fuzzers: import download_refs fuzzer from oss-fuzz

This is a direct copy of the code from google/oss-fuzz, written by
Nelson Elhage (@nelhage). Note that due to the ".cc" ending, the file
will not yet be picked up by the build system. This is intended, as
currently that file is partly written in C++, requiring a conversion to
C.

fuzzers/download_refs_fuzzer.cc

+#include <git2.h>
+#include <git2/sys/transport.h>
+
+#include <cstring>
+#include <cstdlib>
+#include <sys/stat.h>
+
+#include <string>
+
+struct fuzz_buffer {
+    const uint8_t *data;
+    size_t size;
+};
+
+class fuzzer_stream {
+public:
+    git_smart_subtransport_stream base;
+    fuzzer_stream(fuzz_buffer data) : readp(data.data), endp(data.data + data.size) {
+        base.read = fuzzer_stream::read;
+        base.write = fuzzer_stream::write;
+        base.free = fuzzer_stream::free;
+    }
+
+    int do_read(char *buffer, size_t buf_size, size_t *bytes_read) {
+        size_t avail = endp - readp;
+        *bytes_read = std::min(buf_size, avail);
+        memcpy(buffer, readp, *bytes_read);
+        readp += *bytes_read;
+        return 0;
+    }
+
+    static int read(git_smart_subtransport_stream *stream,
+                    char *buffer,
+                    size_t buf_size,
+                    size_t *bytes_read) {
+        fuzzer_stream *fs = reinterpret_cast<fuzzer_stream*>(stream);
+        return fs->do_read(buffer, buf_size, bytes_read);
+    }
+
+    static int write(git_smart_subtransport_stream *stream,
+              const char *buffer,
+              size_t len) {
+        return 0;
+    }
+
+    static void free(git_smart_subtransport_stream *stream) {
+        fuzzer_stream *fs = reinterpret_cast<fuzzer_stream*>(stream);
+        delete fs;
+    }
+private:
+    const uint8_t *readp;
+    const uint8_t *endp;
+};
+
+class fuzzer_subtransport {
+public:
+    git_smart_subtransport base;
+    fuzzer_subtransport(git_transport *owner, fuzz_buffer data) : owner(owner), data(data) {
+        base.action = fuzzer_subtransport::action;
+        base.close = fuzzer_subtransport::close;
+        base.free = fuzzer_subtransport::free;
+    }
+
+    int do_action(git_smart_subtransport_stream **out,
+                  git_smart_subtransport *transport,
+                  const char *url,
+                  git_smart_service_t action) {
+        fuzzer_stream *stream = new fuzzer_stream(this->data);
+        *out = &stream->base;
+        return 0;
+    }
+
+    static int action(git_smart_subtransport_stream **out,
+                      git_smart_subtransport *transport,
+                      const char *url,
+                      git_smart_service_t action) {
+        fuzzer_subtransport *ft = reinterpret_cast<fuzzer_subtransport*>(transport);
+        return ft->do_action(out, transport, url, action);
+    }
+
+    static int close(git_smart_subtransport *transport) {
+        return 0;
+    }
+
+    static void free(git_smart_subtransport *transport) {
+        fuzzer_subtransport *ft = reinterpret_cast<fuzzer_subtransport*>(transport);
+        delete ft;
+    }
+
+private:
+    git_transport *owner;
+    fuzz_buffer data;
+};
+
+int fuzzer_subtransport_cb(git_smart_subtransport **out,
+                           git_transport* owner,
+                           void* param) {
+    fuzz_buffer *buf = static_cast<fuzz_buffer*>(param);
+    fuzzer_subtransport *sub = new fuzzer_subtransport(owner, *buf);
+
+    *out = &sub->base;
+    return 0;
+}
+
+int create_fuzzer_transport(git_transport **out, git_remote *owner, void *param) {
+    git_smart_subtransport_definition fuzzer_subtransport {fuzzer_subtransport_cb, 1, param};
+    return git_transport_smart(out, owner, &fuzzer_subtransport);
+}
+
+void fuzzer_git_abort(const char *op) {
+    const git_error *err  = giterr_last();
+    fprintf(stderr, "unexpected libgit error: %s: %s\n",
+            op, err ? err->message : "<none>");
+    abort();
+}
+
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  static git_repository *repo = nullptr;
+  if (repo == nullptr) {
+      git_libgit2_init();
+      char tmp[] = "/tmp/git2.XXXXXX";
+      if (mkdtemp(tmp) != tmp) {
+          abort();
+      }
+      int err = git_repository_init(&repo, tmp, true);
+      if (err != 0) {
+          fuzzer_git_abort("git_repository_init");
+      }
+  }
+
+  int err;
+  git_remote *remote;
+  err = git_remote_create_anonymous(&remote, repo, "fuzzer://remote-url");
+  if (err != 0) {
+      fuzzer_git_abort("git_remote_create");
+  }
+
+
+  fuzz_buffer buffer = {data, size};
+  git_remote_callbacks callbacks = GIT_REMOTE_CALLBACKS_INIT;
+  callbacks.transport = create_fuzzer_transport;
+  callbacks.payload = &buffer;
+
+  err = git_remote_connect(remote, GIT_DIRECTION_FETCH, &callbacks, nullptr, nullptr);
+  if (err != 0) {
+      goto out;
+  }
+
+  git_remote_download(remote, nullptr, nullptr);
+
+ out:
+  git_remote_free(remote);
+
+  return 0;
+}