Support building projects for FreeBSD

This adds basic FreeBSD support. Basic, because it works for my few
cases. There are two major differences from how things works for Linux
and rest platforms:

The first is that there is no custom suffix for produced `.so` files
and `imp.get_suffixes()` confirms that.

The second is more complicated. There is no universal platform tag which
covers some OS release. In fact, it includes information about:
- OS version (11_2, 12_0),
- release type (RELEASE, STABLE)
- and patch set (p1, p7, etc).

While first two are pretty well known and could be simply hardcoded
to maintain support for a many years, patch set is very generic and
changes once any security or system fixes get issued for specific
release. From point of FreeBSD it's the same system with the same
compatibility guarantees. From point of Python it's a completely
different platform which is not compatible with the others patches
for the same OS version and release.

That means that wheels need to be rebuild (or just renamed) if system
receives any patch set update.

src/get_interpreter_metadata.py

@@ -15,6 +15,8 @@
     "d": sysconfig.get_config_var("Py_DEBUG") == 1,
     # This one isn't technically necessary, but still very useful for sanity checks
     "platform": sys.platform,
+    # This one needs for FreeBSD to get correct platform tag
+    "release": platform.release().replace(".", '_').replace("-", "_"),
 }
 
 print(json.dumps(metadata))

src/python_interpreter.rs

@@ -262,6 +262,7 @@ struct IntepreterMetadataMessage {
     d: bool,
     platform: String,
     abi_tag: Option<String>,
+    release: String,
 }
 
 /// The location and version of an interpreter
@@ -277,8 +278,8 @@ pub struct PythonInterpreter {
     ///
     /// See PEP 261 and PEP 393 for details
     pub abiflags: String,
-    /// Currently just the value of [Target::os()], i.e. "windows", "linux" or
-    /// "macos"
+    /// Currently just the value of [Target::os()], i.e. "windows", "linux",
+    /// "macos" or "freebsd"
     pub target: Target,
     /// Path to the python interpreter, e.g. /usr/bin/python3.6
     ///
@@ -292,6 +293,8 @@ pub struct PythonInterpreter {
     ///
     /// Note that this always `None` on windows
     pub abi_tag: Option<String>,
+    /// Release tag for FreeBSD. May be useful for other non Windows/Linux/Mac platforms.
+    pub release: String,
 }
 
 /// Returns the abiflags that are assembled through the message, with some
@@ -308,6 +311,7 @@ fn fun_with_abiflags(
         "win32" | "win_amd64" => target.is_windows(),
         "linux" | "linux2" | "linux3" => target.is_linux(),
         "darwin" => target.is_macos(),
+        "freebsd11" | "freebsd12" | "freebsd13" => target.is_freebsd(),
         _ => false,
     };
 
@@ -359,7 +363,16 @@ impl PythonInterpreter {
         match self.interpreter {
             Interpreter::CPython => {
                 let platform = self.target.get_platform_tag(manylinux);
-                if self.target.is_unix() {
+                if self.target.is_freebsd() {
+                    let platform_with_release: &str = &platform.replace("{release}", &self.release);
+                    format!(
+                        "cp{major}{minor}-cp{major}{minor}{abiflags}-{platform}",
+                        major = self.major,
+                        minor = self.minor,
+                        abiflags = self.abiflags,
+                        platform = platform_with_release
+                    )
+                } else if self.target.is_unix() {
                     format!(
                         "cp{major}{minor}-cp{major}{minor}{abiflags}-{platform}",
                         major = self.major,
@@ -416,6 +429,7 @@ impl PythonInterpreter {
     /// Linux:   steinlaus.cpython-35m-x86_64-linux-gnu.so
     /// Windows: steinlaus.cp35-win_amd64.pyd
     /// Mac:     steinlaus.cpython-35m-darwin.so
+    /// FreeBSD: steinlaus.cpython-35m.so
     ///
     /// For pypy3, we read sysconfig.get_config_var("EXT_SUFFIX").
     ///
@@ -426,7 +440,15 @@ impl PythonInterpreter {
             Interpreter::CPython => {
                 let platform = self.target.get_shared_platform_tag();
 
-                if self.target.is_unix() {
+                if self.target.is_freebsd() {
+                    format!(
+                        "{base}.cpython-{major}{minor}{abiflags}.so",
+                        base = base,
+                        major = self.major,
+                        minor = self.minor,
+                        abiflags = self.abiflags,
+                    )
+                } else if self.target.is_unix() {
                     format!(
                         "{base}.cpython-{major}{minor}{abiflags}-{platform}.so",
                         base = base,
@@ -519,6 +541,7 @@ impl PythonInterpreter {
             ext_suffix: message.ext_suffix,
             interpreter,
             abi_tag: message.abi_tag,
+            release: message.release,
         }))
     }
 

src/target.rs

@@ -14,6 +14,7 @@ enum OS {
     Linux,
     Windows,
     Macos,
+    FreeBSD,
 }
 
 /// Decides how to handle manylinux compliance
@@ -60,6 +61,7 @@ impl Target {
             "linux" => OS::Linux,
             "windows" => OS::Windows,
             "macos" => OS::Macos,
+            "freebsd" => OS::FreeBSD,
             unsupported => panic!("The platform {} is not supported", unsupported),
         };
 
@@ -89,6 +91,7 @@ impl Target {
             platforms::target::OS::Linux => OS::Linux,
             platforms::target::OS::Windows => OS::Windows,
             platforms::target::OS::MacOS => OS::Macos,
+            platforms::target::OS::FreeBSD => OS::FreeBSD,
             unsupported => bail!("The operating system {:?} is not supported", unsupported),
         };
 
@@ -120,6 +123,11 @@ impl Target {
         self.os == OS::Linux
     }
 
+    /// Returns true if the current platform is freebsd
+    pub fn is_freebsd(&self) -> bool {
+        self.os == OS::FreeBSD
+    }
+
     /// Returns true if the current platform is mac os
     pub fn is_macos(&self) -> bool {
         self.os == OS::Macos
@@ -147,6 +155,8 @@ impl Target {
             (&OS::Windows, false, _) => "win32",
             (&OS::Macos, true, _) => "macosx_10_7_x86_64",
             (&OS::Macos, false, _) => panic!("32-bit wheels are not supported for mac os"),
+            (&OS::FreeBSD, true, _) => "freebsd_{release}_amd64",
+            (&OS::FreeBSD, false, _) => panic!("32-bit wheels are not supported for FreeBSD"),
         }
     }
 
@@ -169,6 +179,7 @@ impl Target {
                 }
             }
             OS::Macos => "darwin",
+            OS::FreeBSD => "", // according imp.get_suffixes(), there are no such
             OS::Windows => {
                 if self.is_64_bit {
                     "win_amd64"