complete out the os.sched_getaffinity error set

lib/std/os.zig

@@ -4432,15 +4432,30 @@ pub fn clock_getres(clk_id: i32, res: *timespec) ClockGetTimeError!void {
     }
 }
 
-pub const SchedGetAffinityError = error{PermissionDenied} || UnexpectedError;
+pub const SchedGetAffinityError = error{
+    /// The calling thread does not have appropriate privileges.
+    PermissionDenied,
+
+    /// Memory address was invalid.
+    InvalidAddress,
+
+    /// The affinity bit mask mask contains no processors that are currently physically on the system and permitted to the process.
+    NoProcessors,
+
+    /// The thread whose ID is pid could not be found.
+    NoThread,
+} || UnexpectedError;
 
+/// Retrieves the affinity mask of a thread.
+///
+/// A thread's CPU affinity mask determines the set of CPUs on which it is eligible to run.
 pub fn sched_getaffinity(pid: pid_t) SchedGetAffinityError!cpu_set_t {
     var set: cpu_set_t = undefined;
     switch (errno(system.sched_getaffinity(pid, @sizeOf(cpu_set_t), &set))) {
         0 => return set,
-        EFAULT => unreachable,
-        EINVAL => unreachable,
-        ESRCH => unreachable,
+        EFAULT => return error.InvalidAddress,
+        EINVAL => return error.NoProcessors,
+        ESRCH => return error.NoThread,
         EPERM => return error.PermissionDenied,
         else => |err| return unexpectedErrno(err),
     }

lib/std/os/test.zig

@@ -601,3 +601,12 @@ test "getrlimit and setrlimit" {
     const cpuLimit = try os.getrlimit(.CPU);
     try os.setrlimit(.CPU, cpuLimit);
 }
+
+test "os.sched_getaffinity" {
+    // TODO enable for other systems when implemented
+    // https://github.com/ziglang/zig/issues/6907
+    if (builtin.os.tag != .linux) {
+        return error.SkipZigTest;
+    }
+    testing.expectError(error.NoThread, os.sched_getaffinity(std.math.maxInt(i32)));
+}

lib/std/thread.zig

@@ -63,10 +63,10 @@ pub const Thread = struct {
             return c.pthread_self();
         } else
             return switch (std.Target.current.os.tag) {
-            .linux => os.linux.gettid(),
-            .windows => windows.kernel32.GetCurrentThreadId(),
-            else => @compileError("Unsupported OS"),
-        };
+                .linux => os.linux.gettid(),
+                .windows => windows.kernel32.GetCurrentThreadId(),
+                else => @compileError("Unsupported OS"),
+            };
     }
 
     /// Returns the handle of this thread.
@@ -479,6 +479,9 @@ pub const Thread = struct {
 
     pub const CpuCountError = error{
         PermissionDenied,
+        InvalidAddress,
+        NoProcessors,
+        NoThread,
         SystemResources,
         Unexpected,
     };