source: trunk/doc/Fork.os2@ 1279

$Id: Fork.os2 1279 2004-02-25 03:26:45Z bird $

Fork Design Draft
--------------------

1.0 Intro
----------

blah.


1.1 The SuS fork() Description
------------------------------

NAME

    fork - create a new process

SYNOPSIS

    #include <unistd.h>

    pid_t fork(void);

DESCRIPTION

    The fork() function shall create a new process. The new process (child process) shall be an exact copy of the calling process (parent process) except as detailed below:

        * The child process shall have a unique process ID.
        * The child process ID also shall not match any active process
                  group ID.
        * The child process shall have a different parent process ID,
                   which shall be the process ID of the calling process.
        * The child process shall have its own copy of the parent's file
                  descriptors. Each of the child's file descriptors shall refer
                  to the same open file description with the corresponding file
                  descriptor of the parent.
        * The child process shall have its own copy of the parent's open
                  directory streams. Each open directory stream in the child process
                  may share directory stream positioning with the corresponding
                  directory stream of the parent.
        * [XSI] The child process shall have its own copy of the parent's
                  message catalog descriptors.
        * The child process' values of tms_utime, tms_stime, tms_cutime, and
                  tms_cstime shall be set to 0.
        * The time left until an alarm clock signal shall be reset to zero,
                  and the alarm, if any, shall be canceled; see alarm() .
        * [XSI] All semadj values shall be cleared.
        * File locks set by the parent process shall not be inherited by
                  the child process.
        * The set of signals pending for the child process shall be
                  initialized to the empty set.
        * [XSI] Interval timers shall be reset in the child process.
        * [SEM] Any semaphores that are open in the parent process shall
                  also be open in the child process.
        * [ML] The child process shall not inherit any address space memory
                  locks established by the parent process via calls to mlockall()
                  or mlock().
        * [MF|SHM] Memory mappings created in the parent shall be retained
                  in the child process. MAP_PRIVATE mappings inherited from the
                  parent shall also be MAP_PRIVATE mappings in the child, and any
                  modifications to the data in these mappings made by the parent
                  prior to calling fork() shall be visible to the child. Any
                  modifications to the data in MAP_PRIVATE mappings made by the
                  parent after fork() returns shall be visible only to the parent.
                  Modifications to the data in MAP_PRIVATE mappings made by the
                  child shall be visible only to the child.
        * [PS] For the SCHED_FIFO and SCHED_RR scheduling policies, the
                  child process shall inherit the policy and priority settings
                  of the parent process during a fork() function. For other s
                  cheduling policies, the policy and priority settings on fork()
                  are implementation-defined.
        * [TMR] Per-process timers created by the parent shall not be
                  inherited by the child process.
        * [MSG] The child process shall have its own copy of the message
                  queue descriptors of the parent. Each of the message descriptors
                  of the child shall refer to the same open message queue
                  description as the corresponding message descriptor of the parent.
        * [AIO] No asynchronous input or asynchronous output operations
                  shall be inherited by the child process.
        * A process shall be created with a single thread. If a
                  multi-threaded process calls fork(), the new process shall contain
                  a replica of the calling thread and its entire address space,
                  possibly including the states of mutexes and other resources.
                  Consequently, to avoid errors, the child process may only execute
                  async-signal-safe operations until such time as one of the exec
                  functions is called. [THR]  Fork handlers may be established by
                  means of the pthread_atfork() function in order to maintain
                  application invariants across fork() calls.

                  When the application calls fork() from a signal handler and any of
                  the fork handlers registered by pthread_atfork() calls a function
                  that is not asynch-signal-safe, the behavior is undefined.
        * [TRC TRI] If the Trace option and the Trace Inherit option are
                  both supported:
                  If the calling process was being traced in a trace stream that
                  had its inheritance policy set to POSIX_TRACE_INHERITED, the
                  child process shall be traced into that trace stream, and the
                  child process shall inherit the parent's mapping of trace event
                  names to trace event type identifiers. If the trace stream in
                  which the calling process was being traced had its inheritance
                  policy set to POSIX_TRACE_CLOSE_FOR_CHILD, the child process
                  shall not be traced into that trace stream. The inheritance
                  policy is set by a call to the posix_trace_attr_setinherited()
                  function.
        * [TRC] If the Trace option is supported, but the Trace Inherit
                  option is not supported:
          The child process shall not be traced into any of the trace
                  streams of its parent process.
        * [TRC] If the Trace option is supported, the child process of
                  a trace controller process shall not control the trace streams
                  controlled by its parent process.
        * [CPT] The initial value of the CPU-time clock of the child
                  process shall be set to zero.
        * [TCT] The initial value of the CPU-time clock of the single
                  thread of the child process shall be set to zero.

