Update the fork-support doc (#28567)

doc/fork_support.md

@@ -1,46 +1,39 @@
-# Background #
+# Background
 
-In Python, multithreading is ineffective at concurrency for CPU bound tasks
-due to the GIL (global interpreter lock).  Extension modules can release
-the GIL in CPU bound tasks, but that isn't an option in pure Python.
-Users use libraries such as multiprocessing, subprocess, concurrent.futures.ProcessPoolExecutor,
-etc, to work around the GIL. These modules call ```fork()``` underneath the hood. Various issues have
-been reported when using these modules with gRPC Python.  gRPC Python wraps
-gRPC core, which uses multithreading for performance, and hence doesn't support ```fork()```.
-Historically, we didn't support forking in gRPC, but some users seemed
-to be doing fine until their code started to break on version 1.6.  This was
-likely caused by the addition of background c-threads and a background
-Python thread.
+In Python, multithreading is ineffective at concurrency for CPU bound tasks due
+to the GIL (global interpreter lock).  Extension modules can release the GIL in
+CPU bound tasks, but that isn't an option in pure Python. Users use libraries
+such as multiprocessing, subprocess, concurrent.futures.ProcessPoolExecutor,
+etc, to work around the GIL. These modules call `fork()` underneath the hood.
+Various issues have been reported when using these modules with gRPC Python.
+gRPC Python wraps gRPC core, which uses multithreading for performance, and
+hence doesn't support `fork()`. Historically, we didn't support forking in gRPC,
+but some users seemed to be doing fine until their code started to break on
+version 1.6.  This was likely caused by the addition of background c-threads and
+a background Python thread.
 
-# Current Status #
+# Current Status
 
-## 1.11 ##
-The background Python thread was removed entirely.  This allows forking
-after creating a channel.  However, the channel must not have issued any
-RPCs prior to the fork.  Attempting to fork with an active channel that
-has been used can result in deadlocks/corrupted wire data.
+gRPC Python applications can enable client-side forking when two environment
+variables are given:
 
-## 1.9 ##
-A regression was noted in cases where users are doing fork/exec. This
-was due to ```pthread_atfork()``` handler that was added in 1.7 to partially
-support forking in gRPC. A deadlock can happen when pthread_atfork
-handler is running, and an application thread is calling into gRPC.
-We have provided a workaround for this issue by allowing users to turn
-off the handler using env flag ```GRPC_ENABLE_FORK_SUPPORT=False```.
-This should be set whenever a user expects to always call exec
-immediately following fork.  It will disable the fork handlers.
+```
+export GRPC_ENABLE_FORK_SUPPORT=true
+export GRPC_POLL_STRATEGY=poll
+```
 
-## 1.7 ##
-A ```pthread_atfork()``` handler was added in 1.7 to automatically shut down
-the background c-threads when fork was called.  This does not shut down the
-background Python thread, so users could not have any open channels when
-forking.
+The fork-support effort only works with "epoll1" and "poll" polling strategy.
+There is no active development to extend fork-support's coverage.
 
-# Future Work #
+For more details about poll strategy setting, see
+https://github.com/grpc/grpc/blob/master/doc/environment_variables.md.
 
-## 1.13 ##
-The workaround when using fork/exec by setting
-```GRPC_ENABLE_FORK_SUPPORT=False``` should no longer be needed.  Following
-[this PR](https://github.com/grpc/grpc/pull/14647), fork
-handlers will not automatically run when multiple threads are calling
-into gRPC.
+# Alternative: use after fork
+
+Complexities mentioned in the background section are inevitable for "pre-fork"
+usage, where the application creates gRPC Python objects (e.g., client channel)
+before invoking `fork()`. However, if the application only instantiate gRPC
+Python objects after calling `fork()`, then `fork()` will work normally, since
+there is no C extension binding at this point. This idea is demonstrated by the
+[multiprocessing
+example](https://github.com/grpc/grpc/tree/master/examples/python/multiprocessing).