Fix string buffer size calculation in Utf8JsonWriter.WriteStringValue

[prozolic]

#103155

Use WriteStringValueSegment for large strings
In StringConverter.Write, this PR use chunk-based WriteStringValueSegment instead of WriteStringValue when the input string length exceeds the safe threshold. This prevents IndexOutOfRangeException that could occur when writing extremely large strings.

This PR precompute maxRequiredBytes before dispatching to Utf8JsonWriter.WriteStringMinimized and Utf8JsonWriter.WriteStringIndented, using the original input length.

• With escaping: value.Length * MaxExpansionFactorWhileEscaping (6)
• Without escaping: value.Length * MaxExpansionFactorWhileTranscoding (3)

The new calculation is safe for inputs up to int.MaxValue / 6 (~357M chars). Utf8JsonWriter.WriteStringValue enforces a maximum input size of 166_666_666 chars (JsonConstants.MaxUnescapedTokenSize), so no overflow occurs in practice.

[Copilot]

Pull request overview

This PR addresses #103155 by preventing IndexOutOfRangeException when serializing extremely large strings. It updates StringConverter.Write to switch to chunked Utf8JsonWriter.WriteStringValueSegment for strings beyond a computed safe length threshold.

Changes:

• Add chunked string writing in StringConverter.Write using WriteStringValueSegment for very large inputs.
• Introduce new OuterLoop tests intended to cover extremely large string serialization (including indented scenarios).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/StringConverter.cs	Uses segmented string writing past a computed safe threshold to avoid overflow/crash paths.
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/Value.WriteTests.cs	Adds new OuterLoop coverage for extremely large string serialization (root + indented + array element).

[Copilot]

Pull request overview

This PR addresses an IndexOutOfRangeException when serializing extremely large strings by switching StringConverter.Write to use chunked Utf8JsonWriter.WriteStringValueSegment once the input string exceeds a computed “safe length” threshold, avoiding overflow-prone size computations in the non-segmented writer path.

Changes:

• Add chunked segmented string writing in StringConverter.Write for very large strings.
• Introduce helper logic to compute a max “safe” string length based on worst-case escaping/transcoding expansion.
• Add OuterLoop tests covering extremely large string serialization (including indented scenarios).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/StringConverter.cs	Routes very large strings through chunked WriteStringValueSegment to avoid overflow/indexing failures in large escaping/transcoding cases.
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/Value.WriteTests.cs	Adds OuterLoop coverage for very large string serialization and indentation scenarios to reproduce/guard against the reported crash.

[Copilot]

Pull request overview

Addresses IndexOutOfRangeException when serializing very large strings by fixing Utf8JsonWriter’s buffer sizing logic for escaped UTF-16 string values, and adds regression tests covering near-limit escaped string writes.

Changes:

• Update Utf8JsonWriter string value write path to use a precomputed max-required-byte count when sizing buffers for escaped strings.
• Add Utf8JsonWriter outerloop tests that write extremely large escaped string values (minimized + indented).
• Add an end-to-end JsonSerializer.Serialize outerloop test for extremely large escaped strings.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.String.cs	Fixes max-buffer-size computation for escaped UTF-16 string values to avoid overflow leading to out-of-range writes.
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Utf8JsonWriterTests.cs	Adds large escaped-string writer regression tests (minimized + indented).
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/Value.WriteTests.cs	Adds end-to-end serializer regression test for extremely large escaped strings.