annotate osx/include/kj/exception.h @ 134:41e769c91eca

Add Capnp and KJ builds for OSX

// Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
// Licensed under the MIT License:
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

#ifndef KJ_EXCEPTION_H_
#define KJ_EXCEPTION_H_

#if defined(__GNUC__) && !KJ_HEADER_WARNINGS
#pragma GCC system_header
#endif

#include "memory.h"
#include "array.h"
#include "string.h"

namespace kj {

class ExceptionImpl;

class Exception {
  // Exception thrown in case of fatal errors.
  //
  // Actually, a subclass of this which also implements std::exception will be thrown, but we hide
  // that fact from the interface to avoid #including <exception>.

public:
  enum class Type {
    // What kind of failure?

    FAILED = 0,
    // Something went wrong. This is the usual error type. KJ_ASSERT and KJ_REQUIRE throw this
    // error type.

    OVERLOADED = 1,
    // The call failed because of a temporary lack of resources. This could be space resources
    // (out of memory, out of disk space) or time resources (request queue overflow, operation
    // timed out).
    //
    // The operation might work if tried again, but it should NOT be repeated immediately as this
    // may simply exacerbate the problem.

    DISCONNECTED = 2,
    // The call required communication over a connection that has been lost. The callee will need
    // to re-establish connections and try again.

    UNIMPLEMENTED = 3
    // The requested method is not implemented. The caller may wish to revert to a fallback
    // approach based on other methods.

    // IF YOU ADD A NEW VALUE:
    // - Update the stringifier.
    // - Update Cap'n Proto's RPC protocol's Exception.Type enum.
  };

  Exception(Type type, const char* file, int line, String description = nullptr) noexcept;
  Exception(Type type, String file, int line, String description = nullptr) noexcept;
  Exception(const Exception& other) noexcept;
  Exception(Exception&& other) = default;
  ~Exception() noexcept;

  const char* getFile() const { return file; }
  int getLine() const { return line; }
  Type getType() const { return type; }
  StringPtr getDescription() const { return description; }
  ArrayPtr<void* const> getStackTrace() const { return arrayPtr(trace, traceCount); }

  struct Context {
    // Describes a bit about what was going on when the exception was thrown.

    const char* file;
    int line;
    String description;
    Maybe<Own<Context>> next;

    Context(const char* file, int line, String&& description, Maybe<Own<Context>>&& next)
        : file(file), line(line), description(mv(description)), next(mv(next)) {}
    Context(const Context& other) noexcept;
  };

  inline Maybe<const Context&> getContext() const {
    KJ_IF_MAYBE(c, context) {
      return **c;
    } else {
      return nullptr;
    }
  }

  void wrapContext(const char* file, int line, String&& description);
  // Wraps the context in a new node.  This becomes the head node returned by getContext() -- it
  // is expected that contexts will be added in reverse order as the exception passes up the
  // callback stack.

  KJ_NOINLINE void extendTrace(uint ignoreCount);
  // Append the current stack trace to the exception's trace, ignoring the first `ignoreCount`
  // frames (see `getStackTrace()` for discussion of `ignoreCount`).

  KJ_NOINLINE void truncateCommonTrace();
  // Remove the part of the stack trace which the exception shares with the caller of this method.
  // This is used by the async library to remove the async infrastructure from the stack trace
  // before replacing it with the async trace.

  void addTrace(void* ptr);
  // Append the given pointer to the backtrace, if it is not already full. This is used by the
  // async library to trace through the promise chain that led to the exception.

private:
  String ownFile;
  const char* file;
  int line;
  Type type;
  String description;
  Maybe<Own<Context>> context;
  void* trace[32];
  uint traceCount;

  friend class ExceptionImpl;
};

StringPtr KJ_STRINGIFY(Exception::Type type);
String KJ_STRINGIFY(const Exception& e);

// =======================================================================================

enum class LogSeverity {
  INFO,      // Information describing what the code is up to, which users may request to see
             // with a flag like `--verbose`.  Does not indicate a problem.  Not printed by
             // default; you must call setLogLevel(INFO) to enable.
  WARNING,   // A problem was detected but execution can continue with correct output.
  ERROR,     // Something is wrong, but execution can continue with garbage output.
  FATAL,     // Something went wrong, and execution cannot continue.
  DBG        // Temporary debug logging.  See KJ_DBG.

