# Problem
We leave too few observability breadcrumbs in the case where wait_lsn is
exceptionally slow.
# Changes
- refactor: extract the monitoring logic out of `log_slow` into
`monitor_slow_future`
- add global + per-timeline counter for time spent waiting for wait_lsn
- It is updated while we're still waiting, similar to what we do for
page_service response flush.
- add per-timeline counterpair for started & finished wait_lsn count
- add slow-logging to leave breadcrumbs in logs, not just metrics
For the slow-logging, we need to consider not flooding the logs during a
broker or network outage/blip.
The solution is a "log-streak-level" concurrency limit per timeline.
At any given time, there is at most one slow wait_lsn that is logging
the "still running" and "completed" sequence of logs.
Other concurrent slow wait_lsn's don't log at all.
This leaves at least one breadcrumb in each timeline's logs if some
wait_lsn was exceptionally slow during a given period.
The full degree of slowness can then be determined by looking at the
per-timeline metric.
# Performance
Reran the `bench_log_slow` benchmark, no difference, so, existing call
sites are fine.
We do use a Semaphore, but only try_acquire it _after_ things have
already been determined to be slow. So, no baseline overhead
anticipated.
# Refs
-
https://github.com/neondatabase/cloud/issues/23486#issuecomment-2711587222
# Fix metric_unit length in test_compute_ctl_api.py
## Description
This PR changes the metric_unit from "microseconds" to "μs" in
test_compute_ctl_api.py to fix the issue where perf test results were
not being stored in the database due to the string exceeding the 10
character limit of the metric_unit column in the perf_test_results
table.
## Problem
As reported in Slack, the perf test results were not being uploaded to
the database because the "microseconds" string (12 characters) exceeds
the 10 character limit of the metric_unit column in the
perf_test_results table.
## Solution
Replace "microseconds" with "μs" in all metric_unit parameters in the
test_compute_ctl_api.py file.
## Testing
The changes have been committed and pushed. The PR is ready for review.
Link to Devin run:
https://app.devin.ai/sessions/e29edd672bd34114b059915820e8a853
Requested by: Peter Bendel
Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: peterbendel@neon.tech <peterbendel@neon.tech>
## Problem
close https://github.com/neondatabase/neon/issues/10310
## Summary of changes
This patch adds a new behavior for the detach_ancestor API: detach with
multi-level ancestor and no reparenting. Though we can potentially
support multi-level + do reparenting / single-level + no-reparenting in
the future, as it's not required for the recovery/snapshot epic, I'd
prefer keeping things simple now that we only handle the old one and the
new one instead of supporting the full feature matrix.
I only added a test case of successful detaching instead of testing
failures. I'd like to make this into staging and add more tests in the
future.
---------
Signed-off-by: Alex Chi Z <chi@neon.tech>
## Problem
When a node becomes active, we query its locations and update the
observed state in-place.
This can race with the observed state updates done when processing
reconcile results.
## Summary of changes
The argument for this reconciliation step is that is reduces the need
for background reconciliations.
I don't think is actually true anymore. There's two cases.
1. Restart of node after drain. Usually the node does not go through the
offline state here, so observed locations
were not marked as none. In any case, there should be a handful of
shards max on the node since we've just drained it.
2. Node comes back online after failure or network partition. When the
node is marked offline, we reschedule everything away from it. When it
later becomes active, the previous observed location is extraneous and
requires a reconciliation anyway.
Closes https://github.com/neondatabase/neon/issues/11148
## Problem
Test `test_timeline_archive` is flaky because it makes requests that are
intended to fail. It sometimes leads to warning in pageserver's logs.
More details are in the issue.
- Closes: https://github.com/neondatabase/neon/issues/11177
## Summary of changes
- Suppress such errors.
Previously, remote extensions were not fetched unless they were used in
some other manner. For instance, loading a BM25 index in pg_search
fetches the pg_search extension. However, if on a fresh compute with
pg_search 0.15.5 installed, the user ran `ALTER EXTENSION pg_search
UPDATE TO '0.15.6'` without first using the pg_search extension, we
would not fetch the extension and fail to find an update path.
Signed-off-by: Tristan Partin <tristan@neon.tech>
We were previously only revoking privileges granted by neon_superuser.
However, we need to do it for all grantors.
Signed-off-by: Tristan Partin <tristan@neon.tech>
## Problem
As part of the disaster recovery tool. Partly for
https://github.com/neondatabase/neon/issues/9114.
