tree-wide: expose "p"-suffix unref calls in public APIs to make gcc cleanup easy

GLIB has recently started to officially support the gcc cleanup
attribute in its public API, hence let's do the same for our APIs.

With this patch we'll define an xyz_unrefp() call for each public
xyz_unref() call, to make it easy to use inside a
__attribute__((cleanup())) expression. Then, all code is ported over to
make use of this.

The new calls are also documented in the man pages, with examples how to
use them (well, I only added docs where the _unref() call itself already
had docs, and the examples, only cover sd_bus_unrefp() and
sd_event_unrefp()).

This also renames sd_lldp_free() to sd_lldp_unref(), since that's how we
tend to call our destructors these days.

Note that this defines no public macro that wraps gcc's attribute and
makes it easier to use. While I think it's our duty in the library to
make our stuff easy to use, I figure it's not our duty to make gcc's own
features easy to use on its own. Most likely, client code which wants to
make use of this should define its own:

       #define _cleanup_(function) __attribute__((cleanup(function)))

Or similar, to make the gcc feature easier to use.

Making this logic public has the benefit that we can remove three header
files whose only purpose was to define these functions internally.

See #2008.

1 files changed, 40 insertions, 8 deletions

diff --git a/man/sd_bus_new.xml b/man/sd_bus_new.xml
index e1cab6e567..d281b5dd44 100644
--- a/man/sd_bus_new.xml
+++ b/man/sd_bus_new.xml
@@ -46,6 +46,7 @@
     <refname>sd_bus_new</refname>
     <refname>sd_bus_ref</refname>
     <refname>sd_bus_unref</refname>
+    <refname>sd_bus_unrefp</refname>
 
     <refpurpose>Create a new bus object and create or destroy references to it</refpurpose>
   </refnamediv>
@@ -68,6 +69,11 @@
         <funcdef>sd_bus *<function>sd_bus_unref</function></funcdef>
         <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
       </funcprototype>
+
+      <funcprototype>
+        <funcdef>void <function>sd_bus_unrefp</function></funcdef>
+        <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
+      </funcprototype>
     </funcsynopsis>
   </refsynopsisdiv>
 
@@ -93,14 +99,40 @@
     only allocate a bus object but also start the connection to a
     well-known bus in a single function invocation.</para>
 
-    <para><function>sd_bus_ref()</function> creates a new reference to
-    <parameter>bus</parameter>.</para>
+    <para><function>sd_bus_ref()</function> increases the reference
+    counter of <parameter>bus</parameter> by one.</para>
 
-    <para><function>sd_bus_unref()</function> destroys a reference to
-    <parameter>bus</parameter>. Once the reference count has dropped
-    to zero, <parameter>bus</parameter> cannot be used anymore, so
-    further calls to <function>sd_bus_ref()</function> or
+    <para><function>sd_bus_unref()</function> decreases the reference
+    counter of <parameter>bus</parameter> by one. Once the reference
+    count has dropped to zero, <parameter>bus</parameter> is destroyed
+    and cannot be used anymore, so further calls to
+    <function>sd_bus_ref()</function> or
     <function>sd_bus_unref()</function> are illegal.</para>
+
+    <para><function>sd_bus_unrefp()</function> is similar to
+    <function>sd_bus_unref()</function> but takes a pointer to a
+    pointer to an <type>sd_bus</type> object. This call is useful in
+    conjunction with GCC's and LLVM's <ulink
+    url="https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html">Clean-up
+    Variable Attribute</ulink>. Note that this function is defined as
+    inline function. Use a declaration like the following, in order to
+    allocate a bus object that is freed automatically as the code
+    block is left:</para>
+
+    <programlisting>{
+        __attribute__((cleanup(sd_bus_unrefp)) sd_bus *bus = NULL;
+        int r;
+        …
+        r = sd_bus_default(&amp;bus);
+        if (r &lt; 0)
+                fprintf(stderr, "Failed to allocate bus: %s\n", strerror(-r));
+        …
+}</programlisting>
+
+    <para><function>sd_bus_ref()</function>,
+    <function>sd_bus_unref()</function> and
+    <function>sd_bus_unrefp()</function> execute no operation if the
+    passed in bus object is <constant>NULL</constant>.</para>
   </refsect1>
 
   <refsect1>
@@ -110,10 +142,10 @@
     positive integer. On failure, it returns a negative errno-style
     error code.</para>
 
-    <para><function>sd_bus_ref</function> always returns the argument.
+    <para><function>sd_bus_ref()</function> always returns the argument.
     </para>
 
-    <para><function>sd_bus_unref</function> always returns
+    <para><function>sd_bus_unref()</function> always returns
     <constant>NULL</constant>.</para>
   </refsect1>