internal/lsp: fix crash completing recursive pointer types

We were recursing infinitely evaluating objects of recursive pointer
types such as "type foo *foo". Now we track named pointer types we
have already seen to avoid trying to dereference such objects forever.
I lazily initialized the "seen" map to avoid the allocation in the
normal case when you aren't dealing with named pointer types.

Fixes golang/go#37104.

Change-Id: I5f294cfc5a641e7b5fd24e1d9dc55520726ea560
Reviewed-on: https://go-review.googlesource.com/c/tools/+/218579
Run-TryBot: Muir Manders <muir@mnd.rs>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/source/completion.go

@@ -324,7 +324,7 @@ func (c *completer) found(cand candidate) {
 		return
 	}
 
-	if c.matchingCandidate(&cand) {
+	if c.matchingCandidate(&cand, nil) {
 		cand.score *= highScore
 	} else if isTypeName(obj) {
 		// If obj is a *types.TypeName that didn't otherwise match, check
@@ -1737,8 +1737,9 @@ func (c *completer) fakeObj(T types.Type) *types.Var {
 }
 
 // matchingCandidate reports whether a candidate matches our type
-// inferences.
-func (c *completer) matchingCandidate(cand *candidate) bool {
+// inferences. seen is used to detect recursive types in certain cases
+// and should be set to nil when calling matchingCandidate.
+func (c *completer) matchingCandidate(cand *candidate, seen map[types.Type]struct{}) bool {
 	if isTypeName(cand.obj) {
 		return c.matchingTypeName(cand)
 	} else if c.wantTypeName() {
@@ -1788,15 +1789,28 @@ func (c *completer) matchingCandidate(cand *candidate) bool {
 
 	// Check if dereferencing cand would match our type inference.
 	if ptr, ok := cand.obj.Type().Underlying().(*types.Pointer); ok {
-		if c.matchingCandidate(&candidate{obj: c.fakeObj(ptr.Elem())}) {
+		// Notice if we have already encountered this pointer type before.
+		_, saw := seen[cand.obj.Type()]
+
+		if _, named := cand.obj.Type().(*types.Named); named {
+			// Lazily allocate "seen" since it isn't used normally.
+			if seen == nil {
+				seen = make(map[types.Type]struct{})
+			}
+
+			// Track named pointer types we have seen to detect cycles.
+			seen[cand.obj.Type()] = struct{}{}
+		}
+
+		if !saw && c.matchingCandidate(&candidate{obj: c.fakeObj(ptr.Elem())}, seen) {
 			// Mark the candidate so we know to prepend "*" when formatting.
 			cand.dereference = true
 			return true
 		}
 	}
 
 	// Check if cand is addressable and a pointer to cand matches our type inference.
-	if cand.addressable && c.matchingCandidate(&candidate{obj: c.fakeObj(types.NewPointer(candType))}) {
+	if cand.addressable && c.matchingCandidate(&candidate{obj: c.fakeObj(types.NewPointer(candType))}, seen) {
 		// Mark the candidate so we know to prepend "&" when formatting.
 		cand.takeAddress = true
 		return true

internal/lsp/source/completion_literal.go

@@ -75,7 +75,7 @@ func (c *completer) literal(literalType types.Type, imp *importInfo) {
 		cand.addressable = true
 	}
 
-	if !c.matchingCandidate(&cand) {
+	if !c.matchingCandidate(&cand, nil) {
 		return
 	}
 

internal/lsp/testdata/lsp/primarymod/address/address.go

@@ -36,6 +36,17 @@ func _() {
 	var _ int = _ //@rank("_ //", addrCPtr, addrA),snippet("_ //", addrCPtr, "*c", "*c")
 
 	wantsVariadic() //@rank(")", addrCPtr, addrA),snippet(")", addrCPtr, "*c", "*c")
+
+	type namedPtr *int
+	var np namedPtr
+	*np           //@item(addrNamedPtr, "*np", "namedPtr", "var")
+	var _ int = _ //@rank("_ //", addrNamedPtr, addrA)
+
+	// don't get tripped up by recursive pointer type
+	type dontMessUp *dontMessUp
+	var dmu *dontMessUp //@item(addrDMU, "dmu", "*dontMessUp", "var")
+
+	var _ int = dmu //@complete(" //", addrDMU)
 }
 
 func (f foo) ptr() *foo { return &f }

internal/lsp/testdata/lsp/summary.txt.golden

@@ -1,10 +1,10 @@
 -- summary --
-CompletionsCount = 226
+CompletionsCount = 227
 CompletionSnippetCount = 66
 UnimportedCompletionsCount = 9
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8
-RankedCompletionsCount = 83
+RankedCompletionsCount = 84
 CaseSensitiveCompletionsCount = 4
 DiagnosticsCount = 38
 FoldingRangesCount = 2