    All other process characteristics defined by IEEE Std 1003.1-2001 shall
        be the same in the parent and child processes. The inheritance of
        process characteristics not defined by IEEE Std 1003.1-2001 is
        unspecified by IEEE Std 1003.1-2001.

    After fork(), both the parent and the child processes shall be capable
        of executing independently before either one terminates.

RETURN VALUE

    Upon successful completion, fork() shall return 0 to the child process
        and shall return the process ID of the child process to the parent
        process. Both processes shall continue to execute from the fork()
        function. Otherwise, -1 shall be returned to the parent process, no
        child process shall be created, and errno shall be set to indicate
        the error.

ERRORS

    The fork() function shall fail if:

    [EAGAIN]
        The system lacked the necessary resources to create another
                process, or the system-imposed limit on the total number of
                processes under execution system-wide or by a single user
                {CHILD_MAX} would be exceeded.

    The fork() function may fail if:

    [ENOMEM]
        Insufficient storage space is available.


                                                                                                        
                                                                                                        
2.0 Requirements and Assumptions Of The Implementation
------------------------------------------------------

The Innotek LIBC fork() implementation will require the following features
in LIBC to work:
        1. A shared process management internal to LIBC for communication to the
           child that a fork() is in progress.
        2. A very generalized and varied set of fork helper functions to archive
           maximum flexibility of the implementation.
        3. Extended versions of some memory related OS/2 APIs must be implemented.
        
The implemenetation will further make the following assumption about the
operation of OS/2:
        1. DosExecPgm will not return till all DLLs are initated successfully.
        

        
3.0     The Shared Process Management
---------------------------------

The fork() implementation requires a method of telling the child process
that it's being forked and must take a very different startup route. For
some other LIBC apis there is need for parent -> child and child -> parent
information exchange. More specifically, the inheritance of sockets,
signals, the different scheduler actions of a posix_spawn[p]() call, and
possibly some process group stuff related to posix_spawn too if we get it
figured out eventually. All this was parent -> child during spawn/fork. A
need exist also for child -> parent notification and possibly exchange for
process termination. It might be necessary to reimplement the different
wait apis and implement SIGCHLD, it's likely that those tasks will make
such demands.

The choice is now whether or not to make this shared process management
specific to each LIBC version or try to make it survive normal LIBC updates.
Making is specific have advantages in code size and memory footprint (no
reserved field), however it have certain disadvantages when LIBC is updated.
The other option is to use a named shared memory object, defining the
content with reserved space for later extensions so several versions of
LIBC with more or less features implemented can co use the memory space.

The latter option is prefered since it allows more applications to
interoperate, it causes less shared memory waste, the shared memory
can be located in high memory and it would be possible to fork
processes using multiple versions of LIBC.

The shared memory must be named \SHAREMEM\INNOTEKLIBC.V01, the version
number being the one of the shared memory layout and contents, it will
only be increased when incompatible changes are made.

The shared memory will be protected by an standard OS/2 mutex semaphore.
It will not use any fast R3 semaphore since the the usage frequency is low
and the result of a messup may be disastrous. Care must be take for
avoiding creation races and owner died scenarios.

The memory will have a fixed size, since adding segments is very hard.
Thus the size must be large enough to cope with a great deal of
processes, but bearing in mind that OS/2 normally doesn't support more
than a 1000 processes, with a theoritical max of some 4000 (being the
max thread count). A very simplistic allocation scheme will be
implemented. Practically speaking a fixed block size pool would do fine
for the process structure, while for the misc structures like socket
lists a linked list based heap would do fine.

The process blocks will be rounded up to in size adding a reasonable
amount of space resevered for future extensions. Reserved space must be
all zeroed.

The fork() specific members of the process block will be a pointer to
the shared memory object for the fork operation (the fork handle) and
list of forkable modules. The fork handle will it self contain
information indicating whether or not another LIBC version have already
started fork() handling in the child. The presense of the fork handle
means that the child is being forked and normal dll init and startup
will not be executed, but a registered callback will be called to do
the forking of each module. (more details in section 4.0)

The parent will before spawn, fork and exec (essentially before DosExecPgm
or DosStartSession) create a process block for the child to be born and
link it into an embryo list in the shared memory block. The child will
find the process block by looking searching an embryo list using the
parent pid as key. All DosExecPgm and DosStartSession calls are
serialized within one LIBC version. (If some empty headed programmer
manages to link together a program which may end up using two or more
LIBC versions and having two or more thread doing DosExecPgm at the
very same time, well then he really deserves what ever trouble he gets!
At least don't blame me!)

