[Qemu-devel] [PATCH v3 34/44] nbd: Less allocation during NBD_OPT_LIST

[Eric Blake <eblake@redhat.com>]

Since we know that the maximum name we are willing to accept
is small enough to stack-allocate, rework the iteration over
NBD_OPT_LIST responses to reuse a stack buffer rather than
allocating every time.  Furthermore, we don't even have to
allocate if we know the server's length doesn't match what
we are searching for.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alex Bligh <alex@alex.org.uk>

---
v3: tweak commit message
---
 nbd/client.c | 130 +++++++++++++++++++++++++++++------------------------------
 1 file changed, 65 insertions(+), 65 deletions(-)

diff --git a/nbd/client.c b/nbd/client.c
index d346c86..4debb5d 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -230,14 +230,17 @@ static int nbd_handle_reply_err(QIOChannel *ioc, nbd_opt_reply *reply,
     return result;
 }

-static int nbd_receive_list(QIOChannel *ioc, char **name, Error **errp)
+/* Return -1 if unrecoverable error occurs, 0 if NBD_OPT_LIST is
+ * unsupported, 1 if iteration is done, 2 to keep looking, and 3 if
+ * this entry matches want. */
+static int nbd_receive_list(QIOChannel *ioc, const char *want, Error **errp)
 {
     nbd_opt_reply reply;
     uint32_t len;
     uint32_t namelen;
+    char name[NBD_MAX_NAME_SIZE + 1];
     int error;

-    *name = NULL;
     if (nbd_receive_option_reply(ioc, NBD_OPT_LIST, &reply, errp) < 0) {
         return -1;
     }
@@ -252,97 +255,94 @@ static int nbd_receive_list(QIOChannel *ioc, char **name, Error **errp)
             error_setg(errp, "length too long for option end");
             return -1;
         }
-    } else if (reply.type == NBD_REP_SERVER) {
-        if (len < sizeof(namelen) || len > NBD_MAX_BUFFER_SIZE) {
-            error_setg(errp, "incorrect option length %"PRIu32, len);
-            return -1;
-        }
-        if (read_sync(ioc, &namelen, sizeof(namelen)) != sizeof(namelen)) {
-            error_setg(errp, "failed to read option name length");
-            return -1;
-        }
-        namelen = be32_to_cpu(namelen);
-        len -= sizeof(namelen);
-        if (len < namelen) {
-            error_setg(errp, "incorrect option name length");
-            return -1;
-        }
-        if (namelen > NBD_MAX_NAME_SIZE) {
-            error_setg(errp, "export name length too long %" PRIu32, namelen);
-            return -1;
-        }
-
-        *name = g_new0(char, namelen + 1);
-        if (read_sync(ioc, *name, namelen) != namelen) {
-            error_setg(errp, "failed to read export name");
-            g_free(*name);
-            *name = NULL;
-            return -1;
-        }
-        (*name)[namelen] = '\0';
-        len -= namelen;
-        if (drop_sync(ioc, len) != len) {
-            error_setg(errp, "failed to read export description");
-            g_free(*name);
-            *name = NULL;
-            return -1;
-        }
-    } else {
+        return 1;
+    } else if (reply.type != NBD_REP_SERVER) {
         error_setg(errp, "Unexpected reply type %" PRIx32 " expected %x",
                    reply.type, NBD_REP_SERVER);
         return -1;
     }
-    return 1;
+
+    if (len < sizeof(namelen) || len > NBD_MAX_BUFFER_SIZE) {
+        error_setg(errp, "incorrect option length %"PRIu32, len);
+        return -1;
+    }
+    if (read_sync(ioc, &namelen, sizeof(namelen)) != sizeof(namelen)) {
+        error_setg(errp, "failed to read option name length");
+        return -1;
+    }
+    namelen = be32_to_cpu(namelen);
+    len -= sizeof(namelen);
+    if (len < namelen) {
+        error_setg(errp, "incorrect option name length");
+        return -1;
+    }
+    if (namelen != strlen(want)) {
+        if (drop_sync(ioc, len) != len) {
+            error_setg(errp, "failed to skip export name with wrong length");
+            return -1;
+        }
+        return 2;
+    }
+
+    assert(namelen < sizeof(name));
+    if (read_sync(ioc, name, namelen) != namelen) {
+        error_setg(errp, "failed to read export name");
+        return -1;
+    }
+    name[namelen] = '\0';
+    len -= namelen;
+    if (drop_sync(ioc, len) != len) {
+        error_setg(errp, "failed to read export description");
+        return -1;
+    }
+    return strcmp(name, want) == 0 ? 3 : 2;
 }


+/* Return -1 on failure, 0 if wantname is an available export. */
 static int nbd_receive_query_exports(QIOChannel *ioc,
                                      const char *wantname,
                                      Error **errp)
 {
     bool foundExport = false;

-    TRACE("Querying export list");
+    TRACE("Querying export list for '%s'", wantname);
     if (nbd_send_option_request(ioc, NBD_OPT_LIST, 0, NULL, errp) < 0) {
         return -1;
     }

     TRACE("Reading available export names");
     while (1) {
-        char *name = NULL;
-        int ret = nbd_receive_list(ioc, &name, errp);
+        int ret = nbd_receive_list(ioc, wantname, errp);

-        if (ret < 0) {
-            g_free(name);
-            name = NULL;
+        switch (ret) {
+        default:
+            /* Server gave unexpected reply */
+            assert(ret < 0);
             return -1;
-        }
-        if (ret == 0) {
+        case 0:
             /* Server doesn't support export listing, so
              * we will just assume an export with our
              * wanted name exists */
-            foundExport = true;
-            break;
-        }
-        if (name == NULL) {
-            TRACE("End of export name list");
+            return 0;
+        case 1:
+            /* Done iterating. */
+            if (!foundExport) {
+                error_setg(errp, "No export with name '%s' available",
+                           wantname);
+                return -1;
+            }
+            return 0;
+        case 2:
+            /* Wasn't this one, keep going. */
             break;
-        }
-        if (g_str_equal(name, wantname)) {
+        case 3:
+            /* Found a match, but must finish parsing reply. */
+            TRACE("Found desired export name '%s'", wantname);
             foundExport = true;
-            TRACE("Found desired export name '%s'", name);
-        } else {
-            TRACE("Ignored export name '%s'", name);
+            break;
         }
-        g_free(name);
-    }
-
-    if (!foundExport) {
-        error_setg(errp, "No export with name '%s' available", wantname);
-        return -1;
     }
-
-    return 0;
 }

 static QIOChannel *nbd_receive_starttls(QIOChannel *ioc,
-- 
2.5.5