tests: Add fuzzing harness testing the locale independence of the strencodings.h functions #18126

Context: C and C++ locale assumptions in bitcoind and bitcoin-qt

Add fuzzing harness for locale independence testing of functions in strencodings.h and tinyformat.h.

Test this PR using:

$ make distclean
$ ./autogen.sh
$ CC=clang CXX=clang++ ./configure --enable-fuzz \
      --with-sanitizers=address,fuzzer,undefined
$ make
$ src/test/fuzz/locale
…

The tested functions (ParseInt32(…), ParseInt64(…), atoi(const std::string&), atoi64(const std::string& str), i64tostr(const char*), itostr(…), strprintf(…)) all call locale dependent functions (such as strtol(…), strtoll(…), atoi(const char*), etc.) but are assumed to do so in a way that the tested functions return same results regardless of the chosen C locale (setlocale).

This fuzzer aims to test that those assumptions hold up also in practice now and over time.

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The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #10443 (Add fee_est tool for debugging fee estimation code by ryanofsky)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Rebased! :)

The OP says "all call locale dependent functions", the title says "locale independence". Which is correct?

What is the point of a test that tests the functions are not locale independent?

@MarcoFalke They all call functions that are known to be locale dependent (see list in lint-locale-dependence.sh) but they are supposed to do so in a way that guarantees that their own output is locale independent. Does that answer your question? :)

This has broken Travis:

Run locale with args ['valgrind', '--quiet', '--error-exitcode=1', '/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-pc-linux-gnu/src/test/fuzz/locale', '-runs=1', '/home/travis/build/bitcoin/bitcoin/ci/scratch//qa-assets/fuzz_seed_corpus/locale']
Output: INFO: Seed: 818858521
INFO: Loaded 1 modules   (1402 inline 8-bit counters): 1402 [0x3a7c40, 0x3a81ba), 
INFO: Loaded 1 PC tables (1402 PCs): 1402 [0x3a81c0,0x3ad960), 
No such file or directory: /home/travis/build/bitcoin/bitcoin/ci/scratch//qa-assets/fuzz_seed_corpus/locale; exiting
INFO: Seed: 818858521
INFO: Loaded 1 modules   (1402 inline 8-bit counters): 1402 [0x3a7c40, 0x3a81ba), 
INFO: Loaded 1 PC tables (1402 PCs): 1402 [0x3a81c0,0x3ad960), 
No such file or directory: /home/travis/build/bitcoin/bitcoin/ci/scratch//qa-assets/fuzz_seed_corpus/locale; exiting
Target "locale" failed with exit code 1: valgrind --quiet --error-exitcode=1 /home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-pc-linux-gnu/src/test/fuzz/locale -runs=1 /home/travis/build/bitcoin/bitcoin/ci/scratch//qa-assets/fuzz_seed_corpus/locale

I assume seeds just need to be added to https://github.com/bitcoin-core/qa-assets?