doc: add documentation for the proc-receive hook

Add documentation for the new "proc-receive" hook.

Signed-off-by: Jiang Xin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jiangxin

Documentation/githooks.txt

@@ -333,6 +333,64 @@ The default 'update' hook, when enabled--and with
 `hooks.allowunannotated` config option unset or set to false--prevents
 unannotated tags to be pushed.
 
+proc-receive
+~~~~~~~~~~~~
+
+This hook is invoked by linkgit:git-receive-pack[1].  If the server has
+set the multi-valued config variable `receive.procReceiveRefs`, and the
+commands sent to 'receive-pack' have matching reference names, these
+commands will be executed by this hook, instead of by the internal
+`execute_commands()` function.  This hook is responsible for updating
+the relevant references and reporting the results back to 'receive-pack'.
+
+This hook executes once for the receive operation.  It takes no
+arguments, but uses a pkt-line format protocol to communicate with
+'receive-pack' to read commands, push-options and send results.  In the
+following example for the protocol, the letter 'S' stands for
+'receive-pack' and the letter 'H' stands for this hook.
+
+    # Version and features negotiation.
+    S: PKT-LINE(version=1\0push-options atomic...)
+    S: flush-pkt
+    H: PKT-LINE(version=1\0push-options...)
+    H: flush-pkt
+
+    # Send commands from server to the hook.
+    S: PKT-LINE(<old-oid> <new-oid> <ref>)
+    S: ... ...
+    S: flush-pkt
+    # Send push-options only if the 'push-options' feature is enabled.
+    S: PKT-LINE(push-option)
+    S: ... ...
+    S: flush-pkt
+
+    # Receive result from the hook.
+    # OK, run this command successfully.
+    H: PKT-LINE(<old-oid> <new-oid> <ref> ok)
+    # NO, I reject it.
+    H: PKT-LINE(<old-oid> <new-oid> <ref> ng <reason>)
+    # Fall through, let 'receive-pack' to execute it.
+    H: PKT-LINE(<old-oid> <new-oid> <ref> ft)
+    # OK, but has an alternate reference.  The alternate reference name
+    # and other status are given in key=value pairs after the null
+    # character.
+    H: PKT-LINE(<old-oid> <new-oid> <ref> ok\0ref=refs/pull/123/head
+		forced-update)
+    H: ... ...
+    H: flush-pkt
+
+Each command for the 'proc-receive' hook may point to a pseudo-reference
+and always has a zero-old as its old-oid, while the 'proc-receive' hook
+may update an alternate reference and the alternate reference may exist
+already with a non-zero old-oid.  For this case, this hook may return
+different OID and different reference name as extended status of the
+report line.
+
+The report of the commands of this hook should have the same order as
+the input.  The exit status of the 'proc-receive' hook only determines
+the success or failure of the group of commands sent to it, unless
+atomic push is in use.
+
 [[post-receive]]
 post-receive
 ~~~~~~~~~~~~