Fully Modified Swift

[Magdalena Liendo]

Toachieve memorysafety,Swift requires exclusive access to a variable in order to modify thatvariable. In essence, a variable cannot be accessed via a differentname for the duration in which the same variable is being modified asan inout argument or as self within a mutating method.

Compile-time (static) diagnostics catch many common exclusivityviolations, but run-time (dynamic) diagnostics are also required tocatch violations involving escaping closures, properties of classtypes, static properties, and global variables. Swift 4.0 providedboth compile-time and run-time enforcement, but run-time enforcementwas only enabled in Debug builds.

While disabling run-time checks may work around a performanceregression, it does not mean that exclusivity violations aresafe. Without enforcement enabled, the programmer must takeresponsibility for obeying exclusivity rules. Disabling run-timechecks in Release builds is strongly discouraged because, if theprogram violates exclusivity, then it could exhibit unpredictablebehavior, including crashes or memory corruption. Even if the programappears to function correctly today, future release of Swift couldcause additional unpredictable behavior to surface, and securityexploits may be exposed.

The runtime diagnostics capture the information that an access startedat the call to modifyX and that a conflicting access occurred withinthe getY closure, along with a backtrace showing the path leading tothe conflict:

At this point, the reader may wonder why the original example isconsidered a violation of exclusivity when two separate properties arewritten and read; point.x and point.y. Because Point is declaredas a struct, it is considered a value type, meaning that all of itsproperties are part of a whole value, and accessing one propertyaccesses the entire value. The compiler makes exception to this rulewhen it can prove safety via a straighforward static analysis. Inparticular, when same statement initiates accesses of two disjointstored properties, the compiler avoids reporting an exclusivityviolation. In the next example, the statement that calls modifyXfirst accesses point in order to immediately pass its property xas inout. The same statement accesses point a second time in orderto capture it in a closure. Since the compiler can immediately see that the captured value is only used to access property y, there is no error.

Only modifications of the first kind of property (instance properties)require exclusivity access to entire storage of the aggregate value asshown in the struct Point example above. The other two kinds ofproperties are enforced separately, as independent storage. If thisexample is converted to a class, the original exclusivity violationgoes away:

As programs scale in size, it becomes increasingly likely for routinesto interact in unexpected ways. The following example is similar inspirit to the Array.append(removingFrom:) example above, whereexclusivity enforcement is needed to prevent the programmer frompassing the same variable as both the source and destination of amove. But notice that, once classes are involved, it becomes mucheasier for programs to unwittingly pass the same instance of Namesin both src and dest position because two variables reference thesame object. Again, this causes an infinite loop:

Prior to Swift 4, exclusivity was necessary for well defined programbehavior, but the rules were unenforced. In practice, it is easy toviolate these rules in subtle ways, leaving programs susceptible tounpredictable behavior, particularly across releases of the compiler.

Failing to fully enforce exclusivity would have an unpredictableimpact on ABI stability. Existing binaries built without fullenforcement may function correctly in one release but behaveincorrectly in future versions of the compiler, standard library,and runtime.

A guarantee of exclusivity on inout parameters and mutatingmethods provides important information to the compiler, which it canuse to optimize memory access and reference countingoperations. Simply declaring an unspecified behavior rule, asdescribed in point #2 above, is an insufficient guarantee for thecompiler given that Swift is a memory safe language. Full exclusivityenforcement allows the compiler to optimize based on memoryexclusivity without sacrificing memory safety.

By shipping with full exclusivity enforcement enabled in Releasebuilds, Swift 5 helps to eliminate bugs and security issues, ensurebinary compatibility, and enable future optimizations and languagefeatures.

This craft harks back to the days of the stupendous P.666 v.Prototype, in the context of it being the perfect AG racer of its time. It certainly seems that way with significantly superior thrust and stability, and its speed and weapon power that rivals quite a few of the top teams (it only loses to Natasha Belmondo's Xios SMF-X in terms of weapon power). Based on its numbers alone, it boasts superlatives with speed, thrust and lateral stability, and is the lightest craft in the entire game.

