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eval: Allow reslicing a slice up to its cap, rather than its length #3796

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Aug 28, 2024
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eval: Allow reslicing a slice up to its cap, rather than its length #3796

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12 changes: 10 additions & 2 deletions pkg/proc/eval.go
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Original file line number Diff line number Diff line change
Expand Up @@ -2712,7 +2712,11 @@ func (v *Variable) reslice(low int64, high int64, trustLen bool) (*Variable, err
wrong := false
cptrNeedsFakeSlice := false
if v.Flags&VariableCPtr == 0 {
wrong = low < 0 || low > v.Len || high < 0 || high > v.Len
if v.Kind == reflect.Slice {
wrong = low < 0 || low > v.Cap || high < 0 || high > v.Cap
} else {
wrong = low < 0 || low > v.Len || high < 0 || high > v.Len
}
} else {
wrong = low < 0 || high < 0
if high == 0 {
Expand Down Expand Up @@ -2742,7 +2746,11 @@ func (v *Variable) reslice(low int64, high int64, trustLen bool) (*Variable, err
}

r := v.newVariable("", 0, typ, mem)
r.Cap = len
if v.Flags&VariableCPtr == 0 {
r.Cap = v.Cap - low
} else {
r.Cap = len
}
r.Len = len
r.Base = base
r.stride = v.stride
Expand Down
2 changes: 1 addition & 1 deletion pkg/proc/variables.go
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Expand Up @@ -690,7 +690,7 @@ func newVariable(name string, addr uint64, dwarfType godwarf.Type, bi *BinaryInf
v.Kind = reflect.Array
v.Base = v.Addr
v.Len = t.Count
v.Cap = -1
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Why are we changing this?

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I thought it was more correct to record the actual capacity of an array rather than -1. Go doesn't allow us to call cap() on a string, but it does allow it on an array, so I thought it'd be cleaner to have it.

Alternatively we can give reslicing a special case for arrays too, but this felt cleaner in the long run.

Let me know if you'd prefer this to remain -1 and I'll change reslice instead.

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Ok, I think it's fine to change it.

v.Cap = t.Count
v.fieldType = t.Type
v.stride = 0

Expand Down
17 changes: 9 additions & 8 deletions pkg/proc/variables_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -119,12 +119,12 @@ func TestVariableEvaluation(t *testing.T) {
childrenlen int
}{
{"a1", reflect.String, "foofoofoofoofoofoo", 18, 0, 0},
{"a11", reflect.Array, nil, 3, -1, 3},
{"a11", reflect.Array, nil, 3, 3, 3},
{"a12", reflect.Slice, nil, 2, 2, 2},
{"a13", reflect.Slice, nil, 3, 3, 3},
{"a2", reflect.Int, int64(6), 0, 0, 0},
{"a3", reflect.Float64, float64(7.23), 0, 0, 0},
{"a4", reflect.Array, nil, 2, -1, 2},
{"a4", reflect.Array, nil, 2, 2, 2},
{"a5", reflect.Slice, nil, 5, 5, 5},
{"a6", reflect.Struct, nil, 2, 0, 2},
{"a7", reflect.Ptr, nil, 1, 0, 1},
Expand All @@ -139,7 +139,7 @@ func TestVariableEvaluation(t *testing.T) {
{"a7.Baz", reflect.Int, int64(5), 0, 0, 0},
{"a8.Baz", reflect.String, "feh", 3, 0, 0},
{"a8", reflect.Struct, nil, 2, 0, 2},
{"i32", reflect.Array, nil, 2, -1, 2},
{"i32", reflect.Array, nil, 2, 2, 2},
{"b1", reflect.Bool, true, 0, 0, 0},
{"b2", reflect.Bool, false, 0, 0, 0},
{"f", reflect.Func, "main.barfoo", 0, 0, 0},
Expand Down Expand Up @@ -295,8 +295,8 @@ func TestSetVariable(t *testing.T) {
{"iface1", "interface {}", "interface {} nil", "iface2", "interface {}(string) \"test\""},
{"iface1", "interface {}", "interface {}(string) \"test\"", "parr", "interface {}(*[4]int) *[0,1,2,3]"},

