Add JSON_PRECISE_STREAM_POSITION to leave the character that terminates a number in the stream (#5344)

* docs: qualify the operator>> stream positioning guarantee

operator>>'s notes state that it leaves the stream positioned right
after the parsed value, so that concatenated JSON values can be read
back to back. That does not hold when the value is a number: a number
is only terminated by the character that follows it, and the lexer's
unget() is simulated (it rewinds only the lexer's own bookkeeping),
so that character stays consumed from the stream.

Document the actual behaviour: the guarantee holds for all value types
except numbers, which must be followed by whitespace. Also qualify the
cross-reference on the JSON Lines page, which repeated the unqualified
claim.

Documentation only; the behaviour itself is tracked in #5340.

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

* fix: restore the character that terminates a number (#5340)

operator>> is documented to leave the stream positioned right after the
parsed value, so that concatenated JSON values can be read back to back.
That did not hold for numbers: a number is only terminated by the
character following it, and lexer::scan_number() reads that character
and calls unget() -- which is simulated and rewinds only the lexer's own
bookkeeping. input_stream_adapter consumes via sbumpc() with no matching
sungetc(), so the terminating character stayed consumed and the next
extraction started one byte too late ('1true' left the stream at 'rue').

Propagating unget() to the adapter directly does not work: next_unget
makes the following get() replay the cached character, so the terminator
would be delivered twice. Instead, restore the still-pending character
once at the end of a non-strict parse, where the input is handed back to
the caller:

- input_stream_adapter gains unget_character() (sungetc()) and advertises
  it via supports_unget, detected the same way as supports_seek.
- lexer::restore_pending_unget() turns a pending simulated unget of a
  real (non-EOF) character into a real one and clears next_unget so the
  character is not also replayed. It is a no-op for adapters that cannot
  unget, and reports failure when sungetc() fails, in which case the
  input is left as it was before.
- parser calls it on the three non-strict paths, i.e. for operator>> and
  sax_parse(strict = false).

Strict parse()/accept() are unaffected: they require the input to end
after the value, so the character is consumed by the end-of-input check
anyway. Parse error messages and reported positions are unchanged.

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

* tests: fix CI failures in the #5340 test helpers

Four CI failures, all in the new test code:

- GCC (-Werror=useless-cast): drop the `json(...)` wrapper around
  `json::parse(...)`, which already returns a `json`.
- GCC (-Werror=unused-result): assign the discarded `json::parse()`
  result to a dummy, the idiom used elsewhere in the test suite, and
  catch `json::parse_error&` for consistency.
- clang-tidy (google-default-arguments): remove the default argument
  from the `pbackfail()` override; `sungetc()` supplies the base
  declaration's default.
- MSVC (bad allocation): `no_putback_streambuf::underflow()` set a
  one-character get area without advancing `m_pos`, so an implementation
  whose `istream::get` peeks before it bumps re-read the same character
  forever. Keep no get area at all: `underflow()` peeks, `uflow()`
  consumes, and `sungetc()` still always lands in `pbackfail()`, which
  is what the test needs.

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

* fix: leave the character that terminates a number in the input

Read the character following a number without consuming it, instead of
consuming it and putting it back. input_stream_adapter now peeks with
sgetc() and only steps over the character when the next one is requested
or when the adapter is destroyed, so releasing it cannot fail - no
putback position is required from the streambuf.

Suggested by gregmarr in #5344.

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

* docs: match the version history wording to the peek-based fix

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

* docs: drop the whitespace-separator caveat from the parsing pages

The caveat added in #5343 describes the behavior this branch fixes: a
number no longer consumes the character that terminates it, so
concatenated values need no separator.

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

* refactor: split the strict and non-strict paths in parser

Folding the release_lookahead() call into the existing strict check left
the "in strict mode" comment on an else-if branch, and made the strict
condition in sax_parse() redundant with the branch it followed.

