#2604 rebased - fork() based garbage collector

changelog/forkgc.dd

@@ -0,0 +1,16 @@
+A concurrent GC for Posix systems
+
+For Posix systems that support the fork() function (or the clone() on linux systems),
+the conservative/precise GC can be made concurrent by enabling the 'fork' GC options in the usual ways, e.g.
+by adding `--DRT-gcopt=fork:1` to the command line or by embedding
+----
+extern(C) __gshared string[] rt_options = [ "gcopt=fork:1" ];
+----
+into your linked binary (see $(LINK $(ROOT_DIR)spec/garbage.html#gc_config)).
+
+The application continues execution and new memory is allocated from the system while the forked
+process is marking heap objects. Parallel marking is disabled for the forked process so it only
+uses a single thread for minimal impact on the concurrent execution of the application.
+
+This reduces "stop the world" time at the cost of needing more memory and page-protection overhead when
+writing to memory currently being scanned.

src/core/exception.d

@@ -453,6 +453,18 @@ unittest
 }
 
 
+/**
+* Thrown on a configuration error.
+*/
+class ForkError : Error
+{
+    this( string file = __FILE__, size_t line = __LINE__, Throwable next = null ) @nogc nothrow pure @safe
+    {
+        super( "fork() failed", file, line, next );
+    }
+}
+
+
 /**
  * Thrown on a switch error.
  */
@@ -716,6 +728,22 @@ extern (C) void onInvalidMemoryOperationError(void* pretend_sideffect = null) @t
     throw staticError!InvalidMemoryOperationError();
 }
 
+
+/**
+ * A callback for errors in the case of a failed fork in D.  A $(LREF ForkError) will be thrown.
+ *
+ * Params:
+ *  file = The name of the file that signaled this error.
+ *  line = The line number on which this error occurred.
+ *
+ * Throws:
+ *  $(LREF ConfigurationError).
+ */
+extern (C) void onForkError( string file = __FILE__, size_t line = __LINE__ ) @trusted pure nothrow @nogc
+{
+    throw staticError!ForkError( file, line, null );
+}
+
 /**
  * A callback for unicode errors in D.  A $(LREF UnicodeException) will be thrown.
  *

src/core/gc/config.d

@@ -15,6 +15,7 @@ __gshared Config config;
 struct Config
 {
     bool disable;            // start disabled
+    bool fork = false;       // optional concurrent behaviour
     ubyte profile;           // enable profiling with summary when terminating program
     string gc = "conservative"; // select gc implementation conservative|precise|manual
 
@@ -39,8 +40,9 @@ struct Config
 
         printf("GC options are specified as white space separated assignments:
     disable:0|1    - start disabled (%d)
+    fork:0|1       - set fork behaviour (%d)
     profile:0|1|2  - enable profiling with summary when terminating program (%d)
-    gc:".ptr, disable, profile);
+    gc:".ptr, disable, fork, profile);
         foreach (i, entry; registeredGCFactories)
         {
             if (i) printf("|");

src/core/internal/gc/bits.d

@@ -7,6 +7,7 @@
  */
 module core.internal.gc.bits;
 
+import core.internal.gc.os : os_mem_map, os_mem_unmap, HaveFork;
 
 import core.bitop;
 import core.stdc.string;
@@ -32,19 +33,29 @@ struct GCBits
     wordtype* data;
     size_t nbits;
 
-    void Dtor() nothrow
+    void Dtor(bool share = false) nothrow
     {
         if (data)
         {
-            free(data);
+            static if (!HaveFork)
+                free(data);
+            else if (share)
+                os_mem_unmap(data, nwords * data[0].sizeof);
+            else
+                free(data);
             data = null;
         }
     }
 
-    void alloc(size_t nbits) nothrow
+    void alloc(size_t nbits, bool share = false) nothrow
     {
         this.nbits = nbits;
-        data = cast(typeof(data[0])*)calloc(nwords, data[0].sizeof);
+        static if (!HaveFork)
+            data = cast(typeof(data[0])*)calloc(nwords, data[0].sizeof);
+        else if (share)
+            data = cast(typeof(data[0])*)os_mem_map(nwords * data[0].sizeof, true); // Allocate as MAP_SHARED
+        else
+            data = cast(typeof(data[0])*)calloc(nwords, data[0].sizeof);
         if (!data)
             onOutOfMemoryError();
     }
@@ -431,6 +442,11 @@ struct GCBits
         memset(data, 0, nwords * wordtype.sizeof);
     }
 
+    void setAll() nothrow
+    {
+        memset(data, 0xFF, nwords * wordtype.sizeof);
+    }
+
     void copy(GCBits *f) nothrow
     in
     {