[PATCH] compat/winansi: fix die_lasterr() argument formatting

From: Yongqiang Tian <yqtian668@gmail.com>
Date: Sep 16, 2026, 04:23 UTC
Message-ID: <20260916042312.35891-1-yqtian668@gmail.com>

During WinANSI initialization, duplicate_handle() reports the handle when DuplicateHandle() fails:

    die_lasterr("DuplicateHandle(%li) failed", ...);

die_lasterr() collects the formatting arguments in a va_list, but passes that va_list to die_errno() as an ordinary variadic argument. die_errno() consequently formats the representation of the va_list instead of the supplied handle, producing an incorrect fatal message. The other current callers pass fixed strings and are unaffected.

Git does not provide a va_list-taking variant of die_errno(), so format the caller's arguments separately with strbuf_vaddf(). This consumes the original va_list correctly and produces the complete diagnostic prefix, including the handle supplied by duplicate_handle().

Save GetLastError() before formatting because calls made while growing the strbuf may change the thread's Windows error value. Convert the saved value to errno only after formatting, then pass the completed message to die_errno() through a literal "%s". This prevents any percent characters in the formatted message from being interpreted a second time, while allowing die_errno() to append the corresponding system error and terminate as before.

The updated compat/winansi.c compiles with MinGW GCC 13. A Win64 probe under Wine prints a value derived from the va_list before this change and the supplied integer afterward.

Signed-off-by: Yongqiang Tian <yqtian668@gmail.com>
---
 compat/winansi.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/compat/winansi.c b/compat/winansi.c
index 3ce190093..5547192a2 100644
--- a/compat/winansi.c
+++ b/compat/winansi.c
@@ -7,6 +7,7 @@
 #define DISABLE_SIGN_COMPARE_WARNINGS
 
 #include "../git-compat-util.h"
+#include "../strbuf.h"
 #include <wingdi.h>
 #include <winreg.h>
 #include "win32.h"
@@ -438,11 +439,15 @@ static void winansi_exit(void)
 
 static void die_lasterr(const char *fmt, ...)
 {
+	DWORD err = GetLastError();
+	struct strbuf message = STRBUF_INIT;
 	va_list params;
+
 	va_start(params, fmt);
-	errno = err_win_to_posix(GetLastError());
-	die_errno(fmt, params);
+	strbuf_vaddf(&message, fmt, params);
 	va_end(params);
+	errno = err_win_to_posix(err);
+	die_errno("%s", message.buf);
 }
 
 #undef dup2

-- 
2.34.1