Panic in protodesc.NewFiles if schema uses dynamic message with an older schema of go_features.proto

[emcfarlane]

What version of protobuf and what language are you using?
v1.36.3

What did you do?
This is a follow on from #1669 . I am loading a dynamic schema that contains an older version of go_features.proto (the version included at protobuf v1.29.1). This does not include the expected field strip_enum_prefix.

What did you expect to see?
Expect the field to act as not present if the dynamic message doesn't include the field.

What did you see instead?
With reflection the field lookup returns a nil field descriptor which panics when calling Has(fd) here: https://github.com/protocolbuffers/protobuf-go/blob/0c3cc2f8edbac7858cdff44cbaaaea2cb415cd0f/reflect/protodesc/editions.go#L143

google.golang.org/protobuf/types/dynamicpb.(*Message).checkField(0x14000739400, {0x0, 0x0})
        /Users/edward/.cache/buf/Darwin/arm64/go/pkg/mod/google.golang.org/[email protected]/types/dynamicpb/dynamic.go:342 +0x28
google.golang.org/protobuf/types/dynamicpb.(*Message).Has(0x14000739400, {0x0, 0x0})
        /Users/edward/.cache/buf/Darwin/arm64/go/pkg/mod/google.golang.org/[email protected]/types/dynamicpb/dynamic.go:156 +0x28
google.golang.org/protobuf/reflect/protodesc.mergeEditionFeatures({0x1061013e0?, 0x140018eef00?}, 0x0?)
        /Users/edward/.cache/buf/Darwin/arm64/go/pkg/mod/google.golang.org/[email protected]/reflect/protodesc/editions.go:143 +0x2d8
google.golang.org/protobuf/reflect/protodesc.initFileDescFromFeatureSet(0x140018eef00, 0x140001acae0)
        /Users/edward/.cache/buf/Darwin/arm64/go/pkg/mod/google.golang.org/[email protected]/reflect/protodesc/editions.go:163 +0x90
google.golang.org/protobuf/reflect/protodesc.FileOptions.New({{}, 0x40?}, 0x1400010f600, {0x1060dc2e8?, 0x14000731b48?})
        /Users/edward/.cache/buf/Darwin/arm64/go/pkg/mod/google.golang.org/[email protected]/reflect/protodesc/desc.go:126 +0x448
google.golang.org/protobuf/reflect/protodesc.FileOptions.addFileDeps({{}, 0x58?}, 0x14000731b48, 0x1400010f600, 0x1400121d228)
        /Users/edward/.cache/buf/Darwin/arm64/go/pkg/mod/google.golang.org/[email protected]/reflect/protodesc/desc.go:287 +0x1ec
google.golang.org/protobuf/reflect/protodesc.FileOptions.NewFiles({{}, 0x80?}, 0x1400121d360)
        /Users/edward/.cache/buf/Darwin/arm64/go/pkg/mod/google.golang.org/[email protected]/reflect/protodesc/desc.go:264 +0x26c
google.golang.org/protobuf/reflect/protodesc.NewFiles(...)
        /Users/edward/.cache/buf/Darwin/arm64/go/pkg/mod/google.golang.org/[email protected]/reflect/protodesc/desc.go:76
buf.build/go/bufplugin/descriptor.FileDescriptorsForProtoFileDescriptors({0x14000731b18, 0x3, 0x140011d9108?})
        /Users/edward/.cache/buf/Darwin/arm64/go/pkg/mod/buf.build/go/[email protected]/descriptor/file_descriptor.go:91 +0x24c

Anything else we should know about your project / environment?
I think a nil check is missing for each field descriptor to ensure the field exists before checking if it has been set:

diff --git a/reflect/protodesc/editions.go b/reflect/protodesc/editions.go
index f55b0369..b334e69a 100644
--- a/reflect/protodesc/editions.go
+++ b/reflect/protodesc/editions.go
@@ -136,15 +136,15 @@ func mergeEditionFeatures(parentDesc protoreflect.Descriptor, child *descriptorp
        gf := goFeatures.Message()
        fields := gf.Descriptor().Fields()

-       if fd := fields.ByName("legacy_unmarshal_json_enum"); gf.Has(fd) {
+       if fd := fields.ByName("legacy_unmarshal_json_enum"); fd != nil && gf.Has(fd) {
                parentFS.GenerateLegacyUnmarshalJSON = gf.Get(fd).Bool()
        }

-       if fd := fields.ByName("strip_enum_prefix"); gf.Has(fd) {
+       if fd := fields.ByName("strip_enum_prefix"); fd != nil && gf.Has(fd) {
                parentFS.StripEnumPrefix = int(gf.Get(fd).Enum())
        }

-       if fd := fields.ByName("api_level"); gf.Has(fd) {
+       if fd := fields.ByName("api_level"); fd != nil && gf.Has(fd) {
                parentFS.APILevel = int(gf.Get(fd).Enum())
        }

[jhump]

I think this CL should do the trick here: https://go-review.googlesource.com/c/protobuf/+/642975