json: vajson: buffer files in memory before parsing

[fbaeuerle]

FromFile parsed directly off the filebuf. The reader's Snap/Restore backtracking does seekg/tellg, which triggers lseek/read syscalls on a filebuf and made FromFile (especially on QNX) significantly slower than in-memory FromBuffer.

Read the file into a std::string once and parse it via std::istringstream, as FromBuffer already does: seeks become O(1) in-memory pointer moves.

Runtime improvement (measured on x86_64 linux) is around 25%:
Before:
BM_RealisticConfig_FromFile/512 30748530 ns 30684328 ns 22 bytes_per_second=1.52507Mi/s
After:
BM_RealisticConfig_FromFile/512 22465309 ns 22427569 ns 30 bytes_per_second=2.08653Mi/s

[github-actions]

The created documentation from the pull request is available at: docu-html

[Copilot]

🟡 Not ready to approve

File reads are not validated for I/O errors and the file is opened without binary mode, which can lead to incorrect behavior on read failures and inconsistent byte handling.

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Pull request overview

This PR optimizes vaJson file parsing by buffering file contents into memory before parsing, avoiding expensive seekg/tellg backtracking operations on file-backed stream buffers (notably improving performance on QNX).

Changes:

• Replace direct parsing from a file stream with a one-time read into std::string and parsing via std::istringstream.
• Adjust includes to support iterator-based buffering.

File summaries

File	Description
score/json/internal/parser/vajson/vajson_impl/reader/json_data.cpp	Buffer JSON file contents in memory before parsing to speed up Snap/Restore backtracking.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 2
• Review effort level: Low

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[Copilot]

🟡 Not ready to approve

The new file-read path needs bounds-checked casting between std::streamoff, std::size_t, and std::streamsize to avoid overflow/narrowing bugs on large files.

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Review details

Comments suppressed due to low confidence (2)

score/json/internal/parser/vajson/vajson_impl/reader/json_data.cpp:97

• FromFile now returns kStreamFailure not only when the file can't be opened but also when size/reading fails. The API documentation in json_data.h currently only mentions the open-failure case; please update it so callers know reads can fail too (and under what conditions).

            return MakeErrorResult<JsonData>(JsonErrc::kStreamFailure, "Could not read file");

score/json/internal/parser/vajson/vajson_impl/reader/json_data.cpp:105

• size is a std::streamoff and is passed directly to std::istream::read, which expects a std::streamsize. For large files this can narrow/overflow and lead to incorrect reads or huge/undefined allocations when casting to std::size_t. Add explicit bounds checks and cast to std::streamsize before calling read().

        file.seekg(0, std::ios::end);
        const std::streamoff size{file.tellg()};
        file.seekg(0, std::ios::beg);
        if (file.fail() || (size < 0))
        {

• Files reviewed: 1/1 changed files
• Comments generated: 0 new
• Review effort level: Low

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