From b93b73bcb08fc981fac11b24218aaf205eb0c656 Mon Sep 17 00:00:00 2001
From: Wang Chencheng <nyte_plus@sjtu.edu.cn>
Date: Sat, 19 Sep 2026 17:24:10 +0800
Subject: [PATCH] operations: fix ignored error in rcat probe reads

Return the first non-EOF input error encountered while Rcat probes whether an
upload is small. Previously that error was ignored and the full probe buffer,
including bytes that were never read, could be passed to Put or PutStream.
---
 fs/operations/operations.go      |  2 ++
 fs/operations/operations_test.go | 29 +++++++++++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/fs/operations/operations.go b/fs/operations/operations.go
index 6b8248b6c..cef999a55 100644
--- a/fs/operations/operations.go
+++ b/fs/operations/operations.go
@@ -1433,6 +1433,8 @@ func rcatSrc(ctx context.Context, fdst fs.Fs, dstFileName string, in io.ReadClos
 	if n, err := io.ReadFull(trackingIn, buf); err == io.EOF || err == io.ErrUnexpectedEOF {
 		fileIsSmall = true
 		buf = buf[:n]
+	} else if err != nil {
+		return nil, fmt.Errorf("failed to read upload input: %w", err)
 	}
 
 	// Read the data we have already read in buf and any further unread
diff --git a/fs/operations/operations_test.go b/fs/operations/operations_test.go
index 6eed8c742..3169d3a37 100644
--- a/fs/operations/operations_test.go
+++ b/fs/operations/operations_test.go
@@ -2168,3 +2168,32 @@ func TestRemoveExisting(t *testing.T) {
 	cleanup(&returnedError)
 	r.CheckRemoteItems(t)
 }
+
+func TestRcatInputFailurePreservesDestination(t *testing.T) {
+	ctx := context.Background()
+	root := t.TempDir()
+	require.NoError(t, os.WriteFile(root+"/target", []byte("original"), 0600))
+	f, err := fs.NewFs(ctx, root)
+	require.NoError(t, err)
+	calls := 0
+	original := f.Features().PutStream
+	f.Features().PutStream = func(ctx context.Context, in io.Reader, src fs.ObjectInfo, opts ...fs.OpenOption) (fs.Object, error) {
+		calls++
+		return original(ctx, in, src, opts...)
+	}
+	inputErr := errors.New("source interrupted")
+	for _, name := range []string{"target", "missing"} {
+		_, err = operations.Rcat(ctx, f, name, io.NopCloser(io.MultiReader(strings.NewReader("prefix"), rcatFailedInput{inputErr})), time.Now(), nil)
+		require.ErrorIs(t, err, inputErr)
+		require.Zero(t, calls)
+		b, readErr := os.ReadFile(root + "/target")
+		require.NoError(t, readErr)
+		require.Equal(t, "original", string(b))
+		_, statErr := os.Stat(root + "/missing")
+		require.True(t, os.IsNotExist(statErr))
+	}
+}
+
+type rcatFailedInput struct{ err error }
+
+func (r rcatFailedInput) Read([]byte) (int, error) { return 0, r.err }