## Summary of changes
* Add a new pageserver API to force patch the fields in index_part and
modify the timeline internal structures.
---------
Signed-off-by: Alex Chi Z <chi@neon.tech>
## Problem
Storage controller uses http for requests to safekeeper management API.
Closes: https://github.com/neondatabase/cloud/issues/24835
## Summary of changes
- Add `use_https_safekeeper_api` option to storcon to use https api
- Use https for requests to safekeeper management API if this option is
enabled
- Add `ssl_ca_file` option to storcon for ability to specify custom root
CA certificate
## Problem
The current migration API does a live migration, but if the destination
doesn't already have a secondary, that live migration is unlikely to be
able to warm up a tenant properly within its timeout (full warmup of a
big tenant can take tens of minutes).
Background optimisation code knows how to do this gracefully by creating
a secondary first, but we don't currently give a human a way to trigger
that.
Closes: https://github.com/neondatabase/neon/issues/10540
## Summary of changes
- Add `prefererred_node` parameter to TenantShard, which is respected by
optimize_attachment
- Modify migration API to have optional prewarm=true mode, in which we
set preferred_node and call optimize_attachment, rather than directly
modifying intentstate
- Require override_scheduler=true flag if migrating somewhere that is a
less-than-optimal scheduling location (e.g. wrong AZ)
- Add `origin_node_id` to migration API so that callers can ensure
they're moving from where they think they're moving from
- Add tests for the above
The storcon_cli wrapper for this has a 'watch' mode that waits for
eventual cutover. This doesn't show the warmth of the secondary evolve
because we don't currently have an API for that in the controller, as
the passthrough API only targets attached locations, not secondaries. It
would be straightforward to add later as a dedicated endpoint for
getting secondary status, then extend the storcon_cli to consume that
and print a nice progress indicator.
## Problem
We just had a regression reported at
https://neondb.slack.com/archives/C08EXUJF554/p1741102467515599, which
clearly came with one of the releases. It's not a huge problem yet, but
it's annoying that we cannot quickly attribute it to a specific commit.
## Summary of changes
Add a very simple `compute_ctl` HTTP API benchmark that does 10k
requests to `/status` and `metrics.json` and reports p50 and p99.
---------
Co-authored-by: Peter Bendel <peterbendel@neon.tech>
## Problem
In f37eeb56, I properly escaped the identifier, but I haven't noticed
that the resulting string is used in the `format('...')`, so it needs
additional escaping. Yet, after looking at it closer and with Heikki's
and Tristan's help, it appeared to be that it's a full can of worms and
we have problems all over the code in places where we use PL/pgSQL
blocks.
## Summary of changes
Add a new `pg_quote_dollar()` helper to deal with it, as dollar-quoting
of strings seems to be the only robust way to escape strings in dynamic
PL/pgSQL blocks. We mimic the Postgres' `pg_get_functiondef` logic here
[1].
While on it, I added more tests and caught a couple of more bugs with
string escaping:
1. `get_existing_dbs_async()` was wrapping `owner` in additional
double-quotes if it contained special characters
2. `construct_superuser_query()` was flawed in even more ways than the
rest of the code. It wasn't realistic to fix it quickly, but after
thinking about it more, I realized that we could drop most of it
altogether. IIUC, it was added as some sort of migration, probably back
when we haven't had migrations yet. So all the complicated code was
needed to properly update existing roles and DBs. In the current Neon,
this code only runs before we create the very first DB and role. When we
create roles and DBs, all `neon_superuser` grants are added in the
different places. So the worst thing that could happen is that there is
an ancient branch somewhere, so when users poke it, they will realize
that not all Neon features work as expected. Yet, the fix is simple and
self-serve -- just create a new role via UI or API, and it will get a
proper `neon_superuser` grant.
[1]:
8b49392b27/src/backend/utils/adt/ruleutils.c (L3153)Closesneondatabase/cloud#25048
## Problem
On unarchival, we update the previous heatmap with all visible layers.
When the primary generates a new heatmap it includes all those layers,
so the secondary will download them. Since they're not actually resident
on the primary (we didn't call the warm up API), they'll never be
evicted,
so they remain in the heatmap.
We want these layers in the heatmap, since we might wish to warm-up an
unarchived timeline after a shard migration. However, we don't want them
to be downloaded on the secondary until we've warmed up the primary.