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

* Put the stream position fix behind JSON_PRECISE_STREAM_POSITION

Leaving the character that terminates a number in the stream is observable:
reading "1,2,3" with repeated operator>> works today only because the comma
after each number is swallowed, and std::getline after a number skips the
line break. Both break with the fix, so make it opt-in for 3.x, as suggested
by @gregmarr in the review.

- JSON_PRECISE_STREAM_POSITION (default 0) selects the peek-based
  input_stream_adapter. Without it, the adapter is the consuming one from
  develop and has no supports_lookahead, so lexer::release_lookahead() and
  the parser's calls to it compile to nothing.
- The macro changes input_stream_adapter's layout and member functions, so
  it gets the ABI tag _psp, after _bics. The ABI config tests, the natvis
  generator, and nlohmann_json.natvis (regenerated) know the tag.
- The tests for the fix move to unit-precise-stream-position.cpp, which
  defines the macro itself and runs in every build, and gain the two cases
  above. unit-deserialization.cpp pins the default behavior instead.
- The docs describe the default behavior again and point to the new macro
  page; version history says "added in 3.13.0, planned default in 4.0.0".

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

---------

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

include/nlohmann/detail/abi_macros.hpp

@@ -38,6 +38,10 @@
     #define JSON_BRACE_INIT_COPY_SEMANTICS 0
 #endif
 
+#ifndef JSON_PRECISE_STREAM_POSITION
+    #define JSON_PRECISE_STREAM_POSITION 0
+#endif
+
 #if JSON_DIAGNOSTICS
     #define NLOHMANN_JSON_ABI_TAG_DIAGNOSTICS _diag
 #else
@@ -62,21 +66,28 @@
     #define NLOHMANN_JSON_ABI_TAG_BRACE_INIT_COPY_SEMANTICS
 #endif
 
+#if JSON_PRECISE_STREAM_POSITION
+    #define NLOHMANN_JSON_ABI_TAG_PRECISE_STREAM_POSITION _psp
+#else
+    #define NLOHMANN_JSON_ABI_TAG_PRECISE_STREAM_POSITION
+#endif
+
 #ifndef NLOHMANN_JSON_NAMESPACE_NO_VERSION
     #define NLOHMANN_JSON_NAMESPACE_NO_VERSION 0
 #endif
 
 // Construct the namespace ABI tags component
-#define NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d) json_abi ## a ## b ## c ## d
-#define NLOHMANN_JSON_ABI_TAGS_CONCAT(a, b, c, d) \
-    NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d)
+#define NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d, e) json_abi ## a ## b ## c ## d ## e
+#define NLOHMANN_JSON_ABI_TAGS_CONCAT(a, b, c, d, e) \
+    NLOHMANN_JSON_ABI_TAGS_CONCAT_EX(a, b, c, d, e)
 
 #define NLOHMANN_JSON_ABI_TAGS                                       \
     NLOHMANN_JSON_ABI_TAGS_CONCAT(                                   \
             NLOHMANN_JSON_ABI_TAG_DIAGNOSTICS,                       \
             NLOHMANN_JSON_ABI_TAG_LEGACY_DISCARDED_VALUE_COMPARISON, \
             NLOHMANN_JSON_ABI_TAG_DIAGNOSTIC_POSITIONS,              \
-            NLOHMANN_JSON_ABI_TAG_BRACE_INIT_COPY_SEMANTICS)
+            NLOHMANN_JSON_ABI_TAG_BRACE_INIT_COPY_SEMANTICS,         \
+            NLOHMANN_JSON_ABI_TAG_PRECISE_STREAM_POSITION)
 
