Fix std::terminate and null pointer access in input_stream_adapter

Parsing from a std::istream crashed in two unusual but valid stream
states, both in input_stream_adapter:

- With eofbit in the stream's exceptions() mask, get_character() sets
  eofbit via is->clear(), which throws std::ios_base::failure. While
  that exception unwinds, ~input_stream_adapter() called clear() again
  to reset eofbit, which is still set and still in the exception mask,
  so it throws a second time out of the (implicitly noexcept)
  destructor and std::terminate() is called. The destructor now only
  calls clear() if a bit other than eofbit remains set, so the first
  exception can propagate normally.
- For an std::istream without a stream buffer (rdbuf() == nullptr,
  e.g. std::istream(nullptr)), the constructor stored the null
  pointer without checking it, and get_character() dereferenced it.
  input_adapter(std::istream&) now throws parse_error.101 for such a
  stream, the same as it already does for a null FILE* or char*.

Added regression tests to unit-deserialization.cpp and, for the
JSON_PRECISE_STREAM_POSITION variant of get_character(), to
unit-precise-stream-position.cpp; both crashed before this fix.
Documented the two exceptions in parse.md and operator_gtgt.md.

Fixes #5646.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/mkdocs/docs/api/basic_json/parse.md

@@ -88,7 +88,12 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
 ## Exceptions
 
 - Throws [`parse_error.101`](../../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token, or
-  empty input like a null `FILE*` or `char*` pointer.
+  empty input like a null `FILE*` or `char*` pointer, or an `std::istream` without a stream buffer
+  (`#!cpp i.rdbuf() == nullptr`, for instance `#!cpp std::istream(nullptr)`).
+- If reading from an `std::istream` reaches the end of the input and `eofbit` is part of the stream's
+  [`exceptions()`](https://en.cppreference.com/w/cpp/io/basic_ios/exceptions) mask, the `std::ios_base::failure`
+  thrown by the stream itself propagates instead of a `parse_error`, the same as it would for the standard library's
+  own extraction operators.
 
 ## Complexity
 
@@ -254,6 +259,8 @@ outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-by
 - Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
 - `JSON_STRICT_NUL_HANDLING` added in version 3.13.0 to optionally reject a NUL byte in the input instead of treating
   it as end of input; planned to become the default in version 4.0.0.
+- Extended empty-input detection to also cover an `std::istream` without a stream buffer, and fixed a crash
+  (`std::terminate`) when parsing from an `std::istream` with `eofbit` in its exception mask, in version 3.13.0.
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/api/operator_gtgt.md

@@ -20,7 +20,12 @@ the stream `i`
 
 ## Exceptions
 
-- Throws [`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token.
+- Throws [`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token, or if
+  `i` has no stream buffer (`#!cpp i.rdbuf() == nullptr`, for instance `#!cpp std::istream(nullptr)`).
+- If reading from `i` reaches the end of the input and `eofbit` is part of `i`'s
+  [`exceptions()`](https://en.cppreference.com/w/cpp/io/basic_ios/exceptions) mask, the `std::ios_base::failure`
+  thrown by `i` itself propagates instead of a `parse_error`, the same as it would for the standard library's own
+  extraction operators.
 
 ## Complexity
 
@@ -118,3 +123,5 @@ being read.
   it as end of input; planned to become the default in version 4.0.0.
 - `JSON_PRECISE_STREAM_POSITION` added in version 3.13.0 to optionally leave the character that terminates a number in
   the stream; planned to become the default in version 4.0.0.
+- Fixed a null pointer dereference for an `std::istream` without a stream buffer (now throws `parse_error.101`), and a
+  crash (`std::terminate`) when `i` has `eofbit` in its exception mask, in version 3.13.0.

include/nlohmann/detail/input/input_adapters.hpp

@@ -106,7 +106,13 @@ class input_stream_adapter
             // was given back with release_lookahead()
             commit_lookahead();
 #endif
-            is->clear(is->rdstate() & std::ios::eofbit);
+            // only call clear() if there is something to clear: it throws
+            // std::ios_base::failure if the stream has exceptions() enabled
+            // for a state bit that remains set, and a destructor must not throw
+            if ((is->rdstate() & ~std::ios::eofbit) != 0)
+            {
+                is->clear(is->rdstate() & std::ios::eofbit);
+            }
         }
     }
 
