Reopen standard handles when they are a TTY (#6518)

* Reopen standard handles when they are a TTY

Reopening the TTY prevents crystal from breaking other programs that are
attached to the same TTY when it sets O_NONBLOCK on its std filehandles.

Due to this change, when exec'ing a new process crystal no longer
unexpectedly sets its own std handles back to 'blocking'.

* Include the std_fd where needed

* Actually add the stdfilehandle file...

* Initialize the static array

* Don't set std handles to nonblocking after fork

Even setting NONBLOCK for a split second is dangerous.
From what I can see the child's stdin/out/err is supposed to be
blocking at all times, so just reopen the original FD's in
blocking mode.

There is still a problem with handling closed filehandles that needs
to be solved. `crystal_program | fast_exiting_child` crashes.. but this
is apparently not a regression.

* Move tty handler logic back to IO::FD

* Undocument `IO::FD.from_stdio` method

* Remove unneeded filehandle openness check

src/crystal/main.cr

@@ -4,10 +4,6 @@ lib LibCrystalMain
 end
 
 module Crystal
-  @@stdin_is_blocking = false
-  @@stdout_is_blocking = false
-  @@stderr_is_blocking = false
-
   # Defines the main routine run by normal Crystal programs:
   #
   # - Initializes the GC
@@ -38,8 +34,6 @@ module Crystal
   def self.main(&block)
     GC.init
 
-    remember_blocking_state
-
     status =
       begin
         yield
@@ -53,7 +47,6 @@ module Crystal
     status = AtExitHandlers.run status
     STDOUT.flush
     STDERR.flush
-    restore_blocking_state
 
     status
   end
@@ -103,26 +96,6 @@ module Crystal
   def self.main_user_code(argc : Int32, argv : UInt8**)
     LibCrystalMain.__crystal_main(argc, argv)
   end
-
-  # :nodoc:
-  def self.remember_blocking_state
-    {% if flag?(:win32) %}
-      @@stdin_is_blocking = true
-      @@stdout_is_blocking = true
-      @@stderr_is_blocking = true
-    {% else %}
-      @@stdin_is_blocking = IO::FileDescriptor.fcntl(0, LibC::F_GETFL) & LibC::O_NONBLOCK == 0
-      @@stdout_is_blocking = IO::FileDescriptor.fcntl(1, LibC::F_GETFL) & LibC::O_NONBLOCK == 0
-      @@stderr_is_blocking = IO::FileDescriptor.fcntl(2, LibC::F_GETFL) & LibC::O_NONBLOCK == 0
-    {% end %}
-  end
-
-  # :nodoc:
-  def self.restore_blocking_state
-    STDIN.blocking = @@stdin_is_blocking
-    STDOUT.blocking = @@stdout_is_blocking
-    STDERR.blocking = @@stderr_is_blocking
-  end
 end
 
 # Main function that acts as C's main function.

src/io/file_descriptor.cr

@@ -18,6 +18,23 @@ class IO::FileDescriptor < IO
     end
   end
 
+  # :nodoc:
+  def self.from_stdio(fd)
+    # If we have a TTY for stdin/out/err, it is a shared terminal.
+    # We need to reopen it to use O_NONBLOCK without causing other programs to break
+
+    # Figure out the terminal TTY name. If ttyname fails we have a non-tty, or something strange.
+    path = uninitialized UInt8[256]
+    if LibC.ttyname_r(fd, path, 256) == 0
+      # Open a fresh handle to the TTY
+      fd = LibC.open(path, LibC::O_RDWR)
+
+      new(fd).tap(&.close_on_exec = true)
+    else
+      new(fd, blocking: true)
+    end
+  end
+
   def blocking
     system_blocking?
   end

src/kernel.cr

@@ -1,13 +1,13 @@
 {% if flag?(:win32) %}
   STDIN  = IO::FileDescriptor.new(0)
-  STDOUT = (IO::FileDescriptor.new(1)).tap { |f| f.flush_on_newline = true }
-  STDERR = (IO::FileDescriptor.new(2)).tap { |f| f.flush_on_newline = true }
+  STDOUT = IO::FileDescriptor.new(1).tap { |f| f.flush_on_newline = true }
+  STDERR = IO::FileDescriptor.new(2).tap { |f| f.flush_on_newline = true }
 {% else %}
   require "c/unistd"
 