 // Construct the namespace version component
 #define NLOHMANN_JSON_NAMESPACE_VERSION_CONCAT_EX(major, minor, patch) \

include/nlohmann/detail/input/input_adapters.hpp

@@ -101,6 +101,11 @@ class input_stream_adapter
         // maintain ifstream flags, except eof
         if (is != nullptr)
         {
+#if JSON_PRECISE_STREAM_POSITION
+            // consume the character last returned by get_character() unless it
+            // was given back with release_lookahead()
+            commit_lookahead();
+#endif
             is->clear(is->rdstate() & std::ios::eofbit);
         }
     }
@@ -114,6 +119,58 @@ class input_stream_adapter
     input_stream_adapter& operator=(input_stream_adapter&) = delete;
     input_stream_adapter& operator=(input_stream_adapter&&) = delete;
 
+#if JSON_PRECISE_STREAM_POSITION
+    input_stream_adapter(input_stream_adapter&& rhs) noexcept
+        : is(rhs.is), sb(rhs.sb), lookahead(rhs.lookahead)
+    {
+        rhs.is = nullptr;
+        rhs.sb = nullptr;
+        rhs.lookahead = false;
+    }
+
+    // Whether the character last returned by get_character() can be given back
+    // to the input with release_lookahead().
+    static constexpr bool supports_lookahead = true;
+
+    // std::istream/std::streambuf use std::char_traits<char>::to_int_type, to
+    // ensure that std::char_traits<char>::eof() and the character 0xFF do not
+    // end up as the same value, e.g., 0xFFFFFFFF.
+    //
+    // The character is peeked rather than consumed: it is only stepped over
+    // once the next character is requested, or when the adapter is destroyed.
+    // Until then, release_lookahead() can leave it in the input.
+    std::char_traits<char>::int_type get_character()
+    {
+        if (lookahead)
+        {
+            // step over the character returned by the previous call
+            sb->sbumpc();
+        }
+
+        auto res = sb->sgetc();
+        // set eof manually, as we don't use the istream interface.
+        if (JSON_HEDLEY_UNLIKELY(res == std::char_traits<char>::eof()))
+        {
+            // there is nothing to step over next time
+            lookahead = false;
+            is->clear(is->rdstate() | std::ios::eofbit);
+        }
+        else
+        {
+            lookahead = true;
+        }
+        return res;
+    }
+
+    // Leave the character last returned by get_character() in the input, so
+    // that the next read from the stream - by this adapter or by the caller
+    // once parsing is done - sees it again. Unlike putting a consumed
+    // character back, this cannot fail.
+    void release_lookahead() noexcept
+    {
+        lookahead = false;
+    }
+#else
     input_stream_adapter(input_stream_adapter&& rhs) noexcept
         : is(rhs.is), sb(rhs.sb)
     {
@@ -124,6 +181,9 @@ class input_stream_adapter
     // std::istream/std::streambuf use std::char_traits<char>::to_int_type, to
     // ensure that std::char_traits<char>::eof() and the character 0xFF do not
     // end up as the same value, e.g., 0xFFFFFFFF.
+    //
+    // The character is consumed, so the character that terminates a number
+    // stays consumed after parsing; see JSON_PRECISE_STREAM_POSITION.
     std::char_traits<char>::int_type get_character()
     {
         auto res = sb->sbumpc();
@@ -134,10 +194,14 @@ class input_stream_adapter
         }
         return res;
     }
+#endif
 
     template<class T>
     std::size_t get_elements(T* dest, std::size_t count = 1)
     {
+#if JSON_PRECISE_STREAM_POSITION
+        commit_lookahead();
+#endif
         auto res = static_cast<std::size_t>(sb->sgetn(reinterpret_cast<char*>(dest), static_cast<std::streamsize>(count * sizeof(T))));
         if (JSON_HEDLEY_UNLIKELY(res < count * sizeof(T)))
         {
@@ -147,9 +211,27 @@ class input_stream_adapter
     }
 
   private:
+#if JSON_PRECISE_STREAM_POSITION
+    // Step over the character last returned by get_character(). The character
+    // has already been peeked successfully, so for every streambuf with a get
+    // area this is a pointer increment that cannot fail.
+    void commit_lookahead()
+    {
+        if (lookahead)
+        {
+            lookahead = false;
+            sb->sbumpc();
+        }
+    }
+#endif
+
     /// the associated input stream
     std::istream* is = nullptr;
     std::streambuf* sb = nullptr;
+#if JSON_PRECISE_STREAM_POSITION
+    /// whether get_character() peeked a character that is not consumed yet
+    bool lookahead = false;
+#endif
 };
 #endif  // JSON_NO_IO
 

include/nlohmann/detail/input/lexer.hpp

@@ -127,6 +127,25 @@ constexpr bool input_adapter_supports_seek(std::false_type /*detected*/)
     return false;
 }
 
