Be a bit more rigorous about how we cache strcoll and strxfrm results.

Commit 0e57b4d8 contained some clever
logic that attempted to make sure that we couldn't get confused about
whether the last thing we cached was a strcoll() result or a strxfrm()
result, but it wasn't quite clever enough, because we can perform
further abbreviations after having already performed some comparisons.
Introduce an explicit flag in the hopes of making this watertight.

Peter Geoghegan, reviewed by me.

src/backend/utils/adt/varlena.c

@@ -65,6 +65,7 @@ typedef struct
 	int			last_len1;		/* Length of last buf1 string/strxfrm() blob */
 	int			last_len2;		/* Length of last buf2 string/strxfrm() blob */
 	int			last_returned;	/* Last comparison result (cache) */
+	bool		cache_blob;		/* Does buf2 contain strxfrm() blob, etc? */
 	bool		collate_c;
 	hyperLogLogState abbr_card; /* Abbreviated key cardinality state */
 	hyperLogLogState full_card; /* Full key cardinality state */
@@ -1838,17 +1839,26 @@ btsortsupport_worker(SortSupport ssup, Oid collid)
 		/* Start with invalid values */
 		tss->last_len1 = -1;
 		tss->last_len2 = -1;
+		/* Initialize */
+		tss->last_returned = 0;
 #ifdef HAVE_LOCALE_T
 		tss->locale = locale;
 #endif
 		/*
-		 * To avoid somehow confusing a strxfrm() blob and an original string
-		 * within bttextfastcmp_locale(), initialize last returned cache to a
-		 * sentinel value.  A platform-specific actual strcoll() return value
-		 * of INT_MIN seems unlikely, but if that occurs it will not cause
-		 * wrong answers.
+		 * To avoid somehow confusing a strxfrm() blob and an original string,
+		 * constantly keep track of the variety of data that buf1 and buf2
+		 * currently contain.
+		 *
+		 * Comparisons may be interleaved with conversion calls.  Frequently,
+		 * conversions and comparisons are batched into two distinct phases,
+		 * but the correctness of caching cannot hinge upon this.  For
+		 * comparison caching, buffer state is only trusted if cache_blob is
+		 * found set to false, whereas strxfrm() caching only trusts the state
+		 * when cache_blob is found set to true.
+		 *
+		 * Arbitrarily initialize cache_blob to true.
 		 */
-		tss->last_returned = INT_MIN;
+		tss->cache_blob = true;
 		tss->collate_c = collate_c;
 		ssup->ssup_extra = tss;
 
@@ -1983,7 +1993,7 @@ bttextfastcmp_locale(Datum x, Datum y, SortSupport ssup)
 		tss->buf2[len2] = '\0';
 		tss->last_len2 = len2;
 	}
-	else if (arg1_match && tss->last_returned != INT_MIN)
+	else if (arg1_match && !tss->cache_blob)
 	{
 		/* Use result cached following last actual strcoll() call */
 		result = tss->last_returned;
@@ -2006,6 +2016,7 @@ bttextfastcmp_locale(Datum x, Datum y, SortSupport ssup)
 		result = strcmp(tss->buf1, tss->buf2);
 
 	/* Cache result, perhaps saving an expensive strcoll() call next time */
+	tss->cache_blob = false;
 	tss->last_returned = result;
 done:
 	/* We can't afford to leak memory here. */
@@ -2086,7 +2097,7 @@ bttext_abbrev_convert(Datum original, SortSupport ssup)
 		}
 
 		/* Might be able to reuse strxfrm() blob from last call */
-		if (tss->last_len1 == len &&
+		if (tss->last_len1 == len && tss->cache_blob &&
 			memcmp(tss->buf1, authoritative_data, len) == 0)
 		{
 			memcpy(pres, tss->buf2, Min(sizeof(Datum), tss->last_len2));
@@ -2167,6 +2178,8 @@ bttext_abbrev_convert(Datum original, SortSupport ssup)
 
 	addHyperLogLog(&tss->abbr_card, hash);
 
+	/* Cache result, perhaps saving an expensive strxfrm() call next time */
+	tss->cache_blob = true;
 done:
 	/*
 	 * Byteswap on little-endian machines.