Pass extra data to bgworkers, and use this to fix parallel contexts.

Up until now, the total amount of data that could be passed to a
background worker at startup was one datum, which can be a small as
4 bytes on some systems.  That's enough to pass a dsm_handle or an
array index, but not much else.  Add a bgw_extra flag to the
BackgroundWorker struct, allowing up to 128 bytes to be passed to
a new worker on any platform.

Use this to fix a problem I recently discovered with the parallel
context machinery added in 9.5: the master assigns each worker an
array index, and each worker subsequently assigns itself an array
index, and there's nothing to guarantee that the two sets of indexes
match, leading to chaos.

Normally, I would not back-patch the change to add bgw_extra, since it
is basically a feature addition.  However, since 9.5 is still in beta
and there seems to be no other sensible way to repair the broken
parallel context machinery, back-patch to 9.5.  Existing background
worker code can ignore the bgw_extra field without a problem, but
might need to be recompiled since the structure size has changed.

Report and patch by me.  Review by Amit Kapila.

diff --git a/doc/src/sgml/bgworker.sgml b/doc/src/sgml/bgworker.sgml
index ef28f7251141603a9158985cb304c8b206c584e6..33ff5269516ad498be8a409a87fe25c1c51843e3 100644 (file)
--- a/doc/src/sgml/bgworker.sgml
+++ b/doc/src/sgml/bgworker.sgml
@@ -58,6 +58,7 @@ typedef struct BackgroundWorker
     char        bgw_library_name[BGW_MAXLEN];   /* only if bgw_main is NULL */
     char        bgw_function_name[BGW_MAXLEN];  /* only if bgw_main is NULL */
     Datum       bgw_main_arg;
+    char        bgw_extra[BGW_EXTRALEN];
     int         bgw_notify_pid;
 } BackgroundWorker;
 </programlisting>
@@ -136,6 +137,13 @@ typedef struct BackgroundWorker
    <structfield>bgw_main</structfield> is NULL.
   </para>
 
+  <para>
+   <structfield>bgw_extra</structfield> can contain extra data to be passed
+   to the background worker.  Unlike <structfield>bgw_main_arg</>, this data
+   is not passed as an argument to the worker's main function, but it can be
+   accessed via <literal>MyBgworkerEntry</literal>, as discussed above.
+  </para>
+
   <para>
    <structfield>bgw_notify_pid</structfield> is the PID of a PostgreSQL
    backend process to which the postmaster should send <literal>SIGUSR1</>

diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index 9c7428f5d6c6efdf4ca274001c1c384f4856c7dc..38a4d65b1f5cf01e764cc9f2a801f6f55005880d 100644 (file)
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -77,10 +77,6 @@ typedef struct FixedParallelState
    /* Mutex protects remaining fields. */
    slock_t     mutex;
 
-   /* Track whether workers have attached. */
-   int         workers_expected;
-   int         workers_attached;
-
    /* Maximum XactLastRecEnd of any worker. */
    XLogRecPtr  last_xlog_end;
 } FixedParallelState;
@@ -286,8 +282,6 @@ InitializeParallelDSM(ParallelContext *pcxt)
    fps->parallel_master_backend_id = MyBackendId;
    fps->entrypoint = pcxt->entrypoint;
    SpinLockInit(&fps->mutex);
-   fps->workers_expected = pcxt->nworkers;
-   fps->workers_attached = 0;
    fps->last_xlog_end = 0;
    shm_toc_insert(pcxt->toc, PARALLEL_KEY_FIXED, fps);
 
@@ -406,6 +400,7 @@ LaunchParallelWorkers(ParallelContext *pcxt)
    worker.bgw_main = ParallelWorkerMain;
    worker.bgw_main_arg = UInt32GetDatum(dsm_segment_handle(pcxt->seg));
    worker.bgw_notify_pid = MyProcPid;
+   memset(&worker.bgw_extra, 0, BGW_EXTRALEN);
 