+// Detect whether an input adapter reads with one character of lookahead that
+// can be left in the input (see input_stream_adapter::supports_lookahead,
+// which is only defined with JSON_PRECISE_STREAM_POSITION), detected like
+// supports_seek above.
+template<typename InputAdapterType>
+using detect_supports_lookahead = decltype(InputAdapterType::supports_lookahead);
+
+template<typename InputAdapterType>
+constexpr bool input_adapter_supports_lookahead(std::true_type /*detected*/)
+{
+    return InputAdapterType::supports_lookahead;
+}
+
+template<typename InputAdapterType>
+constexpr bool input_adapter_supports_lookahead(std::false_type /*detected*/)
+{
+    return false;
+}
+
 // Detect whether an input adapter exposes a contiguous byte block that the
 // lexer can scan directly (see iterator_input_adapter::supports_bulk_scan).
 // Adapters without the flag - file, stream, wide-string, user-defined - fall
@@ -167,6 +186,12 @@ class lexer : public lexer_base<BasicJsonType>
     static constexpr bool lazy_token_string =
         input_adapter_supports_seek<InputAdapterType>(is_detected<detect_supports_seek, InputAdapterType> {});
 
+    /// whether a simulated unget can be passed on to the input adapter, which
+    /// then leaves the character in the input; see
+    /// input_adapter_supports_lookahead
+    static constexpr bool can_release_lookahead =
+        input_adapter_supports_lookahead<InputAdapterType>(is_detected<detect_supports_lookahead, InputAdapterType> {});
+
     /// whether string scanning may bulk-consume runs of ordinary characters
     /// directly from a contiguous input buffer (SWAR fast path). This requires
     /// the token to be reconstructible lazily (lazy_token_string), so bypassing
@@ -1898,6 +1923,21 @@ scan_number_done:
         uncapture_char(std::integral_constant<bool, lazy_token_string> {});
     }
 
+    /// adapter without lookahead: nothing to do (see release_lookahead)
+    void release_lookahead_impl(std::false_type /*can_release*/) const noexcept {}
+
+    /// adapter with lookahead: leave the character in the input instead
+    void release_lookahead_impl(std::true_type /*can_release*/)
+    {
+        if (next_unget)
+        {
+            // the character is read from the input again rather than replayed
+            // from current, so the adapter must not step over it
+            next_unget = false;
+            ia.release_lookahead();
+        }
+    }
+
     /// seekable adapter: nothing was captured, so nothing to undo
     void uncapture_char(std::true_type /*lazy*/) const noexcept {}
 
@@ -1961,6 +2001,31 @@ scan_number_done:
         return position;
     }
 
+    /*!
+    @brief pass a pending simulated unget on to the input
+
+    unget() only rewinds the lexer's own bookkeeping, so the character that
+    terminated the last token (e.g. the character after a number) would still
+    be stepped over when the input adapter is done. Callers that hand the
+    input back to the user afterwards - operator>> and non-strict sax_parse -
+    call this once when scanning is done, so that the input is positioned
+    right after the value.
+
+    Adapters without lookahead (see input_adapter_supports_lookahead) are not
+    handed back to the user, so this is a no-op for them. Without
+    JSON_PRECISE_STREAM_POSITION, no adapter has lookahead, so this is always a
+    no-op and the terminating character stays consumed.
+
+    Scanning may continue after this call: @a next_unget is cleared, and the
+    character is read from the input again instead of being replayed from
+    @a current. A pending unget of EOF needs no special case, because reaching
+    EOF leaves no lookahead to release.
+    */
+    void release_lookahead()
+    {
+        release_lookahead_impl(std::integral_constant<bool, can_release_lookahead> {});
+    }
+
 #if JSON_DIAGNOSTIC_POSITIONS
     /// return the offset of the first character of the last read token; unlike
     /// the token's parsed value, this accounts for escape sequences

