The pool is only used for readonly connections, since writable
connections usually take a lock on the server side and therefore you
cannot open more than one.
---
 plugins/vddk/nbdkit-vddk-plugin.pod |   7 +
 plugins/vddk/vddk.c                 | 201 ++++++++++++++++++++++------
 2 files changed, 164 insertions(+), 44 deletions(-)

diff --git a/plugins/vddk/nbdkit-vddk-plugin.pod b/plugins/vddk/nbdkit-vddk-plugin.pod
index e5539da9..288aed4b 100644
--- a/plugins/vddk/nbdkit-vddk-plugin.pod
+++ b/plugins/vddk/nbdkit-vddk-plugin.pod
@@ -175,6 +175,13 @@ Read the password from file descriptor number C<FD>, inherited from
 the parent process when nbdkit starts up.  This is also a secure
 method to supply a password.

+=item B<pool-max=>N
+
+To improve performance, for read-only connections (see I<-r> option)
+the plugin will open a pool of VixDiskLibHandle disk handles.  You can
+use this option to control the maximum size of the pool.  The default
+is 8.  To disable this feature, set it to 0 or 1.
+
 =item B<port=>PORT

 The port on the VCenter/ESXi host.  Defaults to 443.
diff --git a/plugins/vddk/vddk.c b/plugins/vddk/vddk.c
index 5926e181..173d5031 100644
--- a/plugins/vddk/vddk.c
+++ b/plugins/vddk/vddk.c
@@ -52,6 +52,7 @@
 #include "isaligned.h"
 #include "minmax.h"
 #include "rounding.h"
+#include "vector.h"

 #include "vddk.h"
 #include "vddk-structs.h"
@@ -85,6 +86,7 @@ char *libdir;                              /* libdir */
 static uint16_t nfc_host_port;             /* nfchostport */
 char *password;                            /* password */
 static uint16_t port;                      /* port */
+static unsigned pool_max = 8;              /* pool-max */
 static const char *server_name;            /* server */
 static bool single_link;                   /* single-link */
 static const char *snapshot_moref;         /* snapshot */
@@ -233,6 +235,12 @@ vddk_config (const char *key, const char *value)
     if (nbdkit_parse_uint16_t ("port", value, &port) == -1)
       return -1;
   }
+  else if (strcmp (key, "pool-max") == 0) {
+    if (nbdkit_parse_unsigned ("pool-max", value, &pool_max) == -1)
+      return -1;
+    if (pool_max < 1)
+      pool_max = 1;
+  }
   else if (strcmp (key, "reexeced_") == 0) {
     /* Special name because it is only for internal use. */
     reexeced = (char *)value;
@@ -482,20 +490,37 @@ vddk_dump_plugin (void)
  * https://code.vmware.com/docs/11750/virtual-disk-development-kit-programming-guide/GUID-6BE903E8-DC70-46D9-98E4-E34A2002C2AD.html
  *
  * Before nbdkit 1.22 we used SERIALIZE_ALL_REQUESTS.  Since nbdkit
- * 1.22 we changed this to SERIALIZE_REQUESTS and added a mutex around
- * calls to VixDiskLib_Open and VixDiskLib_Close.  This is not quite
- * within the letter of the rules, but is within the spirit.
+ * 1.22 we changed this to PARALLEL, added a mutex around calls to
+ * VixDiskLib_Open and VixDiskLib_Close, and use a pool of disk
+ * handles.  This is not quite within the letter of the rules, but is
+ * within the spirit.
  */
-#define THREAD_MODEL NBDKIT_THREAD_MODEL_SERIALIZE_REQUESTS
+#define THREAD_MODEL NBDKIT_THREAD_MODEL_PARALLEL

 /* Lock protecting open/close calls - see above. */
 static pthread_mutex_t open_close_lock = PTHREAD_MUTEX_INITIALIZER;

-/* The per-connection handle. */
+struct handle;
 struct vddk_handle {
+  VixDiskLibHandle vddk_handle;
+  bool in_use;
+  struct handle *h;
+};
+DEFINE_VECTOR_TYPE(vddk_handles, struct vddk_handle)
+
+/* The per-connection handle. */
+struct handle {
   VixDiskLibConnectParams *params; /* connection parameters */
   VixDiskLibConnection connection; /* connection */

