hush: fix handling of '' in ${var:+ARG}

This wasn't an ash bug in dollar_altvalue9, it was hush bug (and bash!)

function                                             old     new   delta
expand_one_var                                      2236    2254     +18
expand_vars_to_list                                 1097    1103      +6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 24/0)               Total: 24 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash_test/ash-quoting/dollar_altvalue2.right

@@ -0,0 +1,69 @@
+Unquoted '':
+start:
+||
+end
+start:
+||
+end
+start:
+||
+end
+start:
+||
+end
+start:
+||
+||
+end
+
+Unquoted "":
+start:
+||
+end
+start:
+||
+end
+start:
+||
+end
+start:
+||
+end
+start:
+||
+||
+end
+
+Quoted '':
+start:
+|''|
+end
+start:
+|'' |
+end
+start:
+| ''|
+end
+start:
+| '' |
+end
+start:
+|'' ''|
+end
+
+Quoted "":
+start:
+||
+end
+start:
+| |
+end
+start:
+| |
+end
+start:
+|  |
+end
+start:
+| |
+end

shell/ash_test/ash-quoting/dollar_altvalue2.tests

@@ -0,0 +1,33 @@
+f() { echo start:; for i; do echo "|$i|"; done; echo end; }
+x=a
+
+echo "Unquoted '':"
+f ${x:+''}
+f ${x:+'' }
+f ${x:+ ''}
+f ${x:+ '' }
+f ${x:+'' ''}
+
+echo
+echo 'Unquoted "":'
+f ${x:+""}
+f ${x:+"" }
+f ${x:+ ""}
+f ${x:+ "" }
+f ${x:+"" ""}
+
+echo
+echo "Quoted '':"
+f "${x:+''}"
+f "${x:+'' }"
+f "${x:+ ''}"
+f "${x:+ '' }"
+f "${x:+'' ''}"
+
+echo
+echo 'Quoted "":'
+f "${x:+""}"
+f "${x:+"" }"
+f "${x:+ ""}"
+f "${x:+ "" }"
+f "${x:+"" ""}"

shell/ash_test/ash-quoting/dollar_altvalue9.right

@@ -3,6 +3,7 @@ Unquoted 1:
 |x y|
 |1|
 |2|
+||
 |1 2|
 |A|
 |B|
@@ -14,6 +15,7 @@ Unquoted 2:
 |ax y|
 |1|
 |2|
+||
 |1 2|
 |A|
 |B|

shell/hush.c

@@ -475,7 +475,6 @@
 #endif
 
 #define SPECIAL_VAR_SYMBOL_STR "\3"
-#define SPECIAL_VAR_SYMBOL_CHR '\3'
 #define SPECIAL_VAR_SYMBOL       3
 /* The "variable" with name "\1" emits string "\3". Testcase: "echo ^C" */
 #define SPECIAL_VAR_QUOTED_SVS   1
@@ -5950,6 +5949,8 @@ static int encode_then_append_var_plusminus(o_string *output, int n,
 						break;
 					o_addqchr(&dest, ch);
 				}
+				o_addchr(&dest, SPECIAL_VAR_SYMBOL);
+				o_addchr(&dest, SPECIAL_VAR_SYMBOL);
 				continue;
 			}
 		}
@@ -5959,6 +5960,10 @@ static int encode_then_append_var_plusminus(o_string *output, int n,
 		}
 		if (ch == '"') {
 			dest.o_expflags ^= EXP_FLAG_ESC_GLOB_CHARS;
+			if (dest.o_expflags) {
+				o_addchr(&dest, SPECIAL_VAR_SYMBOL);
+				o_addchr(&dest, SPECIAL_VAR_SYMBOL);
+			}
 			continue;
 		}
 		if (ch == '\\') {
@@ -6565,7 +6570,7 @@ static NOINLINE int expand_vars_to_list(o_string *output, int n, char *arg)
 		case SPECIAL_VAR_QUOTED_SVS:
 			/* <SPECIAL_VAR_SYMBOL><SPECIAL_VAR_QUOTED_SVS><SPECIAL_VAR_SYMBOL> */
 			/* "^C variable", represents literal ^C char (possible in scripts) */
-			o_addchr(output, SPECIAL_VAR_SYMBOL_CHR);
+			o_addchr(output, SPECIAL_VAR_SYMBOL);
 			arg++;
 			break;
 #if ENABLE_HUSH_TICK
@@ -6627,7 +6632,8 @@ static NOINLINE int expand_vars_to_list(o_string *output, int n, char *arg)
 		o_addstr(output, arg);
 		debug_print_list("expand_vars_to_list[b]", output, n);
 	} else if (output->length == o_get_last_ptr(output, n) /* expansion is empty */
-	 && !(cant_be_null & 0x80) /* and all vars were not quoted. */
+	 && !(cant_be_null & 0x80)   /* and all vars were not quoted */
+	 && !output->has_quoted_part
 	) {
 		n--;
 		/* allow to reuse list[n] later without re-growth */

shell/hush_test/hush-quoting/dollar_altvalue2.right

@@ -0,0 +1,69 @@
+Unquoted '':
+start:
+||
+end
+start:
+||
+end
+start:
+||
+end
+start:
+||
+end
+start:
+||
+||
+end
+
+Unquoted "":
+start:
+||
+end
+start:
+||
+end
+start:
+||
+end
+start:
+||
+end
+start:
+||
+||
+end
+
+Quoted '':
+start:
+|''|
+end
+start:
+|'' |
+end
+start:
+| ''|
+end
+start:
+| '' |
+end
+start:
+|'' ''|
+end
+
+Quoted "":
+start:
+||
+end
+start:
+| |
+end
+start:
+| |
+end
+start:
+|  |
+end
+start:
+| |
+end

shell/hush_test/hush-quoting/dollar_altvalue2.tests

@@ -0,0 +1,33 @@
+f() { echo start:; for i; do echo "|$i|"; done; echo end; }
+x=a
+
+echo "Unquoted '':"
+f ${x:+''}
+f ${x:+'' }
+f ${x:+ ''}
+f ${x:+ '' }
+f ${x:+'' ''}
+
+echo
+echo 'Unquoted "":'
+f ${x:+""}
+f ${x:+"" }
+f ${x:+ ""}
+f ${x:+ "" }
+f ${x:+"" ""}
+
+echo
+echo "Quoted '':"
+f "${x:+''}"
+f "${x:+'' }"
+f "${x:+ ''}"
+f "${x:+ '' }"
+f "${x:+'' ''}"
+
+echo
+echo 'Quoted "":'
+f "${x:+""}"
+f "${x:+"" }"
+f "${x:+ ""}"
+f "${x:+ "" }"
+f "${x:+"" ""}"

shell/hush_test/hush-quoting/dollar_altvalue9.right

@@ -3,6 +3,7 @@ Unquoted 1:
 |x y|
 |1|
 |2|
+||
 |1 2|
 |A|
 |B|
@@ -14,6 +15,7 @@ Unquoted 2:
 |ax y|
 |1|
 |2|
+||
 |1 2|
 |A|
 |B|