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impl(spanner): lock mutex in total_sessions accessor #15017

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Merged
scotthart merged 2 commits into from
Mar 6, 2025
Merged

impl(spanner): lock mutex in total_sessions accessor #15017

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256e47a
impl(spanner): lock mutex in total_sessions accessor
scotthart Mar 6, 2025
2f3e758
add snapshot warning
scotthart Mar 6, 2025
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10 changes: 7 additions & 3 deletions google/cloud/spanner/internal/session_pool.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -370,9 +370,13 @@ std::shared_ptr<SpannerStub> SessionPool::GetStub(Session const& session) {
return GetStub(std::unique_lock<std::mutex>(mu_));
}

void SessionPool::DecrementSessionCount(
std::unique_lock<std::mutex> const&,
google::cloud::spanner_internal::Session& session) {
int SessionPool::total_sessions() const {
std::lock_guard<std::mutex> lk(mu_);
return total_sessions_;
}

void SessionPool::DecrementSessionCount(std::unique_lock<std::mutex> const&,
Session const& session) {
--total_sessions_;
auto const& channel = session.channel();
if (channel) {
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9 changes: 6 additions & 3 deletions google/cloud/spanner/internal/session_pool.h
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Expand Up @@ -118,8 +118,11 @@ class SessionPool : public std::enable_shared_from_this<SessionPool> {
/**
* Returns the number of sessions in the session pool plus the number of
* sessions allocated to running transactions.
*
* @note This function should only be used for testing as other threads
* could be modifying the underlying value immediately after it returns.
*/
int total_sessions() const { return total_sessions_; }
int total_sessions() const;
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@devbww devbww Mar 6, 2025

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Given that this is one of those "snapshotting", informational accessors that couldn't really be used in a robust program, it should at least be marked as "for testing purposes only".

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@scotthart scotthart Mar 6, 2025

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Added a bit about using it only for testing.


private:
friend std::shared_ptr<SessionPool> MakeSessionPool(
Expand Down Expand Up @@ -215,7 +218,7 @@ class SessionPool : public std::enable_shared_from_this<SessionPool> {

// Performs the necessary bookkeeping when a session is removed from use.
void DecrementSessionCount(std::unique_lock<std::mutex> const& lk,
Session& session);
Session const& session);

spanner::Database const db_;
google::cloud::CompletionQueue cq_;
Expand All @@ -226,7 +229,7 @@ class SessionPool : public std::enable_shared_from_this<SessionPool> {
int const max_pool_size_;
std::mt19937 random_generator_;

std::mutex mu_;
mutable std::mutex mu_;
std::condition_variable cond_;
SessionHolder multiplexed_session_; // GUARDED_BY(mu_)
std::vector<std::unique_ptr<Session>> sessions_; // GUARDED_BY(mu_)
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