Attempt to fix gsub with zero length matches

Problem: If a regex given to gsub can match a zero length string, it
will never terminate and eventually crash. This is because after
matching once it keeps checking for the same match at the same location
and finding it.

Solution: We can't skip ahead because there might be a non-zero length
match at the same location. Instead, keep track of whether the last
match was zero length, and when it was, ignore any zero length match at
the same offset.

Remaining problem: This fails one of the new tests I added. For some
reason, match doesn't seem to correctly return (some) zero length
matches at the end of the input string. I'm going to report that as a
separate bug, I can't see why it behaves like that.

Ref: jqlang#2148

src/builtin.jq

@@ -132,20 +132,30 @@ def sub($re; s; flags):
   def subg: [explode[] | select(. != 103)] | implode;
   # "fla" should be flags with all occurrences of g removed; gs should be non-nil if flags has a g
   def sub1(fla; gs):
-    def mysub:
+    def find_first_new_match($lastzero):
+      [label $out|
+        match($re; fla + "g")|
+        if $lastzero then
+          # After a zero length match, don't return the same match again!
+          select(.offset != 0 or .length != 0)
+        else . end|
+        ., break $out
+      ];
+    def mysub($lastzero):
       . as $in
-      | [match($re; fla)]
+      | find_first_new_match($lastzero)
       | if length == 0 then $in
-        else .[0] as $edit
+        else (.[0]) as $edit
+        | ($edit | .length == 0) as $zerolength
         | ($edit | .offset + .length) as $len
         # create the "capture" object:
         | reduce ( $edit | .captures | .[] | select(.name != null) | { (.name) : .string } ) as $pair
             ({}; . + $pair)
         | $in[0:$edit.offset]
           + s
-          + ($in[$len:] | if length > 0 and gs then mysub else . end)
+          + ($in[$len:] | if gs then mysub($zerolength) else . end)
         end ;
-    mysub ;
+    mysub(false) ;
     (flags | index("g")) as $gs
     | (flags | if $gs then subg else . end) as $fla
     | sub1($fla; $gs);

tests/onig.test

@@ -75,6 +75,14 @@ gsub( "(.*)"; "";  "x")
 ""
 ""
 
+gsub(""; "_")
+"abc"
+"_a_b_c_"
+
+gsub("\\b"; "X")
+"one two three"
+"XoneX XtwoX XthreeX"
+
 [.[] | scan(", ")]
 ["a,b, c, d, e,f",", a,b, c, d, e,f, "]
 [", ",", ",", ",", ",", ",", ",", ",", "]