Avoid process deadlock in popen by consuming input pipessh-fix-stderr-pipe-consumption

A process that spews a lot of output to stderr or stdout could stall out
due to the pipe buffer being full. As described in https://bugs.ruby-lang.org/issues/9082,
we can use the trick used in Ruby's capture3 function to read the pipes in separate
threads.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/6895

2 files changed, 20 insertions, 7 deletions

diff --git a/lib/gitlab/git/popen.rb b/lib/gitlab/git/popen.rb
index f9f24ecc48d..7426688fc55 100644
--- a/lib/gitlab/git/popen.rb
+++ b/lib/gitlab/git/popen.rb
@@ -21,6 +21,10 @@ module Gitlab
         Open3.popen3(vars, *cmd, options) do |stdin, stdout, stderr, wait_thr|
           stdout.set_encoding(Encoding::ASCII_8BIT)
 
+          # stderr and stdout pipes can block if stderr/stdout aren't drained: https://bugs.ruby-lang.org/issues/9082
+          # Mimic what Ruby does with capture3: https://github.com/ruby/ruby/blob/1ec544695fa02d714180ef9c34e755027b6a2103/lib/open3.rb#L257-L273
+          err_reader = Thread.new { stderr.read }
+
           yield(stdin) if block_given?
           stdin.close
 
@@ -32,7 +36,7 @@ module Gitlab
             cmd_output << stdout.read
           end
 
-          cmd_output << stderr.read
+          cmd_output << err_reader.value
           cmd_status = wait_thr.value.exitstatus
         end
 
@@ -55,16 +59,20 @@ module Gitlab
         rerr, werr = IO.pipe
 
         pid = Process.spawn(vars, *cmd, out: wout, err: werr, chdir: path, pgroup: true)
+        # stderr and stdout pipes can block if stderr/stdout aren't drained: https://bugs.ruby-lang.org/issues/9082
+        # Mimic what Ruby does with capture3: https://github.com/ruby/ruby/blob/1ec544695fa02d714180ef9c34e755027b6a2103/lib/open3.rb#L257-L273
+        out_reader = Thread.new { rout.read }
+        err_reader = Thread.new { rerr.read }
 
         begin
-          status = process_wait_with_timeout(pid, timeout)
-
           # close write ends so we could read them
           wout.close
           werr.close
 
-          cmd_output = rout.readlines.join
-          cmd_output << rerr.readlines.join # Copying the behaviour of `popen` which merges stderr into output
+          status = process_wait_with_timeout(pid, timeout)
+
+          cmd_output = out_reader.value
+          cmd_output << err_reader.value # Copying the behaviour of `popen` which merges stderr into output
 
           [cmd_output, status.exitstatus]
         rescue Timeout::Error => e
diff --git a/lib/gitlab/popen.rb b/lib/gitlab/popen.rb
index b9832a724c4..d0cb7c1a7cf 100644
--- a/lib/gitlab/popen.rb
+++ b/lib/gitlab/popen.rb
@@ -34,11 +34,16 @@ module Gitlab
       start = Time.now
 
       Open3.popen3(vars, *cmd, options) do |stdin, stdout, stderr, wait_thr|
+        # stderr and stdout pipes can block if stderr/stdout aren't drained: https://bugs.ruby-lang.org/issues/9082
+        # Mimic what Ruby does with capture3: https://github.com/ruby/ruby/blob/1ec544695fa02d714180ef9c34e755027b6a2103/lib/open3.rb#L257-L273
+        out_reader = Thread.new { stdout.read }
+        err_reader = Thread.new { stderr.read }
+
         yield(stdin) if block_given?
         stdin.close
 
-        cmd_stdout = stdout.read
-        cmd_stderr = stderr.read
+        cmd_stdout = out_reader.value
+        cmd_stderr = err_reader.value
         cmd_status = wait_thr.value
       end