x/tools/go/analysis/nilness: heuristics for flagging conversions from nil *T to interface #69645

adonovan · 2024-09-26T17:51:00Z

We have forever been discussing ways to make the nilness analyzer report conversions from a definitely-nil pointer of type *T to an interface type. The challenge is how to distinguish the legitimate uses (when a nil *T pointer is valid) from the mistakes. This issue is a place to publicly record some of the raw data we got from running a simple heuristic across the Go module mirror corpus and discuss potential improvements.

The analysis is just this patch to the existing code:

diff --git a/go/analysis/passes/nilness/nilness.go b/go/analysis/passes/nilness/nilness.go
index faaf12a93..8b049a4ea 100644
--- a/go/analysis/passes/nilness/nilness.go
+++ b/go/analysis/passes/nilness/nilness.go
@@ -121,6 +121,15 @@ func runFunc(pass *analysis.Pass, fn *ssa.Function) {
                        case *ssa.Send:
                                // (Not a runtime error, but a likely mistake.)
                                notNil(stack, instr, instr.Chan, "send to nil channel")
+
+                       case *ssa.MakeInterface:
+                               switch instr.X.Type().Underlying().(type) {
+                               case *types.Slice, *types.Map:
+                                       // nils of these types are fine
+                               default:
+                                       notNil(stack, instr, instr.X,
+                                               fmt.Sprintf("converting nil %s to interface %s", instr.X.Type(), instr.Type()))
+                               }
                        }
                }

gabyhelp · 2024-09-26T17:51:10Z

Related Issues and Documentation

reflect: MakeFunc functions cannot use the nil error return idiom #6871 (closed)
x/tools/go/analysis/passes/nilness: bogus "impossible condition: non-nil == nil" that is actually possible #66835 (closed)
cmd/compile: typecheck panic, interface conversion: types.Object is nil, not *ir.Name #50552 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

findleyr · 2024-09-27T13:11:27Z

Thanks for collecting this data! The first 10 links I clicked were false positives.

Arbitrary Examples:

target := btf.Type((*btf.Func)(nil))
RegisterPlugin("TagFilter", PluginTypeFilter, (Filter)((*TagFilter)(nil)))
params: runtime.Object(nilUnstructured)
var dd *DogStatsdSink; _ = metrics.MetricSink(dd)
{"Custom Interface Struct Nil", customInterface(structNil), true} (in a function called TestIsInterfaceNil)
{P{1, 4}, addr(any((**valueStringer)(nil)))}
var i32p *int32 ... TstDecode(bytes.NewReader(buf))(reflect.ValueOf(i32p)) ... ii32p := interface{}(i32p)

This looks like various patterns of:

Intentionally distinguished nils.
Implements checks.
Intentional tests for typed nil handling.
Imprecise analysis due to use of reflect.

Probably some of these can be avoided, but the lack of clear true positives does not bode well.

adonovan · 2024-09-27T14:22:16Z

the lack of clear true positives does not bode well.

To be clear, no-one intends to merge this change; it was just a starting point for other experiments.

I was surprised the corpus contained so few matches. I suspect the much more common case involves maybe nil pointers, e.g.

func maybeOpen(filename string) io.Reader
   var f *os.FIle
   if maybe { f, _ = os.Open(filename) }
   return f // oops
}

ianlancetaylor · 2024-09-27T17:27:57Z

Since the goal here is to identify cases of https://go.dev/doc/faq#nil_error, we shouldn't even be looking at cases where the code explicitly converts to interface type. We should only be looking at cases where the conversion is implicit. Unfortunately the current list has too many irrelevant items to judge whether this is useful or not.

gopherbot added the Tools This label describes issues relating to any tools in the x/tools repository. label Sep 26, 2024

adonovan added Analysis Issues related to static analysis (vet, x/tools/go/analysis) and removed Tools This label describes issues relating to any tools in the x/tools repository. labels Sep 26, 2024

gopherbot added this to the Unreleased milestone Sep 26, 2024

mknyszek added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Sep 30, 2024

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x/tools/go/analysis/nilness: heuristics for flagging conversions from nil *T to interface #69645

x/tools/go/analysis/nilness: heuristics for flagging conversions from nil *T to interface #69645

adonovan commented Sep 26, 2024 •

edited

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gabyhelp commented Sep 26, 2024

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adonovan commented Sep 27, 2024

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ianlancetaylor commented Sep 27, 2024

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x/tools/go/analysis/nilness: heuristics for flagging conversions from nil *T to interface #69645

x/tools/go/analysis/nilness: heuristics for flagging conversions from nil *T to interface #69645

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adonovan commented Sep 26, 2024 •

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