mingw: fix mingw_open_append to work with named pipes

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/compat/mingw.c b/compat/mingw.c
index 858ca14a57351062d2bba147c72ed460fb0aa533..18caf21969a5917e8e6c6d754beffbba2741e6c2 100644 (file)
--- 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 +386,21 @@ static int mingw_open_append(wchar_t const *wfilename, int oflags, ...)
        return fd;
 }
 
+/*
+ * Does the pathname map to the local named pipe filesystem?
+ * That is, does it have a "//./pipe/" prefix?
+ */
+static int is_local_named_pipe_path(const char *filename)
+{
+       return (is_dir_sep(filename[0]) &&
+               is_dir_sep(filename[1]) &&
+               filename[2] == '.'  &&
+               is_dir_sep(filename[3]) &&
+               !strncasecmp(filename+4, "pipe", 4) &&
+               is_dir_sep(filename[8]) &&
+               filename[9]);
+}
+
 int mingw_open (const char *filename, int oflags, ...)
 {
        typedef int (*open_fn_t)(wchar_t const *wfilename, int oflags, ...);
@@ -387,7 +417,7 @@ int mingw_open (const char *filename, int oflags, ...)
        if (filename && !strcmp(filename, "/dev/null"))
                filename = "nul";
 
-       if (oflags & O_APPEND)
+       if ((oflags & O_APPEND) && !is_local_named_pipe_path(filename))
                open_fn = mingw_open_append;
        else
                open_fn = _wopen;

diff --git a/t/t0051-windows-named-pipe.sh b/t/t0051-windows-named-pipe.sh
index e3c36341a0ae032137254225988fb76ec9fdfbcf..10ac92d22507ed839552e5cbec1e5978a00ca51d 100755 (executable)
--- a/t/t0051-windows-named-pipe.sh
+++ b/t/t0051-windows-named-pipe.sh
@@ -4,7 +4,7 @@ test_description='Windows named pipes'
 
 . ./test-lib.sh
 
-test_expect_failure MINGW 'o_append write to named pipe' '
+test_expect_success MINGW 'o_append write to named pipe' '
        GIT_TRACE="$(pwd)/expect" git status >/dev/null 2>&1 &&
        { test-tool windows-named-pipe t0051 >actual 2>&1 & } &&
        pid=$! &&