    /*
     * Start workers.
@@ -417,6 +412,7 @@ LaunchParallelWorkers(ParallelContext *pcxt)
     */
    for (i = 0; i < pcxt->nworkers; ++i)
    {
+       memcpy(worker.bgw_extra, &i, sizeof(int));
        if (!any_registrations_failed &&
            RegisterDynamicBackgroundWorker(&worker,
                                            &pcxt->worker[i].bgwhandle))
@@ -825,6 +821,10 @@ ParallelWorkerMain(Datum main_arg)
    pqsignal(SIGTERM, die);
    BackgroundWorkerUnblockSignals();
 
+   /* Determine and set our parallel worker number. */
+   Assert(ParallelWorkerNumber == -1);
+   memcpy(&ParallelWorkerNumber, MyBgworkerEntry->bgw_extra, sizeof(int));
+
    /* Set up a memory context and resource owner. */
    Assert(CurrentResourceOwner == NULL);
    CurrentResourceOwner = ResourceOwnerCreate(NULL, "parallel toplevel");
@@ -849,18 +849,9 @@ ParallelWorkerMain(Datum main_arg)
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
               errmsg("bad magic number in dynamic shared memory segment")));
 
-   /* Determine and set our worker number. */
+   /* Look up fixed parallel state. */
    fps = shm_toc_lookup(toc, PARALLEL_KEY_FIXED);
    Assert(fps != NULL);
-   Assert(ParallelWorkerNumber == -1);
-   SpinLockAcquire(&fps->mutex);
-   if (fps->workers_attached < fps->workers_expected)
-       ParallelWorkerNumber = fps->workers_attached++;
-   SpinLockRelease(&fps->mutex);
-   if (ParallelWorkerNumber < 0)
-       ereport(ERROR,
-               (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-                errmsg("too many parallel workers already attached")));
    MyFixedParallelState = fps;
 
    /*

diff --git a/src/backend/postmaster/bgworker.c b/src/backend/postmaster/bgworker.c
index 220e599cca2871b47da0e2b5c2d7daf22e6f2c6e..76619e9517cdaaa69994aaefdfd25f8a986849e9 100644 (file)
--- a/src/backend/postmaster/bgworker.c
+++ b/src/backend/postmaster/bgworker.c
@@ -314,6 +314,7 @@ BackgroundWorkerStateChange(void)
        rw->rw_worker.bgw_restart_time = slot->worker.bgw_restart_time;
        rw->rw_worker.bgw_main = slot->worker.bgw_main;
        rw->rw_worker.bgw_main_arg = slot->worker.bgw_main_arg;
+       memcpy(rw->rw_worker.bgw_extra, slot->worker.bgw_extra, BGW_EXTRALEN);
 
        /*
         * Copy the PID to be notified about state changes, but only if the

diff --git a/src/include/postmaster/bgworker.h b/src/include/postmaster/bgworker.h
index f0a9530654532038f9ef2d3ddfd8069a337be34f..6e0b5cd9fc84887e4ddbc2fcda30456f781051dc 100644 (file)
--- a/src/include/postmaster/bgworker.h
+++ b/src/include/postmaster/bgworker.h
@@ -74,6 +74,7 @@ typedef enum
 #define BGW_DEFAULT_RESTART_INTERVAL   60
 #define BGW_NEVER_RESTART              -1
 #define BGW_MAXLEN                     64
+#define BGW_EXTRALEN                   128
 
 typedef struct BackgroundWorker
 {
@@ -85,6 +86,7 @@ typedef struct BackgroundWorker
    char        bgw_library_name[BGW_MAXLEN];   /* only if bgw_main is NULL */
    char        bgw_function_name[BGW_MAXLEN];  /* only if bgw_main is NULL */
    Datum       bgw_main_arg;
+   char        bgw_extra[BGW_EXTRALEN];
    pid_t       bgw_notify_pid; /* SIGUSR1 this backend on start/stop */
 } BackgroundWorker;