include/nlohmann/detail/input/parser.hpp

@@ -100,13 +100,22 @@ class parser
             json_sax_dom_callback_parser<BasicJsonType, InputAdapterType> sdp(result, callback, allow_exceptions, &m_lexer);
             sax_parse_internal(&sdp);
 
-            // in strict mode, input must be completely read
-            if (strict && (get_token() != token_type::end_of_input))
+            if (strict)
             {
-                sdp.parse_error(m_lexer.get_position(),
-                                m_lexer.get_token_string(),
-                                parse_error::create(101, m_lexer.get_position(),
-                                                    exception_message(token_type::end_of_input, "value"), nullptr));
+                // in strict mode, input must be completely read
+                if (get_token() != token_type::end_of_input)
+                {
+                    sdp.parse_error(m_lexer.get_position(),
+                                    m_lexer.get_token_string(),
+                                    parse_error::create(101, m_lexer.get_position(),
+                                                        exception_message(token_type::end_of_input, "value"), nullptr));
+                }
+            }
+            else
+            {
+                // the caller keeps using the input: position it right after
+                // the value by leaving the character that terminated it
+                m_lexer.release_lookahead();
             }
 
             // in case of an error, return a discarded value
@@ -128,12 +137,20 @@ class parser
             json_sax_dom_parser<BasicJsonType, InputAdapterType> sdp(result, allow_exceptions, &m_lexer);
             sax_parse_internal(&sdp);
 
-            // in strict mode, input must be completely read
-            if (strict && (get_token() != token_type::end_of_input))
+            if (strict)
             {
-                sdp.parse_error(m_lexer.get_position(),
-                                m_lexer.get_token_string(),
-                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_of_input, "value"), nullptr));
+                // in strict mode, input must be completely read
+                if (get_token() != token_type::end_of_input)
+                {
+                    sdp.parse_error(m_lexer.get_position(),
+                                    m_lexer.get_token_string(),
+                                    parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_of_input, "value"), nullptr));
+                }
+            }
+            else
+            {
+                // see above
+                m_lexer.release_lookahead();
             }
 
             // in case of an error, return a discarded value
@@ -166,12 +183,24 @@ class parser
         (void)detail::is_sax_static_asserts<SAX, BasicJsonType> {};
         const bool result = sax_parse_internal(sax);
 
-        // strict mode: next byte must be EOF
-        if (result && strict && (get_token() != token_type::end_of_input))
+        if (result)
         {
-            return sax->parse_error(m_lexer.get_position(),
-                                    m_lexer.get_token_string(),
-                                    parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_of_input, "value"), nullptr));
+            if (strict)
+            {
+                // strict mode: next byte must be EOF
+                if (get_token() != token_type::end_of_input)
+                {
+                    return sax->parse_error(m_lexer.get_position(),
+                                            m_lexer.get_token_string(),
+                                            parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_of_input, "value"), nullptr));
+                }
+            }
+            else
+            {
+                // the caller keeps using the input: position it right after
+                // the value by leaving the character that terminated it
+                m_lexer.release_lookahead();
+            }
         }
 
         return result;

include/nlohmann/detail/macro_unscope.hpp

@@ -45,6 +45,7 @@
     #undef JSON_HAS_STATIC_RTTI
     #undef JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
     #undef JSON_BRACE_INIT_COPY_SEMANTICS
+    #undef JSON_PRECISE_STREAM_POSITION
 #endif
 
 #include <nlohmann/thirdparty/hedley/hedley_undef.hpp>