[Libguestfs] [PATCH nbdkit] curl: Enable multi-conn for read-only connections

[Richard W.M. Jones]

Multi-conn is enabled only when we know the connection is read-only:

  $ ./nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run ' nbdinfo $uri ' | grep can_multi_conn
	can_multi_conn: true
  $ ./nbdkit curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run ' nbdinfo $uri ' | grep can_multi_conn
	can_multi_conn: false

This improves performance.

Comparing before and after this commit shows approximate doubling of
performance:

  Benchmark 1:  nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run "nbdcopy -p \$uri null:"
    Time (mean ± σ):     943.8 ms ±  18.8 ms    [User: 316.2 ms, System: 1029.7 ms]
    Range (min … max):   923.7 ms … 989.2 ms    10 runs

  Benchmark 2:  ~/d/nbdkit/nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run "nbdcopy -p \$uri null:"
    Time (mean ± σ):     455.0 ms ±   6.2 ms    [User: 542.2 ms, System: 1824.7 ms]
    Range (min … max):   449.1 ms … 471.6 ms    10 runs

  Summary
    ' ~/d/nbdkit/nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run "nbdcopy -p \$uri null:" ' ran
      2.07 ± 0.05 times faster than ' nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run "nbdcopy -p \$uri null:" '

See also:
https://listman.redhat.com/archives/libguestfs/2023-February/030581.html
---
 plugins/curl/curldefs.h |  1 +
 plugins/curl/curl.c     | 14 ++++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/plugins/curl/curldefs.h b/plugins/curl/curldefs.h
index a2c521c50..16b6071cc 100644
--- a/plugins/curl/curldefs.h
+++ b/plugins/curl/curldefs.h
@@ -68,6 +68,7 @@ extern const char *user_agent;
 /* The per-connection handle. */
 struct curl_handle {
   CURL *c;
+  int readonly;
   bool accept_range;
   int64_t exportsize;
   char errbuf[CURL_ERROR_SIZE];
diff --git a/plugins/curl/curl.c b/plugins/curl/curl.c
index 42aba4216..2f34d1dc1 100644
--- a/plugins/curl/curl.c
+++ b/plugins/curl/curl.c
@@ -465,6 +465,7 @@ curl_open (int readonly)
     nbdkit_error ("calloc: %m");
     return NULL;
   }
+  h->readonly = readonly;
 
   h->c = curl_easy_init ();
   if (h->c == NULL) {
@@ -781,6 +782,18 @@ curl_get_size (void *handle)
   return h->exportsize;
 }
 
+/* Multi-conn is safe for read-only connections, but HTTP does not
+ * have any concept of flushing so we cannot use it for read-write
+ * connections.
+ */
+static int
+curl_can_multi_conn (void *handle)
+{
+  struct curl_handle *h = handle;
+
+  return !! h->readonly;
+}
+
 /* NB: The terminology used by libcurl is confusing!
  *
  * WRITEFUNCTION / write_cb is used when reading from the remote server
@@ -924,6 +937,7 @@ static struct nbdkit_plugin plugin = {
   .open              = curl_open,
   .close             = curl_close,
   .get_size          = curl_get_size,
+  .can_multi_conn    = curl_can_multi_conn,
   .pread             = curl_pread,
   .pwrite            = curl_pwrite,
 };
-- 
2.39.0