@@ -811,12 +817,16 @@ inline file_input_adapter input_adapter(std::FILE* file)
 
 inline input_stream_adapter input_adapter(std::istream& stream)
 {
+    if (stream.rdbuf() == nullptr)
+    {
+        JSON_THROW(parse_error::create(101, 0, "attempting to parse an empty input; check that your input string or stream contains the expected JSON", nullptr));
+    }
     return input_stream_adapter(stream);
 }
 
 inline input_stream_adapter input_adapter(std::istream&& stream)
 {
-    return input_stream_adapter(stream);
+    return input_adapter(stream);
 }
 #endif  // JSON_NO_IO
 

single_include/nlohmann/json.hpp

@@ -7650,7 +7650,13 @@ class input_stream_adapter
             // was given back with release_lookahead()
             commit_lookahead();
 #endif
-            is->clear(is->rdstate() & std::ios::eofbit);
+            // only call clear() if there is something to clear: it throws
+            // std::ios_base::failure if the stream has exceptions() enabled
+            // for a state bit that remains set, and a destructor must not throw
+            if ((is->rdstate() & ~std::ios::eofbit) != 0)
+            {
+                is->clear(is->rdstate() & std::ios::eofbit);
+            }
         }
     }
 
@@ -8355,12 +8361,16 @@ inline file_input_adapter input_adapter(std::FILE* file)
 
 inline input_stream_adapter input_adapter(std::istream& stream)
 {
+    if (stream.rdbuf() == nullptr)
+    {
+        JSON_THROW(parse_error::create(101, 0, "attempting to parse an empty input; check that your input string or stream contains the expected JSON", nullptr));
+    }
     return input_stream_adapter(stream);
 }
 
 inline input_stream_adapter input_adapter(std::istream&& stream)
 {
-    return input_stream_adapter(stream);
+    return input_adapter(stream);
 }
 #endif  // JSON_NO_IO
 

tests/src/unit-deserialization.cpp

@@ -388,6 +388,37 @@ TEST_CASE("deserialization")
             }));
         }
 
+        SECTION("stream with eofbit in its exception mask (issue #5646)")
+        {
+            // reaching EOF while parsing a value that fills the whole input
+            // (e.g., a number, or any value under strict parsing) makes
+            // get_character() call std::istream::clear() to record eofbit;
+            // with eofbit in the exception mask, that clear() itself throws
+            // std::ios_base::failure - it must propagate to the caller instead
+            // of ~input_stream_adapter() throwing a second exception while the
+            // first is still unwinding, which would call std::terminate
+            json _;
+
+            std::istringstream is1("1");
+            is1.exceptions(std::ios::eofbit);
+            CHECK_THROWS_AS(_ = json::parse(is1), std::ios_base::failure&);
+
+            // the same holds for the common std::ifstream::exceptions(failbit |
+            // badbit | eofbit) pattern, because only eofbit ends up set
+            std::istringstream is2("1");
+            is2.exceptions(std::ios::failbit | std::ios::badbit | std::ios::eofbit);
+            CHECK_THROWS_AS(_ = json::parse(is2), std::ios_base::failure&);
+        }
+
+        SECTION("stream without a streambuf (issue #5646)")
+        {
+            // std::istream(nullptr) has badbit set and rdbuf() == nullptr;
+            // get_character() must not dereference that null streambuf
+            std::istream is(nullptr);
+            json _;
+            CHECK_THROWS_WITH_AS(_ = json::parse(is), "[json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON", json::parse_error&);
+        }
+
         SECTION("string")
         {
             json::string_t const s = R"(["foo",1,2,3,false,{"one":1})";

tests/src/unit-precise-stream-position.cpp

@@ -234,4 +234,33 @@ TEST_CASE("JSON_PRECISE_STREAM_POSITION")
         CHECK(j == json(1));
         CHECK(remaining(is) == "true");
     }
+
+    SECTION("stream with eofbit in its exception mask (issue #5646)")
+    {
+        // with JSON_PRECISE_STREAM_POSITION, get_character() peeks via
+        // sb->sgetc() rather than consuming via sb->sbumpc(), but it still
+        // calls std::istream::clear() to record eofbit once the streambuf is
+        // exhausted; with eofbit in the exception mask, that clear() itself
+        // throws std::ios_base::failure, which must propagate to the caller
+        // instead of ~input_stream_adapter() throwing a second exception
+        // while the first is still unwinding (which would call std::terminate)
+        json _;
+
+        std::istringstream is1("1");
+        is1.exceptions(std::ios::eofbit);
+        CHECK_THROWS_AS(_ = json::parse(is1), std::ios_base::failure&);
+
+        std::istringstream is2("1");
+        is2.exceptions(std::ios::failbit | std::ios::badbit | std::ios::eofbit);
+        CHECK_THROWS_AS(_ = json::parse(is2), std::ios_base::failure&);
+    }
+
+    SECTION("stream without a streambuf (issue #5646)")
+    {
+        // std::istream(nullptr) has badbit set and rdbuf() == nullptr;
+        // get_character() must not dereference that null streambuf
+        std::istream is(nullptr);
+        json _;
+        CHECK_THROWS_WITH_AS(_ = json::parse(is), "[json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON", json::parse_error&);
+    }
 }