-  VixDiskLibHandle handle;         /* disk handle */
+  int readonly;                    /* readonly flag for this connection */
+  uint32_t flags;                  /* open flags */
+
+  /* Pool of VDDK disk handles.  Do not access this directly, use
+   * GET_VDDK_HANDLE_FOR_CURRENT_SCOPE macro to get a free handle.
+   */
+  pthread_mutex_t vddk_handles_lock;
+  pthread_cond_t vddk_handles_cond;
+  vddk_handles vddk_handles;
 };

 static inline VixDiskLibConnectParams *
@@ -531,17 +556,28 @@ free_connect_params (VixDiskLibConnectParams *params)
 static void *
 vddk_open (int readonly)
 {
-  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&open_close_lock);
-  struct vddk_handle *h;
+  struct handle *h;
   VixError err;
-  uint32_t flags;

-  h = malloc (sizeof *h);
+  h = calloc (1, sizeof *h);
   if (h == NULL) {
-    nbdkit_error ("malloc: %m");
+    nbdkit_error ("calloc: %m");
     return NULL;
   }

+  h->readonly = readonly;
+  pthread_mutex_init (&h->vddk_handles_lock, NULL);
+  pthread_cond_init (&h->vddk_handles_cond, NULL);
+
+  /* We have to reserve this vector to ensure that it is not
+   * reallocated, as that would make the pointer returned by
+   * get_vddk_handle in another thread invalid.
+   */
+  if (vddk_handles_reserve (&h->vddk_handles, pool_max) == -1) {
+    nbdkit_error ("realloc: %m");
+    goto err0;
+  }
+
   h->params = allocate_connect_params ();
   if (h->params == NULL) {
     nbdkit_error ("allocate VixDiskLibConnectParams: %m");
@@ -589,49 +625,120 @@ vddk_open (int readonly)
     goto err1;
   }

-  flags = 0;
+  h->flags = 0;
   if (readonly)
-    flags |= VIXDISKLIB_FLAG_OPEN_READ_ONLY;
+    h->flags |= VIXDISKLIB_FLAG_OPEN_READ_ONLY;
   if (single_link)
-    flags |= VIXDISKLIB_FLAG_OPEN_SINGLE_LINK;
+    h->flags |= VIXDISKLIB_FLAG_OPEN_SINGLE_LINK;
   if (unbuffered)
-    flags |= VIXDISKLIB_FLAG_OPEN_UNBUFFERED;
-
-  DEBUG_CALL ("VixDiskLib_Open",
-              "connection, %s, %d, &handle", filename, flags);
-  err = VixDiskLib_Open (h->connection, filename, flags, &h->handle);
-  if (err != VIX_OK) {
-    VDDK_ERROR (err, "VixDiskLib_Open: %s", filename);
-    goto err2;
-  }
-
-  nbdkit_debug ("transport mode: %s",
-                VixDiskLib_GetTransportMode (h->handle));
+    h->flags |= VIXDISKLIB_FLAG_OPEN_UNBUFFERED;

   return h;

- err2:
-  DEBUG_CALL ("VixDiskLib_Disconnect", "connection");
-  VixDiskLib_Disconnect (h->connection);
  err1:
   free_connect_params (h->params);
  err0:
+  free (h->vddk_handles.ptr);
+  pthread_cond_destroy (&h->vddk_handles_cond);
+  pthread_mutex_destroy (&h->vddk_handles_lock);
   free (h);
   return NULL;
 }

