mpm_event,mod_http2: Keep compatibility with CONN_STATE_PROCESSING + OK

Before r1918022, returning OK with CONN_STATE_PROCESSING to mpm_event was
handled like/by CONN_STATE_LINGER "to not break old or third-party modules
which might return OK w/o touching the state and expect lingering close,
like with worker or prefork MPMs".

So we need a new return code to be allowed to apply the new POLLIN/POLLOUT
behaviour for CONN_STATE_PROCESSING, thus revive AGAIN as introduced by
Graham some times ago for a nonblocking WIP (moved to a branch/PR since then).

MPM event will advertise its ability to handle CONN_STATE_PROCESSING + AGAIN
with AP_MPMQ_CAN_AGAIN, and mod_http2 can use that to know how to return to
the MPM as expected. When !AP_MPMQ_CAN_AGAIN modules/mod_http2 can still use
CONN_STATE_WRITE_COMPLETION + CONN_SENSE_WANT_READ + c->clogging_input_filters
which will work in mpm_even-2.4.x still.

* include/ap_mmn.h:
  Bump MMN minor for AP_MPMQ_CAN_AGAIN and AGAIN.

* include/ap_mpm.h:
  Define AP_MPMQ_CAN_AGAIN.

* include/httpd.h:
  Define AGAIN.

* modules/http2/h2.h:
  No need for H2_USE_STATE_PROCESSING anymore with AP_MPMQ_CAN_AGAIN.

* modules/http2/h2_c1.c:
  For !keepalive case return to the MPM using CONN_STATE_PROCESSING + AGAIN
  or CONN_STATE_WRITE_COMPLETION + c->clogging_input_filters depending on
  AP_MPMQ_CAN_AGAIN only.

* modules/http2/h2_session.c:
  Can return to the MPM for h2_send_flow_blocked() provided it's async only.

* server/mpm/event/event.c:
  Rework process_socket()'s CONN_STATE_PROCESSING to handle AGAIN and preserve
  compatibility. Have a lingering_close label to goto there faster when
  process_lingering_close() is to be called. Improve relevant comments.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1918257 13f79535-47bb-0310-9956-ffa450edef68

3 files changed, 19 insertions, 10 deletions

diff --git a/include/ap_mmn.h b/include/ap_mmn.h
index 34abf61654..fcf98e8908 100644
--- a/include/ap_mmn.h
+++ b/include/ap_mmn.h
@@ -725,6 +725,7 @@
  * 20211221.19 (2.5.1-dev) Add AP_REG_NOTEMPTY_ATSTART
  * 20211221.20 (2.5.1-dev) Add CONN_STATE_KEEPALIVE and CONN_STATE_PROCESSING
  * 20211221.21 (2.5.1-dev) Add processing field struct process_score
+ * 20211221.22 (2.5.1-dev) Add AGAIN, AP_MPMQ_CAN_AGAIN.
  */
 
 #define MODULE_MAGIC_COOKIE 0x41503235UL /* "AP25" */
@@ -732,7 +733,7 @@
 #ifndef MODULE_MAGIC_NUMBER_MAJOR
 #define MODULE_MAGIC_NUMBER_MAJOR 20211221
 #endif
-#define MODULE_MAGIC_NUMBER_MINOR 21             /* 0...n */
+#define MODULE_MAGIC_NUMBER_MINOR 22             /* 0...n */
 
 /**
  * Determine if the server's current MODULE_MAGIC_NUMBER is at least a
diff --git a/include/ap_mpm.h b/include/ap_mpm.h
index 6698d0e7c6..82075a4488 100644
--- a/include/ap_mpm.h
+++ b/include/ap_mpm.h
@@ -182,6 +182,8 @@ AP_DECLARE(apr_status_t) ap_os_create_privileged_process(
 #define AP_MPMQ_CAN_SUSPEND          17
 /** MPM supports additional pollfds */
 #define AP_MPMQ_CAN_POLL             18
+/** MPM reacts to AGAIN response */
+#define AP_MPMQ_CAN_AGAIN            19
 /** @} */
 
 /**
diff --git a/include/httpd.h b/include/httpd.h
index f2e3795f5a..588e092c48 100644
--- a/include/httpd.h
+++ b/include/httpd.h
@@ -458,13 +458,16 @@ AP_DECLARE(const char *) ap_get_server_built(void);
 
 /* non-HTTP status codes returned by hooks */
 
-#define OK 0                    /**< Module has handled this stage. */
-#define DECLINED -1             /**< Module declines to handle */
-#define DONE -2                 /**< Module has served the response completely
-                                 *  - it's safe to die() with no more output
-                                 */
-#define SUSPENDED -3 /**< Module will handle the remainder of the request.
-                      * The core will never invoke the request again, */
+#define OK           0  /**< Module has handled this stage. */
+#define DECLINED    -1  /**< Module declines to handle */
+#define DONE        -2  /**< Module has served the response completely
+                         *   - it's safe to die() with no more output
+                         */
+#define SUSPENDED   -3  /**< Module will handle the remainder of the request.
+                         *   The core will never invoke the request again */
+#define AGAIN       -4  /**< Module wants to be called again when more
+                         *   data is available.
+                         */
 
 /** Returned by the bottom-most filter if no data was written.
  *  @see ap_pass_brigade(). */
@@ -1319,8 +1322,11 @@ struct conn_slave_rec {
  */
 typedef enum  {
     CONN_STATE_KEEPALIVE,           /* Kept alive in the MPM (using KeepAliveTimeout) */
-    CONN_STATE_PROCESSING,          /* Handled by process_connection() hooks, may be returned
-                                       to the MPM for POLLIN/POLLOUT (using Timeout) */
+    CONN_STATE_PROCESSING,          /* Processed by process_connection() hooks, returning
+                                     * AGAIN to the MPM in this state will make it wait for
+                                     * the connection to be readable or writable according to
+                                     * CONN_SENSE_WANT_READ or CONN_SENSE_WANT_WRITE respectively,
+                                     * where Timeout applies */
     CONN_STATE_HANDLER,             /* Processed by the modules handlers */
     CONN_STATE_WRITE_COMPLETION,    /* Flushed by the MPM before entering CONN_STATE_KEEPALIVE */
     CONN_STATE_SUSPENDED,           /* Suspended in the MPM until ap_run_resume_suspended() */