Robust STDIN pass-through to sub-process.

My previous attempt at preventing explosive thread growth didn't work because I couldn't reliably interrupt a .gets call.  The timeout in the thread did cause the thread to begin aborting, but  would still be blocked.  Starting multiple such threads meant any user input could end up being consumed by any one of those threads.  Moving towards a single thread for getting user input and communicating to a new pass-through thread via a Queue avoids those problems and allows us to stop the pass-through threads when they're no longer needed.

Author: nirvdrum

lib/svn2git/migration.rb

@@ -342,7 +342,15 @@ def run_command(cmd, exit_on_error=true, printout_output=false)
       log "Running command: #{cmd}"
 
       ret = ''
-      mutex = Mutex.new
+      @mutex ||= Mutex.new
+      @stdin_queue ||= Queue.new
+
+      # We need to fetch input from the user to pass through to the underlying sub-process.  We'll constantly listen
+      # for input and place any received values on a queue for consumption by a pass-through thread that will forward
+      # the contents to the underlying sub-process's stdin pipe.
+      @stdin_thread ||= Thread.new do
+        loop { @stdin_queue << $stdin.gets.chomp }
+      end
 
       # Open4 forks, which JRuby doesn't support.  But JRuby added a popen4-compatible method on the IO class,
       # so we can use that instead.
@@ -351,7 +359,7 @@ def run_command(cmd, exit_on_error=true, printout_output=false)
 
         threads << Thread.new(stdout) do |stdout|
           stdout.each do |line|
-            mutex.synchronize do
+            @mutex.synchronize do
               ret << line
 
               if printout_output
@@ -364,8 +372,12 @@ def run_command(cmd, exit_on_error=true, printout_output=false)
         end
 
         threads << Thread.new(stderr) do |stderr|
+          # git-svn seems to do all of its prompting for user input via STDERR.  When it prompts for input, it will
+          # not terminate the line with a newline character, so we can't split the input up by newline.  It will,
+          # however, use a space to separate the user input from the prompt.  So we split on word boundaries here
+          # while draining STDERR.
           stderr.each(' ') do |word|
-            mutex.synchronize do
+            @mutex.synchronize do
               ret << word
 
               if printout_output
@@ -377,15 +389,24 @@ def run_command(cmd, exit_on_error=true, printout_output=false)
           end
         end
 
+        # Simple pass-through thread to take anything the user types via STDIN and passes it through to the
+        # sub-process's stdin pipe.
         Thread.new(stdin) do |stdin|
-          until $stdin.closed?
-            user_reply = Timeout.timeout(5) { $stdin.gets.chomp }
+          loop do
+            user_reply = @stdin_queue.pop
+
+            # nil is our cue to stop looping (pun intended).
+            break if user_reply.nil?
+
             stdin.puts user_reply
             stdin.close
           end
         end
 
         threads.each(&:join)
+
+        # Push nil to the stdin_queue to gracefully exit the STDIN pass-through thread.
+        @stdin_queue << nil
       end
 
       # JRuby's open4 doesn't return a Process::Status object when invoked with a block, but rather returns the