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/fuzzed_stream.h

@@ -0,0 +1,84 @@
+#pragma once
+
+#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; };
+
+  const std::string &response() const { return response_; }
+
+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:
+  // A server that is not listening counts as shutting down, and then writes
+  // no file. The socket is never used.
+  void MarkAsRunning() { svr_sock_ = 0; }
+
+  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);
+  }
+};