From a432d45d59ee07dc7e49a11bf1973c97e94ca184 Mon Sep 17 00:00:00 2001
From: Matt Toohey <mtoohey@block.xyz>
Date: Tue, 5 Mar 2024 13:35:32 +1100
Subject: [PATCH] fix: prevent schema dependency cycles (#894) (#1015)

---
 backend/schema/schema_test.go | 110 ++++++++++++++++++++++++++++++++++
 backend/schema/validate.go    |  78 ++++++++++++++++++++++++
 2 files changed, 188 insertions(+)

diff --git a/backend/schema/schema_test.go b/backend/schema/schema_test.go
index 086e7862ef..5545ad6ab9 100644
--- a/backend/schema/schema_test.go
+++ b/backend/schema/schema_test.go
@@ -481,3 +481,113 @@ var testSchema = MustValidate(&Schema{
 		},
 	},
 })
+
+func TestValidateDependencies(t *testing.T) {
+	tests := []struct {
+		name   string
+		schema string
+		err    string
+	}{
+		{
+			// one <--> two, cyclical
+			name: "TwoModuleCycle",
+			schema: `
+				module one {
+					verb one(builtin.Empty) builtin.Empty
+						calls two.two
+				}
+
+				module two {
+					verb two(builtin.Empty) builtin.Empty
+						calls one.one
+				}
+				`,
+			err: "found cycle in dependencies: two -> one -> two",
+		},
+		{
+			// one --> two --> three, noncyclical
+			name: "ThreeModulesNoCycle",
+			schema: `
+				module one {
+					verb one(builtin.Empty) builtin.Empty
+						calls two.two
+				}
+
+				module two {
+					verb two(builtin.Empty) builtin.Empty
+						calls three.three
+				}
+
+				module three {
+					verb three(builtin.Empty) builtin.Empty
+				}
+				`,
+			err: "",
+		},
+		{
+			// one --> two --> three -> one, cyclical
+			name: "ThreeModulesCycle",
+			schema: `
+				module one {
+					verb one(builtin.Empty) builtin.Empty
+						calls two.two
+				}
+
+				module two {
+					verb two(builtin.Empty) builtin.Empty
+						calls three.three
+				}
+
+				module three {
+					verb three(builtin.Empty) builtin.Empty
+					calls one.one
+				}
+				`,
+			err: "found cycle in dependencies: two -> three -> one -> two",
+		},
+		{
+			// one.a --> two.a
+			// one.b <---
+			// cyclical (does not depend on verbs used)
+			name: "TwoModuleCycleDiffVerbs",
+			schema: `
+				module one {
+					verb a(builtin.Empty) builtin.Empty
+						calls two.a
+					verb b(builtin.Empty) builtin.Empty
+				}
+
+				module two {
+					verb a(builtin.Empty) builtin.Empty
+						calls one.b
+				}
+				`,
+			err: "found cycle in dependencies: two -> one -> two",
+		},
+		{
+			// one --> one, this is allowed
+			name: "SelfReference",
+			schema: `
+				module one {
+					verb a(builtin.Empty) builtin.Empty
+						calls one.b
+
+					verb b(builtin.Empty) builtin.Empty
+						calls one.a
+				}
+			`,
+			err: "",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			_, err := ParseString("", test.schema)
+			if test.err == "" {
+				assert.NoError(t, err)
+			} else {
+				assert.EqualError(t, err, test.err)
+			}
+		})
+	}
+}
diff --git a/backend/schema/validate.go b/backend/schema/validate.go
index 5bc866a9dd..109b62b57b 100644
--- a/backend/schema/validate.go
+++ b/backend/schema/validate.go
@@ -58,6 +58,11 @@ func Validate(schema *Schema) (*Schema, error) {
 
 	scopes := NewScopes()
 
+	// Validate dependencies
+	if err := validateDependencies(schema); err != nil {
+		merr = append(merr, err)
+	}
+
 	// First pass, add all the modules.
 	for _, module := range schema.Modules {
 		if module == builtins {
@@ -309,3 +314,76 @@ func cleanErrors(merr []error) []error {
 	})
 	return merr
 }
+
+type dependencyVertex struct {
+	from, to string
+}
+
+type dependencyVertexState int
+
+const (
+	notExplored dependencyVertexState = iota
+	exploring
+	fullyExplored
+)
+
+func validateDependencies(schema *Schema) error {
+	// go through schema's modules, find cycles in modules' dependencies
+
+	// First pass, set up direct imports and vertex states for each module
+	// We need each import array and vertex array to be sorted to make the output deterministic
+	imports := map[string][]string{}
+	vertexes := []dependencyVertex{}
+	vertexStates := map[dependencyVertex]dependencyVertexState{}
+
+	for _, module := range schema.Modules {
+		currentImports := module.Imports()
+		sort.Strings(currentImports)
+		imports[module.Name] = currentImports
+
+		for _, imp := range currentImports {
+			v := dependencyVertex{module.Name, imp}
+			vertexes = append(vertexes, v)
+			vertexStates[v] = notExplored
+		}
+	}
+
+	sort.Slice(vertexes, func(i, j int) bool {
+		lhs := vertexes[i]
+		rhs := vertexes[j]
+		return lhs.from < rhs.from || (lhs.from == rhs.from && lhs.to < rhs.to)
+	})
+
+	// DFS to find cycles
+	for _, v := range vertexes {
+		if cycle := dfsForDependencyCycle(imports, vertexStates, v); cycle != nil {
+			return fmt.Errorf("found cycle in dependencies: %s", strings.Join(cycle, " -> "))
+		}
+	}
+
+	return nil
+}
+
+func dfsForDependencyCycle(imports map[string][]string, vertexStates map[dependencyVertex]dependencyVertexState, v dependencyVertex) []string {
+	switch vertexStates[v] {
+	case notExplored:
+		vertexStates[v] = exploring
+
+		for _, toModule := range imports[v.to] {
+			nextV := dependencyVertex{v.to, toModule}
+			if cycle := dfsForDependencyCycle(imports, vertexStates, nextV); cycle != nil {
+				// found cycle. prepend current module to cycle and return
+				cycle = append([]string{nextV.from}, cycle...)
+				return cycle
+			}
+		}
+		vertexStates[v] = fullyExplored
+		return nil
+	case exploring:
+		return []string{v.to}
+	case fullyExplored:
+		return nil
+	}
+
+	return nil
+}