Row cache updates do not provide strong exception safety guarantees

[bhalevy]

The row_cache update path uses an external_updater class with two api functions: prepare and execute.
The current design calls for:

• prepare to create only temporary state, so if it fails, the failure is handled gracefully and the error is propagated to row_cache::update caller leaving no state changes behind (to the row cache nor to the updated table).
• execute role is to apply the temporary state prepare left behind.

Currently (as of 5.4-dev 652153c), we let errors from execute escape row_cache::update, but there is no guarantee that the execute implementation provides strong exception safety gurantees, i.e. either succeed in whole or be exception safe leaving the state unchanged in case an exception is thrown.

See

scylladb/row_cache.hh

Lines 172 to 182 in 6d34f99

	// A function which adds new writes to the underlying mutation source.
	// All invocations of external_updater on given cache instance are serialized internally.
	// Must have strong exception guarantees. If throws, the underlying mutation source
	// must be left in the state in which it was before the call.
	class external_updater_impl {
	public:
	virtual ~external_updater_impl() {}
	virtual future<> prepare() { return make_ready_future<>(); }
	// FIXME: make execute() noexcept, that will require every updater to make execution exception safe,
	// also change function signature.
	virtual void execute() = 0;

See also #13937 (comment)

[bhalevy]

Backport should be applicable to all live versions

[bhalevy]

@scylladb/scylla-maint please backport

[denesb]

There are conflicts, please provide backport PRs. Actually, a single PR against branch-5.4 should be enough, I can try to cherr-pick that backport to the older branches too.

[denesb]

The 5.4 backport PR was good for 5.2 but it doesn't apply to 5.1.

[mykaul]

The 5.4 backport PR was good for 5.2 but it doesn't apply to 5.1.

Once 5.4 is released, we only support it and 5.2. There's no need for a 5.1 backport.

[denesb]

@bhalevy please provide a backport for enterprise, if needed.