gstreamer: Directly format into a NUL-terminated C string for debug log messages

And also replace `%` with `%%` for < 1.20 inline, and for >= 1.20 use
the new `gst_debug_log_literal()` function that doesn't require this and
also reduces string allocations further.

gstreamer/src/log.rs

@@ -155,6 +155,7 @@ impl DebugCategory {
 
     #[inline]
     #[doc(alias = "gst_debug_log")]
+    #[doc(alias = "gst_debug_log_literal")]
     pub fn log<O: IsA<glib::Object>>(
         self,
         obj: Option<&O>,
@@ -180,16 +181,70 @@ impl DebugCategory {
             None => ptr::null_mut(),
         };
 
-        unsafe {
+        #[cfg(feature = "v1_20")]
-            ffi::gst_debug_log(
+        {
-                cat.as_ptr(),
+            let mut w = glib::GStringBuilder::default();
-                level.into_glib(),
+
-                file.to_glib_none().0,
+            // Can't really happen but better safe than sorry
-                module.to_glib_none().0,
+            if fmt::write(&mut w, args).is_err() {
-                line as i32,
+                return;
-                obj_ptr,
+            }
-                fmt::format(args).replace("%", "%%").to_glib_none().0,
+
-            );
+            unsafe {
+                ffi::gst_debug_log_literal(
+                    cat.as_ptr(),
+                    level.into_glib(),
+                    file.to_glib_none().0,
+                    module.to_glib_none().0,
+                    line as i32,
+                    obj_ptr,
+                    w.into_string().to_glib_none().0,
+                );
+            }
+        }
+        #[cfg(not(feature = "v1_20"))]
+        {
+            // Replace literal percentage signs with two so that they are not interpreted as printf
+            // format specifiers
+            struct Write(glib::GStringBuilder);
+            impl fmt::Write for Write {
+                fn write_str(&mut self, mut s: &str) -> Result<(), fmt::Error> {
+                    while let Some((prefix, suffix)) = s.split_once('%') {
+                        self.0.append(prefix);
+                        self.0.append("%%");
+                        s = suffix;
+                    }
+                    self.0.append(s);
+                    Ok(())
+                }
+
+                fn write_char(&mut self, c: char) -> fmt::Result {
+                    if c == '%' {
+                        self.0.append("%%");
+                    } else {
+                        self.0.append_c(c);
+                    }
+                    Ok(())
+                }
+            }
+            let mut w = Write(glib::GStringBuilder::default());
+
+            // Can't really happen but better safe than sorry
+            if fmt::write(&mut w, args).is_err() {
+                return;
+            }
+
+            unsafe {
+                ffi::gst_debug_log(
+                    cat.as_ptr(),
+                    level.into_glib(),
+                    file.to_glib_none().0,
+                    module.to_glib_none().0,
+                    line as i32,
+                    obj_ptr,
+                    w.0.into_string().to_glib_none().0,
+                );
+            }
         }
     }