From 0b7d93e2964ab5e2adb56ac7f36977eb3b64b96c Mon Sep 17 00:00:00 2001
From: Anthonin Bonnefoy <anthonin.bonnefoy@datadoghq.com>
Date: Fri, 26 Jun 2026 11:06:16 +0200
Subject: Add PQTRACE env to enable protocol tracing in libpq

Add 2 env vars, PQTRACE and PQTRACEFLAGS.

When PQTRACE is set, PQtrace will be called when connection options are
processed. This allows to enable protocol tracing on application using
libpq without requiring specific code. Setting PQTRACE to "-" will
output protocol tracing on stdout.

PQTRACEFLAGS is the bitwise value of the PQTRACE_SUPPRESS_TIMESTAMPS(1)
and PQTRACE_REGRESS_MODE(2) flags.
---
 doc/src/sgml/libpq.sgml           | 48 +++++++++++++++++++++++
 src/interfaces/libpq/fe-connect.c | 65 ++++++++++++++++++++++++++++++-
 src/interfaces/libpq/libpq-int.h  |  4 ++
 3 files changed, 116 insertions(+), 1 deletion(-)

diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml
index 7d3c3bb66d8..b8a43380652 100644
--- a/doc/src/sgml/libpq.sgml
+++ b/doc/src/sgml/libpq.sgml
@@ -2424,6 +2424,34 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
       </listitem>
      </varlistentry>
 
+     <varlistentry id="libpq-connect-trace-file" xreflabel="trace_file">
+      <term><literal>trace_file</literal></term>
+      <listitem>
+       <para>
+        If set, enables tracing of the client/server protocol exchange to
+        the named file. The special value <literal>-</literal> sends the
+        trace to <literal>stdout</literal>. Multiple connections using
+        the same trace file is currently not supported.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry id="libpq-connect-trace-flags" xreflabel="trace_flags">
+      <term><literal>trace_flags</literal></term>
+      <listitem>
+       <para>
+        Specifies the trace flags applied when
+        <xref linkend="libpq-connect-trace-file"/> is set.  The value is
+        an integer that is the bitwise OR of the flag values accepted by
+        <xref linkend="libpq-PQsetTraceFlags"/>:
+        <literal>1</literal> for <literal>PQTRACE_SUPPRESS_TIMESTAMPS</literal>,
+        <literal>2</literal> for <literal>PQTRACE_REGRESS_MODE</literal>.
+        The default is <literal>0</literal>.  This option has no effect
+        unless <literal>trace_file</literal> is also set.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="libpq-connect-load-balance-hosts" xreflabel="load_balance_hosts">
       <term><literal>load_balance_hosts</literal></term>
       <listitem>
@@ -9454,6 +9482,26 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
       linkend="libpq-connect-oauth-ca-file"/> connection parameter.
      </para>
     </listitem>
+
+    <listitem>
+     <para>
+      <indexterm>
+       <primary><envar>PQTRACE</envar></primary>
+      </indexterm>
+      <envar>PQTRACE</envar> behaves the same as the <xref
+      linkend="libpq-connect-trace-file"/> connection parameter.
+     </para>
+    </listitem>
+
+    <listitem>
+     <para>
+      <indexterm>
+       <primary><envar>PQTRACEFLAGS</envar></primary>
+      </indexterm>
+      <envar>PQTRACEFLAGS</envar> behaves the same as the <xref
+      linkend="libpq-connect-trace-flags"/> connection parameter.
+     </para>
+    </listitem>
    </itemizedlist>
   </para>
 
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 17c2288e9bc..204b5c535a2 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -385,6 +385,15 @@ static const internalPQconninfoOption PQconninfoOptions[] = {
 		"Target-Session-Attrs", "", 15, /* sizeof("prefer-standby") = 15 */
 	offsetof(struct pg_conn, target_session_attrs)},
 
