Remove old libicu max collation sentinel hack

For old libicu versions < 59 on RHEL 6 and 7 we had to add a hack in the NIF
driver to handle the max sortable sentinel `<<255,255,255,255>>` [1]. We don't
support those versions any longer, and since libicu version 59 the library will
automatically sort those as the highest values [2], so we can clean up our
collator NIF.

[1] #3491

[2] https://www.unicode.org/reports/tr35/tr35-collation.html#tailored-noncharacter-weights

> U+FFFF: This code point is tailored to have a primary weight higher than all
other characters. This allows the reliable specification of a range, such as
“Sch” ≤ X ≤ “Sch\uFFFF”, to include all strings starting with "sch" or
equivalent.

Author: nickva

src/couch/priv/couch_ejson_compare/couch_ejson_compare.c

@@ -18,6 +18,15 @@
 #include "erl_nif.h"
 #include "unicode/ucol.h"
 #include "unicode/ucasemap.h"
+#include "unicode/uvernum.h"
+
+/* {255,255,255,255} max-key sentinel relies on libicu sorting FFFF byte
+ * sequence w/ the highest collation weight. However that happens starting with
+ * libicu 59, so we'll explicitly check the version to avoid silently
+ * mis-collating data */
+#if U_ICU_VERSION_MAJOR_NUM < 59
+#error "CouchDB requires libicu >= 59 for correct max key collation"
+#endif
 
 #define MAX_DEPTH 10
 
@@ -73,14 +82,8 @@ static __inline int atom_sort_order(ErlNifEnv*, ERL_NIF_TERM);
 static __inline int compare_strings(ctx_t*, ErlNifBinary, ErlNifBinary);
 static __inline int compare_lists(int, ctx_t*, ERL_NIF_TERM, ERL_NIF_TERM);
 static __inline int compare_props(int, ctx_t*, ERL_NIF_TERM, ERL_NIF_TERM);
-static __inline int is_max_utf8_marker(ErlNifBinary);
 static __inline UCollator* get_collator(void);
 
-/* Should match the <<255,255,255,255>> in:
- *  - src/mango/src/mango_idx_view.hrl#L13
- *  - src/couch_mrview/src/couch_mrview_util.erl#L40 */
-static const unsigned char max_utf8_marker[]  = {255, 255, 255, 255};
-
 
 UCollator*
 get_collator(void)
@@ -469,47 +472,13 @@ compare_props(int depth, ctx_t* ctx, ERL_NIF_TERM a, ERL_NIF_TERM b)
 }
 
 
-int
-is_max_utf8_marker(ErlNifBinary bin)
-{
-    if (bin.size == sizeof(max_utf8_marker)) {
-        if(memcmp(bin.data, max_utf8_marker, sizeof(max_utf8_marker)) == 0) {
-            return 1;
-        }
-        return 0;
-    }
-    return 0;
-}
-
-
 int
 compare_strings(ctx_t* ctx, ErlNifBinary a, ErlNifBinary b)
 {
     UErrorCode status = U_ZERO_ERROR;
     UCharIterator iterA, iterB;
     int result;
 
-    /* libicu versions earlier than 59 (at least) don't consider the
-     * {255,255,255,255} to be the highest sortable string as CouchDB expects.
-     * While we are still shipping CentOS 7 packages with libicu 50, we should
-     * explicitly check for the marker, later on we can remove the max
-     * logic */
-
-    int a_is_max = is_max_utf8_marker(a);
-    int b_is_max = is_max_utf8_marker(b);
-
-    if(a_is_max && b_is_max) {
-        return 0;
-    }
-
-    if(a_is_max) {
-        return 1;
-    }
-
-    if(b_is_max) {
-        return -1;
-    }
-
     uiter_setUTF8(&iterA, (const char *) a.data, (uint32_t) a.size);
     uiter_setUTF8(&iterB, (const char *) b.data, (uint32_t) b.size);
 

src/couch/test/eunit/couch_ejson_compare_tests.erl

@@ -36,7 +36,8 @@
     <<"B">>,
     <<"ba">>,
     <<"bb">>,
-    % Highest sorting unicode value. Special case in the nif
+    % Highest sorting unicode value
+    % libicu >= 59 sorts it highest natively
     ?MAX_UNICODE_STRING,
     [<<"a">>],
     [<<"b">>],