  // Make sure to update the stringifier if you add a new severity level.
};

StringPtr KJ_STRINGIFY(LogSeverity severity);

class ExceptionCallback {
  // If you don't like C++ exceptions, you may implement and register an ExceptionCallback in order
  // to perform your own exception handling.  For example, a reasonable thing to do is to have
  // onRecoverableException() set a flag indicating that an error occurred, and then check for that
  // flag just before writing to storage and/or returning results to the user.  If the flag is set,
  // discard whatever you have and return an error instead.
  //
  // ExceptionCallbacks must always be allocated on the stack.  When an exception is thrown, the
  // newest ExceptionCallback on the calling thread's stack is called.  The default implementation
  // of each method calls the next-oldest ExceptionCallback for that thread.  Thus the callbacks
  // behave a lot like try/catch blocks, except that they are called before any stack unwinding
  // occurs.

public:
  ExceptionCallback();
  KJ_DISALLOW_COPY(ExceptionCallback);
  virtual ~ExceptionCallback() noexcept(false);

  virtual void onRecoverableException(Exception&& exception);
  // Called when an exception has been raised, but the calling code has the ability to continue by
  // producing garbage output.  This method _should_ throw the exception, but is allowed to simply
  // return if garbage output is acceptable.
  //
  // The global default implementation throws an exception unless the library was compiled with
  // -fno-exceptions, in which case it logs an error and returns.

  virtual void onFatalException(Exception&& exception);
  // Called when an exception has been raised and the calling code cannot continue.  If this method
  // returns normally, abort() will be called.  The method must throw the exception to avoid
  // aborting.
  //
  // The global default implementation throws an exception unless the library was compiled with
  // -fno-exceptions, in which case it logs an error and returns.

  virtual void logMessage(LogSeverity severity, const char* file, int line, int contextDepth,
                          String&& text);
  // Called when something wants to log some debug text.  `contextDepth` indicates how many levels
  // of context the message passed through; it may make sense to indent the message accordingly.
  //
  // The global default implementation writes the text to stderr.

protected:
  ExceptionCallback& next;

private:
  ExceptionCallback(ExceptionCallback& next);

  class RootExceptionCallback;
  friend ExceptionCallback& getExceptionCallback();
};

ExceptionCallback& getExceptionCallback();
// Returns the current exception callback.

KJ_NOINLINE KJ_NORETURN(void throwFatalException(kj::Exception&& exception, uint ignoreCount = 0));
// Invoke the exception callback to throw the given fatal exception.  If the exception callback
// returns, abort.

KJ_NOINLINE void throwRecoverableException(kj::Exception&& exception, uint ignoreCount = 0);
// Invoke the exception callback to throw the given recoverable exception.  If the exception
// callback returns, return normally.

// =======================================================================================

namespace _ { class Runnable; }

template <typename Func>
Maybe<Exception> runCatchingExceptions(Func&& func) noexcept;
// Executes the given function (usually, a lambda returning nothing) catching any exceptions that
// are thrown.  Returns the Exception if there was one, or null if the operation completed normally.
// Non-KJ exceptions will be wrapped.
//
// If exception are disabled (e.g. with -fno-exceptions), this will still detect whether any
// recoverable exceptions occurred while running the function and will return those.

class UnwindDetector {
  // Utility for detecting when a destructor is called due to unwind.  Useful for:
  // - Avoiding throwing exceptions in this case, which would terminate the program.
  // - Detecting whether to commit or roll back a transaction.
  //
  // To use this class, either inherit privately from it or declare it as a member.  The detector
  // works by comparing the exception state against that when the constructor was called, so for
  // an object that was actually constructed during exception unwind, it will behave as if no
  // unwind is taking place.  This is usually the desired behavior.