+/* Get a VDDK handle on demand. */
+static VixDiskLibHandle
+open_vddk_handle (struct handle *h)
+{
+  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&open_close_lock);
+  VixDiskLibHandle vddk_handle;
+  VixError err;
+
+  DEBUG_CALL ("VixDiskLib_Open",
+              "connection, %s, %d, &handle", filename, h->flags);
+  err = VixDiskLib_Open (h->connection, filename, h->flags, &vddk_handle);
+  if (err != VIX_OK) {
+    VDDK_ERROR (err, "VixDiskLib_Open: %s", filename);
+    return NULL;
+  }
+
+  nbdkit_debug ("transport mode: %s",
+                VixDiskLib_GetTransportMode (vddk_handle));
+  return vddk_handle;
+}
+
+static struct vddk_handle *
+get_vddk_handle (struct handle *h)
+{
+  const unsigned max = h->readonly ? pool_max : 1;
+  ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&h->vddk_handles_lock);
+  VixDiskLibHandle vddk_handle;
+  size_t i;
+
+ again:
+  /* See if there's a handle in the pool which is not in use. */
+  for (i = 0; i < h->vddk_handles.size; ++i) {
+    if (!h->vddk_handles.ptr[i].in_use) {
+      h->vddk_handles.ptr[i].in_use = true;
+      return &h->vddk_handles.ptr[i];
+    }
+  }
+
+  /* If the pool is too big we have to wait for another thread to
+   * finish using its handle and try again.
+   */
+  if (h->vddk_handles.size >= max) {
+    pthread_cond_wait (&h->vddk_handles_cond, &h->vddk_handles_lock);
+    goto again;
+  }
+
+  /* Open another handle and add it to the pool.  Note that
+   * vddk_handles_append cannot fail because we reserved space in
+   * vddk_open.
+   */
+  vddk_handle = open_vddk_handle (h);
+  if (vddk_handle == NULL)
+    return NULL;
+  vddk_handles_append (&h->vddk_handles,
+                       (struct vddk_handle){ vddk_handle, true, h });
+  return &h->vddk_handles.ptr[h->vddk_handles.size-1];
+}
+
+static void
+put_vddk_handle (struct vddk_handle **p)
+{
+  if (*p) {
+    ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&(*p)->h->vddk_handles_lock);
+    assert ((*p)->in_use);
+    (*p)->in_use = false;
+    pthread_cond_signal (&(*p)->h->vddk_handles_cond);
+  }
+}
+
+/* Wrap get/put_vddk_handle in nicer macro. */
+#define GET_VDDK_HANDLE_FOR_CURRENT_SCOPE(h, name)      \
+  __attribute__((cleanup (put_vddk_handle)))            \
+  struct vddk_handle *name##_h = get_vddk_handle (h);   \
+  if (name##_h == NULL) return -1;                      \
+  VixDiskLibHandle name = name##_h->vddk_handle
+
 /* Free up the per-connection handle. */
 static void
 vddk_close (void *handle)
 {
   ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&open_close_lock);
-  struct vddk_handle *h = handle;
+  struct handle *h = handle;
+  size_t i;

-  DEBUG_CALL ("VixDiskLib_Close", "handle");
-  VixDiskLib_Close (h->handle);
+  for (i = 0; i < h->vddk_handles.size; ++i) {
+    DEBUG_CALL ("VixDiskLib_Close", "handle");
+    VixDiskLib_Close (h->vddk_handles.ptr[i].vddk_handle);
+  }
+  free (h->vddk_handles.ptr);
   DEBUG_CALL ("VixDiskLib_Disconnect", "connection");
   VixDiskLib_Disconnect (h->connection);
   free_connect_params (h->params);
+  pthread_cond_destroy (&h->vddk_handles_cond);
+  pthread_mutex_destroy (&h->vddk_handles_lock);
   free (h);
 }

@@ -639,13 +746,14 @@ vddk_close (void *handle)
 static int64_t
 vddk_get_size (void *handle)
 {
-  struct vddk_handle *h = handle;
+  struct handle *h = handle;
+  GET_VDDK_HANDLE_FOR_CURRENT_SCOPE (h, vddk_handle);
   VixDiskLibInfo *info;
   VixError err;
   uint64_t size;

   DEBUG_CALL ("VixDiskLib_GetInfo", "handle, &info");
-  err = VixDiskLib_GetInfo (h->handle, &info);
+  err = VixDiskLib_GetInfo (vddk_handle, &info);
   if (err != VIX_OK) {
     VDDK_ERROR (err, "VixDiskLib_GetInfo");
     return -1;
@@ -687,7 +795,8 @@ static int
 vddk_pread (void *handle, void *buf, uint32_t count, uint64_t offset,
             uint32_t flags)
 {
-  struct vddk_handle *h = handle;
+  struct handle *h = handle;
+  GET_VDDK_HANDLE_FOR_CURRENT_SCOPE (h, vddk_handle);
   VixError err;

