v1-0001-Improve-log_statement_max_length-truncation.patch

From a47ccafc334e8b508f117afdfc5ce3266e0ed8fa Mon Sep 17 00:00:00 2001
From: Jim Jones <jim.jones@uni-muenster.de>
Date: Wed, 8 Jul 2026 18:29:09 +0200
Subject: [PATCH v1 1/2] Improve log_statement_max_length truncation

Apply log_statement_max_length consistently when logging prepared
statements. Previously, the prepared query emitted in the DETAIL
message for EXECUTE was not truncated, even though the corresponding
statement logged by log_statement was.

Also append an ellipsis to truncated statements, so it is immediately
apparent when a statement has been truncated.

This also avoid scanning the entire query string when determining
if truncation is needed by using strnlen() instead of strlen() in
truncate_query_log().

Author: Jim Jones <jim.jones@uni-muenster.de>
Co-authored-by: Fujii Masao <masao.fujii@gmail.com>
---
 doc/src/sgml/config.sgml                      |  4 +-
 src/backend/tcop/postgres.c                   | 16 +++++---
 .../t/014_log_statement_max_length.pl         | 37 +++++++++++++++----
 3 files changed, 42 insertions(+), 15 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 9172a4c5c95..f4782261e17 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8525,7 +8525,9 @@ log_line_prefix = '%m [%p] %q%u@%d/%a '
         <xref linkend="guc-log-min-duration-statement"/>,
         <xref linkend="guc-log-min-duration-sample"/>, or
         <xref linkend="guc-log-transaction-sample-rate"/>
-        is truncated to at most this many bytes.
+        is truncated to at most this many bytes. When a statement is
+        truncated, an ellipsis (<literal>...</literal>) is appended to
+        indicate that truncation has occurred.
         A value of zero causes statements to be logged with an empty body.
         <literal>-1</literal> (the default) logs statements in full.
         If this value is specified without units, it is taken as bytes.
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index ce18df820cd..b0947fd7298 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -2553,8 +2553,9 @@ check_log_duration(char *msec_str, bool was_logged)
  * truncate_query_log
  *		Truncate query string if needed for logging
  *
- * Returns a palloc'd truncated copy if truncation is needed,
- * or NULL if no truncation is required.
+ * Returns a palloc'd copy of the query truncated for logging, with an
+ * ellipsis appended if truncation occurs, or NULL if no truncation is
+ * required.
  */
 static char *
 truncate_query_log(const char *query)
@@ -2567,7 +2568,7 @@ truncate_query_log(const char *query)
 	if (!query || log_statement_max_length < 0)
 		return NULL;
 
-	query_len = strlen(query);
+	query_len = strnlen(query, (size_t) log_statement_max_length + MAX_MULTIBYTE_CHAR_LEN);
 
 	/*
 	 * No need to allocate a truncated copy if the query is shorter than
@@ -2578,9 +2579,10 @@ truncate_query_log(const char *query)
 
 	/* Truncate at a multibyte character boundary */
 	truncated_len = pg_mbcliplen(query, query_len, log_statement_max_length);
-	truncated_query = (char *) palloc(truncated_len + 1);
+	truncated_query = (char *) palloc(truncated_len + 4);
 	memcpy(truncated_query, query, truncated_len);
-	truncated_query[truncated_len] = '\0';
+	memcpy(truncated_query + truncated_len, "...", 3);
+	truncated_query[truncated_len + 3] = '\0';
 
 	return truncated_query;
 }
@@ -2608,7 +2610,9 @@ errdetail_execute(List *raw_parsetree_list)
 			pstmt = FetchPreparedStatement(stmt->name, false);
 			if (pstmt)
 			{
-				errdetail("prepare: %s", pstmt->plansource->query_string);
+				char *truncated_stmt = truncate_query_log(pstmt->plansource->query_string);
+
+				errdetail("prepare: %s", truncated_stmt ? truncated_stmt : pstmt->plansource->query_string);
 				return 0;
 			}
 		}
diff --git a/src/test/modules/test_misc/t/014_log_statement_max_length.pl b/src/test/modules/test_misc/t/014_log_statement_max_length.pl
index b1ce6068f5d..1239eff4a93 100644
--- a/src/test/modules/test_misc/t/014_log_statement_max_length.pl
+++ b/src/test/modules/test_misc/t/014_log_statement_max_length.pl
@@ -22,7 +22,7 @@ $node->psql(
 	'postgres', "
 	SET log_statement_max_length TO 20;
 	SELECT '123456789ABCDEF';");
-ok($node->log_contains(qr/statement: SELECT '123456789ABC$/m, $log_offset),
+ok($node->log_contains(qr/statement: SELECT '123456789ABC...$/m, $log_offset),
 	"ASCII query truncated at 20 bytes");
 
