diff --git a/compat/mingw.c b/compat/mingw.c
index 858ca14a57..f87376b26a 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -341,6 +341,19 @@ int mingw_mkdir(const char *path, int mode)
return ret;
}
+/*
+ * Calling CreateFile() using FILE_APPEND_DATA and without
FILE_WRITE_DATA
+ * is documented in [1] as opening a writable file handle in append
mode.
+ * (It is believed that) this is atomic since it is maintained by the
+ * kernel unlike the O_APPEND flag which is racily maintained by the
CRT.
+ *
+ * [1]
https://docs.microsoft.com/en-us/windows/desktop/fileio/file-access-rights-constants
+ *
+ * This trick does not appear to work for named pipes. Instead it
creates
+ * a named pipe client handle that cannot be written to. Callers should
+ * just use the regular _wopen() for them. (And since client handle
gets
+ * bound to a unique server handle, it isn't really an issue.)
+ */
static int mingw_open_append(wchar_t const *wfilename, int oflags, ...)
{
HANDLE handle;
@@ -360,10 +373,12 @@ static int mingw_open_append(wchar_t const
*wfilename, int oflags, ...)
NULL, create, FILE_ATTRIBUTE_NORMAL, NULL);
if (handle == INVALID_HANDLE_VALUE)
return errno = err_win_to_posix(GetLastError()), -1;
+
/*
* No O_APPEND here, because the CRT uses it only to reset the
- * file pointer to EOF on write(); but that is not necessary
- * for a file created with FILE_APPEND_DATA.
+ * file pointer to EOF before each write(); but that is not
+ * necessary (and may lead to races) for a file created with
+ * FILE_APPEND_DATA.
*/
fd = _open_osfhandle((intptr_t)handle, O_BINARY);
if (fd < 0)
@@ -371,6 +386,23 @@ static int mingw_open_append(wchar_t const
*wfilename, int oflags, ...)
return fd;
}
+#define IS_SBS(ch) (((ch) == '/') || ((ch) == '\\'))
+/*
+ * Does the pathname map to the local named pipe filesystem?
+ * That is, does it have a "//./pipe/" prefix?
+ */
+static int mingw_is_local_named_pipe_path(const char *filename)
+{
+ return (IS_SBS(filename[0]) &&
+ IS_SBS(filename[1]) &&
+ filename[2] == '.' &&
+ IS_SBS(filename[3]) &&
+ !strncasecmp(filename+4, "pipe", 4) &&
+ IS_SBS(filename[8]) &&
+ filename[9]);
+}
+#undef IS_SBS
+
int mingw_open (const char *filename, int oflags, ...)
{
typedef int (*open_fn_t)(wchar_t const *wfilename, int oflags,
...);
@@ -387,7 +419,7 @@ int mingw_open (const char *filename, int oflags,
...)
if (filename && !strcmp(filename, "/dev/null"))
filename = "nul";
- if (oflags & O_APPEND)
+ if ((oflags & O_APPEND) &&
!mingw_is_local_named_pipe_path(filename))
open_fn = mingw_open_append;
else
open_fn = _wopen;
