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GH-33823: [C++] Improve err msgs when opening files that are the wrong format #49771

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GH-33823: [C++] Improve err msgs when opening files that are the wrong format #49771

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#33823 Improve err msgs when opening files that are the wrong format
RobertLD Apr 16, 2026
db7dd63
33823 Formatting + linting
RobertLD Apr 16, 2026
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11 changes: 11 additions & 0 deletions cpp/src/arrow/ipc/message.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -565,6 +565,17 @@ Status DecodeMessage(MessageDecoder* decoder, io::InputStream* file) {
auto metadata_length = decoder->next_required_size();
ARROW_ASSIGN_OR_RAISE(auto metadata, file->Read(metadata_length));
if (metadata->size() != metadata_length) {
// The first sizeof(int32_t) bytes of the Arrow file magic ("ARRO") may have been
// misread as metadata_length. Check if the remaining bytes complete the magic.
const auto remaining_magic = internal::kArrowMagicBytes.substr(sizeof(int32_t));
if (metadata->size() >= static_cast<int64_t>(remaining_magic.size()) &&
std::string_view(reinterpret_cast<const char*>(metadata->data()),
remaining_magic.size()) == remaining_magic) {
return Status::Invalid("Expected to read ", metadata_length,
" metadata bytes, but only read ", metadata->size(),
". This appears to be an Arrow IPC File format file. "
"Try open_file() instead of open_stream().");
Comment on lines +576 to +577
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Copilot AI Apr 19, 2026

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This error text is emitted from the C++ IPC decoder but recommends open_file() / open_stream(), which are binding-specific names (they don't exist in the C++ API). Consider API-neutral guidance (IPC file reader vs IPC stream reader) or include the C++ entry points (e.g., RecordBatchFileReader::Open / RecordBatchStreamReader::Open). Also, "Arrow IPC File format file" repeats "file"; reword to avoid the redundancy.

Suggested change
". This appears to be an Arrow IPC File format file. "
"Try open_file() instead of open_stream().");
". This appears to be an Arrow IPC file. "
"Try the IPC file reader instead of the IPC stream reader.");

Copilot uses AI. Check for mistakes.
}
return Status::Invalid("Expected to read ", metadata_length, " metadata bytes, but ",
"only read ", metadata->size());
}
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4 changes: 3 additions & 1 deletion cpp/src/arrow/ipc/reader.cc
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Expand Up @@ -1890,7 +1890,9 @@ class RecordBatchFileReaderImpl : public RecordBatchFileReader {
const auto magic_start = buffer->data() + sizeof(int32_t);
if (std::string_view(reinterpret_cast<const char*>(magic_start), kMagicSize) !=
kArrowMagicBytes) {
return Status::Invalid("Not an Arrow file");
return Status::Invalid(
"Not an Arrow file. If this is an Arrow IPC Streaming format file, try "
"open_stream() instead.");
Comment on lines +1894 to +1895
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Copilot AI Apr 19, 2026

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The error message suggests calling open_stream(), but this is not a C++ API entry point (C++ uses RecordBatchStreamReader::Open / stream reader types). Since this Status message is emitted from the C++ library and may be surfaced to C++ callers as well as bindings, consider making the guidance API-neutral (e.g., refer to the IPC stream reader) or mention the C++ API name explicitly.

Suggested change
"Not an Arrow file. If this is an Arrow IPC Streaming format file, try "
"open_stream() instead.");
"Not an Arrow file. If this is an Arrow IPC streaming format file, use "
"the IPC stream reader instead.");

Copilot uses AI. Check for mistakes.
}

int32_t footer_length = bit_util::FromLittleEndian(
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