Feedback-ID: ie3de48e3:Fastmail
Date: Mon, 23 Feb 2026 21:16:50 +0100
From: =?utf-8?Q?=C3=81lvaro?= Herrera <alvherre@kurilemu.de>
To: Bryan Green <dbryan.green@gmail.com>
Cc: Euler Taveira <euler@eulerto.com>,
	Jakub Wartak <jakub.wartak@enterprisedb.com>,
 Michael Paquier <michael@paquier.xyz>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: [PATCH] Add Windows support for backtrace_functions (MSVC only)
Message-ID: <202602232005.2cmbj5ougsfv@alvherre.pgsql>
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Hello, thanks for following up.

However, I was going over the commit message one final time after your
reply, and I noticed that SymInitialize()'s documentation says quite
explicitly:

"Do not use the handle returned by GetCurrentProcess"
https://learn.microsoft.com/en-us/windows/win32/api/dbghelp/nf-dbghelp-syminitialize

which is exactly what we do with this patch.

I think we should DuplicateHandle() on the GetCurrentProcess() handle
(which then obviously needs CloseHandle afterwards).  Would you agree?

Here's the whole thing again with edited commit message and some trivial
stylistic changes.


I was amused to discover that Solaris, macOS and FreeBSD all copied the
backtrace() interface from glibc, and the only platform in the buildfarm
other than Windows that seems to lack support for backtraces is Alpine
Linux.

Regards

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"Ed is the standard text editor."
      http://groups.google.com/group/alt.religion.emacs/msg/8d94ddab6a9b0ad3

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From a1666e89d771c036df71fc70155ce706114657d8 Mon Sep 17 00:00:00 2001
From: Bryan Green <dbryan.green@gmail.com>
Date: Sat, 1 Nov 2025 23:04:24 -0600
Subject: [PATCH v8] Add backtrace support for Windows using DbgHelp API

Previously, backtrace generation on Windows would return an "unsupported"
message.  With this commit, we rely on CaptureStackBackTrace() to capture
the call stack and the DbgHelp API (SymFromAddrW, SymGetLineFromAddrW64)
for symbol resolution.

Symbol handler initialization (SymInitialize) is performed once per
process and cached.  If initialization fails, the report for it is
returned as the backtrace output.  The symbol handler is cleaned up via
on_proc_exit() to release DbgHelp resources.

The implementation provides symbol names, offsets, and addresses.  When
PDB files are available, it also includes source file names and line
numbers.  Symbol names and file paths are converted from UTF-16 to the
database encoding using wchar2char(), which properly handles both UTF-8
and non-UTF-8 databases on Windows.  When symbol information is
unavailable or encoding conversion fails, it falls back to displaying raw
addresses.

The implementation uses the explicit Unicode versions of the DbgHelp
functions (SYMBOL_INFOW, SymFromAddrW, IMAGEHLP_LINEW64,
SymGetLineFromAddrW64) rather than the generic versions.  This allows us
to rely on predictable encoding conversion from Unicode, rather than using
the haphazard ANSI codepage that we'd get otherwise.

DbgHelp is apparently available on all Windows platforms we support, so
there are no version number checks.

Author: Bryan Green <dbryan.green@gmail.com>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Reviewed-by: Jakub Wartak <jakub.wartak@enterprisedb.com>
Reviewed-by: Greg Burd <greg@burd.me>
Discussion: https://postgr.es/m/a692c0fe-caca-4c08-9c5d-debfd0ef2504@gmail.com
---
 src/backend/meson.build        |   6 ++
 src/backend/utils/error/elog.c | 153 ++++++++++++++++++++++++++++++++-
 2 files changed, 156 insertions(+), 3 deletions(-)

diff --git a/src/backend/meson.build b/src/backend/meson.build
index 712a857cdb4..9041cde59fd 100644
--- a/src/backend/meson.build
+++ b/src/backend/meson.build
@@ -41,6 +41,12 @@ backend_link_args = []
 backend_link_depends = []
 
