light client: Address panic in response to empty signed header (backp…

…ort #577) (#700)

* light client: Address panic in response to empty signed header (#577)

Closes #575.

This at least addresses the panic in the light client, instead turning it into a temporary error (i.e. that the node does not yet have the signed header for the requisite height, which is true here).

I've tested this patch on the `v0.34.x` branch and it prevents the panic, but instead causes the E2E tests to hang indefinitely. As such, I've logged #576.

The code change is identical across `main`, `v0.37.x` and `v0.34.x`, so backports should be clean. I'll add changelog entries on the backport branches prior to merging them.

---

#### PR checklist

- [ ] Tests written/updated
- [ ] Changelog entry added in `.changelog` (we use [unclog](https://github.com/informalsystems/unclog) to manage our changelog)
- [ ] Updated relevant documentation (`docs/` or `spec/`) and code comments

(cherry picked from commit ea2020ffc5c752b5631ee1b769dc5dd78624f1bc)

* Add changelog entry

Signed-off-by: Thane Thomson <[email protected]>

---------

Signed-off-by: Thane Thomson <[email protected]>
Co-authored-by: Thane Thomson <[email protected]>

.changelog/unreleased/bug-fixes/575-fix-light-client-panic.md

@@ -0,0 +1,6 @@
+- `[light]` Fixed an edge case where a light client would panic when attempting
+  to query a node that (1) has started from a non-zero height and (2) does
+  not yet have any data. The light client will now, correctly, not panic
+  _and_ keep the node in its list of providers in the same way it would if
+  it queried a node starting from height zero that does not yet have data
+  ([\#575](https://github.com/cometbft/cometbft/issues/575))

light/provider/http/http.go

@@ -180,6 +180,15 @@ func (p *http) signedHeader(ctx context.Context, height *int64) (*types.SignedHe
 		commit, err := p.client.Commit(ctx, height)
 		switch {
 		case err == nil:
+			// See https://github.com/cometbft/cometbft/issues/575
+			// If the node is starting at a non-zero height, but does not yet
+			// have any blocks, it can return an empty signed header without
+			// returning an error.
+			if commit.SignedHeader.IsEmpty() {
+				// Technically this means that the provider still needs to
+				// catch up.
+				return nil, provider.ErrHeightTooHigh
+			}
 			return &commit.SignedHeader, nil
 
 		case regexpTooHigh.MatchString(err.Error()):

types/light.go

@@ -120,6 +120,11 @@ type SignedHeader struct {
 	Commit *Commit `json:"commit"`
 }
 
+// IsEmpty returns true if both the header and commit are nil.
+func (sh SignedHeader) IsEmpty() bool {
+	return sh.Header == nil && sh.Commit == nil
+}
+
 // ValidateBasic does basic consistency checks and makes sure the header
 // and commit are consistent.
 //