-  STDIN  = IO::FileDescriptor.new(0, blocking: LibC.isatty(0) == 0)
-  STDOUT = (IO::FileDescriptor.new(1, blocking: LibC.isatty(1) == 0)).tap { |f| f.flush_on_newline = true }
-  STDERR = (IO::FileDescriptor.new(2, blocking: LibC.isatty(2) == 0)).tap { |f| f.flush_on_newline = true }
+  STDIN  = IO::FileDescriptor.from_stdio(0)
+  STDOUT = IO::FileDescriptor.from_stdio(1).tap { |f| f.flush_on_newline = true }
+  STDERR = IO::FileDescriptor.from_stdio(2).tap { |f| f.flush_on_newline = true }
 {% end %}
 
 PROGRAM_NAME = String.new(ARGV_UNSAFE.value)
@@ -445,7 +445,6 @@ def exit(status = 0) : NoReturn
   status = AtExitHandlers.run status
   STDOUT.flush
   STDERR.flush
-  Crystal.restore_blocking_state
   Process.exit(status)
 end
 

src/lib_c/aarch64-linux-gnu/c/unistd.cr

@@ -26,6 +26,7 @@ lib LibC
   fun getpid : PidT
   fun getppid : PidT
   fun isatty(fd : Int) : Int
+  fun ttyname_r(fd : Int, buf : Char*, buffersize : SizeT) : Int
   fun lchown(file : Char*, owner : UidT, group : GidT) : Int
   fun link(from : Char*, to : Char*) : Int
   fun lockf(fd : Int, cmd : Int, len : OffT) : Int

src/lib_c/aarch64-linux-musl/c/unistd.cr

@@ -26,6 +26,7 @@ lib LibC
   fun getpid : PidT
   fun getppid : PidT
   fun isatty(x0 : Int) : Int
+  fun ttyname_r(fd : Int, buf : Char*, buffersize : SizeT) : Int
   fun lchown(x0 : Char*, x1 : UidT, x2 : GidT) : Int
   fun link(x0 : Char*, x1 : Char*) : Int
   fun lockf(x0 : Int, x1 : Int, x2 : OffT) : Int

src/lib_c/amd64-unknown-openbsd/c/unistd.cr

@@ -25,6 +25,7 @@ lib LibC
   fun getpid : PidT
   fun getppid : PidT
   fun isatty(x0 : Int) : Int
+  fun ttyname_r(fd : Int, buf : Char*, buffersize : SizeT) : Int
   fun lchown(x0 : Char*, x1 : UidT, x2 : GidT) : Int
   fun link(x0 : Char*, x1 : Char*) : Int
   fun lockf(x0 : Int, x1 : Int, x2 : OffT) : Int

src/lib_c/arm-linux-gnueabihf/c/unistd.cr

@@ -26,6 +26,7 @@ lib LibC
   fun getpid : PidT
   fun getppid : PidT
   fun isatty(fd : Int) : Int
+  fun ttyname_r(fd : Int, buf : Char*, buffersize : SizeT) : Int
   fun lchown(file : Char*, owner : UidT, group : GidT) : Int
   fun link(from : Char*, to : Char*) : Int
   fun lockf(fd : Int, cmd : Int, len : OffT) : Int

src/lib_c/i686-linux-gnu/c/unistd.cr

@@ -26,6 +26,7 @@ lib LibC
   fun getpid : PidT
   fun getppid : PidT
   fun isatty(fd : Int) : Int
+  fun ttyname_r(fd : Int, buf : Char*, buffersize : SizeT) : Int
   fun lchown(file : Char*, owner : UidT, group : GidT) : Int
   fun link(from : Char*, to : Char*) : Int
   fun lockf(fd : Int, cmd : Int, len : OffT) : Int