## Summary of Changes
Include these layers in the heatmap and mark them as cold. All heatmap
operations act on non-cold layers apart from the attached location
warming up API,
which will download the cold layers. Once the cold layers are downloaded
on the primary,
they'll be included in the next heatmap as hot and the secondary starts
fetching them too.
## Problem
For pg_regress test, we do both v1 and v2; for all the rest, we default
to v2.
part of https://github.com/neondatabase/neon/issues/9516
## Summary of changes
Use reldir v2 across test cases by default.
---------
Signed-off-by: Alex Chi Z <chi@neon.tech>
## Problem
Our benchmarking workflows contain links to grafana dashboards to
troubleshoot problems.
This works fine for non-pooled endpoints.
For pooled endpoints we need to remove the `-pooler` suffix from the
endpoint's hostname to get a valid endpoint ID.
Example link that doesn't work in this run
https://github.com/neondatabase/neon/actions/runs/13678933253/job/38246028316#step:8:311
## Summary of changes
Check if connection string is a -pooler connection string and if so
remove this suffix from the endpoint ID.
---------
Co-authored-by: Alexander Bayandin <alexander@neon.tech>
## Problem
We realized that we may use this metric for more 'live' info about
extension installations vs. what we have with installed extensions
metric, which is only updated at start, atm.
## Summary of changes
Add `filename` label to `compute_ctl_remote_ext_requests_total`. Note
that it contains the raw archive name with `.tar.zst` at the end, so the
consumer may need to strip this suffix.
Closes https://github.com/neondatabase/cloud/issues/24694
## Problem
`test_check_visibility_map` is the slowest test in CI, and can cause
timeouts under particularly slow configurations (`debug` and
`without-lfc`).
## Summary of changes
* Reduce the `pgbench` scale factor from 10 to 8.
* Omit a redundant vacuum during `pgbench` init.
* Remove a final `vacuum freeze` + `pg_check_visible` pass, which has
questionable value (we've already done a vacuum freeze previously, and
we don't flush the compute cache before checking anyway).
## Problem
On unarchival, we update the previous heatmap with all visible layers.
When the primary generates a new heatmap it includes all those layers,
so the secondary will download them. Since they're not actually resident
on the primary (we didn't call the warm up API), they'll never be
evicted, so they remain in the heatmap.
This leads to oversized secondary locations like we saw in pre-prod.
## Summary of changes
Gate the loading of the previous heatmaps and the heatmap generation on
unarchival behind configuration
flags. They are disabled by default, but enabled in tests.
## Problem
part of https://github.com/neondatabase/neon/issues/11067
My observation is that with the current value of settings, x86-v1
usually takes 30s, arm-v1 1m30s, x86-v2 1m, arm-v2 3m. But sometimes the
system could run too slow and cause test to timeout on arm with reldir
v2.
While I investigate what's going on and further improve the performance,
I'd like to set both of them to use the same test input, so that it
doesn't timeout and we don't abuse this test case as a performance test.
## Summary of changes
Use the same settings for both test cases.
Signed-off-by: Alex Chi Z <chi@neon.tech>
## Problem
Incoming requests often take the service lock, and sometimes even do
database transactions. That creates a risk that a rogue client can
starve the controller of the ability to do its primary job of
reconciling tenants to an available state.
## Summary of changes
* Use the `governor` crate to rate limit tenant requests at 10 requests
per second. This is ~10-100x lower than the worst "attack" we've seen
from a client bug. Admin APIs are not rate limited.
* Add a `storage_controller_http_request_rate_limited` histogram for
rate limited requests.
* Log a warning every 10 seconds for rate limited tenants.
The rate limiter is parametrized on TenantId, because the kinds of
client bug we're protecting against generally happen within tenant
scope, and the rates should be somewhat stable: we expect the global
rate of requests to increase as we do more work, but we do not expect
the rate of requests to one tenant to increase.
---------
Co-authored-by: John Spray <john@neon.tech>
## Problem
part of https://github.com/neondatabase/neon/issues/9516
## Summary of changes
Similar to the aux v2 migration, we persist the relv2 migration status
into index_part, so that even the config item is set to false, we will
still read from the v2 storage to avoid loss of data.
Note that only the two variants `None` and
`Some(RelSizeMigration::Migrating)` are used for now. We don't have full
migration implemented so it will never be set to
`RelSizeMigration::Migrated`.