 
+# On Windows also make the backend depend on dbghelp, for backtrace support
+if host_system == 'windows' and cc.get_id() == 'msvc'
+  backend_build_deps += cc.find_library('dbghelp')
+endif
+
+
 # On windows when compiling with msvc we need to make postgres export all its
 # symbols so that extension libraries can use them. For that we need to scan
 # the constituting objects and generate a file specifying all the functions as
diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index cb1c9d85ffe..d406f18f9d7 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -66,6 +66,10 @@
 #include <execinfo.h>
 #endif
 
+#ifdef _MSC_VER
+#include <dbghelp.h>
+#endif
+
 #include "access/xact.h"
 #include "common/ip.h"
 #include "libpq/libpq.h"
@@ -140,6 +144,11 @@ static void write_syslog(int level, const char *line);
 static void write_eventlog(int level, const char *line, int len);
 #endif
 
+#ifdef _MSC_VER
+static bool backtrace_symbols_initialized = false;
+static HANDLE backtrace_process = NULL;
+#endif
+
 /* We provide a small stack of ErrorData records for re-entrant cases */
 #define ERRORDATA_STACK_SIZE  5
 
@@ -180,6 +189,7 @@ static void set_stack_entry_location(ErrorData *edata,
 									 const char *funcname);
 static bool matches_backtrace_functions(const char *funcname);
 static pg_noinline void set_backtrace(ErrorData *edata, int num_skip);
+static void backtrace_cleanup(int code, Datum arg);
 static void set_errdata_field(MemoryContextData *cxt, char **ptr, const char *str);
 static void FreeErrorDataContents(ErrorData *edata);
 static int	log_min_messages_cmp(const ListCell *a, const ListCell *b);
@@ -1122,6 +1132,13 @@ errbacktrace(void)
  * specifies how many inner frames to skip.  Use this to avoid showing the
  * internal backtrace support functions in the backtrace.  This requires that
  * this and related functions are not inlined.
+ *
+ * The implementation is, unsurprisingly, platform-specific:
+ * - GNU libc and copycats: Uses backtrace() and backtrace_symbols()
+ * - Windows: Uses CaptureStackBackTrace() with DbgHelp for symbol resolution
+ * 	 (requires PDB files; falls back to exported functions/raw addresses if
+ * 	 unavailable)
+ * - Others (musl libc): unsupported
  */
 static void
 set_backtrace(ErrorData *edata, int num_skip)
@@ -1132,12 +1149,12 @@ set_backtrace(ErrorData *edata, int num_skip)
 
 #ifdef HAVE_BACKTRACE_SYMBOLS
 	{
-		void	   *buf[100];
+		void	   *frames[100];
 		int			nframes;
 		char	  **strfrms;
 
