libostree: Add multiple getter/setter support to OstreeAsyncProgress

OstreeAsyncProgress is thread-safe: it can have keys changed by one
thread while another is getting the same keys (modulo some locking
contention). However, the thread safety is done at the function call
level: if some code calls an OstreeAsyncProgress getter several times,
the key fetches are not atomic with respect to each other.

In the case of contention on the lock, this can result in consumers of
OstreeAsyncProgress data seeing an inconsistent state between the
properties they query, which could result in progress reporting
inaccuracies.

In the uncontested case, this results in the OstreeAsyncProgress lock
being locked and unlocked many times more than necessary.

Try to improve this by adding new API, which supports getting and
setting multiple keys atomically:
 • ostree_async_progress_get()
 • ostree_async_progress_set()

The new API uses GVariants and varargs: keys are passed as a
GVariantType string followed by arguments as for g_variant_new() or
g_variant_get(), followed by the next key, etc.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #819
Approved by: cgwalters

1 files changed, 134 insertions, 0 deletions

diff --git a/src/libostree/ostree-async-progress.c b/src/libostree/ostree-async-progress.c
index 68f4c834..4927a9de 100644
--- a/src/libostree/ostree-async-progress.c
+++ b/src/libostree/ostree-async-progress.c
@@ -157,6 +157,68 @@ ostree_async_progress_get_uint64 (OstreeAsyncProgress       *self,
   return (rval != NULL) ? g_variant_get_uint64 (rval) : 0;
 }
 
+/**
+ * ostree_async_progress_get:
+ * @self: an #OstreeAsyncProgress
+ * @...: key name, format string, #GVariant return locations, …, followed by %NULL
+ *
+ * Get the values corresponding to zero or more keys from the
+ * #OstreeAsyncProgress. Each key is specified in @... as the key name, followed
+ * by a #GVariant format string, followed by the necessary arguments for that
+ * format string, just as for g_variant_get(). After those arguments is the
+ * next key name. The varargs list must be %NULL-terminated.
+ *
+ * Each format string must make deep copies of its value, as the values stored
+ * in the #OstreeAsyncProgress may be freed from another thread after this
+ * function returns.
+ *
+ * This operation is thread-safe, and all the keys are queried atomically.
+ *
+ * |[<!-- language="C" -->
+ * guint32 outstanding_fetches;
+ * guint64 bytes_received;
+ * g_autofree gchar *status = NULL;
+ * g_autoptr(GVariant) refs_variant = NULL;
+ *
+ * ostree_async_progress_get (progress,
+ *                            "outstanding-fetches", "u", &outstanding_fetches,
+ *                            "bytes-received", "t", &bytes_received,
+ *                            "status", "s", &status,
+ *                            "refs", "@a{ss}", &refs_variant,
+ *                            NULL);
+ * ]|
+ *
+ * Since: 2017.6
+ */
+void
+ostree_async_progress_get (OstreeAsyncProgress *self,
+                           ...)
+{
+  va_list ap;
+  const char *key, *format_string;
+
+  g_mutex_lock (&self->lock);
+  va_start (ap, self);
+
+  for (key = va_arg (ap, const char *), format_string = va_arg (ap, const char *);
+       key != NULL;
+       key = va_arg (ap, const char *), format_string = va_arg (ap, const char *))
+    {
+      GVariant *variant;
+
+      g_assert (format_string != NULL);
+
+      variant = g_hash_table_lookup (self->values, GUINT_TO_POINTER (g_quark_from_string (key)));
+      g_assert (variant != NULL);
+      g_assert (g_variant_check_format_string (variant, format_string, TRUE));
+
+      g_variant_get_va (variant, format_string, NULL, &ap);
+    }
+
+  va_end (ap);
+  g_mutex_unlock (&self->lock);
+}
+
 static gboolean
 idle_invoke_async_progress (gpointer user_data)
 {
@@ -206,6 +268,78 @@ ostree_async_progress_get_status (OstreeAsyncProgress       *self)
 }
 
 /**
+ * ostree_async_progress_set:
+ * @self: an #OstreeAsyncProgress
+ * @...: key name, format string, #GVariant parameters, …, followed by %NULL
+ *
+ * Set the values for zero or more keys in the #OstreeAsyncProgress. Each key is
+ * specified in @... as the key name, followed by a #GVariant format string,
+ * followed by the necessary arguments for that format string, just as for
+ * g_variant_new(). After those arguments is the next key name. The varargs list
+ * must be %NULL-terminated.
+ *
+ * g_variant_ref_sink() will be called as appropriate on the #GVariant
+ * parameters, so they may be floating.
+ *
+ * This operation is thread-safe, and all the keys are set atomically.
+ *
+ * |[<!-- language="C" -->
+ * guint32 outstanding_fetches = 15;
+ * guint64 bytes_received = 1000;
+ *
+ * ostree_async_progress_set (progress,
+ *                            "outstanding-fetches", "u", outstanding_fetches,
+ *                            "bytes-received", "t", bytes_received,
+ *                            "status", "s", "Updated status",
+ *                            "refs", "@a{ss}", g_variant_new_parsed ("@a{ss} {}"),
+ *                            NULL);
+ * ]|
+ *
+ * Since: 2017.6
+ */
+void
+ostree_async_progress_set (OstreeAsyncProgress *self,
+                           ...)
+{
+  va_list ap;
+  const char *key, *format_string;
+  gboolean changed;
+
+  g_mutex_lock (&self->lock);
+
+  if (self->dead)
+    goto out;
+
+  va_start (ap, self);
+
+  for (key = va_arg (ap, const char *), format_string = va_arg (ap, const char *);
+       key != NULL;
+       key = va_arg (ap, const char *), format_string = va_arg (ap, const char *))
+    {
+      GVariant *orig_value;
+      g_autoptr(GVariant) new_value = NULL;
+      gpointer qkey = GUINT_TO_POINTER (g_quark_from_string (key));
+
+      new_value = g_variant_ref_sink (g_variant_new_va (format_string, NULL, &ap));
+
+      if (g_hash_table_lookup_extended (self->values, qkey, NULL, (gpointer *) &orig_value) &&
+          g_variant_equal (orig_value, new_value))
+        continue;
+
+      g_hash_table_replace (self->values, qkey, g_steal_pointer (&new_value));
+      changed = TRUE;
+    }
+
+  va_end (ap);
+
+  if (changed)
+    ensure_callback_locked (self);
+
+out:
+  g_mutex_unlock (&self->lock);
+}
+
+/**
  * ostree_async_progress_set_variant:
  * @self: an #OstreeAsyncProgress
  * @key: a key to set