serde/proto: seastar friendly protobuf parser

In support of the iceberg project which needs to be able to parse
arbitrary protocol buffers, create a parser that works within the
constraints of seastar - namely it yields cooperatively when parsing
very large (or complex) protocol buffers, it is zero copy
for string and byte types, and supports non-contiguous allocations of
data for `repeated` and `map` types.

MODULE.bazel

@@ -72,6 +72,7 @@ use_repo(non_module_dependencies, "hwloc")
 use_repo(non_module_dependencies, "jsoncons")
 use_repo(non_module_dependencies, "krb5")
 use_repo(non_module_dependencies, "libpciaccess")
+use_repo(non_module_dependencies, "libprotobuf_mutator")
 use_repo(non_module_dependencies, "libxml2")
 use_repo(non_module_dependencies, "lksctp")
 use_repo(non_module_dependencies, "numactl")

bazel/repositories.bzl

@@ -80,6 +80,14 @@ def data_dependency():
         url = "https://gitlab.freedesktop.org/xorg/lib/libpciaccess/-/archive/2ec2576cabefef1eaa5dd9307c97de2e887fc347/libpciaccess-2ec2576cabefef1eaa5dd9307c97de2e887fc347.tar.gz",
     )
 
+    http_archive(
+        name = "libprotobuf_mutator",
+        build_file = "//bazel/thirdparty:libprotobuf-mutator.BUILD",
+        integrity = "sha256-KWUbFgNpDJtAO6Kr0eTo1v6iczEOta72jSle9oivFhg=",
+        strip_prefix = "libprotobuf-mutator-b922c8ab9004ef9944982e4f165e2747b13223fa",
+        url = "/~https://github.com/google/libprotobuf-mutator/archive/b922c8ab9004ef9944982e4f165e2747b13223fa.zip",
+    )
+
     http_archive(
         name = "libxml2",
         build_file = "//bazel/thirdparty:libxml2.BUILD",

bazel/thirdparty/libprotobuf-mutator.BUILD

@@ -0,0 +1,25 @@
+# See google/libprotobuf-mutator#91
+
+cc_library(
+    name = "libprotobuf_mutator",
+    testonly = 1,
+    srcs = glob(
+        [
+            "src/*.cc",
+            "src/*.h",
+        ],
+        exclude = [
+            "**/*_test.cc",
+            "src/mutator.h",
+        ],
+    ) + [
+        "port/protobuf.h",
+    ],
+    hdrs = [
+        "src/mutator.h",
+    ],
+    include_prefix = "protobuf_mutator",
+    strip_include_prefix = "src",
+    visibility = ["//visibility:public"],
+    deps = ["@protobuf"],
+)

src/v/serde/protobuf/BUILD

@@ -0,0 +1,21 @@
+load("//bazel:build.bzl", "redpanda_cc_library")
+
+redpanda_cc_library(
+    name = "protobuf",
+    srcs = [
+        "parser.cc",
+    ],
+    hdrs = [
+        "parser.h",
+    ],
+    include_prefix = "serde/protobuf",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//src/v/bytes:iobuf",
+        "//src/v/bytes:iobuf_parser",
+        "//src/v/container:chunked_hash_map",
+        "//src/v/container:fragmented_vector",
+        "//src/v/utils:vint",
+        "@protobuf",
+    ],
+)

src/v/serde/protobuf/README.md

@@ -0,0 +1,19 @@
+# Protobuf Parser
+
+This directory contains a seastar friendly protobuf parser.
+It should adhere to the same compatibility guarantees as the offical C++ library
+but has a few notable differences:
+
+1. Does not make contiguous allocations of repeated fields, maps or strings/bytes types
+2. Is reactor friendly on deeply nested or large protobufs in that it will yield control
+3. Is a stackless parser, so it is not bound by the smallish 1MB stacks that seastar uses for threads
+
+## Development
+
+If you are tasked with updating this code, here are a few helpful links:
+
+1. [Encoding spec](https://protobuf.dev/programming-guides/encoding/) (note this elides some important details about how invalid/corrupted data is handled)
+2. [Golang protobuf parser](/~https://github.com/protocolbuffers/protobuf-go/blob/master/proto/decode.go)
+3. [Java protobuf parser](/~https://github.com/protocolbuffers/protobuf/tree/main/java/core/src/main/java/com/google/protobuf)
+4. [C++ protobuf parser](/~https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/wire_format_lite.cc)
+5. [Protobuf Zero](/~https://github.com/mapbox/protozero/blob/master/include/protozero/pbf_reader.hpp)