`Client#execute_query` always routes to leader, even when unnecessary

[michaelherold]

Problem

When using the single_use specifier for execute_query, the Spanner gem creates a read-only transaction. However, execute_query is hard-coded to always route requests to the leader, which introduces unnecessary latency depending on the region of the leader and the querist.

I believe that we should be able to check if a read-only transaction is open when determining whether to route to the leader, like so:

          single_use_tx = single_use_transaction single_use
-          route_to_leader = LARHeaders.execute_query true
+          route_to_leader = LARHeaders.execute_query !single_use_tx

but am unsure if I have the semantics correct.

This, however, has the underlying effect of setting the x-goog-spanner-route-to-leader value to "false" instead of just not setting it.

Environment details

• OS: Any
• Ruby version: Any
• Gem name and version: google-cloud-spanner = 2.28.0 and current main branch

Steps to reproduce

1. Create a client
2. Run execute_query(sql, single_use: { bounded_staleness: 10 })
3. See the query route through the leader even when specifying a different placement

Code example

require "google/cloud/spanner"

puts "Starting"
emulator_host = ENV.fetch("SPANNER_EMULATOR_HOST", nil)
project_id = ENV.fetch("SPANNER_EMULATOR_PROJECT", "")
instance_id = ENV.fetch("SPANNER_EMULATOR_INSTANCE", "")
database_id = ENV.fetch("SPANNER_EMULATOR_DATABASE", "")

puts "Creating project"
project = Google::Cloud::Spanner.new(
  project_id: project_id,
  emulator_host: emulator_host,
)

puts "Creating client"
client = project.client(instance_id, database_id)

puts "Executing sql"
puts client.execute_sql("SELECT * FROM my_table", single_use: { bounded_staleness: 10 })

Full backtrace

There is no backtrace, the query runs just not at the location one would expect.