From a32e8871acc1922f8bfd8057c08a97e504b1dacc Mon Sep 17 00:00:00 2001
From: Christian Schwarz <christian@neon.tech>
Date: Fri, 14 Feb 2025 21:11:42 +0100
Subject: [PATCH] compute/pageserver: correlation of logs through backend PID
 (via `application_name`) (#10810)

This PR makes compute set the `application_name` field to the
PG backend process PID which is also included in each compute log line.

This allows correlation of Pageserver connection logs with compute logs
in a way that was guesswork before this PR.

In future, we can switch for a more unique identifier for a page_service
session.

Refs
- discussion in
https://neondb.slack.com/archives/C08DE6Q9C3B/p1739465208296169?thread_ts=1739462628.361019&cid=C08DE6Q9C3B
- fixes https://github.com/neondatabase/neon/issues/10808
---
 pageserver/src/page_service.rs | 11 +++++++++--
 pgxn/neon/libpagestore.c       | 31 ++++++++++++++++++++++++-------
 2 files changed, 33 insertions(+), 9 deletions(-)

diff --git a/pageserver/src/page_service.rs b/pageserver/src/page_service.rs
index e9d87dec71..53a6a7124d 100644
--- a/pageserver/src/page_service.rs
+++ b/pageserver/src/page_service.rs
@@ -237,7 +237,7 @@ pub async fn libpq_listener_main(
 
 type ConnectionHandlerResult = anyhow::Result<()>;
 
-#[instrument(skip_all, fields(peer_addr))]
+#[instrument(skip_all, fields(peer_addr, application_name))]
 #[allow(clippy::too_many_arguments)]
 async fn page_service_conn_main(
     conf: &'static PageServerConf,
@@ -2463,9 +2463,16 @@ where
     fn startup(
         &mut self,
         _pgb: &mut PostgresBackend<IO>,
-        _sm: &FeStartupPacket,
+        sm: &FeStartupPacket,
     ) -> Result<(), QueryError> {
         fail::fail_point!("ps::connection-start::startup-packet");
+
+        if let FeStartupPacket::StartupMessage { params, .. } = sm {
+            if let Some(app_name) = params.get("application_name") {
+                Span::current().record("application_name", field::display(app_name));
+            }
+        };
+
         Ok(())
     }
 
diff --git a/pgxn/neon/libpagestore.c b/pgxn/neon/libpagestore.c
index 22aeb2e2d6..fc1aecd340 100644
--- a/pgxn/neon/libpagestore.c
+++ b/pgxn/neon/libpagestore.c
@@ -378,8 +378,9 @@ pageserver_connect(shardno_t shard_no, int elevel)
 	{
 	case PS_Disconnected:
 	{
-		const char *keywords[3];
-		const char *values[3];
+		const char *keywords[4];
+		const char *values[4];
+		char pid_str[16];
 		int			n_pgsql_params;
 		TimestampTz	now;
 		int64		us_since_last_attempt;
@@ -424,14 +425,30 @@ pageserver_connect(shardno_t shard_no, int elevel)
 		 * can override the password from the env variable. Seems useful, although
 		 * we don't currently use that capability anywhere.
 		 */
-		keywords[0] = "dbname";
-		values[0] = connstr;
-		n_pgsql_params = 1;
+		n_pgsql_params = 0;
+
+		/*
+		 * Pageserver logs include this in the connection's tracing span.
+		 * This allows for reasier log correlation between compute and pageserver.
+		 */
+		keywords[n_pgsql_params] = "application_name";
+		{
+			int ret = snprintf(pid_str, sizeof(pid_str), "%d", MyProcPid);
+			if (ret < 0 || ret >= (int)(sizeof(pid_str)))
+				elog(FATAL, "stack-allocated buffer too small to hold pid");
+		}
+		/* lifetime: PQconnectStartParams strdups internally */
+		values[n_pgsql_params] = (const char*) pid_str;
+		n_pgsql_params++;
+
+		keywords[n_pgsql_params] = "dbname";
+		values[n_pgsql_params] = connstr;
+		n_pgsql_params++;
 
 		if (neon_auth_token)
 		{
-			keywords[1] = "password";
-			values[1] = neon_auth_token;
+			keywords[n_pgsql_params] = "password";
+			values[n_pgsql_params] = neon_auth_token;
 			n_pgsql_params++;
 		}