server mode crash frequently.

call stack:
0 __memmove_avx_unaligned_erms () at ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:527
#1 0x00007f96341bb76a in std::basic_streambuf<char, std::char_traits >::xsgetn(char*, long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#2 0x00007f96341872e3 in std::basic_filebuf<char, std::char_traits >::xsgetn(char*, long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3 0x00007f96341944bd in std::istream::read(char*, long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#4 0x0000565324d916a8 in SPTAG::FileMetadataSet::GetMetadata(int) const ()
#5 0x0000565324d3977f in SPTAG::BKT::Index::SearchIndex(SPTAG::QueryResult&) const ()
#6 0x0000565324dab607 in SPTAG::Service::SearchExecutor::ExecuteInternal() ()
#7 0x0000565324dab21c in SPTAG::Service::SearchExecutor::Execute() ()
#8 0x0000565324daeb71 in SPTAG::Service::SearchService::SearchHanlder(unsigned int, SPTAG::Socket::Packet) ()
#9 0x0000565324dc08c5 in void std::__invoke_impl<void, void (SPTAG::Service::SearchService::&)(unsigned int, SPTAG::Socket::Packet), SPTAG::Service::SearchService&, unsigned int&, SPTAG::Socket::Packet&>(std::__invoke_memfun_deref, void (SPTAG::Service::SearchService::&)(unsigned int, SPTAG::Socket::Packet), SPTAG::Service::SearchService&, unsigned int&, SPTAG::Socket::Packet&) ()

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To Reproduce
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4. See error

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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
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Start with the server-mode search path shown in the stack trace, following SearchService::SearchHanlder through SearchExecutor::ExecuteInternal and BKT::Index::SearchIndex. Inspect FileMetadataSet::GetMetadata, especially its file-read operation, and reproduce the crash with a server-mode search. Done means identifying and fixing the crash and confirming that the same search path no longer fails.