From 5df6ce6f54a9f4edb472ab829d11bf10eb7eece1 Mon Sep 17 00:00:00 2001
From: Yage Hu <me@huyage.dev>
Date: Mon, 11 Dec 2023 20:37:30 -0800
Subject: [PATCH] Fix `fd_fdstat_set_flags` truncating with O_TRUNC

This commit fixes a buf when `fd_fdstat_set_flags` would erroneously
truncate an open file if it is opened with the O_TRUNC flag.  This
happens because the O_TRUNC flag causes the file to be truncated when it
is opened.

Signed-off-by: Yage Hu <me@huyage.dev>
---
 imports/wasi_snapshot_preview1/fs_test.go | 36 +++++++++++++++++++++++
 internal/sysfs/osfile.go                  |  3 ++
 2 files changed, 39 insertions(+)

diff --git a/imports/wasi_snapshot_preview1/fs_test.go b/imports/wasi_snapshot_preview1/fs_test.go
index fc23e897536..1ef0f7e0a9c 100644
--- a/imports/wasi_snapshot_preview1/fs_test.go
+++ b/imports/wasi_snapshot_preview1/fs_test.go
@@ -480,6 +480,42 @@ func Test_fdFdstatGet_StdioNonblock(t *testing.T) {
 	}
 }
 
+func Test_fdFdstatSetFlagsWithTrunc(t *testing.T) {
+	tmpDir := t.TempDir()
+	fileName := "test"
+
+	mod, r, log := requireProxyModule(t, wazero.NewModuleConfig().
+		WithFSConfig(wazero.NewFSConfig().WithDirMount(tmpDir, "/")))
+	defer r.Close(testCtx)
+
+	fsc := mod.(*wasm.ModuleInstance).Sys.FS()
+	preopen := fsc.RootFS()
+
+	fd, errno := fsc.OpenFile(preopen, fileName, experimentalsys.O_RDWR|experimentalsys.O_CREAT|experimentalsys.O_EXCL|experimentalsys.O_TRUNC, 0o600)
+	require.EqualErrno(t, 0, errno)
+
+	// Write the initial text to the file.
+	f, ok := fsc.LookupFile(fd)
+	require.True(t, ok)
+	n, errno := f.File.Write([]byte("abc"))
+	require.Equal(t, n, 3)
+
+	buf, err := os.ReadFile(joinPath(tmpDir, fileName))
+	require.NoError(t, err)
+	require.Equal(t, "abc", string(buf))
+
+	requireErrnoResult(t, wasip1.ErrnoSuccess, mod, wasip1.FdFdstatSetFlagsName, uint64(fd), uint64(0))
+	require.Equal(t, `
+==> wasi_snapshot_preview1.fd_fdstat_set_flags(fd=4,flags=)
+<== errno=ESUCCESS
+`, "\n"+log.String())
+	log.Reset()
+
+	buf, err = os.ReadFile(joinPath(tmpDir, fileName))
+	require.NoError(t, err)
+	require.Equal(t, "abc", string(buf))
+}
+
 func Test_fdFdstatSetFlags(t *testing.T) {
 	tmpDir := t.TempDir() // open before loop to ensure no locking problems.
 
diff --git a/internal/sysfs/osfile.go b/internal/sysfs/osfile.go
index 13bd418f242..40df119a009 100644
--- a/internal/sysfs/osfile.go
+++ b/internal/sysfs/osfile.go
@@ -86,6 +86,9 @@ func (f *osFile) SetAppend(enable bool) (errno experimentalsys.Errno) {
 	// Clear any create flag, as we are re-opening, not re-creating.
 	f.flag &= ^experimentalsys.O_CREAT
 
+	// Clear trunc flag, as we are re-opening.
+	f.flag &= ^experimentalsys.O_TRUNC
+
 	// appendMode (bool) cannot be changed later, so we have to re-open the
 	// file. https://github.com/golang/go/blob/go1.20/src/os/file_unix.go#L60
 	return fileError(f, f.closed, f.reopen())