Fix recently-introduced performance problem in ts_headline().

The new hlCover() algorithm that I introduced in commit c9b0c67
turns out to potentially take O(N^2) or worse time on long documents,
if there are many occurrences of individual query words but few or no
substrings that actually satisfy the query.  (One way to hit this
behavior is with a "common_word & rare_word" type of query.)  This
seems unavoidable given the original goal of checking every substring
of the document, so we have to back off that idea.  Fortunately, it
seems unlikely that anyone would really want headlines spanning all of
a long document, so we can avoid the worse-than-linear behavior by
imposing a maximum length of substring that we'll consider.

For now, just hard-wire that maximum length as a multiple of max_words
times max_fragments.  Perhaps at some point somebody will argue for
exposing it as a ts_headline parameter, but I'm hesitant to make such
a feature addition in a back-patched bug fix.

I also noted that the hlFirstIndex() function I'd added in that
commit was unnecessarily stupid: it really only needs to check whether
a HeadlineWordEntry's item pointer is null or not.  This wouldn't make
all that much difference in typical cases with queries having just
a few terms, but a cycle shaved is a cycle earned.

In addition, add a CHECK_FOR_INTERRUPTS call in TS_execute_recurse.
This ensures that hlCover's loop is cancellable if it manages to take
a long time, and it may protect some other TS_execute callers as well.

Back-patch to 9.6 as the previous commit was.  I also chose to add the
CHECK_FOR_INTERRUPTS call to 9.5.  The old hlCover() algorithm seems
to avoid the O(N^2) behavior, at least on the test case I tried, but
nonetheless it's not very quick on a long document.

Per report from Stephen Frost.

Discussion: https://postgr.es/m/20200724160535.GW12375@tamriel.snowman.net

Author: tglsfdc

src/backend/tsearch/wparser_def.c

@@ -2010,33 +2010,29 @@ checkcondition_HL(void *opaque, QueryOperand *val, ExecPhraseData *data)
  * Returns -1 if no such index
  */
 static int
-hlFirstIndex(HeadlineParsedText *prs, TSQuery query, int pos)
+hlFirstIndex(HeadlineParsedText *prs, int pos)
 {
 	int			i;
 
-	/* For each word ... */
 	for (i = pos; i < prs->curwords; i++)
 	{
-		/* ... scan the query to see if this word matches any operand */
-		QueryItem  *item = GETQUERY(query);
-		int			j;
-
-		for (j = 0; j < query->size; j++)
-		{
-			if (item->type == QI_VAL &&
-				prs->words[i].item == &item->qoperand)
-				return i;
-			item++;
-		}
+		if (prs->words[i].item != NULL)
+			return i;
 	}
 	return -1;
 }
 
 /*
  * hlCover: try to find a substring of prs' word list that satisfies query
  *
- * At entry, *p must be the first word index to consider (initialize this to
- * zero, or to the next index after a previous successful search).
+ * At entry, *p must be the first word index to consider (initialize this
+ * to zero, or to the next index after a previous successful search).
+ * We will consider all substrings starting at or after that word, and
+ * containing no more than max_cover words.  (We need a length limit to
+ * keep this from taking O(N^2) time for a long document with many query
+ * words but few complete matches.  Actually, since checkcondition_HL is
+ * roughly O(N) in the length of the substring being checked, it's even
+ * worse than that.)
  *
  * On success, sets *p to first word index and *q to last word index of the
  * cover substring, and returns true.
@@ -2045,7 +2041,8 @@ hlFirstIndex(HeadlineParsedText *prs, TSQuery query, int pos)
  * words used in the query.
  */
 static bool
-hlCover(HeadlineParsedText *prs, TSQuery query, int *p, int *q)
+hlCover(HeadlineParsedText *prs, TSQuery query, int max_cover,
+		int *p, int *q)
 {
 	int			pmin,
 				pmax,
@@ -2059,7 +2056,7 @@ hlCover(HeadlineParsedText *prs, TSQuery query, int *p, int *q)
 	 * appearing in the query; there's no point in trying endpoints in between
 	 * such points.
 	 */
-	pmin = hlFirstIndex(prs, query, *p);
+	pmin = hlFirstIndex(prs, *p);
 	while (pmin >= 0)
 	{
 		/* This useless assignment just keeps stupider compilers quiet */
@@ -2080,7 +2077,7 @@ hlCover(HeadlineParsedText *prs, TSQuery query, int *p, int *q)
 				return true;
 			}
 			/* Nope, so advance pmax to next feasible endpoint */
-			nextpmax = hlFirstIndex(prs, query, pmax + 1);
+			nextpmax = hlFirstIndex(prs, pmax + 1);
 