   /* Align to sectors. */
@@ -706,7 +815,7 @@ vddk_pread (void *handle, void *buf, uint32_t count, uint64_t offset,
                        "handle, %" PRIu64 " sectors, "
                        "%" PRIu32 " sectors, buffer",
                        offset, count);
-  err = VixDiskLib_Read (h->handle, offset, count, buf);
+  err = VixDiskLib_Read (vddk_handle, offset, count, buf);
   if (err != VIX_OK) {
     VDDK_ERROR (err, "VixDiskLib_Read");
     return -1;
@@ -726,7 +835,8 @@ vddk_pwrite (void *handle, const void *buf, uint32_t count, uint64_t offset,
              uint32_t flags)
 {
   const bool fua = flags & NBDKIT_FLAG_FUA;
-  struct vddk_handle *h = handle;
+  struct handle *h = handle;
+  GET_VDDK_HANDLE_FOR_CURRENT_SCOPE (h, vddk_handle);
   VixError err;

   /* Align to sectors. */
@@ -745,7 +855,7 @@ vddk_pwrite (void *handle, const void *buf, uint32_t count, uint64_t offset,
                        "handle, %" PRIu64 " sectors, "
                        "%" PRIu32 " sectors, buffer",
                        offset, count);
-  err = VixDiskLib_Write (h->handle, offset, count, buf);
+  err = VixDiskLib_Write (vddk_handle, offset, count, buf);
   if (err != VIX_OK) {
     VDDK_ERROR (err, "VixDiskLib_Write");
     return -1;
@@ -761,7 +871,8 @@ vddk_pwrite (void *handle, const void *buf, uint32_t count, uint64_t offset,
 static int
 vddk_flush (void *handle, uint32_t flags)
 {
-  struct vddk_handle *h = handle;
+  struct handle *h = handle;
+  GET_VDDK_HANDLE_FOR_CURRENT_SCOPE (h, vddk_handle);
   VixError err;

   /* The Flush call was not available in VDDK < 6.0 so this is simply
@@ -771,7 +882,7 @@ vddk_flush (void *handle, uint32_t flags)
     return 0;

   DEBUG_CALL ("VixDiskLib_Flush", "handle");
-  err = VixDiskLib_Flush (h->handle);
+  err = VixDiskLib_Flush (vddk_handle);
   if (err != VIX_OK) {
     VDDK_ERROR (err, "VixDiskLib_Flush");
     return -1;
@@ -783,7 +894,8 @@ vddk_flush (void *handle, uint32_t flags)
 static int
 vddk_can_extents (void *handle)
 {
-  struct vddk_handle *h = handle;
+  struct handle *h = handle;
+  GET_VDDK_HANDLE_FOR_CURRENT_SCOPE (h, vddk_handle);
   VixError err;
   VixDiskLibBlockList *block_list;

@@ -808,7 +920,7 @@ vddk_can_extents (void *handle)
   DEBUG_CALL ("VixDiskLib_QueryAllocatedBlocks",
               "handle, 0, %d sectors, %d sectors",
               VIXDISKLIB_MIN_CHUNK_SIZE, VIXDISKLIB_MIN_CHUNK_SIZE);
-  err = VixDiskLib_QueryAllocatedBlocks (h->handle,
+  err = VixDiskLib_QueryAllocatedBlocks (vddk_handle,
                                          0, VIXDISKLIB_MIN_CHUNK_SIZE,
                                          VIXDISKLIB_MIN_CHUNK_SIZE,
                                          &block_list);
@@ -864,7 +976,8 @@ static int
 vddk_extents (void *handle, uint32_t count, uint64_t offset, uint32_t flags,
               struct nbdkit_extents *extents)
 {
-  struct vddk_handle *h = handle;
+  struct handle *h = handle;
+  GET_VDDK_HANDLE_FOR_CURRENT_SCOPE (h, vddk_handle);
   bool req_one = flags & NBDKIT_FLAG_REQ_ONE;
   uint64_t position, end, start_sector;

@@ -896,7 +1009,7 @@ vddk_extents (void *handle, uint32_t count, uint64_t offset, uint32_t flags,
                 "handle, %" PRIu64 " sectors, %" PRIu64 " sectors, "
                 "%d sectors",
                 start_sector, nr_sectors, VIXDISKLIB_MIN_CHUNK_SIZE);
-    err = VixDiskLib_QueryAllocatedBlocks (h->handle,
+    err = VixDiskLib_QueryAllocatedBlocks (vddk_handle,
                                            start_sector, nr_sectors,
                                            VIXDISKLIB_MIN_CHUNK_SIZE,
                                            &block_list);
--
2.27.0