 # Verify -1 logs statement in full (closing quote must be present).
@@ -51,7 +51,7 @@ SKIP:
 		SET client_encoding TO 'UTF8';
 		SET log_statement_max_length TO 11;
 		$mbquery");
-	ok($node->log_contains(qr/statement: SELECT 'AA$/m, $log_offset),
+	ok($node->log_contains(qr/statement: SELECT 'AA...$/m, $log_offset),
 		"multibyte truncation at character boundary");
 }
 
@@ -62,8 +62,8 @@ $node->psql(
 	'postgres', "
 	SET log_statement_max_length TO 0;
 	SELECT '123456789ABCDEF';");
-ok($node->log_contains(qr/statement:\s*$/m, $log_offset),
-	"0 logs an empty statement body");
+ok($node->log_contains(qr/statement: ...\s*$/m, $log_offset),
+	"0 logs statement body with ellipsis");
 
 # Verify truncation via the extended query protocol (execute message).
 # With log_statement_max_length = 20, a 24-byte query should end
@@ -75,7 +75,7 @@ $node->psql(
 	SET log_statement_max_length TO 20;
 	SELECT '123456789ABCDEF' \\bind \\g");
 ok( $node->log_contains(
-		qr/execute <unnamed>: SELECT '123456789ABC$/m, $log_offset),
+		qr/execute <unnamed>: SELECT '123456789ABC...$/m, $log_offset),
 	"extended protocol execute truncated at 20 bytes");
 
 # Verify extended protocol also respects -1 (no truncation; closing quote
@@ -102,14 +102,35 @@ $node->psql(
 	SET log_statement_max_length TO 20;
 	SELECT '123456789ABCDEF' \\bind \\g");
 ok( $node->log_contains(
-		qr/parse <unnamed>: SELECT '123456789ABC$/m, $log_offset),
+		qr/parse <unnamed>: SELECT '123456789ABC...$/m, $log_offset),
 	"parse duration entry truncated");
 ok( $node->log_contains(
-		qr/bind <unnamed>: SELECT '123456789ABC$/m, $log_offset),
+		qr/bind <unnamed>: SELECT '123456789ABC...$/m, $log_offset),
 	"bind duration entry truncated");
 ok( $node->log_contains(
-		qr/execute <unnamed>: SELECT '123456789ABC$/m, $log_offset),
+		qr/execute <unnamed>: SELECT '123456789ABC...$/m, $log_offset),
 	"execute duration entry truncated");
 
+note "Truncate prepared statement query in DETAIL";
+$log_offset = -s $node->logfile;
+$node->psql(
+	'postgres', "
+	SET log_statement_max_length TO 12;
+	PREPARE stmt AS SELECT * FROM pg_hba_file_rules WHERE address = \$1;
+	EXECUTE stmt('127.0.0.1');");
+ok( $node->log_contains(
+		qr/prepare: PREPARE stmt...$/m, $log_offset),
+	"Truncate prepared statement query in DETAIL");
+
+note "Truncate prepared statement query in DETAIL (0 length)";
+$log_offset = -s $node->logfile;
+$node->psql(
+	'postgres', "
+	SET log_statement_max_length TO 0;
+	PREPARE stmt AS SELECT * FROM pg_hba_file_rules WHERE address = \$1;
+	EXECUTE stmt('127.0.0.1');");
+ok( $node->log_contains(
+		qr/prepare: ...$/m, $log_offset),
+	"Truncate prepared statement query in DETAIL (0 length)");
 $node->stop;
 done_testing();
-- 
2.25.1