The only front that lets it down is the shielding. Unusually for a fully modified craft, neither one exceeds 100 collision points, making it quite fragile. The brake force is a similar problem, but unnecessary due to the craft's tight handling on its own. It is also ironic as Piranha once manufactured airbrakes back in the earlier days of the sport.

How much time per sq. meter do your machines take to clean solar panels?

To provide the fastest possible ROI to our customers, we developed a range of robots to best address the needs of various solar plant layouts. A large utility-scale project with high level of soiling losses in a desert environment will need a very fast and reactive cleaning solution such as our SolarBridge B1, which can clean 24/7/365 fully autonomously. The most suitable solution for a farm rooftop in Germany that needs to be cleaned three to four times a year might be our F1 model, which can clean the equivalent of up to two soccer fields a day. It is designed for rooftops, floating panels and midsize plants up to 50 MW. While the speed of cleaning is a very important variable, the quality of cleaning is often considered as the driver to performance, which is why we propose different types of brushes depending on the soiling types. Plus, the robot speed can be modified according to the soiling level.

What spatial accuracy requirements do the robots have for this task?

Safety is our absolute priority. Therefore, our robots need an accuracy of less than 3 cm. They also need to be aware, in real time, of changes in their surroundings, such as maintenance teams, animals and uneven ground.

On large solar farms, GNSS receivers always have a clear line of sight to the satellites and do not suffer from multipath. So, what are the key technical challenges?

Our robots have the additional advantage that they do not need to drive very fast. However, we need to manage fleets of robots on the other side of the world in regions difficult to access and with harsh weather conditions, such as very high or low temperatures and the accumulation of dust behind panels due to air vortices. We need to be able to perform remote maintenance and solve any issue from our control center in Luxembourg. These challenges make our robots increasingly robust. With a current fleet of more than 300 robots around the world, we collect lessons every day to ensure a greater reliability for our upcoming generations of robots.

Auto-sears and other gun conversion devices can beinstalled in about a minute, and 3-D printed or purchased online for less than$20. An investigation by The Trace and VICE News this March found that federal prosecutions involving gunconversion devices have increased over eight-fold from 2017-2021. Additionallythe investigation reported that the Bureau of Alcohol, Tobacco, Firearms andExplosives (ATF) seized 1,500 weapons modified with auto sears in 2021, afive-fold increase from the year before .

"As communities across the country are sufferingunprecedented levels of gun violence, it is critical that Congress take swiftaction to address the proliferation of autosears and other conversion devices.These small devices are capable of grave damage and destruction. When used witha semiautomatic firearm, conversion devices enable shooters to replicatemachine gun fire, making them a deadly threat to public safety. We applaudSenator Klobuchar for her leadership and unwavering commitment to ending gunviolence," said Adzi Vokhiwa, Giffords Federal Affairs Director.

The frozen copy is an immutable collection which contains the same data as this collectioncurrently contains, but will not update when writes are made to the containing Realm. Unlikelive collections, frozen collections can be accessed from any thread.

The block will be asynchronously called with the initial sectioned results collection, and then called again after each writetransaction which changes either any of the objects in the sectioned results collection, or which objects are in the sectioned results collection.

The change parameter that is passed to the block reports, in the form of indices within the collection, which ofthe objects were added, removed, or modified during each write transaction. See the SectionedResultsChangedocumentation for more information on the change information supplied and an example of how to use it to update aUITableView.

At the time when the block is called, the collection will be fully evaluated and up-to-date, and as long as you donot perform a write transaction on the same thread or explicitly call realm.refresh(), accessing it will neverperform blocking work.

For example, the following code performs a write transaction immediately after adding the notification block, sothere is no opportunity for the initial notification to be delivered first. As a result, the initial notificationwill reflect the state of the Realm after the write transaction.

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