-		nframes = backtrace(buf, lengthof(buf));
-		strfrms = backtrace_symbols(buf, nframes);
+		nframes = backtrace(frames, lengthof(frames));
+		strfrms = backtrace_symbols(frames, nframes);
 		if (strfrms != NULL)
 		{
 			for (int i = num_skip; i < nframes; i++)
@@ -1148,6 +1165,123 @@ set_backtrace(ErrorData *edata, int num_skip)
 			appendStringInfoString(&errtrace,
 								   "insufficient memory for backtrace generation");
 	}
+#elif defined(_MSC_VER)
+	{
+		void	   *frames[100];
+		int			nframes;
+		char		buffer[sizeof(SYMBOL_INFOW) + MAX_SYM_NAME * sizeof(wchar_t)];
+		PSYMBOL_INFOW psymbol;
+
+		if (!backtrace_symbols_initialized)
+		{
+			backtrace_process = GetCurrentProcess();
+
+			SymSetOptions(SYMOPT_UNDNAME |
+						  SYMOPT_DEFERRED_LOADS |
+						  SYMOPT_LOAD_LINES |
+						  SYMOPT_FAIL_CRITICAL_ERRORS);
+
+			if (SymInitialize(backtrace_process, NULL, TRUE))
+			{
+				backtrace_symbols_initialized = true;
+				on_proc_exit(backtrace_cleanup, 0);
+			}
+			else
+			{
+				appendStringInfo(&errtrace,
+								 "could not initialize symbol handler: error code %lu",
+								 GetLastError());
+				edata->backtrace = errtrace.data;
+				return;
+			}
+		}
+
+		nframes = CaptureStackBackTrace(num_skip, lengthof(frames), frames, NULL);
+
+		if (nframes == 0)
+		{
+			appendStringInfoString(&errtrace, "zero stack frames captured");
+			edata->backtrace = errtrace.data;
+			return;
+		}
+
+		psymbol = (PSYMBOL_INFOW) buffer;
+		psymbol->MaxNameLen = MAX_SYM_NAME;
+		psymbol->SizeOfStruct = sizeof(SYMBOL_INFOW);
+
+		for (int i = 0; i < nframes; i++)
+		{
+			DWORD64		address = (DWORD64) frames[i];
+			DWORD64		displacement = 0;
+			BOOL		sym_result;
+
+			sym_result = SymFromAddrW(backtrace_process,
+									  address,
+									  &displacement,
+									  psymbol);
+			if (sym_result)
+			{
+				char		symbol_name[MAX_SYM_NAME];
+				size_t		result;
+
+				/*
+				 * Convert symbol name from UTF-16 to database encoding using
+				 * wchar2char(), which handles both UTF-8 and non-UTF-8
+				 * databases correctly on Windows.
+				 */
+				result = wchar2char(symbol_name, (const wchar_t *) psymbol->Name,
+									sizeof(symbol_name), NULL);
+
+				if (result == (size_t) -1)
+				{
+					/* Conversion failed, use address only */
+					appendStringInfo(&errtrace,
+									 "\n[0x%llx]",
+									 (unsigned long long) address);
+				}
+				else
+				{
+					IMAGEHLP_LINEW64 line;
+					DWORD		line_displacement = 0;
+					char		filename[MAX_PATH];
+
+					line.SizeOfStruct = sizeof(IMAGEHLP_LINEW64);
+
+					/* Start with the common part: symbol+offset [address] */
+					appendStringInfo(&errtrace,
+									 "\n%s+0x%llx [0x%llx]",
+									 symbol_name,
+									 (unsigned long long) displacement,
+									 (unsigned long long) address);
+
+					/* Try to append line info if available */
+					if (SymGetLineFromAddrW64(backtrace_process,
+											  address,
+											  &line_displacement,
+											  &line))
+					{
+						result = wchar2char(filename, (const wchar_t *) line.FileName,
+											sizeof(filename), NULL);
+
+						if (result != (size_t) -1)
+						{
+							appendStringInfo(&errtrace,
+											 " [%s:%lu]",
+											 filename,
+											 (unsigned long) line.LineNumber);
+						}
+					}
+				}
+			}
+			else
+			{
+				appendStringInfo(&errtrace,
+								 "\n[0x%llx] (symbol lookup failed: error code %lu)",
+								 (unsigned long long) address,
+								 GetLastError());
+			}
+		}
+	}
 #else
 	appendStringInfoString(&errtrace,
 						   "backtrace generation is not supported by this installation");
@@ -1156,6 +1290,19 @@ set_backtrace(ErrorData *edata, int num_skip)
 	edata->backtrace = errtrace.data;
 }
 
+/*
+ * Cleanup function for DbgHelp resources.
+ * Called via on_proc_exit() to release resources allocated by SymInitialize().
+ */
+pg_attribute_unused()
+static void
+backtrace_cleanup(int code, Datum arg)
+{
+#ifdef _MSC_VER
+	SymCleanup(backtrace_process);
+#endif
+}
+
 /*
  * errmsg_internal --- add a primary error message text to the current error
  *
-- 
2.47.3


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