[strict_errexit] Allow assignments as conditionals.

For example

    if x=$(false); then ...

and

    while x=$(true); do ...

The error message had the wrong location, which is #1107.  But we don't
need this error message at all.

I don't personally like this idiom.  But maybe someone uses it.

https://utcc.utoronto.ca/~cks/space/blog/programming/BourneIfCanSetVars?showcomments

Also fix location.SpanForCommand() function, although it's not tickled
because of the change above.

frontend/location.py

@@ -9,9 +9,10 @@
 from __future__ import print_function
 
 from _devbuild.gen.syntax_asdl import (
-    command_e, command_t, command__Simple, command__Pipeline, command__AndOr,
-    command__DoGroup, command__Sentence, command__Subshell,
-    command__WhileUntil, command__If, command__Case, command__TimeBlock,
+    command_e, command_t, command__Simple, command__ShAssignment,
+    command__Pipeline, command__AndOr, command__DoGroup, command__Sentence,
+    command__Subshell, command__WhileUntil, command__If, command__Case,
+    command__TimeBlock,
     BraceGroup,
 
     arith_expr_e, arith_expr_t, compound_word, Token,
@@ -45,6 +46,10 @@ def SpanForCommand(node):
     elif len(node.redirects):
       return node.redirects[0].op.span_id
 
+  if tag == command_e.ShAssignment:
+    node = cast(command__ShAssignment, UP_node)
+    return node.spids[0]
+
   if tag == command_e.Pipeline:
     node = cast(command__Pipeline, UP_node)
     return node.spids[0]  # first |

osh/cmd_eval.py

@@ -162,7 +162,8 @@ def _HasManyStatuses(node):
   UP_node = node
   with tagswitch(node) as case:
     # command subs in words are detected by allow_command_sub in ctx_ErrExit
-    if case(command_e.Simple, command_e.DBracket, command_e.DParen):
+    if case(command_e.Simple, command_e.ShAssignment, command_e.DBracket,
+        command_e.DParen):
       return False
 
     elif case(command_e.Pipeline):

spec/assign.test.sh

@@ -669,3 +669,4 @@ dict:3
 ## END
 ## N-I dash/mksh status: 99
 ## N-I dash/mksh stdout-json: ""
+

spec/errexit.test.sh

@@ -390,3 +390,62 @@ status=0
 ft
 status=0
 ## END
+
+#### ShAssignment used as conditional
+
+while x=$(false)
+do   
+  echo while
+done
+
+if x=$(false)
+then
+  echo if
+fi
+
+if x=$(true)
+then
+  echo yes
+fi
+
+# Same thing with errexit -- NOT affected
+set -o errexit
+
+while x=$(false)
+do   
+  echo while
+done
+
+if x=$(false)
+then
+  echo if
+fi
+
+if x=$(true)
+then
+  echo yes
+fi
+
+# Same thing with strict_errexit -- NOT affected
+shopt -s strict_errexit || true
+
+while x=$(false)
+do   
+  echo while
+done
+
+if x=$(false)
+then
+  echo if
+fi
+
+if x=$(true)
+then
+  echo yes
+fi
+
+## STDOUT:
+yes
+yes
+yes
+## END

test/runtime-errors.sh

@@ -291,9 +291,32 @@ strict_errexit_1() {
   _strict-errexit-case 'shopt -s oil:basic
                         proc p { echo p }
                         if p { echo hi }'
+
+  # issue #1107
+  _strict-errexit-case '
+myfunc() {
+  return 1
+}
+
+foo=$(true)
+
+# test assignment without proc
+while bar=$(false)
+do
+  echo yes
+done
+
+# issue 1007 was caused using command.ShAssignment, rather than the more common
+# command.Sentence with ;
+while spam=$(myfunc)
+do
+  echo yes
+done
+'
 }
 
 # OLD WAY OF BLAMING
+# Note: most of these don't fail
 strict_errexit_2() {
   # Test out all the location info