[core] Use structured _error for purity errors

So users can catch the error

Author: Andy C

builtin/func_misc.py

@@ -544,6 +544,10 @@ def Call(self, rd):
         return value.List(l)
 
 
+# status code 4 is special, for encode/decode errors.
+_CODEC_STATUS = 4
+
+
 class ToJson8(vm._Callable):
 
     def __init__(self, is_j8):
@@ -571,8 +575,8 @@ def Call(self, rd):
             else:
                 j8.PrintJsonMessage(val, buf, indent, type_errors)
         except error.Encode as e:
-            # status code 4 is special, for encode/decode errors.
-            raise error.Structured(4, e.Message(), rd.LeftParenToken())
+            raise error.Structured(_CODEC_STATUS, e.Message(),
+                                   rd.LeftParenToken())
 
         return value.Str(buf.getvalue())
 
@@ -601,7 +605,7 @@ def Call(self, rd):
                 'start_pos': num.ToBig(e.start_pos),
                 'end_pos': num.ToBig(e.end_pos),
             }  # type: Dict[str, value_t]
-            # status code 4 is special, for encode/decode errors.
-            raise error.Structured(4, e.Message(), rd.LeftParenToken(), props)
+            raise error.Structured(_CODEC_STATUS, e.Message(),
+                                   rd.LeftParenToken(), props)
 
         return val

core/executor.py

@@ -208,6 +208,8 @@ def MaybeWaitOnProcessSubs(self, waiter, status_array):
     '/bin'
 ]
 
+_PURITY_STATUS = 5
+
 
 class PureExecutor(vm._Executor):
 
@@ -226,9 +228,14 @@ def __init__(
 
     def RunBuiltin(self, builtin_id, cmd_val):
         # type: (int, cmd_value.Argv) -> int
-        """Called by the 'builtin' builtin in osh/builtin_meta.py."""
-        e_die("Can't run builtin in pure mode", cmd_val.arg_locs[0])
-        return 1
+        """Called by the 'builtin' builtin in builtin/meta_oils.py."""
+
+        # TODO: RunSimpleCommand will allow 'true false'
+        #
+        # Then this function should allow
+        #   builtin true
+        #   builtin false
+        raise AssertionError()
 
     def RunSimpleCommand(self, cmd_val, cmd_st, run_flags):
         # type: (cmd_value.Argv, CommandStatus, int) -> int
@@ -241,8 +248,10 @@ def RunBackgroundJob(self, node):
 
     def RunPipeline(self, node, status_out):
         # type: (command.Pipeline, CommandStatus) -> None
-        e_die("Pipelines aren't allowed in pure mode (OILS-ERR-204)",
-              loc.Command(node))
+        raise error.Structured(
+            _PURITY_STATUS,
+            "Pipelines aren't allowed in pure mode (OILS-ERR-204)",
+            loc.Command(node))
 
     def RunSubshell(self, node):
         # type: (command_t) -> int
@@ -257,9 +266,10 @@ def CaptureStdout(self, node):
 
     def RunCommandSub(self, cs_part):
         # type: (CommandSub) -> str
-        e_die("Command subs aren't allowed in pure mode (OILS-ERR-204)",
-              loc.WordPart(cs_part))
-        return ''
+        raise error.Structured(
+            _PURITY_STATUS,
+            "Command subs aren't allowed in pure mode (OILS-ERR-204)",
+            loc.WordPart(cs_part))
 
     def RunProcessSub(self, cs_part):
         # type: (CommandSub) -> str

spec/ysh-purity.test.sh

@@ -16,25 +16,41 @@ pp test_ (d)
 
 var impure = ^(seq 3 | wc -l)
 
-call eval(impure)
+try {
+  call eval(impure)
+}
+#= _error
+echo impure code=$[_error.code]
+
+# Can run impure code after pure code
+call io->eval(impure)
 
-## status: 1
 ## STDOUT:
 (Dict)   {"a":1,"b":2}
+impure code=5
+3
 ## END
 
 #### evalExpr() is a pure function
+shopt --set ysh:upgrade
 
 var x = 42
 var pure = ^[x + 1]
 echo pure=$[evalExpr(pure)]
 
-var impure = ^[x + $(echo 1)]
-echo impure=$[evalExpr(impure)]
+var impure = ^[x + $(echo 3)]
+try {
+  echo impure code=$[evalExpr(impure)]
+}
+echo impure code=$[_error.code]
+
+# Can run impure code after pure code
+echo impure=$[io->evalExpr(impure)]
 
-## status: 1
 ## STDOUT:
 pure=43
+impure code=5
+impure=45
 ## END
 
 #### Idiom to handle purity errors from untrusted config files
@@ -118,20 +134,29 @@ $SH --eval-pure pipeline.sh -c 'echo pipeline.sh=$?'
 ## status: 0
 ## STDOUT:
 command sub
-command-sub.sh=1
-command-sub.ysh=1
+command-sub.sh=5
+command-sub.ysh=5
 
 3
 eval
-pipeline.sh=1
+pipeline.sh=5
 ## END
 
 #### Executor: Builtins not allowed
 
-mapfile lines < <(seq 3)
-echo "${lines[@]}"
+var cmd = ^(
+  mapfile lines < <(seq 3)
+  echo "${lines[@]}"
+)
+
+var cmd = ^(
+  builtin echo hi
+)
+
+call eval(cmd)
 
 ## STDOUT:
+what
 ## END
 
 #### Are source or use builtins allowed?