---------
Signed-off-by: Alex Chi Z <chi@neon.tech>
## Problem
If a caller times out on safekeeper timeline deletion on a large
timeline, and waits a while before retrying, the deletion will not
progress while the retry is waiting. The net effect is very very slow
deletion as it only proceeds in 30 second bursts across 5 minute idle
periods.
Related: https://github.com/neondatabase/neon/issues/10265
## Summary of changes
- Run remote deletion in a background task
- Carry a watch::Receiver on the Timeline for other callers to join the
wait
- Restart deletion if the API is called again and the previous attempt
failed
## Problem
We want to support larger tenants (regarding logical database size,
number of transactions per second etc.) and should increase our test
coverage of OLTP transactions at larger scale.
## Summary of changes
Start a new benchmark that over time will add more OLTP tests at larger
scale.
This PR covers the first version and will be extended in further PRs.
Also fix some infrastructure:
- default for new connections and large tenants is to use connection
pooler pgbouncer, however our fixture always added
`statement_timeout=120` which is not compatible with pooler
[see](https://neon.tech/docs/connect/connection-errors#unsupported-startup-parameter)
- action to create branch timed out after 10 seconds and 10 retries but
for large tenants it can take longer so use increasing back-off for
retries
## Test run
https://github.com/neondatabase/neon/actions/runs/13593446706
## Problem
Preparation for https://github.com/neondatabase/neon/issues/10851
## Summary of changes
Add walproposer `safekeepers_generations` field which can be set by
prefixing `neon.safekeepers` GUC with `g#n:`. Non zero value (n) forces
walproposer to use generations. In particular, this also disables
implicit timeline creation as timeline will be created by storcon. Add
test checking this. Also add missing infra: `--safekeepers-generation`
flag to neon_local endpoint start + fix `--start-timeout` flag: it
existed but value wasn't used.
## Problem
In multi-character keys, the GIN index creates a CRC Hash of the first 3
bytes of the key.
The hash can have the first bit to be set or unset, needing to have a
consistent representation
of `char` across architectures for consistent results. GIN stores these
keys by their hashes
which determines the order in which the keys are obtained from the GIN
index.
By default, chars are signed in x86 and unsigned in arm, leading to
inconsistent behavior across different platform architectures. Adding
the `-fsigned-char` flag to the GCC compiler forces chars to be treated
as signed across platforms, ensuring the ordering in which the keys are
obtained consistent.
## Summary of changes
Added `-fsigned-char` to the `CFLAGS` to force GCC to use signed chars
across platforms.
Added a test to check this across platforms.
Fixes: https://github.com/neondatabase/cloud/issues/23199
## Problem
JWT tokens aren't in place, so all SK heartbeats fail. This is
equivalent to a wait before applying the PS heartbeats and makes things
more flaky.
## Summary of Changes
Add a flag that skips loading SKs from the db on start-up and at
runtime.
https://github.com/neondatabase/cloud/issues/23008
For TLS between proxy and compute, we are using an internally
provisioned CA to sign the compute certificates. This change ensures
that proxy will load them from a supplied env var pointing to the
correct file - this file and env var will be configured later, using a
kubernetes secret.
Control plane responds with a `server_name` field if and only if the
compute uses TLS. This server name is the name we use to validate the
certificate. Control plane still sends us the IP to connect to as well
(to support overlay IP).
To support this change, I'd had to split `host` and `host_addr` into
separate fields. Using `host_addr` and bypassing `lookup_addr` if
possible (which is what happens in production). `host` then is only used
for the TLS connection.
There's no blocker to merging this. The code paths will not be triggered
until the new control plane is deployed and the `enableTLS` compute flag
is enabled on a project.
## Problem
We intend for cplane to use the heatmap layer download API to warm up
timelines after unarchival. It's tricky for them to recurse in the
ancestors,
and the current implementation doesn't work well when unarchiving a
chain
of branches and warming them up.
## Summary of changes
* Add a `recurse` flag to the API. When the flag is set, the operation
recurses into the parent
timeline after the current one is done.
* Be resilient to warming up a chain of unarchived branches. Let's say
we unarchived `B` and `C` from
the `A -> B -> C` branch hierarchy. `B` got unarchived first. We
generated the unarchival heatmaps
and stash them in `A` and `B`. When `C` unarchived, it dropped it's
unarchival heatmap since `A` and `B`
already had one. If `C` needed layers from `A` and `B`, it was out of
luck. Now, when choosing whether
to keep an unarchival heatmap we look at its end LSN. If it's more
inclusive than what we currently have,
keep it.
