runner: clear cache when shift is not possible (#9433)

Clear KV cache when shift operation is not supported by model.
Added KvCacheCanShift() check to handle models that can't perform cache shifts,
falling back to full cache clear while preserving logical token history to
maintain expected behavior when context window fills up.

runner/ollamarunner/cache_test.go

@@ -1,10 +1,13 @@
 package ollamarunner
 
 import (
+	"errors"
+	"fmt"
 	"image"
 	"testing"
 	"time"
 
+	"github.com/ollama/ollama/ml"
 	"github.com/ollama/ollama/model/input"
 )
 
@@ -425,3 +428,91 @@ func TestLoadCacheSlot(t *testing.T) {
 		})
 	}
 }
+
+// Mock implementation of the Cache interface
+type mockCache struct {
+	shouldFail bool
+}
+
+// Implement only the methods needed for the test
+func (m *mockCache) Remove(seq int, beginIndex, endIndex int32) error {
+	if m.shouldFail {
+		return fmt.Errorf("mock cache removal error")
+	}
+	return nil
+}
+
+// Stub implementations for other interface methods
+func (m *mockCache) SetLayer(layer int)                                                            {}
+func (m *mockCache) Get(ctx ml.Context) (ml.Tensor, ml.Tensor, ml.Tensor)                          { return nil, nil, nil }
+func (m *mockCache) Put(ctx ml.Context, key, value ml.Tensor)                                      {}
+func (m *mockCache) Init(backend ml.Backend, dtype ml.DType, maxSequences, capacity, maxBatch int) {}
+func (m *mockCache) Close()                                                                        {}
+func (m *mockCache) StartForward(ctx ml.Context, batch input.Batch) error                          { return nil }
+func (m *mockCache) CopyPrefix(srcSeq, dstSeq int, len int32)                                      {}
+func (m *mockCache) SetConfig(ml.CacheConfig)                                                      {}
+
+func TestShiftCacheSlot(t *testing.T) {
+	tests := []struct {
+		name          string
+		numCtx        int32
+		inputs        []input.Input
+		numKeep       int32
+		cacheErr      bool
+		wantErr       any
+		wantInputsLen int
+	}{
+		{
+			name:          "Normal shift",
+			numCtx:        10,
+			inputs:        []input.Input{{Token: 1}, {Token: 2}, {Token: 3}, {Token: 4}, {Token: 5}, {Token: 6}, {Token: 7}, {Token: 8}, {Token: 9}, {Token: 10}},
+			numKeep:       2,
+			cacheErr:      false, // No error
+			wantErr:       nil,
+			wantInputsLen: 6, // After discarding 4 tokens
+		},
+		{
+			name:          "Cache removal fails",
+			numCtx:        10,
+			inputs:        []input.Input{{Token: 1}, {Token: 2}, {Token: 3}, {Token: 4}, {Token: 5}, {Token: 6}, {Token: 7}, {Token: 8}, {Token: 9}, {Token: 10}},
+			numKeep:       2,
+			cacheErr:      true,
+			wantErr:       &ErrReprocessInputs{},
+			wantInputsLen: 0, // Original inputs should be cleared
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			mock := &mockCache{shouldFail: tt.cacheErr}
+			c := InputCache{
+				numCtx: tt.numCtx,
+				cache:  mock,
+			}
+			slot := &InputCacheSlot{
+				Id:     123,
+				Inputs: make([]input.Input, len(tt.inputs)),
+			}
+			copy(slot.Inputs, tt.inputs)
+
+			err := c.ShiftCacheSlot(slot, tt.numKeep)
+
+			if tt.wantErr != nil {
+				if err == nil {
+					t.Errorf("Expected error but got nil")
+					return
+				}
+
+				if !errors.As(err, &tt.wantErr) {
+					t.Errorf("Expected error of type %T but got %T: %v", tt.wantErr, err, err)
+				}
+			} else if err != nil {
+				t.Errorf("Unexpected error: %v", err)
+			}
+
+			if len(slot.Inputs) != tt.wantInputsLen {
+				t.Errorf("Slot inputs length after operation: got %v, want %v", len(slot.Inputs), tt.wantInputsLen)
+			}
+		})
+	}
+}