src/lib_c/i686-linux-musl/c/unistd.cr

@@ -26,6 +26,7 @@ lib LibC
   fun getpid : PidT
   fun getppid : PidT
   fun isatty(x0 : Int) : Int
+  fun ttyname_r(fd : Int, buf : Char*, buffersize : SizeT) : Int
   fun lchown(x0 : Char*, x1 : UidT, x2 : GidT) : Int
   fun link(x0 : Char*, x1 : Char*) : Int
   fun lockf(x0 : Int, x1 : Int, x2 : OffT) : Int

src/lib_c/x86_64-linux-gnu/c/unistd.cr

@@ -26,6 +26,7 @@ lib LibC
   fun getpid : PidT
   fun getppid : PidT
   fun isatty(fd : Int) : Int
+  fun ttyname_r(fd : Int, buf : Char*, buffersize : SizeT) : Int
   fun lchown(file : Char*, owner : UidT, group : GidT) : Int
   fun link(from : Char*, to : Char*) : Int
   fun lockf(fd : Int, cmd : Int, len : OffT) : Int

src/lib_c/x86_64-linux-musl/c/unistd.cr

@@ -26,6 +26,7 @@ lib LibC
   fun getpid : PidT
   fun getppid : PidT
   fun isatty(x0 : Int) : Int
+  fun ttyname_r(fd : Int, buf : Char*, buffersize : SizeT) : Int
   fun lchown(x0 : Char*, x1 : UidT, x2 : GidT) : Int
   fun link(x0 : Char*, x1 : Char*) : Int
   fun lockf(x0 : Int, x1 : Int, x2 : OffT) : Int

src/lib_c/x86_64-macosx-darwin/c/unistd.cr

@@ -26,6 +26,7 @@ lib LibC
   fun getpid : PidT
   fun getppid : PidT
   fun isatty(x0 : Int) : Int
+  fun ttyname_r(fd : Int, buf : Char*, buffersize : SizeT) : Int
   fun lchown(x0 : Char*, x1 : UidT, x2 : GidT) : Int
   fun link(x0 : Char*, x1 : Char*) : Int
   fun lockf(x0 : Int, x1 : Int, x2 : OffT) : Int

src/lib_c/x86_64-portbld-freebsd/c/unistd.cr

@@ -25,6 +25,7 @@ lib LibC
   fun getpid : PidT
   fun getppid : PidT
   fun isatty(x0 : Int) : Int
+  fun ttyname_r(fd : Int, buf : Char*, buffersize : SizeT) : Int
   fun lchown(x0 : Char*, x1 : UidT, x2 : GidT) : Int
   fun link(x0 : Char*, x1 : Char*) : Int
   fun lockf(x0 : Int, x1 : Int, x2 : OffT) : Int

src/process.cr

@@ -391,9 +391,10 @@ class Process
 
   # :nodoc:
   protected def self.exec_internal(command : String, argv, env, clear_env, input, output, error, chdir)
-    reopen_io(input, STDIN, "r")
-    reopen_io(output, STDOUT, "w")
-    reopen_io(error, STDERR, "w")
+    # Reopen handles if the child is being redirected
+    reopen_io(input, IO::FileDescriptor.new(0, blocking: true), "r")
+    reopen_io(output, IO::FileDescriptor.new(1, blocking: true), "w")
+    reopen_io(error, IO::FileDescriptor.new(2, blocking: true), "w")
 
     ENV.clear if clear_env
     env.try &.each do |key, val|

src/signal.cr

@@ -200,8 +200,6 @@ module Crystal::Signal
   end
 
   private def self.fatal(message : String)
-    Crystal.restore_blocking_state
-
     STDERR.puts("FATAL: #{message}, exiting")
     STDERR.flush
     LibC._exit(1)
@@ -269,8 +267,6 @@ end
 
 # :nodoc:
 fun __crystal_sigfault_handler(sig : LibC::Int, addr : Void*)
-  Crystal.restore_blocking_state
-
   # Capture fault signals (SEGV, BUS) and finish the process printing a backtrace first
   LibC.dprintf 2, "Invalid memory access (signal %d) at address 0x%lx\n", sig, addr
   CallStack.print_backtrace