The offsetof trick is not compatible with an incoming Clang change.
Clang has started to enforce that it is UB to declare types inside offsetof. See e327b52766.
```
third_party/upb/upb/mem/arena.c:166:25: error: 'struct (unnamed at third_party/upb/upb/mem/arena.c:166:7)' cannot be defined in '__builtin_offsetof'
n = UPB_ALIGN_DOWN(n, UPB_ALIGN_OF(upb_Arena));
^
./third_party/upb/upb/port/def.inc:113:38: note: expanded from macro 'UPB_ALIGN_OF'
#define UPB_ALIGN_OF(type) offsetof (struct { char c; type member; }, member)
^
third_party/upb/upb/mem/arena.c:166:25: error: 'struct (unnamed at third_party/upb/upb/mem/arena.c:166:7)' cannot be defined in '__builtin_offsetof'
./third_party/upb/upb/port/def.inc:113:38: note: expanded from macro 'UPB_ALIGN_OF'
#define UPB_ALIGN_OF(type) offsetof (struct { char c; type member; }, member)
```
PiperOrigin-RevId: 501872556
This CL changes the upb compiler to no longer depend on C++ protobuf libraries. upb now uses its own reflection libraries to implement its code generator.
# Key Benefits
1. upb can now use its own reflection libraries throughout the compiler. This makes upb more consistent and principled, and gives us more chances to dogfood our own C++ reflection API. This highlighted several parts of the C++ reflection API that were incomplete.
2. This CL removes code duplication that previously existed in the compiler. The upb reflection library has code to build MiniDescriptors and MiniTables out of descriptors, but prior to this CL the upb compiler could not use it. The upb compiler had a separate copy of this logic, and the compiler's copy of this logic was especially tricky and hard to maintain. This CL removes the separate copy of that logic.
3. This CL (mostly) removes upb's dependency on the C++ protobuf library. We still depend on `protoc` (the binary), but the runtime and compiler no longer link against C++'s libraries. This opens up the possibility of speeding up some builds significantly if we can use a prebuilt `protoc` binary.
# Bootstrap Stages
To bootstrap, we check in a copy of our generated code for `descriptor.proto` and `plugin.proto`. This allows the compiler to depend on the generated code for these two protos without creating a circular dependency. This code is checked in to the `stage0` directory.
The bootstrapping process is divided into a few stages. All `cc_library()`, `upb_proto_library()`, and `cc_binary()` targets that would otherwise be circular participate in this staging process. That currently includes:
* `//third_party/upb:descriptor_upb_proto`
* `//third_party/upb:plugin_upb_proto`
* `//third_party/upb:reflection`
* `//third_party/upb:reflection_internal`
* `//third_party/upbc:common`
* `//third_party/upbc:file_layout`
* `//third_party/upbc:plugin`
* `//third_party/upbc:protoc-gen-upb`
For each of these targets, we produce a rule for each stage (the logic for this is nicely encapsulated in Blaze/Bazel macros like `bootstrap_cc_library()` and `bootstrap_upb_proto_library()`, so the `BUILD` file remains readable). For example:
* `//third_party/upb:descriptor_upb_proto_stage0`
* `//third_party/upb:descriptor_upb_proto_stage1`
* `//third_party/upb:descriptor_upb_proto`
The stages are:
1. `stage0`: This uses the checked-in version of the generated code. The stage0 compiler is correct and outputs the same code as all other compilers, but it is unnecessarily slow because its protos were compiled in bootstrap mode. The stage0 compiler is used to generate protos for stage1.
2. `stage1`: The stage1 compiler is correct and fast, and therefore we use it in almost all cases (eg. `upb_proto_library()`). However its own protos were not generated using `upb_proto_library()`, so its `cc_library()` targets cannot be safely mixed with `upb_proto_library()`, as this would lead to duplicate symbols.
3. final (no stage): The final compiler is identical to the `stage1` compiler. The only difference is that its protos were built with `upb_proto_library()`. This doesn't matter very much for the compiler binary, but for the `cc_library()` targets like `//third_party/upb:reflection`, only the final targets can be safely linked in by other applications.
# "Bootstrap Mode" Protos
The checked-in generated code is generated in a special "bootstrap" mode that is a bit different than normal generated code. Bootstrap mode avoids depending on the internal representation of MiniTables or the messages, at the cost of slower runtime performance.
Bootstrap mode only interacts with MiniTables and messages using public APIs such as `upb_MiniTable_Build()`, `upb_Message_GetInt32()`, etc. This is very important as it allows us to change the internal representation without needing to regenerate our bootstrap protos. This will make it far easier to write CLs that change the internal representation, because it avoids the awkward dance of trying to regenerate the bootstrap protos when the compiler itself is broken due to bootstrap protos being out of date.
The bootstrap generated code does have two downsides:
1. The accessors are less efficient, because they look up MiniTable fields by number instead of hard-coding the MiniTableField into the generated code.
2. It requires runtime initialization of the MiniTables, which costs CPU cycles at startup, and also allocates memory which is never freed. Per google3 rules this is not really a leak, since this memory is still reachable via static variables, but it is undesirable in many contexts. We could fix this part by introducing the equivalent of `google::protobuf::ShutdownProtobufLibrary()`).
These downsides are fine for the bootstrapping process, but they are reason enough not to enable bootstrap mode in general for all protos.
# Bootstrapping Always Uses OSS Protos
To enable smooth syncing between Google3 and OSS, we always use an OSS version of the checked in generated code for `stage0`, even in google3.
This requires that the google3 code can be switched to reference the OSS proto names using a preprocessor define. We introduce the `UPB_DESC(xyz)` macro for this, which will expand into either `proto2_xyz` or `google_protobuf_xyz`. Any libraries used in `stage0` must use `UPB_DESC(xyz)` rather than refer to the symbol names directly.
PiperOrigin-RevId: 501458451
Implemented in java, c++, python and upb. Also added conformance test.
https://developers.google.com/protocol-buffers/docs/reference/google.protobuf#google.protobuf.Value
where it says:
attempting to serialize NaN or Infinity results in error. (We can't serialize these as string "NaN" or "Infinity" values like we do for regular fields, because they would parse as string_value, not number_value).
PiperOrigin-RevId: 500828964
We have previously been using Copybara to rewrite these names, but for bootstrapping we will want to be able to sometimes use OSS names inside google3.
PiperOrigin-RevId: 500294974
Implemented in java, c++, python and upb. Also added conformance test.
https://developers.google.com/protocol-buffers/docs/reference/google.protobuf#google.protobuf.Value
where it says:
attempting to serialize NaN or Infinity results in error. (We can't serialize these as string "NaN" or "Infinity" values like we do for regular fields, because they would parse as string_value, not number_value).
PiperOrigin-RevId: 500139380
This simplifies the code generation by making output agnostic to whether fasttables will be used or not.
This grows the generated code in the common case, but when fasttables are not being used the preprocessor will strip away the unused tables.
PiperOrigin-RevId: 499340805
We would like for upb_Map_Delete() to optionally return the deleted value.
Unfortunately this will require several steps since we are crossing repos.
Step #3: Give the new footprint to the original function and switch back to it.
Since we're already touching map.h, also mark UPB_API as appropriate.
PiperOrigin-RevId: 498398474
Mike Kruskal
Eric Salo
Protobuf Team Bot
Joshua Haberman
Copybara-Service
Dmitri Gribenko
Jie Luo
Tom Hughes
Sandy Zhang
Deanna Garcia
deannagarcia