Ingore DISK_FULL error when performing availability check for client (#5010)

See #5001

No space is what's expected if we're at size limit.
Of course if SK incorrectly returned "no space", the availability check
wouldn't fire.
But  users would notice such a bug quite soon anyways.
So  ignoring "no space" is the right trade-off.


## Problem

## Summary of changes

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
Co-authored-by: Joonas Koivunen <joonas@neon.tech>

compute_tools/src/checker.rs

@@ -1,7 +1,7 @@
 use anyhow::{anyhow, Ok, Result};
 use postgres::Client;
 use tokio_postgres::NoTls;
-use tracing::{error, instrument};
+use tracing::{error, instrument, warn};
 
 use crate::compute::ComputeNode;
 
@@ -55,13 +55,24 @@ pub async fn check_writability(compute: &ComputeNode) -> Result<()> {
         ON CONFLICT (id) DO UPDATE
          SET updated_at = now();";
 
-    let result = client.simple_query(query).await?;
-
-    if result.len() != 1 {
-        return Err(anyhow::format_err!(
-            "expected 1 query result, but got {}",
-            result.len()
-        ));
+    match client.simple_query(query).await {
+        Result::Ok(result) => {
+            if result.len() != 1 {
+                return Err(anyhow::anyhow!(
+                    "expected 1 query results, but got {}",
+                    result.len()
+                ));
+            }
+        }
+        Err(err) => {
+            if let Some(state) = err.code() {
+                if state == &tokio_postgres::error::SqlState::DISK_FULL {
+                    warn!("Tenant disk is full");
+                    return Ok(());
+                }
+            }
+            return Err(err.into());
+        }
     }
 
     Ok(())