Before this PR, re-attach and validate would log the same warning
```
calling control plane generation validation API failed
```
on retry errors.
This can be confusing.
This PR makes the message generically valid for any upcall and adds
additional tracing spans to capture context.
Along the way, clean up some copy-pasta variable naming.
refs
-
https://github.com/neondatabase/neon/issues/10381#issuecomment-2684755827
---------
Co-authored-by: Alexander Lakhin <alexander.lakhin@neon.tech>
## Problem
So far cumulative statistics have not been persisted when Neon scales to
zero (suspends endpoint).
With PR https://github.com/neondatabase/neon/pull/6560 the cumulative
statistics should now survive endpoint restarts and correctly trigger
the auto- vacuum and auto analyze maintenance
So far we did not have a testcase that validates that improvement in our
dev cloud environment with a real project.
## Summary of changes
Introduce testcase `test_cumulative_statistics_persistence`in the
benchmarking workflow running daily to verify:
- Verifies that the cumulative statistics are correctly persisted across
restarts.
- Cumulative statistics are important to persist across restarts because
they are used
- when auto-vacuum an auto-analyze trigger conditions are met.
- The test performs the following steps:
- Seed a new project using pgbench
- insert tuples that by itself are not enough to trigger auto-vacuum
- suspend the endpoint
- resume the endpoint
- insert additional tuples that by itself are not enough to trigger
auto-vacuum but in combination with the previous tuples are
- verify that autovacuum is triggered by the combination of tuples
inserted before and after endpoint suspension
## Test run
https://github.com/neondatabase/neon/actions/runs/13546879714/job/37860609089#step:6:282
## Problem
Yet another source of flakyness for
https://github.com/neondatabase/neon/issues/10517
## Summary of changes
The test scenario we want to create is that we have an image layer in
index_part and then overwrite it, so we have to ensure it gets persisted
in index_part by doing a force checkpoint.
Signed-off-by: Alex Chi Z <chi@neon.tech>
## Problem
https://github.com/neondatabase/neon/pull/10241 added configuration
switch endpoint, but it didn't delete timeline if node was excluded.
## Summary of changes
Add separate /exclude API endpoint which similarly accepts membership
configuration where sk is supposed by be excluded. Implementation
deletes the timeline locally.
Some more small related tweaks:
- make mconf switch API PUT instead of POST as it is idempotent;
- return 409 if switch was refused instead of 200 with requested &
current;
- remove unused was_active flag from delete response;
- remove meaningless _force suffix from delete functions names;
- reuse timeline.rs delete_dir function in timelines_global_map instead
of its own copy.
part of https://github.com/neondatabase/neon/issues/9965
## Problem
I see a lot of timeout errors, which indicates that this test is too
slow. It seems that create relations are fast, but the subsequent
truncating step is slow.
## Summary of changes
Reduce number of relations for now, and investigate later.
Signed-off-by: Alex Chi Z <chi@neon.tech>
## Problem
json_ctrl.rs is an obsolete attempt to have tests with fine control of
feeding messages into safekeeper superseded by desim framework.
## Summary of changes
Drop it.
## Problem
part of https://github.com/neondatabase/neon/issues/9114
## Summary of changes
Add the auto trigger for gc-compaction. It computes two values: L1 size
and L2 size. When L1 size >= initial trigger threshold, we will trigger
an initial gc-compaction. When l1_size / l2_size >=
gc_compaction_ratio_percent, we will trigger the "tiered" gc-compaction.
---------
Signed-off-by: Alex Chi Z <chi@neon.tech>
## Problem
We have `test_perf_many_relations` but it only runs on remote clusters,
and we cannot directly modify tenant config. Therefore, I patched one of
the current tests to benchmark relv2 performance.
close https://github.com/neondatabase/neon/issues/9986
## Summary of changes
* Add `v1/v2` selector to `test_tx_abort_with_many_relations`.
---------
Signed-off-by: Alex Chi Z <chi@neon.tech>
## Problem
As part of https://github.com/neondatabase/neon/issues/8614 we need to
pass membership configurations between compute and safekeepers.
## Summary of changes
Add version 3 of the protocol carrying membership configurations.
Greeting message in both sides gets full conf, and other messages
generation number only. Use protocol bump to include other accumulated
changes:
- stop packing whole structs on the wire as is;
- make the tag u8 instead of u64;
- send all ints in network order;
- drop proposer_uuid, we can pass it in START_WAL_PUSH and it wasn't
much useful anyway.
