Libguestfs - Jul 2023 - [PATCH nbdkit] curl: Try to share as much as possible between handles in the pool

I'm not necessarily proposing this as a patch for nbdkit, so this is
mainly posting as a back-up on the mailing list.

Previous version of the same patch with discussion:
https://listman.redhat.com/archives/libguestfs/2023-February/thread.html#30838

Rich.

Using the libcurl share interface we can share data between the
separate curl easy handles in the pool.  For more about this see:

https://curl.se/libcurl/c/CURLSHOPT_SHARE.html
https://gist.github.com/bagder/7eccf74f8b6d70b5abefeb7f288dba9b
https://everything.curl.dev/libcurl/sharing
---
 plugins/curl/pool.c | 65 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)

diff --git a/plugins/curl/pool.c b/plugins/curl/pool.c
index 10f9011d7..e9f387c38 100644
--- a/plugins/curl/pool.c
+++ b/plugins/curl/pool.c
@@ -52,6 +52,7 @@
 
 #include <nbdkit-plugin.h>
 
+#include "array-size.h"
 #include "ascii-ctype.h"
 #include "ascii-string.h"
 #include "cleanup.h"
@@ -72,6 +73,18 @@ static int get_content_length_accept_range (struct
curl_handle *ch);
 static bool try_fallback_GET_method (struct curl_handle *ch);
 static size_t header_cb (void *ptr, size_t size, size_t nmemb, void *opaque);
 static size_t error_cb (char *ptr, size_t size, size_t nmemb, void *opaque);
+static void lock_cb (CURL *handle, curl_lock_data data,
+                     curl_lock_access access, void *userptr);
+static void unlock_cb (CURL *handle, curl_lock_data data,
+                       void *userptr);
+
+/* These locks protect access to the curl share data.  See:
+ * https://gist.github.com/bagder/7eccf74f8b6d70b5abefeb7f288dba9b
+ */
+static pthread_rwlock_t sharelocks[CURL_LOCK_DATA_LAST];
+
+/* Curl share data. */
+static CURLSH *share;
 
 /* This lock protects access to the curl_handles vector below. */
 static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
@@ -93,6 +106,22 @@ static size_t in_use = 0, waiting = 0;
 void
 load_pool (void)
 {
+  size_t i;
+
+  for (i = 0; i < ARRAY_SIZE (sharelocks); ++i)
+    pthread_rwlock_init (&sharelocks[i], NULL);
+
+  share = curl_share_init ();
+  if (share == NULL) {
+    nbdkit_error ("curl_share_init: %m");
+    exit (EXIT_FAILURE);
+  }
+  curl_share_setopt (share, CURLSHOPT_LOCKFUNC, lock_cb);
+  curl_share_setopt (share, CURLSHOPT_UNLOCKFUNC, unlock_cb);
+  curl_share_setopt (share, CURLSHOPT_SHARE, CURL_LOCK_DATA_CONNECT);
+  curl_share_setopt (share, CURLSHOPT_SHARE, CURL_LOCK_DATA_COOKIE);
+  curl_share_setopt (share, CURLSHOPT_SHARE, CURL_LOCK_DATA_DNS);
+  curl_share_setopt (share, CURLSHOPT_SHARE, CURL_LOCK_DATA_SSL_SESSION);
 }
 
 /* Close and free all handles in the pool. */
@@ -108,6 +137,12 @@ unload_pool (void)
   for (i = 0; i < curl_handles.len; ++i)
     free_handle (curl_handles.ptr[i]);
   curl_handle_list_reset (&curl_handles);
+
+  /* Free share data. */
+  curl_share_cleanup (share);
+
+  for (i = 0; i < ARRAY_SIZE (sharelocks); ++i)
+    pthread_rwlock_destroy (&sharelocks[i]);
 }
 
 /* Get a handle from the pool.
@@ -230,6 +265,9 @@ allocate_handle (void)
     goto err;
   }
 
+  /* Share settings with other handles. */
+  curl_easy_setopt (ch->c, CURLOPT_SHARE, share);
+
   /* Various options we always set.
    *
    * NB: Both here and below constants must be explicitly long because
@@ -621,3 +659,30 @@ error_cb (char *ptr, size_t size, size_t nmemb, void
*opaque)
   return 0; /* in older curl, any size < requested will also be an error */
 #endif
 }
+
+static void
+lock_cb (CURL *handle, curl_lock_data data, curl_lock_access access,
+         void *userptr)
+{
+  assert (data < ARRAY_SIZE (sharelocks));
+
+  switch (access) {
+  case CURL_LOCK_ACCESS_SHARED:
+    pthread_rwlock_rdlock (&sharelocks[data]);
+    break;
+  case CURL_LOCK_ACCESS_SINGLE:
+    pthread_rwlock_wrlock (&sharelocks[data]);
+    break;
+  default:
+    /* CURL_LOCK_ACCESS_NONE is never used in the current libcurl code. */
+    abort ();
+  }
+}
+
+static void
+unlock_cb (CURL *handle, curl_lock_data data, void *userptr)
+{
+  assert (data < ARRAY_SIZE (sharelocks));
+
+  pthread_rwlock_unlock (&sharelocks[data]);
+}
-- 
2.41.0