+	{"trace_file", "PQTRACE",
+		NULL, NULL,
+		"Trace-File", "D", 64,
+	offsetof(struct pg_conn, trace_file_str)},
+
+	{"trace_flags", "PQTRACEFLAGS", NULL, NULL,
+		"Trace-Flags", "D", 1,
+	offsetof(struct pg_conn, trace_flags_str)},
+
 	{"load_balance_hosts", "PGLOADBALANCEHOSTS",
 		DefaultLoadBalanceHosts, NULL,
 		"Load-Balance-Hosts", "", 8,	/* sizeof("disable") = 8 */
@@ -2062,6 +2071,45 @@ pqConnectOptions2(PGconn *conn)
 		conn->scram_client_key_len = len;
 	}
 
+	/*
+	 * If a trace file was provided and there's no active Pfdebug, enable
+	 * PQtrace
+	 */
+	if (conn->trace_file_str && !conn->Pfdebug)
+	{
+		int			trace_flags = 0;
+
+		if (conn->trace_flags_str)
+		{
+			if (!pqParseIntParam(conn->trace_flags_str, &trace_flags, conn,
+								 "trace_flags"))
+				return false;
+		}
+
+		if (strcmp(conn->trace_file_str, "-") == 0)
+			conn->trace_file = stdout;
+
+		/*
+		 * If the connection is created for a cancel request, and trace_file
+		 * is not stdout, don't try to trace it as the source connection has
+		 * already an opened fd on this trace file
+		 */
+		else if (!conn->cancelRequest)
+		{
+			conn->trace_file = fopen(conn->trace_file_str, "w");
+			if (conn->trace_file == NULL)
+			{
+				libpq_append_conn_error(conn, "could not open trace file \"%s\": %m", conn->trace_file_str);
+				return false;
+			}
+			/* Make it line-buffered */
+			setvbuf(conn->trace_file, NULL, PG_IOLBF, 0);
+		}
+
+		PQtrace(conn, conn->trace_file);
+		PQsetTraceFlags(conn, trace_flags);
+	}
+
 	if (conn->scram_server_key)
 	{
 		int			len;
@@ -5048,6 +5096,7 @@ pqMakeEmptyPGconn(void)
 	conn->sock = PGINVALID_SOCKET;
 	conn->altsock = PGINVALID_SOCKET;
 	conn->Pfdebug = NULL;
+	conn->trace_file = NULL;
 
 	/*
 	 * We try to send at least 8K at a time, which is the usual size of pipe
@@ -5169,7 +5218,21 @@ freePGconn(PGconn *conn)
 	free(conn->oauth_client_secret);
 	free(conn->oauth_ca_file);
 	free(conn->oauth_scope);
-	/* Note that conn->Pfdebug is not ours to close or free */
+	free(conn->trace_file_str);
+	free(conn->trace_flags_str);
+
+	/* Untrace the connection to flush Pfdebug */
+	PQuntrace(conn);
+
+	/*
+	 * Note that conn->Pfdebug is not ours to close or free if provided
+	 * externally. However, if tracing was enabled through PQTRACE, we need to
+	 * close conn->trace_file.
+	 */
+	if (conn->trace_file && conn->trace_file != stdout)
+	{
+		fclose(conn->trace_file);
+	}
 	free(conn->events);
 	pqReleaseConnHosts(conn);
 	free(conn->connip);
diff --git a/src/interfaces/libpq/libpq-int.h b/src/interfaces/libpq/libpq-int.h
index 3f921207a14..8c926483b43 100644
--- a/src/interfaces/libpq/libpq-int.h
+++ b/src/interfaces/libpq/libpq-int.h
@@ -449,6 +449,10 @@ struct pg_conn
 	char	   *oauth_ca_file;	/* CA file path */
 	bool		oauth_want_retry;	/* should we retry on failure? */
 
+	FILE	   *trace_file;
+	char	   *trace_file_str;
+	char	   *trace_flags_str;
+
 	/* Optional file to write trace info to */
 	FILE	   *Pfdebug;
 	int			traceFlags;
-- 
2.55.0