Per message changes, apart from mconf:
- ProposerGreeting: tenant / timeline id is sent now as hex cstring.
Remove proto version, it is passed outside in START_WAL_PUSH. Remove
postgres timeline, it is unused. Reorder fields a bit.
- AcceptorGreeting: reorder fields
- VoteResponse: timeline_start_lsn is removed. It can be taken from
first member of term history, and later we won't need it at all when all
timelines will be explicitly created. Vote itself is u8 instead of u64.
- ProposerElected: timeline_start_lsn is removed for the same reasons.
- AppendRequest: epoch_start_lsn removed, it is known from term history
in ProposerElected.
Both compute and sk are able to talk v2 and v3 to make rollbacks (in
case we need them) easier; neon.safekeeper_proto_version GUC sets the
client version. v2 code can be dropped later.
So far empty conf is passed everywhere, future PRs will handle them.
To test, add param to some tests choosing proto version; we want to test
both 2 and 3 until we fully migrate.
ref https://github.com/neondatabase/neon/issues/10326
---------
Co-authored-by: Arthur Petukhovsky <petuhovskiy@yandex.ru>
We've seen some cases in production where a compute doesn't get a
response to a pageserver request for several minutes, or even more. We
haven't found the root cause for that yet, but whatever the reason is,
it seems overly optimistic to think that if the pageserver hasn't
responded for 2 minutes, we'd get a response if we just wait patiently a
little longer. More likely, the pageserver is dead or there's some kind
of a network glitch so that the TCP connection is dead, or at least
stuck for a long time. Either way, it's better to disconnect and
reconnect. I set the default timeout to 2 minutes, which should be
enough for any GetPage request under normal circumstances, even if the
pageserver has to download several layer files from remote storage.
Make the disconnect timeout configurable. Also make the "log interval",
after which we print a message to the log configurable, so that if you
change the disconnect timeout, you can set the log timeout
correspondingly. The default log interval is still 10 s. The new GUCs
are called "neon.pageserver_response_log_timeout" and
"neon.pageserver_response_disconnect_timeout".
Includes a basic test for the log and disconnect timeouts.
Implements issue #10857
## Problem
https://github.com/neondatabase/neon/pull/10788 introduced an API for
warming up attached locations
by downloading all layers in the heatmap. We intend to use it for
warming up timelines after unarchival too,
but it doesn't work. Any heatmap generated after the unarchival will not
include our timeline, so we've lost
all those layers.
## Summary of changes
Generate a cheeky heatmap on unarchival. It includes all the visible
layers.
Use that as the `PreviousHeatmap` which inputs into actual heatmap
generation.
Closes: https://github.com/neondatabase/neon/issues/10541
## Problem
We failed to detect https://github.com/neondatabase/neon/pull/10845
before merging, because the tests we run with a matrix of component
versions didn't include the ones that did live migrations.
## Summary of changes
- Do a live migration during the storage controller smoke test, since
this is a pretty core piece of functionality
- Apply a compat version matrix to the graceful cluster restart test,
since this is the functionality that we most urgently need to work
across versions to make deploys work.
I expect the first CI run of this to fail, because
https://github.com/neondatabase/neon/pull/10845 isn't merged yet.
There was a race condition with compute_ctl and the metric being
collected related to whether the neon extension had been updated or not.
compute_ctl will run `ALTER EXTENSION neon UPDATE` on compute start in
the postgres database.
Fixes: https://github.com/neondatabase/neon/issues/10932
Signed-off-by: Tristan Partin <tristan@neon.tech>
## Problem
Prefetch is performed locally, so different backers can request the same
pages form PS.
Such duplicated request increase load of page server and network
traffic.
Making prefetch global seems to be very difficult and undesirable,
because different queries can access chunks on different speed. Storing
prefetch chunks in LFC will not completely eliminate duplicates, but can
minimise such requests.
The problem with storing prefetch result in LFC is that in this case
page is not protected by share buffer lock.
So we will have to perform extra synchronisation at LFC side.
See:
https://neondb.slack.com/archives/C0875PUD0LC/p1736772890602029?thread_ts=1736762541.116949&cid=C0875PUD0LC
@MMeent implementation of prewarm:
See https://github.com/neondatabase/neon/pull/10312/
## Summary of changes
Use conditional variables to sycnhronize access to LFC entry.
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Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
