Report a missing 0xFF after a final BON8 string also without strict

A BON8 string ends at the first byte that cannot continue it. When a
string ends a value and is not terminated by 0xFF, that byte belongs to
whatever follows the value, but the reader has already taken it from the
input and keeps it in its push-back buffer. With strict = true the
end-of-input check reports it; with strict = false the buffer was
discarded, so a following value in a stream lost its first byte or bytes
without an error ({0x61, 0x90} read "a" and then dropped the 0 that
follows).

Report parse_error.110 "string at the end of the value is not terminated
by 0xFF" when the push-back buffer is not empty once a BON8 value is
complete and strict is false, as bon8.md requires the final 0xFF. Output
of to_bon8 always has it, so canonical input is not affected.

Fixes #6110.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-bon8.cpp

@@ -608,14 +608,26 @@ TEST_CASE("BON8")
             CHECK_THROWS_WITH_AS(_ = json::from_bon8(vec), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing BON8 value: expected end of input; last byte: 0x90", json::parse_error&);
             CHECK(json::from_bon8(vec, true, false).is_discarded());
 
-            // the byte after a string that ends a container
+            // the byte after a string that ends a container: without strict, the
+            // missing 0xFF is reported, as the byte has been taken from the input
             const bytes vec2 = {0x81, 'a', 0x90};
-            CHECK(json::from_bon8(vec2, false) == json({"a"}));
+            CHECK_THROWS_WITH_AS(_ = json::from_bon8(vec2, false), "[json.exception.parse_error.110] parse error at byte 3: syntax error while parsing BON8 value: string at the end of the value is not terminated by 0xFF; last byte: 0x90", json::parse_error&);
+            CHECK(json::from_bon8(vec2, false, false).is_discarded());
             CHECK_THROWS_WITH_AS(_ = json::from_bon8(vec2), "[json.exception.parse_error.110] parse error at byte 3: syntax error while parsing BON8 value: expected end of input; last byte: 0x90", json::parse_error&);
 
             const bytes vec3 = {0x81, 'a', 0xC2, 0x00};
-            CHECK(json::from_bon8(vec3, false) == json({"a"}));
+            CHECK_THROWS_WITH_AS(_ = json::from_bon8(vec3, false), "[json.exception.parse_error.110] parse error at byte 3: syntax error while parsing BON8 value: string at the end of the value is not terminated by 0xFF; last byte: 0xC2", json::parse_error&);
             CHECK_THROWS_WITH_AS(_ = json::from_bon8(vec3), "[json.exception.parse_error.110] parse error at byte 3: syntax error while parsing BON8 value: expected end of input; last byte: 0xC2", json::parse_error&);
+
+            // with a terminated string, strict = false leaves the next value in the stream
+            std::istringstream terminated(std::string("\x61\xFF\x90", 3));
+            CHECK(json::from_bon8(terminated, false) == "a");
+            CHECK(terminated.tellg() == 2);
+            CHECK(json::from_bon8(terminated, false) == 0);
+
+            // without the terminator, the next value would be lost
+            std::istringstream unterminated(std::string("\x61\x90", 2));
+            CHECK_THROWS_WITH_AS(_ = json::from_bon8(unterminated, false), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing BON8 value: string at the end of the value is not terminated by 0xFF; last byte: 0x90", json::parse_error&);
         }
     }