compute_ctl: database_schema should keep process::Child as part of returned value (#10273)

## Problem

/database_schema endpoint returns incomplete output from `pg_dump`

## Summary of changes

The Tokio process was not used properly. The returned stream does not
include `process::Child`, and the process is scheduled to be killed
immediately after the `get_database_schema` call when `cmd` goes out of
scope.

The solution in this PR is to return a special Stream implementation
that retains `process::Child`.

compute_tools/src/catalog.rs

@@ -140,5 +140,34 @@ pub async fn get_database_schema(
             warn!("pg_dump stderr: {}", line)
         }
     });
-    Ok(initial_stream.chain(stdout_reader.map(|res| res.map(|b| b.freeze()))))
+
+    #[allow(dead_code)]
+    struct SchemaStream<S> {
+        // We keep a reference to the child process to ensure it stays alive
+        // while the stream is being consumed. When SchemaStream is dropped,
+        // cmd will be dropped, which triggers kill_on_drop and terminates pg_dump
+        cmd: tokio::process::Child,
+        stream: S,
+    }
+
+    impl<S> Stream for SchemaStream<S>
+    where
+        S: Stream<Item = Result<bytes::Bytes, std::io::Error>> + Unpin,
+    {
+        type Item = Result<bytes::Bytes, std::io::Error>;
+
+        fn poll_next(
+            mut self: std::pin::Pin<&mut Self>,
+            cx: &mut std::task::Context<'_>,
+        ) -> std::task::Poll<Option<Self::Item>> {
+            Stream::poll_next(std::pin::Pin::new(&mut self.stream), cx)
+        }
+    }
+
+    let schema_stream = SchemaStream {
+        cmd,
+        stream: initial_stream.chain(stdout_reader.map(|res| res.map(|b| b.freeze()))),
+    };
+
+    Ok(schema_stream)
 }

test_runner/regress/test_compute_catalog.py

@@ -82,7 +82,7 @@ def test_compute_catalog(neon_simple_env: NeonEnv):
         ddl = client.database_schema(database=test_db["name"])
 
         # Check that it looks like a valid PostgreSQL dump
-        assert "-- PostgreSQL database dump" in ddl
+        assert "-- PostgreSQL database dump complete" in ddl
 
         # Check that it doesn't contain health_check and migration traces.
         # They are only created in system `postgres` database, so by checking