Add a Getting Started document for the Static Linux SDK. (#686)

* Add a Getting Started document for the Static Linux SDK.

First pass at a Getting Started document for the Static Linux SDK.

rdar://129217209

* Add a link to the Articles section

* Fixed a missing line, and use console everywhere.

* Added a note about versions

* Add SDK URL

* Updated after review comments

Author: al45tair

_data/documentation.yaml

@@ -58,6 +58,11 @@
     url: /documentation/concurrency/
     description: |
         Prepare for Swift 6 by enabling complete concurrency checking in your SwiftPM packages, Xcode projects, and CI scripts.
+  - title: Getting Started with the Static Linux SDK
+    url: /documentation/articles/static-linux-getting-started.html
+    description: |
+      Learn how to get started building binaries for Linux with no system dependencies (not even the Swift runtime or C library).
+      Even better, you can do this from any system with a Swift toolchain, allowing you to develop on macOS or Windows and easily deploy to Linux when you go to production.
 #----------------------------------------------------
 - header: Contributing
   pages:

documentation/articles/static-linux-getting-started.md

@@ -0,0 +1,263 @@
+---
+layout: page
+date: 2024-06-04 12:00:00
+title: Getting Started with the Static Linux SDK
+author: [al45tair]
+---
+
+It's well known that Swift can be used to build software for Apple
+platforms such as macOS or iOS, but Swift is also supported on other
+platforms, including Linux and Windows.
+
+Building for Linux is especially interesting because, historically,
+Linux programs written in Swift needed to ensure that a copy of the
+Swift runtime---and all of its dependencies---was installed on the
+target system.  Additionally, a program built for a particular
+distribution, or even a particular major version of a particular
+distribution, would not necessarily run on any other distribution or
+in some cases even on a different major version of the same
+distribution.
+
+The Swift Static Linux SDK solves both of these problems by allowing
+you to build your program as a _fully statically linked_ executable,
+with no external dependencies at all (not even the C library), which
+means that it will run on _any_ Linux distribution as the only thing
+it depends on is the Linux system call interface.
+
+Additionally, the Static Linux SDK can be used from any platform
+supported by the Swift compiler and package manager; this means that
+you can develop and test your program on macOS before building and
+deploying it to a Linux-based server, whether running locally or
+somewhere in the cloud.
+
+### Static vs Dynamic Linking
+
+_Linking_ is the process of taking different pieces of a computer
+program and wiring up any references between those pieces.  For
+_static_ linking, generally speaking those pieces are _object files_,
+or _static libraries_ (which are really just collections of object
+files).
+
+For _dynamic_ linking, the pieces are _executables_ and _dynamic
+libraries_ (aka dylibs, shared objects, or DLLs).
+
+There are two key differences between dynamic and static linking:
+
+* The time at which linking takes place.  Static linking happens when
+  you build your program; dynamic linking happens at runtime.
+
+* The fact that a static library (or _archive_) is really a collection
+  of individual object files, whereas a dynamic library is monolithic.
+
+The latter is important because traditionally, the static linker will
+include every object explicitly listed on its command line, but it
+will _only_ include an object from a static library if doing so lets
+it resolve an unresolved symbolic reference.  If you statically link
+against a library that you do not actually use, a traditional static
+linker will completely discard that library and not include any code
+from it in your final binary.
+
+In practice, things can be more complicated---the static linker may
+actually work on the basis of individual _sections_ or _atoms_ from
+your object files, so it may in fact be able to discard individual
+functions or pieces of data rather than just whole objects.
+
+### Pros and Cons of Static Linking
+
+Pros of static linking:
+
+* No runtime overhead.
+
+* Only include code from libraries that is actually needed.
+
+* No need for separately installed dynamic libraries.
+
+* No versioning issues at runtime.
+
+Cons of static linking:
+
+* Programs cannot share code (higher overall memory usage).
+
+* No way to update dependencies without rebuilding program.
+
+* Larger executables (though this can be offset by not having to
+  install separate dynamic libraries).
+
+On Linux in particular, it's also possible to use static linking to
+completely eliminate dependencies on system libraries supplied by the
+distribution, resulting in executables that work on any distribution
+and can be installed by simply copying.
+
+### Installing the SDK
+
+Before you start, it's important to note:
+
+* You will need to [install an Open Source toolchain from
+  swift.org](https://www.swift.org/install/).
+
+* You cannot use the toolchain provided with Xcode to build programs
+  using the SDK.
+
+* If you are using macOS, you will also need to ensure that you use
+  the Swift compiler from this toolchain by [following the
+  instructions
+  here](https://www.swift.org/install/macos/#installation-via-swiftorg-package-installer).
+
+* The toolchain must match the version of the Static Linux SDK that
+  you install.  The Static Linux SDK includes the corresponding Swift
+  version in its filename to help identify the correct version of the
+  SDK.
+
+Once that is out of the way, actually installing the Static Linux SDK
+is easy; at a prompt, enter
+
+```console
+$ swift sdk install <URL-or-filename-here>
+```
+
+giving the URL or filename at which the SDK can be found.
+
+For instance, assuming you have installed the
+`swift-6.0-DEVELOPMENT-SNAPSHOT-2024-06-06-a` toolchain, you would
+need to enter
+
+```console
+$ swift sdk install https://download.swift.org/development/static-sdk/swift-DEVELOPMENT-SNAPSHOT-2024-06-06-a/swift-DEVELOPMENT-SNAPSHOT-2024-06-06-a_static-linux-0.0.1.artifactbundle.tar.gz
+```
+
+to install the corresponding Static Linux SDK.
+
+Swift will download and install the SDK on your system.  You can get a
+list of installed SDKs with
+
+```console
+$ swift sdk list
+```
+
+and it's also possible to remove them using
+
+```console
+$ swift sdk remove <name-of-SDK>
+```
+
+### Your first statically linked Linux program
+
+First, create a directory to hold your code:
+
+```console
+$ mkdir hello
+$ cd hello
+```
+
+Next, ask Swift to create a new program package for you:
+
+```console
+$ swift package init --type executable
+```
+
+You can build and run this locally:
+
+```console
+$ swift build
+Building for debugging...
+[8/8] Applying hello
+Build complete! (15.29s)
+$ .build/debug/hello
+Hello, world!
+```
+
+But with the Static Linux SDK installed, you can also build Linux
+binaries for x86-64 and ARM64 machines:
+
+```console
+$ swift build --sdk x86_64-swift-linux-musl
+Building for debugging...
+[8/8] Linking hello
+Build complete! (2.04s)
+$ file .build/x86_64-swift-linux-musl/debug/hello
+.build/x86_64-swift-linux-musl/debug/hello: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, with debug_info, not stripped
+```
+
+```console
+$ swift build --sdk aarch64-swift-linux-musl
+Building for debugging...
+[8/8] Linking hello
+Build complete! (2.00s)
+$ file .build/aarch64-swift-linux-musl/debug/hello
+.build/aarch64-swift-linux-musl/debug/hello: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, with debug_info, not stripped
+```
+
+These can be copied to an appropriate Linux-based system and executed:
+
+```console
+$ scp .build/x86_64-swift-linux-musl/debug/hello linux:~/hello
+$ ssh linux ~/hello
+Hello, world!
+```
+
+### What about package dependencies?
+
+Swift packages that make use of Foundation or Swift NIO should just
+work.  If you try to use a package that uses the C library, however,
+you may have a little work to do.  Such packages often contain files
+with code like the following:
+
+```swift
+#if os(macOS) || os(iOS)
+import Darwin
+#elseif os(Linux)
+import Glibc
+#elseif os(Windows)
+import ucrt
+#else
+#error(Unknown platform)
+#endif
+```
+
+The Static Linux SDK does not use Glibc; instead, it is built on top
+of an alternative C library for Linux called
+[Musl](https://musl-libc.org).  We chose this approach for two
+reasons:
+
+1. Musl has excellent support for static linking.
+
+2. Musl is permissively licensed, which makes it easy to distribute
+   executables statically linked with it.
+
+If you are using such a dependency, you will therefore need to adjust
+it to import the `Musl` module instead of the `Glibc` module:
+
+```swift
+#if os(macOS) || os(iOS)
+import Darwin
+#elseif canImport(Glibc)
+import Glibc
+#elseif canImport(Musl)
+import Musl
+#elseif os(Windows)
+import ucrt
+#else
+#error(Unknown platform)
+#endif
+```
+
+Occasionally there might be a difference between the way a C library
+type gets imported between Musl and Glibc; this sometimes happens if
+someone has added nullability annotations, or where a pointer type is
+using a forward-declared `struct` for which no actual definition is
+ever provided.  Usually the problem will be obvious---a function
+argument or result will be `Optional` in one case and non-`Optional`
+in another, or a pointer type will be imported as `OpaquePointer`
+rather than `UnsafePointer<FOO>`.
+
+If you do find yourself needing to make these kinds of adjustments,
+you can make your local copy of the package dependency editable by
+doing
+
+```console
+$ swift package edit SomePackage
+```
+
+and then editing the files in the `Packages` directory that appears in
+your program's source directory.  You may wish to consider raising PRs
+upstream with any fixes you may have.