{"s3", "[]int", `[]int len: 0, cap: 6, []`, "s4[2:5]", "[]int len: 3, cap: 3, [3,4,5]"},
{"s3", "[]int", "[]int len: 3, cap: 3, [3,4,5]", "arr1[:]", "[]int len: 4, cap: 4, [0,1,2,3]"},
{"s3", "[]int", `[]int len: 0, cap: 6, []`, "s4[2:5]", "[]int len: 3, cap: 8, [3,4,5]"},
{"s3", "[]int", "[]int len: 3, cap: 8, [3,4,5]", "arr1[:]", "[]int len: 4, cap: 4, [0,1,2,3]"},
{"str1", "string", `"01234567890"`, `"new value"`, errorPrefix + "literal string can not be allocated because function calls are not allowed without using 'call'"},
}

Expand Down Expand Up @@ -646,16 +646,17 @@ func getEvalExpressionTestCases() []varTest {
{"str1[11]", false, "", "", "byte", errors.New("index out of bounds")},

// slice/array/string reslicing
{"a1[2:4]", false, "[]string len: 2, cap: 2, [\"three\",\"four\"]", "[]string len: 2, cap: 2, [\"three\",\"four\"]", "[]string", nil},
{"s1[2:4]", false, "[]string len: 2, cap: 2, [\"three\",\"four\"]", "[]string len: 2, cap: 2, [\"three\",\"four\"]", "[]string", nil},
{"a1[2:4]", false, "[]string len: 2, cap: 3, [\"three\",\"four\"]", "[]string len: 2, cap: 3, [\"three\",\"four\"]", "[]string", nil},
{"s1[2:4]", false, "[]string len: 2, cap: 3, [\"three\",\"four\"]", "[]string len: 2, cap: 3, [\"three\",\"four\"]", "[]string", nil},
{"str1[2:4]", false, "\"23\"", "\"23\"", "string", nil},
{"str1[0:11]", false, "\"01234567890\"", "\"01234567890\"", "string", nil},
{"str1[:3]", false, "\"012\"", "\"012\"", "string", nil},
{"str1[3:]", false, "\"34567890\"", "\"34567890\"", "string", nil},
{"str1[0:12]", false, "", "", "string", errors.New("index out of bounds")},
{"str1[5:3]", false, "", "", "string", errors.New("index out of bounds")},
{"str1[11:]", false, "\"\"", "\"\"", "string", nil},
{"longbyteslice[:70]", false, "[]uint8 len: 70, cap: 70, [118,101,114,121,32,108,111,110,103,32,115,116,114,105,110,103,32,48,49,50,51,52,53,54,55,56,57,97,48,49,50,51,52,53,54,55,56,57,98,48,49,50,51,52,53,54,55,56,57,99,48,49,50,51,52,53,54,55,56,57,100,48,49,50,51,52,53,54,55,56]", "[]uint8 len: 70, cap: 70, [118,101,114,121,32,108,111,110,103,32,115,116,114,105,110,103,32,48,49,50,51,52,53,54,55,56,57,97,48,49,50,51,52,53,54,55,56,57,98,48,49,50,51,52,53,54,55,56,57,99,48,49,50,51,52,53,54,55,56,57,100,48,49,50,51,52,53,54,55,56]", "[]uint8", nil},
{"longbyteslice[:70]", false, "[]uint8 len: 70, cap: 144, [118,101,114,121,32,108,111,110,103,32,115,116,114,105,110,103,32,48,49,50,51,52,53,54,55,56,57,97,48,49,50,51,52,53,54,55,56,57,98,48,49,50,51,52,53,54,55,56,57,99,48,49,50,51,52,53,54,55,56,57,100,48,49,50,51,52,53,54,55,56]", "[]uint8 len: 70, cap: 144, [118,101,114,121,32,108,111,110,103,32,115,116,114,105,110,103,32,48,49,50,51,52,53,54,55,56,57,97,48,49,50,51,52,53,54,55,56,57,98,48,49,50,51,52,53,54,55,56,57,99,48,49,50,51,52,53,54,55,56,57,100,48,49,50,51,52,53,54,55,56]", "[]uint8", nil},
{"longbyteslice[:3][:5]", false, "[]uint8 len: 5, cap: 144, [118,101,114,121,32]", "[]uint8 len: 5, cap: 144, [118,101,114,121,32]", "[]uint8", nil},

// NaN and Inf floats
{"pinf", false, "+Inf", "+Inf", "float64", nil},
Expand Down