Horton

Broken PQtrace CopyData display

Ran Benita <ran@unusedvar.com>

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From: "Ran Benita" <ran@unusedvar.com>
To: pgsql-bugs@lists.postgresql.org
Date: 2025-08-29T19:45:18Z
Lists: pgsql-bugs

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  1. libpq: Fix PQtrace() format for non-printable characters

I used PQtrace to trace a logical decoding session, and was confused to see
lines like this:

2025-08-29 22:09:57.633980	F	38	CopyData	 'r\x00\x00\x00\x00\xffffff8e\x07\xffffff85x\x00\x00\x00\x00\xffffff8e\x07\xffffff85x\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\xffffffe0\xffffff84\xffffff89R\xffffffd7\xffffffb8\x00'

The data doesn't make sense and has the wrong length. A quick look shows that
the \xffffff85 parts are a signedness issue (on my compiler char is signed).

I instead expect to see output like this:

2025-08-29 22:39:17.041261	F	38	CopyData	 'r\x00\x00\x00\x00\x8e\x07\x89\xf8\x00\x00\x00\x00\x8e\x07\x89\xf8\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\xe0\x84\xf21Dh\x00'

The diff below fixes this particular problem, though they say bugs are like
mushrooms, if you find one then there are probably others nearby, so a more
comprehensive fix may be warranted.

diff --git i/src/interfaces/libpq/fe-trace.c w/src/interfaces/libpq/fe-trace.c
index a45f0d85587..f487c111fc7 100644
--- i/src/interfaces/libpq/fe-trace.c
+++ w/src/interfaces/libpq/fe-trace.c
@@ -212,7 +212,7 @@ pqTraceOutputNchar(FILE *pfdebug, int len, const char *data, int *cursor, bool s
 		else
 		{
 			fwrite(v + next, 1, i - next, pfdebug);
-			fprintf(pfdebug, "\\x%02x", v[i]);
+			fprintf(pfdebug, "\\x%02x", (unsigned char) v[i]);
 			next = i + 1;
 		}
 	}