public:
  UnwindDetector();

  bool isUnwinding() const;
  // Returns true if the current thread is in a stack unwind that it wasn't in at the time the
  // object was constructed.

  template <typename Func>
  void catchExceptionsIfUnwinding(Func&& func) const;
  // Runs the given function (e.g., a lambda).  If isUnwinding() is true, any exceptions are
  // caught and treated as secondary faults, meaning they are considered to be side-effects of the
  // exception that is unwinding the stack.  Otherwise, exceptions are passed through normally.

private:
  uint uncaughtCount;

  void catchExceptionsAsSecondaryFaults(_::Runnable& runnable) const;
};

namespace _ {  // private

class Runnable {
public:
  virtual void run() = 0;
};

template <typename Func>
class RunnableImpl: public Runnable {
public:
  RunnableImpl(Func&& func): func(kj::mv(func)) {}
  void run() override {
    func();
  }
private:
  Func func;
};

Maybe<Exception> runCatchingExceptions(Runnable& runnable) noexcept;

}  // namespace _ (private)

template <typename Func>
Maybe<Exception> runCatchingExceptions(Func&& func) noexcept {
  _::RunnableImpl<Decay<Func>> runnable(kj::fwd<Func>(func));
  return _::runCatchingExceptions(runnable);
}

template <typename Func>
void UnwindDetector::catchExceptionsIfUnwinding(Func&& func) const {
  if (isUnwinding()) {
    _::RunnableImpl<Decay<Func>> runnable(kj::fwd<Func>(func));
    catchExceptionsAsSecondaryFaults(runnable);
  } else {
    func();
  }
}

#define KJ_ON_SCOPE_SUCCESS(code) \
  ::kj::UnwindDetector KJ_UNIQUE_NAME(_kjUnwindDetector); \
  KJ_DEFER(if (!KJ_UNIQUE_NAME(_kjUnwindDetector).isUnwinding()) { code; })
// Runs `code` if the current scope is exited normally (not due to an exception).

#define KJ_ON_SCOPE_FAILURE(code) \
  ::kj::UnwindDetector KJ_UNIQUE_NAME(_kjUnwindDetector); \
  KJ_DEFER(if (KJ_UNIQUE_NAME(_kjUnwindDetector).isUnwinding()) { code; })
// Runs `code` if the current scope is exited due to an exception.

// =======================================================================================

KJ_NOINLINE ArrayPtr<void* const> getStackTrace(ArrayPtr<void*> space, uint ignoreCount);
// Attempt to get the current stack trace, returning a list of pointers to instructions. The
// returned array is a slice of `space`. Provide a larger `space` to get a deeper stack trace.
// If the platform doesn't support stack traces, returns an empty array.
//
// `ignoreCount` items will be truncated from the front of the trace. This is useful for chopping
// off a prefix of the trace that is uninteresting to the developer because it's just locations
// inside the debug infrastructure that is requesting the trace. Be careful to mark functions as
// KJ_NOINLINE if you intend to count them in `ignoreCount`. Note that, unfortunately, the
// ignored entries will still waste space in the `space` array (and the returned array's `begin()`
// is never exactly equal to `space.begin()` due to this effect, even if `ignoreCount` is zero
// since `getStackTrace()` needs to ignore its own internal frames).

String stringifyStackTrace(ArrayPtr<void* const>);
// Convert the stack trace to a string with file names and line numbers. This may involve executing
// suprocesses.

void printStackTraceOnCrash();
// Registers signal handlers on common "crash" signals like SIGSEGV that will (attempt to) print
// a stack trace. You should call this as early as possible on program startup. Programs using
// KJ_MAIN get this automatically.

kj::StringPtr trimSourceFilename(kj::StringPtr filename);
// Given a source code file name, trim off noisy prefixes like "src/" or
// "/ekam-provider/canonical/".

}  // namespace kj

#endif  // KJ_EXCEPTION_H_