Here's how RFC 5280 defines an X.509 extension field:
Extension ::= SEQUENCE {
extnID OBJECT IDENTIFIER,
critical BOOLEAN DEFAULT FALSE,
extnValue OCTET STRING
-- contains the DER encoding of an ASN.1 value
-- corresponding to the extension type identified
-- by extnID
}
When I use the pyasn1 decoder, I'll get an object foo whose foo['extnValue'].asOctets() can be further decoded, with another call to the decoder, using the schema appropriate to foo['extnID'].
Question: supposing that there's one special extnID in my application, is it possible in pyasn1 to define a schema that (a) doesn't accept any OBJECT IDENTIFIER, only the special one; and (b) steps past the OCTET STRING wrapper to decode the payload according to the appropriate special "sub-schema" ?
I can do this by special-case logic in the code, but I'd prefer to define a special-case schema if that is supported.
You should use OpenType for extnValue type definition. When you pass decoder the map of extnID -> ASN.1 type, the decoder then will step past the outermost extnValue wrapper trying to decode its contents.
Here's a quick example:
openType = opentype.OpenType(
'id',
{1: univ.Integer(),
2: univ.OctetString()}
)
self.s = univ.Sequence(
componentType=namedtype.NamedTypes(
namedtype.NamedType('id', univ.Integer()),
namedtype.NamedType('blob', univ.Any(), openType=openType)
)
)
Make sure to pass decodeOpenTypes=True argument to the decode() function if you want the decoder to unwrap these open types.
BTW, if you look at the pyasn1-modules, that map is already there. If you use these definitions and just pass decodeOpenTypes=True keyword argument to decoder, you should get your extensions unwrapped.
There is no ready-made way to fail the decoder on unknown extnID. You can probably model that my giving it your own map (perhaps based on dict) which would fail on unsuccessful key in my_map operation.
Related
am using a protobuf definition file to create golang classes
file.proto
message my_message {
<what type to use?> data = 1 [json_name = "data"]
}
generated.go
type MyMessage struct {
data any
}
I have checked out Struct , Value and Any, though none of them actually mark the type as “any” in generated classes.
The closes I get is Value type which can accomodate primitives as well as structs and lists.
As far as I'm aware the Go code generator doesn't have any circumstances where any/interface{} will be used for a generated field.
google.protobuf.Any is likely what you want here, but the field in the generated code will of type anypb.Any. If you absolutely require the any type be used, you'll unfortunately have to manually map to another struct.
The code below contains a reference to Enum::name (notice no type parameter).
public static <T extends Enum<T>> ColumnType<T, String> enumColumn(Class<T> klazz) {
return simpleColumn((row, label) -> valueOf(klazz, row.getString(label)), Enum::name);
}
public static <T, R> ColumnType<T, R> simpleColumn(BiFunction<JsonObject, String, T> readFromJson,
Function<T, R> writeToDb) {
// ...
}
Javac reports a warning during compilation:
[WARNING] found raw type: java.lang.Enum missing type arguments for
generic class java.lang.Enum
Changing the expression to Enum<T>::name causes the warning to go away.
However Idea flags the Enum<T>::name version with a warning that:
Explicit type arguments can be inferred
In turn Eclipse (ECJ) doesn't report any problems with either formulation.
Which of the three approaches is correct?
On one hand raw types are rather nasty. If you try to put some other type argument e.g. Enum<Clause>::name will cause the compilation to fails so it's some extra protection.
On the other hand the above reference is equivalent to e -> e.name() lambda, and this formulation doesn't require type arguments.
Enviorment:
Java 8u91
IDEA 15.0.3 Community
ECJ 4.5.2
There is no such thing as a “raw method reference”. Whilst raw types exist to help the migration of pre-Generics code, there can’t be any pre-Generics usage of method references, hence there is no “compatibility mode” and type inference is the norm. The Java Language Specification §15.13. Method Reference Expressions states:
If a method or constructor is generic, the appropriate type arguments may either be inferred or provided explicitly. Similarly, the type arguments of a generic type mentioned by the method reference expression may be provided explicitly or inferred.
Method reference expressions are always poly expressions
So while you may call the type before the :: a “raw type” when it referes to a generic class without specifying type arguments, the compiler will still infer the generic type signature according to the target function type. That’s why producing a warning about “raw type usage” makes no sense here.
Note that, e.g.
BiFunction<List<String>,Integer,String> f1 = List::get;
Function<Enum<Thread.State>,String> f2 = Enum::name;
can be compiled with javac without any warning (the specification names similar examples where the type should get inferred), whereas
Function<Thread.State,String> f3 = Enum::name;
generates a warning. The specification says about this case:
In the second search, if P1, ..., Pn is not empty and P1 is a subtype of ReferenceType, then the method reference expression is treated as if it were a method invocation expression with argument expressions of types P2, ..., Pn. If ReferenceType is a raw type, and there exists a parameterization of this type, G<...>, that is a supertype of P1, the type to search is the result of capture conversion (§5.1.10) applied to G<...>;…
So in the above example, the compiler should infer Enum<Thread.State> as the parametrization of Enum that is a supertype of Thread.State to search for an appropriate method and come to the same result as for the f2 example. It somehow does work, though it generates the nonsensical raw type warning.