Process blocks will have to stay around after the process terminated
(for child -> parent term exchange), a cleanup mechanism will be invoked
whenever a free memory threshold is reached. All processes will register
exit list handlers to mark the process block as zombie (and later
perhaps setting error codes and notifying waiters/child-listeners).

                                        

4.0 The fork() Implementation
-----------------------------


The implementation will be based on a fork handle and a set of primitives.
The fork handle is a pointer to an shared memory object allocated for the
occation and which will be freed before fork() returns. The primitives
all operates on this handle and will be provided using a callback table
in order to fully support multiple LIBC versions.


4.1 Forkable Executable and DLLs
--------------------------------

The support for fork() is an optional feature of LIBC. The default
executable produced with LIBC and GCC will not be forkable. The fork
support will be based on registration of the DLLs and EXEs in their
LIBC supplied startup code (crt0/dll0). A set of fork versions of these
modules will be made.

The big differnece between the ordinary crt0/dll0 and the forkable
crt0/dll0 is a per module structure, a call to register this, and the
handling of the return code of that call.

The structure will contain these fields:
        - chain pointer.
        - data segment base address.
        - data segment end address.
        - fork callback function.
        
The fork callback function is called _atfork_callback, it takes the fork
handle, module structure, and an operation enum as arguments. LIBC will
contain a default implementation of _atfork_callback() which simply
duplicates the data segment.

The register call, __libc_ForkRegisterModule(), will return:
        - 0 if normal process startup. no forking.
        - 1 if fork() is in progress. The crt0/dll0 code will then
          not call any standard initiation code, but let the
          _atfork_callback() do all necessary stuff.


4.2 Fork Primitives
-------------------

These primitives are provided by the fork implementation in the fork
handle structure. We will define a set of these primitives now, if
later new ones are added the users of these must check that they are
actually present.

Example:
        rc = pForkHandle->pOps->pfnDuplicatePages(pModule->pvDataBase, pModule->pvDataEnd, __LIBC_FORK_ONLY_DIRTY);
        if (rc)
                return rc; /* failure */
        
Prototypes:
        /**
         * Duplicating a number of pages from pvStart to pvEnd.
         * @returns     0 on success.
         * @returns appropriate non-zero error code on failure.
         * @param   pForkHandle Handle of the current fork operation.
         * @param       pvStart         Pointer to start of the pages. Rounded down.
         * @param       pvEnd           Pointer to end of the pages. Rounded up.
         * @param       fFlags          __LIBC_FORK_ONLY_DIRTY means checking whether the
         *                                              pages are actually dirty before bothering touching
         *                                              and copying them. (Using the partically broken
         *                                              DosQueryMemState() API.)
         *                                              __LIBC_FORK_ALL means not to bother checking, but
         *                                              just go ahead copying all the pages.
         */
        int pfnDuplicatePages(__LIBC_FORKHANDLE *pForkHandle, void *pvStart, void *pvEnd, unsigned fFlags);
        
        /**
         * Invoke a function in the child process giving it an chunk of input.
         * The function is invoked the next time the fork buffer is flushed,
         * call pfnFlush() if the return code is desired.
         *
         * @returns     0 on success.
         * @returns appropriate non-zero error code on failure.
         * @param   pForkHandle Handle of the current fork operation.
         * @param       pfn                     Pointer to the function to invoke in the child.
         *                                              The function gets the fork handle, pointer to
         *                                              the argument memory chunk and the size of that.
         *                                              The function must return 0 on success, and non-zero
         *                                              on failure.
         * @param       pvArg           Pointer to a block of memory of size cbArg containing
         *                                              input to be copied to the child and given to pfn upon
         *                                              invocation.
         */
        int pfnInvoke(int *(pfn)(__LIBC_FORKHANDLE *pForkHandle, void *pvArg, size_t cbArg), void *pvArg, size_t cbArg);
        
        /**
         * Flush the fork() buffer. Meaning taking what ever is in the fork buffer
         * and let the child process it.
         * This might be desired to get the result of a pfnInvoke() in a near
         * synchornous way.
         * @returns     0 on success.
         * @returns appropriate non-zero error code on failure.
         * @param   pForkHandle Handle of the current fork operation.
         */
        int pfnFlush(__LIBC_FORKHANDLE *pForkHandle);
        ...