Module API for Forking

* create module API for forking child processes.
* refactor duplicate code around creating and tracking forks by AOF and RDB.
* child processes listen to SIGUSR1 and dies exitFromChild in order to
  eliminate a valgrind warning of unhandled signal.
* note that BGSAVE error reply has changed.

valgrind error is:
  Process terminating with default action of signal 10 (SIGUSR1)

tests/modules/Makefile

@@ -13,12 +13,16 @@ endif
 
 .SUFFIXES: .c .so .xo .o
 
-all: commandfilter.so
+all: commandfilter.so fork.so
 
 .c.xo:
 	$(CC) -I../../src $(CFLAGS) $(SHOBJ_CFLAGS) -fPIC -c $< -o $@
 
 commandfilter.xo: ../../src/redismodule.h
+fork.xo: ../../src/redismodule.h
 
 commandfilter.so: commandfilter.xo
 	$(LD) -o $@ $< $(SHOBJ_LDFLAGS) $(LIBS) -lc
+
+fork.so: fork.xo
+	$(LD) -o $@ $< $(SHOBJ_LDFLAGS) $(LIBS) -lc

tests/modules/fork.c

@@ -0,0 +1,84 @@
+#define REDISMODULE_EXPERIMENTAL_API
+#include "redismodule.h"
+
+#include <string.h>
+#include <assert.h>
+#include <unistd.h>
+
+#define UNUSED(V) ((void) V)
+
+int child_pid = -1;
+int exitted_with_code = -1;
+
+void done_handler(int exitcode, int bysignal, void *user_data) {
+    child_pid = -1;
+    exitted_with_code = exitcode;
+    assert(user_data==(void*)0xdeadbeef);
+    UNUSED(bysignal);
+}
+
+int fork_create(RedisModuleCtx *ctx, RedisModuleString **argv, int argc)
+{
+    long long code_to_exit_with;
+    if (argc != 2) {
+        RedisModule_WrongArity(ctx);
+        return REDISMODULE_OK;
+    }
+    RedisModule_StringToLongLong(argv[1], &code_to_exit_with);
+    exitted_with_code = -1;
+    child_pid = RedisModule_Fork(done_handler, (void*)0xdeadbeef);
+    if (child_pid < 0) {
+        RedisModule_ReplyWithError(ctx, "Fork failed");
+        return REDISMODULE_OK;
+    } else if (child_pid > 0) {
+        /* parent */
+        RedisModule_ReplyWithLongLong(ctx, child_pid);
+        return REDISMODULE_OK;
+    }
+
+    /* child */
+    RedisModule_Log(ctx, "notice", "fork child started");
+    usleep(200000);
+    RedisModule_Log(ctx, "notice", "fork child exiting");
+    RedisModule_ExitFromChild(code_to_exit_with);
+    /* unreachable */
+    return 0;
+}
+
+int fork_exitcode(RedisModuleCtx *ctx, RedisModuleString **argv, int argc)
+{
+    UNUSED(argv);
+    UNUSED(argc);
+    RedisModule_ReplyWithLongLong(ctx, exitted_with_code);
+    return REDISMODULE_OK;
+}
+
+int fork_kill(RedisModuleCtx *ctx, RedisModuleString **argv, int argc)
+{
+    UNUSED(argv);
+    UNUSED(argc);
+    if (RedisModule_KillForkChild(child_pid) != REDISMODULE_OK)
+        RedisModule_ReplyWithError(ctx, "KillForkChild failed");
+    else
+        RedisModule_ReplyWithLongLong(ctx, 1);
+    child_pid = -1;
+    return REDISMODULE_OK;
+}
+
+int RedisModule_OnLoad(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
+    UNUSED(argv);
+    UNUSED(argc);
+    if (RedisModule_Init(ctx,"fork",1,REDISMODULE_APIVER_1)== REDISMODULE_ERR)
+        return REDISMODULE_ERR;
+
+    if (RedisModule_CreateCommand(ctx,"fork.create", fork_create,"",0,0,0) == REDISMODULE_ERR)
+        return REDISMODULE_ERR;
+
+    if (RedisModule_CreateCommand(ctx,"fork.exitcode", fork_exitcode,"",0,0,0) == REDISMODULE_ERR)
+        return REDISMODULE_ERR;
+
+    if (RedisModule_CreateCommand(ctx,"fork.kill", fork_kill,"",0,0,0) == REDISMODULE_ERR)
+        return REDISMODULE_ERR;
+
+    return REDISMODULE_OK;
+}

tests/unit/moduleapi/fork.tcl

@@ -0,0 +1,32 @@
+set testmodule [file normalize tests/modules/fork.so]
+
+proc count_log_message {pattern} {
+    set result [exec grep -c $pattern < [srv 0 stdout]]
+}
+
+start_server {tags {"modules"}} {
+    r module load $testmodule
+
+    test {Module fork} {
+        # the argument to fork.create is the exitcode on termination
+        r fork.create 3
+        wait_for_condition 20 100 {
+            [r fork.exitcode] != -1
+        } else {
+            fail "fork didn't terminate"
+        }
+        r fork.exitcode
+    } {3}
+
+    test {Module fork kill} {
+        r fork.create 3
+        after 20
+        r fork.kill
+        after 100
+
+        assert {[count_log_message "fork child started"] eq "2"}
+        assert {[count_log_message "Received SIGUSR1 in child"] eq "1"}
+        assert {[count_log_message "fork child exiting"] eq "1"}
+    }
+
+}