@ unsetfunc(const char *name)
 	return (0);
 }
 
+
+/*
+ * Check if a function by a certain name exists.
+ */
+int
+isfunc(const char *name)
+{
+	struct tblentry *cmdp;
+	cmdp = cmdlookup(name, 0);
+	return (cmdp != NULL && cmdp->cmdtype == CMDFUNCTION);
+}
+
+
 /*
  * Shared code for the following builtin commands:
  *    type, command -v, command -V

bin/sh/exec.h

@@ -72,5 +72,6 @@ void hashcd(void);
 void changepath(const char *);
 void defun(const char *, union node *);
 int unsetfunc(const char *);
+int isfunc(const char *);
 int typecmd_impl(int, char **, int, const char *);
 void clearcmdentry(void);

bin/sh/sh.1

@@ -32,7 +32,7 @@
 .\"	from: @(#)sh.1	8.6 (Berkeley) 5/4/95
 .\" $FreeBSD$
 .\"
-.Dd November 5, 2011
+.Dd July 15, 2012
 .Dt SH 1
 .Os
 .Sh NAME
@@ -1164,6 +1164,20 @@ Assignments are expanded differently from other words:
 tilde expansion is also performed after the equals sign and after any colon
 and usernames are also terminated by colons,
 and field splitting and pathname expansion are not performed.
+.Pp
+This special expansion applies not only to assignments that form a simple
+command by themselves or precede a command word,
+but also to words passed to the
+.Ic export ,
+.Ic local
+or
+.Ic readonly
+built-in commands that have this form.
+For this, the builtin's name must be literal
+(not the result of an expansion)
+and may optionally be preceded by one or more literal instances of
+.Ic command
+without options.
 .Ss Positional Parameters
 A positional parameter is a parameter denoted by a number greater than zero.
 The shell sets these initially to the values of its command line

tools/regression/bin/sh/expansion/export2.0

@@ -0,0 +1,24 @@
+# $FreeBSD$
+
+w='@ @'
+check() {
+	[ "$v" = "$w" ] || echo "Expected $w got $v"
+}
+
+export v=$w
+check
+
+HOME=/known/value
+check() {
+	[ "$v" = ~ ] || echo "Expected $HOME got $v"
+}
+
+export v=~
+check
+
+check() {
+	[ "$v" = "x:$HOME" ] || echo "Expected x:$HOME got $v"
+}
+
+export v=x:~
+check

tools/regression/bin/sh/expansion/export3.0

@@ -0,0 +1,30 @@
+# $FreeBSD$
+
+w='@ @'
+check() {
+	[ "$v" = "$w" ] || echo "Expected $w got $v"
+}
+
+command export v=$w
+check
+command command export v=$w
+check
+
+HOME=/known/value
+check() {
+	[ "$v" = ~ ] || echo "Expected $HOME got $v"
+}
+
+command export v=~
+check
+command command export v=~
+check
+
+check() {
+	[ "$v" = "x:$HOME" ] || echo "Expected x:$HOME got $v"
+}
+
+command export v=x:~
+check
+command command export v=x:~
+check

tools/regression/bin/sh/expansion/local1.0

@@ -0,0 +1,28 @@
+# $FreeBSD$
+
+run_test() {
+	w='@ @'
+	check() {
+		[ "$v" = "$w" ] || echo "Expected $w got $v"
+	}
+
+	local v=$w
+	check
+
+	HOME=/known/value
+	check() {
+		[ "$v" = ~ ] || echo "Expected $HOME got $v"
+	}
+
+	local v=~
+	check
+
+	check() {
+		[ "$v" = "x:$HOME" ] || echo "Expected x:$HOME got $v"
+	}
+
+	local v=x:~
+	check
+}
+
+run_test

tools/regression/bin/sh/expansion/local2.0

@@ -0,0 +1,34 @@
+# $FreeBSD$
+
+run_test() {
+	w='@ @'
+	check() {
+		[ "$v" = "$w" ] || echo "Expected $w got $v"
+	}
+
+	command local v=$w
+	check
+	command command local v=$w
+	check
+
+	HOME=/known/value
+	check() {
+		[ "$v" = ~ ] || echo "Expected $HOME got $v"
+	}
+
+	command local v=~
+	check
+	command command local v=~
+	check
+
+	check() {
+		[ "$v" = "x:$HOME" ] || echo "Expected x:$HOME got $v"
+	}
+
+	command local v=x:~
+	check
+	command command local v=x:~
+	check
+}
+
+run_test

tools/regression/bin/sh/expansion/readonly1.0

@@ -0,0 +1,7 @@
+# $FreeBSD$
+
+w='@ @'
+
+v=0 HOME=/known/value
+readonly v=~:~/:$w
+[ "$v" = "$HOME:$HOME/:$w" ] || echo "Expected $HOME/:$w got $v"