Add server_mount_fuzzer for static file serving (#2666)

Fuzz Server::handle_file_request through a mount point backed by a temporary directory that holds symlinks escaping the mount root, one of them into a sibling directory sharing its name prefix. Abort if a token stored outside the root appears in a response. The seeds are based on those of #2661.

Move FuzzedStream into fuzzed_stream.h so that both server fuzzers share it, and add FuzzableServer::MarkAsRunning(), without which a server that is not listening writes no file.

test/fuzzing/server_fuzzer.cc

@@ -1,79 +1,4 @@
-#include <cstdint>
-
-#include <httplib.h>
-
-class FuzzedStream : public httplib::Stream {
-public:
-  FuzzedStream(const uint8_t *data, size_t size)
-      : data_(data), size_(size), read_pos_(0) {}
-
-  ssize_t read(char *ptr, size_t size) override {
-    if (size + read_pos_ > size_) { size = size_ - read_pos_; }
-    memcpy(ptr, data_ + read_pos_, size);
-    read_pos_ += size;
-    // Like SocketStream, pull in more than was asked for, which the line
-    // reader then takes through buffered_data(), so both of its paths run.
-    buffered_end_ =
-        (std::max)(buffered_end_, (std::min)(size_, read_pos_ + 4096));
-    return static_cast<ssize_t>(size);
-  }
-
-  const char *buffered_data(size_t &size) const override {
-    size = buffered_end_ > read_pos_ ? buffered_end_ - read_pos_ : 0;
-    return size ? reinterpret_cast<const char *>(data_ + read_pos_) : nullptr;
-  }
-
-  void consume_buffered(size_t size) override { read_pos_ += size; }
-
-  ssize_t write(const char *ptr, size_t size) override {
-    response_.append(ptr, size);
-    return static_cast<int>(size);
-  }
-
-  ssize_t write(const char *ptr) { return write(ptr, strlen(ptr)); }
-
-  ssize_t write(const std::string &s) { return write(s.data(), s.size()); }
-
-  bool is_readable() const override { return true; }
-
-  bool wait_readable() const override { return true; }
-
-  bool wait_writable() const override { return true; }
-
-  void get_remote_ip_and_port(std::string &ip, int &port) const override {
-    ip = "127.0.0.1";
-    port = 8080;
-  }
-
-  void get_local_ip_and_port(std::string &ip, int &port) const override {
-    ip = "127.0.0.1";
-    port = 8080;
-  }
-
-  socket_t socket() const override { return 0; }
-
-  time_t duration() const override { return 0; };
-
-private:
-  const uint8_t *data_;
-  size_t size_;
-  size_t read_pos_;
-  size_t buffered_end_ = 0;
-  std::string response_;
-};
-
-class FuzzableServer : public httplib::Server {
-public:
-  void ProcessFuzzedRequest(FuzzedStream &stream) {
-    bool connection_close = false;
-    process_request(stream,
-                    /*remote_addr=*/"",
-                    /*remote_port =*/0,
-                    /*local_addr=*/"",
-                    /*local_port =*/0,
-                    /*last_connection=*/false, connection_close, nullptr);
-  }
-};
+#include "fuzzed_stream.h"
 
 static FuzzableServer g_server;