Reject Duration strings carrying more than nanosecond precision (#30446)
TimeUtil::FromString accepted fractional seconds with more than nine digits and silently produced a wrong Duration. The scaling step computes Pow(10, 9 - nanos_part.length()); when the fraction is longer than nine digits the exponent is negative, and Pow() treats a negative exponent as zero and returns 1. The unscaled digits are then handed to set_nanos, which truncates them to 32 bits, and IsDurationValid is happy with the result.
Concretely, before the change:
TimeUtil::FromString("0.3294967297s", &d) -> true, d = { seconds: 0, nanos: -999999999 }
TimeUtil::FromString("1.0000000001s", &d) -> true, d = { seconds: 1, nanos: 1 }
The first case flips the sign of the parsed value outright, since 3294967297 truncated to int32 is -999999999. The second case silently moves a digit: 1.0000000001s becomes 1.000000001s. A caller that parses attacker supplied duration text gets a valid looking message with an arbitrary value.
The fix rejects any fraction longer than nine digits, the most a Duration can represent, matching the JSON mapping rule that caps fractional seconds at nine digits. Values with nine digits or fewer are unchanged.
Testing: added assertions to DurationStringFormat in src/google/protobuf/util/time_util_test.cc. On unpatched main the five new FromString calls all return true (the EXPECT_FALSE lines fail); with the fix the full TimeUtilTest suite passes, 14 tests locally.
Closes #30446
COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/30446 from ManoharPaturi:fix-timeutil-duration-precision 7cd00b90a52fead245e8cc7dead270dba1b08887
PiperOrigin-RevId: 1002869518
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