#16545 refactor: Implement missing error checking for ArgsManager flags

Implements parsing and startup validation for `ALLOW_BOOL`, `ALLOW_INT`, `ALLOW_STRING`, and `ALLOW_LIST` argument flags in `ArgsManager`. It rejects malformed typed inputs (such as double negations or duplicate config keys for non-list arguments) during configuration parsing and enforces flag consistency across `GetArg` accessors at runtime. Existing production arguments remain on `ALLOW_ANY`, leaving current node behavior unchanged until options adopt the new flags.

Problem: Command-line arguments and configuration settings historically underwent minimal type validation during parsing, resulting in type-coercion footguns (such as `-nofoo` returning the string `"0"`, which previously caused `-nowallet` to create a wallet file named `"0"`). Additionally, duplicate non-list assignments in configuration files are silently ignored, and developers can invoke mismatched accessors (such as `GetIntArg` on a non-integer argument) without error enforcement.

Evidence of importance:

• Commit 99c35134de notes historical footguns: "negated settings will return '' empty strings instead of '0' strings (in the past the '0' strings caused strangeness like '-nowallet' options creating wallet files named '0')"
• Commit 97071b909f states: "if a non-list setting is assigned multiple times in the same section of a configuration file, the later assignments trigger errors instead of being silently ignored"

Risk of a mistake: medium (other, tests). A mistake in configuration parsing or argument flag validation could prevent nodes from starting or throw unhandled logic errors on valid CLI/config settings.

Changed since the previous assessment: Added test coverage for settings.json parsing with type flags, updated fuzz harness exception handling for stricter type checks, and applied documentation and grammar cleanups.

Ready

Nothing known that would invalidate a review now.

Waiting on (from the thread read):

• reviewer: review of the latest rebases and updates 2024-12-09

Mild

objection that needs a change was answered, no reply from the objector (ajtowns (NACK): interpreting empty setting values as resetting to defaults alters existing configuration semantics)

Thread notes: The author updated and rebased the PR through multiple iterations fixing conflicts and CI issues.

Objections, suggestions, and questions:

Support:

• promag (contributor) (2019-08-07) [both]: Looks good, read the code and tried locally [not substantive]
• hebasto (member) (Concept ACK, 2019-08-10): supports adding error checking for ArgsManager flags [not substantive]
• laanwj (member) (Concept ACK, 2019-10-23): consistent argument error checking is good
• hebasto (member) (ACK, 2020-01-06) [tested]: tested on Linux Mint 19.3
• hebasto (member) (ACK, 2020-01-07) [tested]: re-ACK tested on Linux Mint 19.3
• hebasto (member) (ACK, 2020-01-07): just fix a typo [not substantive]
• hodlinator (member) (Concept ACK, 2024-08-15) [read]: incremental approach of this PR + gradually activating it for existing args will achieve a better baseline to approach future compile-time initiatives from
• l0rinc (member) (Approach ACK, 2024-09-25) [read]: did a first scan over the implementation and left nits and refactoring ideas [not substantive]

Participants:

• hebasto (member, support, 13 statements 2019-08-04 to 2020-01-07): Concept ACK and repeated ACKs after testing on Linux Mint
• promag (contributor, support, 11 statements 2019-08-06 to 2020-01-09): read code and tested locally, suggested assertion and usage examples
• laanwj (member, support, 1 statements 2019-10-23 to 2019-10-23): Concept ACK for consistent argument error checking
• ajtowns (member, objection, 13 statements 2020-01-09 to 2021-09-01): objected to resetting empty values to defaults altering existing config semantics
• fanquake (member, neutral, 1 statements 2021-03-15 to 2021-03-15): requested using std::optional in new code
• maflcko (member, objection, 7 statements 2021-08-19 to 2024-09-23): objected to combining flags without starting from single types like bool
• achow101 (member, neutral, 1 statements 2023-09-23 to 2023-09-23): commented during omitted discussion period
• hodlinator (member, support, 30 statements 2024-07-23 to 2024-10-02): Concept ACK praising incremental argument typing, tested CLI flags thoroughly
• l0rinc (member, support, 28 statements 2024-08-09 to 2024-10-02): Approach ACK, proposed code structure refactorings and tightening fuzz exceptions

Enables:

• #17580
• #17783
• #17493
• #17581
• #22978

Base for: #17493, #17580, #17581, #17783

564 lines under test/bench/ci.

• src/test/argsman_tests.cpp +425/-9
• src/common/args.cpp +160/-31
• src/common/args.h +144/-16
• src/test/getarg_tests.cpp +61/-3
• src/test/fuzz/system.cpp +48/-7
• test/lint/check-doc.py +3/-2
• src/common/config.cpp +4/-0
• src/test/util/common.h +4/-0
• test/functional/feature_config_args.py +1/-1

The short text the ranking pass compares across PRs.

Implements runtime parsing and startup validation for ALLOW_BOOL, ALLOW_INT, ALLOW_STRING, and ALLOW_LIST argument flags in ArgsManager. Currently, configuration settings lack strict type validation, causing type-coercion footguns like negated options resolving to string "0" (which previously caused -nowallet to create a wallet named "0") and duplicate configuration file assignments being silently ignored. This change rejects malformed typed inputs at startup and raises internal errors on mismatched getter calls, while keeping existing arguments under ALLOW_ANY until migrated. It serves as the prerequisite foundation for several follow-up PRs adding compile-time and runtime option checking (#17580, #17783, #22978).

P2 · unblocks #17580

Implements argument flag type validation to eliminate configuration footguns and unblocks subsequent PRs transitioning options to typed flags. Commit 99c35134de highlights historical footguns where '-nowallet' created a wallet file named '0', and commit 97071b909f prevents duplicate non-list configuration settings from being silently ignored, laying foundational safety for argument parsing across the node.

Membership: Carries the Utils/log/libs label and directly modifies ArgsManager in src/common/args.cpp and src/common/args.h.

Factors: security/stability 1, bug 1, performance 0, user value 1, leverage 2