Reserve the shared memory region during backend startup on Windows, so

that memory allocated by starting third party DLLs doesn't end up
conflicting with it.

Hopefully this solves the long-time issue with "could not reattach
to shared memory" errors on Win32.

Patch from Tsutomu Yamada and me, based on idea from Trevor Talbot.

diff --git a/src/backend/port/win32_shmem.c b/src/backend/port/win32_shmem.c
index f263948ec660da80f8482d925d9ce447ceafef2e..fc8e51b487089edb77bf9f3602378813b7f9d16f 100644 (file)
--- a/src/backend/port/win32_shmem.c
+++ b/src/backend/port/win32_shmem.c
@@ -18,6 +18,7 @@
 
 unsigned long UsedShmemSegID = 0;
 void      *UsedShmemSegAddr = NULL;
+static Size UsedShmemSegSize = 0;
 
 static void pgwin32_SharedMemoryDelete(int status, Datum shmId);
 
@@ -233,6 +234,7 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port)
 
        /* Save info for possible future use */
        UsedShmemSegAddr = memAddress;
+       UsedShmemSegSize = size;
        UsedShmemSegID = (unsigned long) hmap2;
 
        return hdr;
@@ -257,6 +259,13 @@ PGSharedMemoryReAttach(void)
        Assert(UsedShmemSegAddr != NULL);
        Assert(IsUnderPostmaster);
 
+       /*
+        * Release memory region reservation that was made by the postmaster
+        */
+       if (VirtualFree(UsedShmemSegAddr, 0, MEM_RELEASE) == 0)
+               elog(FATAL, "failed to release reserved memory region (addr=%p): %lu",
+                        UsedShmemSegAddr, GetLastError());
+
        hdr = (PGShmemHeader *) MapViewOfFileEx((HANDLE) UsedShmemSegID, FILE_MAP_READ | FILE_MAP_WRITE, 0, 0, 0, UsedShmemSegAddr);
        if (!hdr)
                elog(FATAL, "could not reattach to shared memory (key=%d, addr=%p): %lu",
@@ -302,3 +311,53 @@ pgwin32_SharedMemoryDelete(int status, Datum shmId)
        if (!CloseHandle((HANDLE) DatumGetInt32(shmId)))
                elog(LOG, "could not close handle to shared memory: %lu", GetLastError());
 }
+
+/*
+ * pgwin32_ReserveSharedMemoryRegion(hChild)
+ *
+ * Reserve the memory region that will be used for shared memory in a child
+ * process. It is called before the child process starts, to make sure the
+ * memory is available.
+ *
+ * Once the child starts, DLLs loading in different order or threads getting
+ * scheduled differently may allocate memory which can conflict with the
+ * address space we need for our shared memory. By reserving the shared
+ * memory region before the child starts, and freeing it only just before we
+ * attempt to get access to the shared memory forces these allocations to
+ * be given different address ranges that don't conflict.
+ *
+ * NOTE! This function executes in the postmaster, and should for this
+ * reason not use elog(FATAL) since that would take down the postmaster.
+ */
+int
+pgwin32_ReserveSharedMemoryRegion(HANDLE hChild)
+{
+       void *address;
+
+       Assert(UsedShmemSegAddr != NULL);
+       Assert(UsedShmemSegSize != 0);
+
+       address = VirtualAllocEx(hChild, UsedShmemSegAddr, UsedShmemSegSize,
+                                                               MEM_RESERVE, PAGE_READWRITE);
+       if (address == NULL) {
+               /* Don't use FATAL since we're running in the postmaster */
+               elog(LOG, "could not reserve shared memory region (addr=%p) for child %lu: %lu",
+                        UsedShmemSegAddr, hChild, GetLastError());
+               return false;
+       }
+       if (address != UsedShmemSegAddr)
+       {
+               /*
+                * Should never happen - in theory if allocation granularity causes strange
+                * effects it could, so check just in case.
+                *
+                * Don't use FATAL since we're running in the postmaster.
+                */
+           elog(LOG, "reserved shared memory region got incorrect address %p, expected %p",
+                        address, UsedShmemSegAddr);
+               VirtualFreeEx(hChild, address, 0, MEM_RELEASE);
+               return false;
+       }
+
+       return true;
+}

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index a453d742d2ead1a38888f79e30711bdad91e1850..1bd19cb7a1b88853d3bb4c931edb66f45641dac6 100644 (file)
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -3635,7 +3635,7 @@ internal_forkexec(int argc, char *argv[], Port *port)
                return -1;                              /* log made by save_backend_variables */
        }
 
-       /* Drop the shared memory that is now inherited to the backend */
+       /* Drop the parameter shared memory that is now inherited to the backend */
        if (!UnmapViewOfFile(param))
                elog(LOG, "could not unmap view of backend parameter file: error code %d",
                         (int) GetLastError());
@@ -3643,6 +3643,25 @@ internal_forkexec(int argc, char *argv[], Port *port)
                elog(LOG, "could not close handle to backend parameter file: error code %d",
                         (int) GetLastError());
 
+       /*
+        * Reserve the memory region used by our main shared memory segment before we
+        * resume the child process.
+        */
+       if (!pgwin32_ReserveSharedMemoryRegion(pi.hProcess))
+       {
+               /*
+                * Failed to reserve the memory, so terminate the newly created
+                * process and give up.
+                */
+               if (!TerminateProcess(pi.hProcess, 255))
+                       ereport(ERROR,
+                                       (errmsg_internal("could not terminate process that failed to reserve memory: error code %d",
+                                                                        (int) GetLastError())));
+               CloseHandle(pi.hProcess);
+               CloseHandle(pi.hThread);
+               return -1;                      /* logging done made by pgwin32_ReserveSharedMemoryRegion() */
+       }
+
        /*
         * Now that the backend variables are written out, we start the child
         * thread so it can start initializing while we set up the rest of the

diff --git a/src/include/port/win32.h b/src/include/port/win32.h
index 76ff7435ec828d7d1c5eea3b1991207fcd2c05db..91171d49b0bb988f21c0dea957ffdd49790608c7 100644 (file)
--- a/src/include/port/win32.h
+++ b/src/include/port/win32.h
@@ -288,6 +288,9 @@ extern int  pgwin32_is_admin(void);
 extern int     pgwin32_is_service(void);
 #endif
 
+/* in backend/port/win32_shmem.c */
+extern int     pgwin32_ReserveSharedMemoryRegion(HANDLE);
+
 /* in port/win32error.c */
 extern void _dosmaperr(unsigned long);