fix WeightedRandomizedLoadBalancer skipping the last server

[Nas01010101]

What problem does this PR solve?

WeightedRandomizedLoadBalancer::SelectServer() never selects the server that was added last, and over-selects the first one.

Add() fills Server::current_weight_sum with an inclusive prefix sum. SelectServer() draws random_weight from butil::fast_rand_less_than(weight_sum), i.e. from [0, weight_sum - 1], and then lower_bound()s that value against those prefix sums. lower_bound() returns the first server whose prefix sum is >= random_weight, but a server owns the half-open range [prefix(i-1), prefix(i)), so the predicate has to be > random_weight.

Two consequences: the first server also serves random_weight == prefix(0), so it gets one slot too many; and the last server is never selected at all, because random_weight can never reach weight_sum.

With four servers of equal weight the measured distribution is 49.8 / 25.1 / 25.1 / 0.0 percent instead of 25 percent each. The last server only receives traffic through the fallback loop, when every server picked at random happens to be unavailable.

What is changed and the side effects?

Changed: search for random_weight + 1, so lower_bound() lands on the first prefix sum strictly greater than random_weight. random_weight + 1 is at most weight_sum, which is exactly the last prefix sum, so the iterator is still always valid.

Side effects:

• Performance effects: none, the comparison is unchanged.
• Breaking backward compatibility: no. Traffic shifts towards the configured weights.

The existing weighted_randomized test does not catch this. Its servers have weights 3/2/5/10 and it only asserts that every rate is within 0.5x~2x of the expected one. The weight-10 server measures 0.453 before this change and 0.5055 after it, and both are inside that band. The off-by-one predicts that number exactly: the server should own 10 of 20 slots but owns 9, i.e. 0.45.

This PR adds weighted_randomized_equal_weight, which uses equal weights so a single misplaced slot is visible, and checks the rates within 0.9x1.1x. On master the last server is selected 0 times out of 40000 and the test fails; with this change all four land within 0.24760.25435.

Check List:

• brpc_load_balancer_unittest passes in full: [ PASSED ] 17 tests.
• The new test was run with --gtest_repeat=40 without a failure.
• Verified the new test fails without the one-line change and passes with it.

Tested on macOS/arm64 with clang. I was not able to build on Linux/gcc locally; CI will cover that.

[Copilot]

Pull request overview

This PR fixes an off-by-one error in brpc::policy::WeightedRandomizedLoadBalancer::SelectServer() that prevented the last server in the weighted prefix-sum list from ever being selected (and slightly over-selected the first), and adds a targeted unit test to catch this regression.

Changes:

• Fix selection logic by searching lower_bound(random_weight + 1) against inclusive prefix sums.
• Add weighted_randomized_equal_weight unit test with equal weights and tighter distribution bounds to reliably detect the bug.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/brpc/policy/weighted_randomized_load_balancer.cpp	Adjusts the weighted random selection to correctly map random draws onto prefix-sum ranges.
test/brpc_load_balancer_unittest.cpp	Adds a new equal-weight distribution test to expose the prior off-by-one behavior.

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