swift run: Speed up fd closes on macOS/Linux

Today, for everything that isn't the BSDs, we grab the
maximum number of open fds the process supports and then
loop through from 3 -> max, calling close(2) on everything.
Even for a low open fd count of 65k this will typically result
in 99+% of these close's being EBADF. At a 65k nofile RLIMIT
the sluggishness is not really felt, but on systems that may
have this in the millions it is extremely stark. `swift run`
on a hello world program can take minutes before the program
is actually ran.

There's a couple ways to work around this, but there's also another issue
in that actually closing the fds poses a problem in some cases with debug
builds of libdispatch. There can be a race between libdispatch going to
use the kqueue fd(s) and us closing them before the execve. Because of this,
the most sane thing to do is instead of closing we can set all of the open
fds as CLOEXEC. To do this efficiently on linux and macOS we can read /dev/fd
and /proc/self/fd respectively and only close what's actually open.

Below is the delta between two runs of `swift run` on a simple hello world
program. The shell I'm running these in has a nofile rlimit of 1 billion.
At 100 million it falls to about 20 seconds on my machine, and gets progressively
smaller until the two approaches aren't really any different at all.

With the patch:
```
Build of product 'fdwoo' complete! (0.23s)
Hello, world!

real    0m0.925s
user    0m0.698s
sys     0m0.129s
```

Without:
```
Build of product 'closerange' complete! (0.15s)
Hello, world!

real	2m43.203s
user	0m47.357s
sys	1m55.344s
```

Signed-off-by: Danny Canter <[email protected]>

Author: dcantah

Sources/Commands/SwiftRunCommand.swift

@@ -331,36 +331,58 @@ public struct SwiftRunCommand: AsyncSwiftCommand {
         #if !os(Windows)
         // Dispatch will disable almost all asynchronous signals on its worker threads, and this is called from `async`
         // context. To correctly `exec` a freshly built binary, we will need to:
-        // 1. reset the signal masks
+        // 1. Reset the signal masks
         for i in 1..<NSIG {
             signal(i, SIG_DFL)
         }
         var sig_set_all = sigset_t()
         sigfillset(&sig_set_all)
         sigprocmask(SIG_UNBLOCK, &sig_set_all, nil)
 
+        // 2. Close or set CLOEXEC on all fds above stdio.
+        //
+        // On platforms that have pthread_suspend_all_np, lets suspend all threads before
+        // closing fds above stderr. This is mostly to handle a race that can occur with debug
+        // builds of libdispatch, where we could be closing kqueue fds right before the exec.
+        // pthread_suspend_all_np is not portable however, and is really only available readily
+        // to call on FreeBSD.
         #if os(FreeBSD) || os(OpenBSD)
         #if os(FreeBSD)
         pthread_suspend_all_np()
         #endif
         closefrom(3)
         #else
-        #if os(Android)
-        let number_fds = Int32(sysconf(_SC_OPEN_MAX))
-        #else
-        let number_fds = getdtablesize()
-        #endif /* os(Android) */
-        
-        // 2. close all file descriptors.
-        for i in 3..<number_fds {
-            close(i)
-        }
+        // On other platforms lets do CLOEXEC and try /dev/fd and /proc/self/fd, and
+        // fall back to sysconf if those fail.
+        cloexecFDsFrom(3)
         #endif /* os(FreeBSD) || os(OpenBSD) */
-        #endif
+        #endif /* !os(Windows) */
 
+        // All of the swift run modes expect to replace the process itself, with no fork in between.
         try TSCBasic.exec(path: path, args: args)
     }
 
+    #if !os(Windows)
+    private func cloexecFDsFrom(_ from: Int32) {
+        let fdDirs = ["/proc/self/fd", "/dev/fd"]
+        for fdDir in fdDirs {
+            if let fds = try? FileManager.default.contentsOfDirectory(atPath: fdDir) {
+                for fdStr in fds {
+                    if let fd = Int32(fdStr), fd >= from {
+                        _ = fcntl(fd, F_SETFD, FD_CLOEXEC)
+                    }
+                }
+                return
+            }
+        }
+
+        let numFDs = sysconf(Int32(_SC_OPEN_MAX))
+        for fd in from..<Int32(numFDs) {
+            _ = fcntl(fd, F_SETFD, FD_CLOEXEC)
+        }
+    }
+    #endif
+
     public init() {}
 }
 
@@ -369,4 +391,3 @@ private extension Basics.Diagnostic {
         .warning("'swift run file.swift' command to interpret swift files is deprecated; use 'swift file.swift' instead")
     }
 }
-