Since apparently, javac only generates this warning when it has to search for an appropriate supertype, there is a simple solution for your case. Just use the exact type to search:
public static <T extends Enum<T>> ColumnType<T, String> enumColumn(Class<T> klazz) {
return simpleColumn((row, label) -> valueOf(klazz, row.getString(label)), T::name);
}
This compiles without any warning.
type Config struct {
CommitIndex uint64 `json:"commitIndex"`
// TODO decide what we need to store in peer struct
Peers []*Peer `json:"peers"`
}
I understand what the first two columns are,but what is json:"commitIndex"?
It's called a struct tag, they can be parsed using the reflect package at runtime.
From https://golang.org/ref/spec#Struct_types:
A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration.
The tags are made visible through a reflection interface and take part in type identity for structs but are otherwise ignored.
Some packages that use reflection like json and xml use tags to handle special cases better.
What you are referring to is called a tag, and the Go specification states:
A field declaration may be followed by an optional string literal tag,
which becomes an attribute for all the fields in the corresponding
field declaration. The tags are made visible through a reflection
interface and take part in type identity for structs but are otherwise
ignored.
// A struct corresponding to the TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
microsec uint64 "field 1"
serverIP6 uint64 "field 2"
process string "field 3"
}
This does nothing at compile time, but is used by different packages when doing runtime reflection on the struct. As Amit already pointed out, the encoding/json package is using it to specify marshalling/unmarshalling behaviour. The same goes with encoding/xml, gopkg.in/mgo.v2/bson, etc.
The tag string is by convention a space separated string. As stated in the reflect package:
By convention, tag strings are a concatenation of optionally
space-separated key:"value" pairs. Each key is a non-empty string
consisting of non-control characters other than space (U+0020 ' '),
quote (U+0022 '"'), and colon (U+003A ':'). Each value is quoted using
U+0022 '"' characters and Go string literal syntax.
the code at https://code.google.com/p/goauth2/source/browse/oauth/oauth.go#99 declares this type:
package oauth
...
type Config struct {...}
...
the suggested use of this is following:
var config = &oauth.Config{...}
I do not understand why this code takes the address of this type and why this is even possible in Go. I am a newbie. I thought that types are for the compiler, no? Please help.
The Go Programming Language Specification
Composite literals
Composite literals construct values for structs, arrays, slices, and
maps and create a new value each time they are evaluated. They consist
of the type of the value followed by a brace-bound list of composite
elements. An element may be a single expression or a key-value pair.
Given the declaration
type Point3D struct { x, y, z float64 }
one may write
origin := Point3D{} // zero value for Point3D
Taking the address of a composite literal generates a pointer to a
unique instance of the literal's value.
var pointer *Point3D = &Point3D{y: 1000}
It's an example of the use of a pointer to a composite literal.
This is taking the address of a new instance of the Config type, not the address of the type itself.
Whats the correct way in go to distinguish between when a value in a struct was never set, or is just empty, for example, given the following:
type Organisation struct {
Category string
Code string
Name string
}
I need to know (for example) if the category was never set, or was saved as blank by the user, should I be doing this:
type Organisation struct {
Category *string
Code *string
Name *string
}
I also need to ensure I correctly persist either null or an empty string to the database
I'm still learning GO so it is entirely possible my question needs more info.
The zero value for a string is an empty string, and you can't distinguish between the two.
If you are using the database/sql package, and need to distinguish between NULL and empty strings, consider using the sql.NullString type. It is a simple struct that keeps track of the NULL state:
type NullString struct {
String string
Valid bool // Valid is true if String is not NULL
}
You can scan into this type and use it as a query parameter, and the package will handle the NULL state for you.
Google's protocol buffers (https://code.google.com/p/goprotobuf/) use pointers to describe optional fields.
The generated objects provide GetFoo methods which take the pain away from testing for nil (a.GetFoo() returns an empty string if a.Foo is nil, otherwise it returns *a.Foo).
It introduces a nuisance when you want to write literal structs (in tests, for example), because &"something" is not valid syntax to generate a pointer to a string, so you need a helper function (see, for example, the source code of the protocol buffer library for proto.String).
// String is a helper routine that allocates a new string value
// to store v and returns a pointer to it.
func String(v string) *string {
return &v
}
Overall, using pointers to represent optional fields is not without drawbacks, but it's certainly a viable design choice.
The standard database/sql package provides a NullString struct (members are just String string and Valid bool). To take care of some of the repetitive work of persistence, you could look at an object-relational manager like gorp.
I looked into whether there was some way to distinguish two kinds of empty string just out of curiosity, and couldn't find one. With []bytes, []byte{} == []byte(nil) currently returns false, but I'm not sure if the spec guarantees that to always remain true. In any case, it seems like the most practical thing to do is to go with the flow and use NullString.