 			/*
 			 * If this is our first advance past pmin, then the result is also
@@ -2091,7 +2088,7 @@ hlCover(HeadlineParsedText *prs, TSQuery query, int *p, int *q)
 				nextpmin = nextpmax;
 			pmax = nextpmax;
 		}
-		while (pmax >= 0);
+		while (pmax >= 0 && pmax - pmin < max_cover);
 		/* No luck here, so try next feasible startpoint */
 		pmin = nextpmin;
 	}
@@ -2193,7 +2190,7 @@ get_next_fragment(HeadlineParsedText *prs, int *startpos, int *endpos,
 static void
 mark_hl_fragments(HeadlineParsedText *prs, TSQuery query, bool highlightall,
 				  int shortword, int min_words,
-				  int max_words, int max_fragments)
+				  int max_words, int max_fragments, int max_cover)
 {
 	int32		poslen,
 				curlen,
@@ -2220,7 +2217,7 @@ mark_hl_fragments(HeadlineParsedText *prs, TSQuery query, bool highlightall,
 	covers = palloc(maxcovers * sizeof(CoverPos));
 
 	/* get all covers */
-	while (hlCover(prs, query, &p, &q))
+	while (hlCover(prs, query, max_cover, &p, &q))
 	{
 		startpos = p;
 		endpos = q;
@@ -2375,7 +2372,7 @@ mark_hl_fragments(HeadlineParsedText *prs, TSQuery query, bool highlightall,
  */
 static void
 mark_hl_words(HeadlineParsedText *prs, TSQuery query, bool highlightall,
-			  int shortword, int min_words, int max_words)
+			  int shortword, int min_words, int max_words, int max_cover)
 {
 	int			p = 0,
 				q = 0;
@@ -2393,7 +2390,7 @@ mark_hl_words(HeadlineParsedText *prs, TSQuery query, bool highlightall,
 	if (!highlightall)
 	{
 		/* examine all covers, select a headline using the best one */
-		while (hlCover(prs, query, &p, &q))
+		while (hlCover(prs, query, max_cover, &p, &q))
 		{
 			/*
 			 * Count words (curlen) and interesting words (poslen) within
@@ -2549,6 +2546,7 @@ prsd_headline(PG_FUNCTION_ARGS)
 	int			shortword = 3;
 	int			max_fragments = 0;
 	bool		highlightall = false;
+	int			max_cover;
 	ListCell   *l;
 
 	/* Extract configuration option values */
@@ -2588,6 +2586,15 @@ prsd_headline(PG_FUNCTION_ARGS)
 							defel->defname)));
 	}
 
+	/*
+	 * We might eventually make max_cover a user-settable parameter, but for
+	 * now, just compute a reasonable value based on max_words and
+	 * max_fragments.
+	 */
+	max_cover = Max(max_words * 10, 100);
+	if (max_fragments > 0)
+		max_cover *= max_fragments;
+
 	/* in HighlightAll mode these parameters are ignored */
 	if (!highlightall)
 	{
@@ -2612,10 +2619,10 @@ prsd_headline(PG_FUNCTION_ARGS)
 	/* Apply appropriate headline selector */
 	if (max_fragments == 0)
 		mark_hl_words(prs, query, highlightall, shortword,
-					  min_words, max_words);
+					  min_words, max_words, max_cover);
 	else
 		mark_hl_fragments(prs, query, highlightall, shortword,
-						  min_words, max_words, max_fragments);
+						  min_words, max_words, max_fragments, max_cover);
 
 	/* Fill in default values for string options */
 	if (!prs->startsel)

src/backend/utils/adt/tsvector_op.c

@@ -1868,6 +1868,9 @@ TS_execute_recurse(QueryItem *curitem, void *arg, uint32 flags,
 	/* since this function recurses, it could be driven to stack overflow */
 	check_stack_depth();
 
+	/* ... and let's check for query cancel while we're at it */
+	CHECK_FOR_INTERRUPTS();
+
 	if (curitem->type == QI_VAL)
 		return chkcond(arg, (QueryOperand *) curitem,
 					   NULL /* don't need position info */ );