Added some "address space slack" under FullDebugMode. This reserves a…

… block of address space on startup (currently 5MB) that is released just before the first time an EOutOfMemory exception is raised, allowing some GetMem calls following the initial EOutOfMemory to succeed. This allows the application to perform any error logging and other shutdown operations successfully that would have failed if the address space was actually completely exhausted. (Under FullDebugMode address space is never released back to the operating system so once the address space has been exhausted there is very little room to manoeuvre.)

FastMM4.pas

@@ -849,7 +849,16 @@
     immediately during a FreeMem call the block will added to a list of blocks
     that will be freed later, either in the background cleanup thread or during
     the next call to FreeMem.
-
+  Version 4.??? (unreleased)
+  - Added some "address space slack" under FullDebugMode. This reserves a
+    block of address space on startup (currently 5MB) that is released just
+    before the first time an EOutOfMemory exception is raised, allowing some
+    GetMem calls following the initial EOutOfMemory to succeed. This allows
+    the application to perform any error logging and other shutdown operations
+    successfully that would have failed it the address space was actually
+    completely exhausted. (Under FullDebugMode address space is never released
+    back to the operating system so once the address space has been exhausted
+    there is very little room to manoeuvre.)
 *)
 
 unit FastMM4;
@@ -1461,6 +1470,11 @@ procedure LogReleaseStackUsage;
 {$else}
   DebugFillPattern = $8080808080808080;
 {$endif}
+  {The number of bytes of address space that cannot be allocated under FullDebugMode.  This block is reserved on
+  startup and freed the first time the system runs out of address space.  This allows some subsequent memory allocation
+  requests to succeed in order to allow the application to allocate some memory for error handling, etc. in response to
+  the first EOutOfMemory exception.}
+  FullDebugModeAddressSpaceSlack = 5 * 1024 * 1024;
 
 {-------------------------FullDebugMode structures--------------------}
 type
@@ -2128,6 +2142,10 @@   TExpectedMemoryLeaks = record
   {The 64K block of reserved memory used to trap invalid memory accesses using
    fields in a freed object.}
   ReservedBlock: Pointer;
+  {Points to a block of size FullDebugModeAddressSpaceSlack that is freed the first time the system runs out of memory.
+  Memory is never release under FullDebugMode, so this allows the application to continue to function for a short while
+  after the first EOutOfMemory exception.}
+  AddressSpaceSlackPtr: Pointer;
   {The virtual method index count - used to get the virtual method index for a
    virtual method call on a freed object.}
   VMIndex: Integer;
@@ -9652,6 +9670,16 @@ function DebugGetMem(ASize: {$ifdef XE2AndUp}NativeInt{$else}Integer{$endif}): P
 {$endif LogLockContention}
         Result := nil;
       end;
+    end
+    else
+    begin
+      {The process ran out of address space:  Release the address space slack so that some subsequent GetMem calls will
+      succeed in order for any error logging, etc. to complete successfully.}
+      if AddressSpaceSlackPtr <> nil then
+      begin
+        VirtualFree(AddressSpaceSlackPtr, 0, MEM_RELEASE);
+        AddressSpaceSlackPtr := nil;
+      end;
     end;
   finally
     {Leaving the memory manager routine: Block scans may be performed again.}
@@ -12601,6 +12629,8 @@ procedure InstallMemoryManager;
   {$ifdef 32Bit}
     {Try to reserve the 64K block covering address $80808080}
     ReservedBlock := VirtualAlloc(Pointer(DebugReservedAddress), 65536, MEM_RESERVE, PAGE_NOACCESS);
+    {Allocate the address space slack.}
+    AddressSpaceSlackPtr := VirtualAlloc(nil, FullDebugModeAddressSpaceSlack, MEM_RESERVE or MEM_TOP_DOWN, PAGE_NOACCESS);
   {$endif}
 {$endif}
 {$ifdef MMSharingEnabled}
@@ -12803,6 +12833,12 @@ procedure UninstallMemoryManager;
       VirtualFree(ReservedBlock, 0, MEM_RELEASE);
       ReservedBlock := nil;
     end;
+    {Release the address space slack}
+    if AddressSpaceSlackPtr <> nil then
+    begin
+      VirtualFree(AddressSpaceSlackPtr, 0, MEM_RELEASE);
+      AddressSpaceSlackPtr := nil;
+    end;
 {$endif}
   end;
 {$ifndef DetectMMOperationsAfterUninstall}