encoding/jsonschema: ignore empty fragment when comparing schema versions

It's a very common error for the presence or absence of an empty
fragment in schema version URI not to match the presence or absence of
the empty fragment in the canonical version URI, particularly as earlier
versions included that empty fragment but later versions do not.

From a technical point of view, a URI with an empty fragment _should_ be
considered identical to a URI with no fragment (as witness Go's
`net/url` package which always omits an empty fragment).

The specification says [1] of `$schema`:

> The value of this keyword MUST be a URI (containing a scheme) and this
> URI MUST be normalized. The current schema MUST be valid against the
> meta-schema identified by this URI.

I think it can reasonably be argued that a URI with a mismatching empty
fragment "identifies" the same URL its counterpart.

So compare versions while ignoring a trailing empty fragment.

[1]: https://json-schema.org/draft/2020-12/draft-bhutton-json-schema-00#rfc.section.8.1.1

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I28c6e767f7be7a2832b42124ffe5f0ec37f83635
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201677
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: rogpeppe

encoding/jsonschema/testdata/txtar/extrafragment_version.txtar

@@ -7,6 +7,6 @@
   "type": "string"
 }
 -- out/decode/extract --
-ERROR:
-invalid $schema URL "https://json-schema.org/draft/2019-09/schema#": $schema URI not recognized:
-    schema.json:2:3
+@jsonschema(schema="https://json-schema.org/draft/2019-09/schema")
+@jsonschema(id="http://example.test")
+string

encoding/jsonschema/testdata/txtar/missingfragment_version.txtar

@@ -7,6 +7,6 @@
   "type": "string"
 }
 -- out/decode/extract --
-ERROR:
-invalid $schema URL "http://json-schema.org/draft-04/schema": $schema URI not recognized:
-    schema.json:2:3
+@jsonschema(schema="http://json-schema.org/draft-04/schema#")
+@jsonschema(id="http://example.test")
+string

encoding/jsonschema/version.go

@@ -16,6 +16,7 @@ package jsonschema
 
 import (
 	"fmt"
+	"strings"
 )
 
 //go:generate go run golang.org/x/tools/cmd/stringer -type=Version -linecomment
@@ -74,10 +75,15 @@ func vto(v Version) versionSet {
 
 // ParseVersion parses a version URI that defines a JSON Schema version.
 func ParseVersion(sv string) (Version, error) {
+	// Ignore a trailing empty fragment: it's a common error
+	// to omit or supply such a fragment and it's not entirely
+	// clear whether comparison should or should not
+	// be sensitive to its presence or absence.
+	sv = strings.TrimSuffix(sv, "#")
 	// If this linear search is ever a performance issue, we could
 	// build a map, but it doesn't seem worthwhile for now.
 	for i := Version(1); i < numJSONSchemaVersions; i++ {
-		if sv == i.String() {
+		if sv == strings.TrimSuffix(i.String(), "#") {
 			return i, nil
 		}
 	}