[builtins/printf] Validate the var name in the -v flag.

Relevant to the conversation in issue #300.

osh/builtin_printf.py

@@ -18,6 +18,7 @@
 from core import util
 from core.util import p_die
 from frontend import args
+from frontend import match
 from frontend import reader
 from osh import builtin
 from osh import state
@@ -250,7 +251,15 @@ def __call__(self, arg_vec):
 
     result = ''.join(out)
     if arg.v:
-      state.SetStringDynamic(self.mem, arg.v, result)
+      var_name = arg.v
+
+      # Notes:
+      # - bash allows a[i] here (as in unset and ${!x}), but we haven't
+      # implemented it.
+      # - TODO: get the span_id for arg.v!
+      if not match.IsValidVarName(var_name):
+        raise args.UsageError('got invalid variable name %r' % var_name)
+      state.SetStringDynamic(self.mem, var_name, result)
     else:
       sys.stdout.write(result)
     return 0

spec/builtin-printf.test.sh

@@ -50,6 +50,26 @@ OK
 ## N-I dash stdout-json: ""
 ## N-I dash status: 1
 
+#### printf -v a[1]
+a=(a b c)
+printf -v 'a[1]' %s 'foo'
+echo status=$?
+argv.py "${a[@]}"
+## STDOUT:
+status=0
+['a', 'foo', 'c']
+## END
+## N-I mksh/zsh STDOUT:
+-vstatus=0
+['a', 'b', 'c']
+## END
+## N-I dash/ash stdout-json: ""
+## N-I dash/ash status: 2
+## N-I osh STDOUT:
+status=2
+['a', 'b', 'c']
+## END
+
 #### dynamic declare instead of %s
 var=foo
 declare $var='hello there'

test/parse-errors.sh

@@ -242,6 +242,8 @@ printf-builtin() {
   set +o errexit
   _error-case 'printf %'
   _error-case 'printf [%Z]'
+
+  _error-case 'printf -v "-invalid-" %s foo'
 }
 
 other-builtins() {