encoding/jsonschema: allow impossible subschemas

Some real-world schemas contain self-contradictory elements, despite
being useful in practice. We change `encoding/jsonschema` so that it's
not necessarily an error to have a self-contradictory schema as long as
the top level schema itself is not self-contradictory.

This allows us to generate CUE for more real-world schemas, such as this
one:

https://github.com/SchemaStore/schemastore/blob/c084075dbfa7eb7da2c4c81456d04543b7be5744/src/schemas/json/stylelintrc.json#L173

We might add functionality to enable stricter vetting of such schemas in
the future, but for now, schema linting is not what we're aiming for.

Fixes #3455

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I60f5b56b7afd59d9e42ca93906f6f335b4500a39
Dispatch-Trailer: {"type":"trybot","CL":1201396,"patchset":4,"ref":"refs/changes/96/1201396/4","targetBranch":"master"}

cue.mod/pkg/github.com/SchemaStore/schemastore/src/schemas/json/github-workflow.cue

@@ -778,7 +778,7 @@ import (
 
 		// The type of machine to run the job on. The machine can be
 		// either a GitHub-hosted runner, or a self-hosted runner.
-		"runs-on"!: matchN(>=1, [string, matchN(>=1, [[string] & [_, ...]]), {
+		"runs-on"!: matchN(>=1, [string, [string] & [_, ...] & [...], {
 			group?: string
 			labels?: matchN(1, [string, [...string]])
 			...

encoding/jsonschema/constraints_combinator.go

@@ -75,10 +75,23 @@ func constraintAnyOf(key string, n cue.Value, s *state) {
 	a := make([]ast.Expr, 0, len(items))
 	for _, v := range items {
 		x, sub := s.schemaState(v, s.allowedTypes, nil, true)
+		if sub.allowedTypes == 0 {
+			// Nothing is allowed; omit.
+			continue
+		}
 		types |= sub.allowedTypes
 		knownTypes |= sub.knownTypes
 		a = append(a, x)
 	}
+	if len(a) == 0 {
+		// Nothing at all is allowed.
+		s.allowedTypes = 0
+		return
+	}
+	if len(a) == 1 {
+		s.all.add(n, a[0])
+		return
+	}
 	s.allowedTypes &= types
 	s.knownTypes &= knownTypes
 	s.all.add(n, ast.NewCall(
@@ -106,6 +119,10 @@ func constraintOneOf(key string, n cue.Value, s *state) {
 	a := make([]ast.Expr, 0, len(items))
 	for _, v := range items {
 		x, sub := s.schemaState(v, s.allowedTypes, nil, true)
+		if sub.allowedTypes == 0 {
+			// Nothing is allowed; omit
+			continue
+		}
 		types |= sub.allowedTypes
 
 		// TODO: make more finegrained by making it two pass.

encoding/jsonschema/decode.go

@@ -126,6 +126,9 @@ func (d *decoder) schema(ref []ast.Label, v cue.Value) (a []ast.Decl) {
 	}
 
 	expr, state := root.schemaState(v, allTypes, nil, false)
+	if state.allowedTypes == 0 {
+		d.addErr(errors.Newf(state.pos.Pos(), "constraints are not possible to satisfy"))
+	}
 
 	tags := []string{}
 	if state.schemaVersionPresent {
@@ -485,8 +488,8 @@ const allTypes = cue.NullKind | cue.BoolKind | cue.NumberKind | cue.IntKind |
 // finalize constructs a CUE type from the collected constraints.
 func (s *state) finalize() (e ast.Expr) {
 	if s.allowedTypes == 0 {
-		// Nothing is possible.
-		s.addErr(errors.Newf(s.pos.Pos(), "constraints are not possible to satisfy"))
+		// Nothing is possible. This isn't a necessarily a problem, as
+		// we might be inside an allOf or oneOf with other valid constraints.
 		return bottom()
 	}
 	s.addIfThenElse()

encoding/jsonschema/testdata/external/tests/draft2019-09/anchor.json

@@ -141,8 +141,8 @@
 			"$ref": "child1#my_anchor"
 		},
 		"skip": {
-			"v2": "extract error: keyword \"$anchor\" not yet implemented (and 4 more errors)",
-			"v3": "extract error: keyword \"$anchor\" not yet implemented (and 4 more errors)"
+			"v2": "extract error: keyword \"$anchor\" not yet implemented (and 2 more errors)",
+			"v3": "extract error: keyword \"$anchor\" not yet implemented (and 2 more errors)"
 		},
 		"tests": [
 			{

encoding/jsonschema/testdata/external/tests/draft2020-12/anchor.json

@@ -141,8 +141,8 @@
 			"$ref": "child1#my_anchor"
 		},
 		"skip": {
-			"v2": "extract error: keyword \"$anchor\" not yet implemented (and 4 more errors)",
-			"v3": "extract error: keyword \"$anchor\" not yet implemented (and 4 more errors)"
+			"v2": "extract error: keyword \"$anchor\" not yet implemented (and 2 more errors)",
+			"v3": "extract error: keyword \"$anchor\" not yet implemented (and 2 more errors)"
 		},
 		"tests": [
 			{

encoding/jsonschema/testdata/txtar/anyof_with_one_possible.txtar

@@ -0,0 +1,17 @@
+-- schema.json --
+{
+    "anyOf": [
+        {
+            "type": "string",
+            "enum": [
+                1,
+                2
+            ]
+        },
+        {
+            "type": "boolean"
+        }
+    ]
+}
+-- out/decode/extract --
+bool

encoding/jsonschema/testdata/txtar/anyofof_with_impossible_elements.txtar

@@ -0,0 +1,20 @@
+-- schema.json --
+{
+    "anyOf": [
+        {
+            "type": "string",
+            "enum": [
+                1,
+                2
+            ]
+        },
+        {
+            "type": "boolean"
+        },
+        {
+            "type": "number"
+        }
+    ]
+}
+-- out/decode/extract --
+matchN(>=1, [bool, number])

encoding/jsonschema/testdata/txtar/impossibleallof.txtar

@@ -9,5 +9,3 @@
 ERROR:
 constraints are not possible to satisfy:
     schema.json:1:1
-constraints are not possible to satisfy:
-    schema.json:4:9

encoding/jsonschema/testdata/txtar/oneof_with_one_possible.txtar

@@ -0,0 +1,17 @@
+-- schema.json --
+{
+    "oneOf": [
+        {
+            "type": "string",
+            "enum": [
+                1,
+                2
+            ]
+        },
+        {
+            "type": "boolean"
+        }
+    ]
+}
+-- out/decode/extract --
+bool